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Properties of Lanthanide Hydroxide Molecules Produced in Reactions of Lanthanide Atoms with HO and H + O Mixtures: Roles of the +I, +II, +III and +IV Oxidation States 2

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Xuefeng Wang, Lester Andrews, Zongtang Fang, Kanchana Sahan Thanthiriwatte, Mingyang Chen, and David A Dixon J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b12607 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on February 15, 2017

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The Journal of Physical Chemistry

Properties of Lanthanide Hydroxide Molecules Produced in Reactions of Lanthanide Atoms with H2O2 and H2 + O2 Mixtures: Roles of the +I, +II, +III and +IV Oxidation States Xuefeng Wang,a,b Lester Andrews,a,*,† Zongtang Fang,c K. Sahan Thanthiriwatte,c Mingyang Chen,c and David A. Dixonc*,† a

Department of Chemistry, Box 400319, University of Virginia, Charlottesville, Virginia 22904-

4319 USA; bChemistry Department, Tongji University, Shanghai 200093, China; c

Chemistry Department, University of Alabama, Tuscaloosa Alabama 34487-0336, USA

Abstract The reactions of laser-ablated lanthanide metal atoms with hydrogen peroxide or hydrogen plus oxygen mixtures have been studied experimentally in a solid argon matrix and theoretically with the ab initio MP2 and CCSD(T) methods. The Ln(OH)3 and Ln(OH)2 molecules and Ln(OH)2+ cations are the major products and the reactions to form those hydroxides are predicted to be highly exothermic at the CCSD(T) level. Vibronic interactions are hypothesized to contribute to the abnormalities in deuterium shifts for Ln-OH(D) stretching modes for several hydroxides, consistent with CASSCF calculations. Additional new absorptions were assigned as HLnO or LnOH and OLnOH molecules. The tetrahydroxides of Ce, Pr, and Tb have also been observed. These reactive intermediates were identified from their matrix infrared spectra by using D2O2, HD, D2, 16,18O2 and

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O2 isotopic substitution, by matching observed

frequencies with values calculated by electronic structure methods, and by following the trends observed in frequencies going through different lanthanide metal hydroxide series across the Periodic Table. The lanthanides are in the +II oxidation state for Ln(OH)2 and are in the +III oxidation state for Ln(OH)3 and Ln(OH)2+.

†

Email: [email protected]; [email protected]

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Introduction Lanthanide (Ln) metal chemistry is dominated by the trivalent state.

Oxides of

stoichiometry Ln2O3 are formed in the solid state, and these hydrolyze to produce distinct trihydroxide Ln(OH)3 compounds. 1 The simple molecular oxides LnO and LnO2 are major products in the reaction of laser ablated lanthanide metal atoms with oxygen molecules, 2,3 and molecular lanthanide hydrides with LnH1,2,3,4 examples have been reported in the analogous reactions. 4,5,6 Subsequent investigations of lanthanum and rare earth metal reactions with water molecules under matrix isolation conditions reported HLnOH insertion products upon red visible photolysis of the metal-water complex, and evidence was presented for Ln(OH)2 molecules upon photodissociation of Ln(H2O)2 complexes. 7,8,9 Reactions with methanol produced the analogous CH3OLn-H molecules. 10 Oxygen difluoride reactions with the lanthanide series have produced the oxidative addition products OLnF2 and OLnF where the latter is more ionic with greater formal negative charge on the oxygen center.

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Lanthanide metal atoms also undergo

chemiionization reactions with oxygen atoms and molecules in the gas phase, which have been investigated by chemielectron spectroscopy. 12 Structures and properties of the reaction products of lanthanide metal atoms with H2O have been investigated theoretically at the density functional level including NBO analysis, and comparisons with lanthanide metal and OF2 reaction products have been made.11,13 In our prior work on the reactions of Ln with H2O,13 density functional theory calculations at the B3LYP level showed that the most exothermic reactions for most of the Ln with H2O are formation of HLnOH rather than H2LnO, HLnO + H and LnOH + H, showing the presence of the +II oxidation state. In addition, CCSD(T) and CASPT2 (complete active space-MP2) results were reported13 for selected HLnOH to obtain agreement with the experimental frequencies. There is substantial interest in the formation of novel oxidation states

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of the lanthanides, notably through the work of Evans and co-workers for complexes of Ln in the +II formal oxidation state in contrast to the usual +III oxidation state. 14,15,16 In addition, Riedel and coworkers 17 found two new +IV oxidation state molecules 3NdF4 and 7DyF4, in addition to the known tetrafluorides of Ce, Pr and Tb in the reactions of F2 with Ln under matrix isolation conditions. Another source of oxidative addition products is the reaction with hydrogen peroxide molecules, which has become straightforward through the use of a urea-hydrogen peroxide complex as a source of H2O2 molecules at room temperature. 18,19,20,21,22,23,24 (In reference 21, the captions for Figures 8 and 9 should state La instead of Y.) Owing to the relatively weak O-O bond (50 kcal/mol) in H2O2, 25 metal atom reactions to form M(OH)2 are highly exothermic. Lanthanum provides a starting point for the lanthanide family of metals, and its major reaction products are the La(OH)2,3 molecules and La(OH)2+ cations. In addition, Ce, the next atom in the period, as well as Ti, Zr, Hf , and Th metal atoms also form tetrahydroxide molecules. 26,27,28 In our prior work,27 the observed major species for the reaction of Ce with H2O2 or H2 and O2 in an Ar matrix are Ce(OH)3 and Ce(OH)2, and Ce(OH)2+ in +III or +II oxidation states and the minor product, Ce(OH)4 in the +IV oxidation state and additional minor products HCeO and OCeOH in the formal +III oxidation state. Experimental and Computational Methods Laser-ablated lanthanide metal atoms were co-deposited with H2O2 molecules diluted in argon during condensation onto a 4 K cesium iodide window.19,20,21,22,23,24,26,27,28,29 Because it is more difficult to trap small reactive species in Ne than in Ar because of the much lower freezing temperature for Ne particularly when adding the energy from laser ablation, we have chosen to study all of the Ln metals (excluding radioactive Pm) in argon only. A hydrogen peroxide

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complex with urea (Aldrich) at room temperature behind a Chemglass Pyrex-Teflon valve evaporated H2O2 molecules into the flowing argon (4 mmol in 1 h) deposition stream directed at the cold window. The H2O2 concentration cannot be determined, but our infrared spectra exhibit mostly monomeric H2O2 based on comparison with spectra from the Helsinki laboratory,18 and for this degree of matrix isolation, argon flow, and peroxide complex temperature, the concentration of H2O2 is estimated to be less than or on the order of 0.2 %. Deuterium substituted urea-D2O2 was prepared through exchanging the urea-H2O2 complex with D2O as described previously.19,21,24 Matrix infrared spectra of the enriched vapor showed approximately 90% D2O2 and 10% HDO2 in these samples by comparing O-O-D bending mode absorptions of HDO2 (981 cm-1) and D2O2 (952 cm-1), and the virtual absence of H2O2 (1275 cm1 18

).

Matrix infrared spectra were recorded on a Nicolet 750 spectrometer after sample

deposition, after annealing, and after irradiation using a mercury arc street lamp for 15-20 min periods. The observed peaks shift slightly (0.3 cm-1) during this treatment, but they are reproduced within 0.2 cm-1 accuracy on deposition, first annealing and first photolysis. Parallel experiments employed hydrogen/oxygen mixtures in order to incorporate

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O into the product

molecules, and the frequencies from H2 and O2 mixtures were within 1 cm-1 of the H2O2 values.19,20,21,22,23,24 The initial structures and vibrational frequencies of lanthanide M(OH)2 and M(OH)3 molecules and the M(OH)2+ molecular cation and additional minor products were calculated using the B3LYP hybrid functional 30,31 with the DZVP2 basis set 32 for H and O and the Stuttgart small core relativistic effective core potential (ECP) with its accompanying segmented basis set for the lanthanides 33,34 using Gaussian 09. 35Atomic orbital occupancies were determined using NBO6 36, 37 for the natural bond orbital (NBO) 38, 39, 40, 41 population analysis at the DFT level.

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Subsequent geometry optimizations and frequency calculations starting from the DFT geometries were done at the UMP2 level 42,43 with the same basis set and ECP used in the DFT calculations for the Ln and the aug-cc-pVDZ basis set for O and H. 44 The MP2 level provided better agreement with the experimental frequencies for LnF3 and LnF4 than did DFT/B3LYP.17 Only the valence electrons were correlated. The NBO population analysis was done with the geometries optimized at the MP2 level and the Hartree-Fock density. Reaction energies were calculated at the B3LYP, MP2 and at the coupled cluster R/UCCSD(T)

45,46,47,48,49,50,51

levels

with the same basis set used in the MP2 calculations. In the R/UCCSD(T) approach, a restricted open shell Hartree-Fock (ROHF) calculation is initially performed and the spin constraint was then relaxed in the coupled cluster calculation. The CCSD(T) calculations were done with MOLPRO. 52,53 Results and Discussion Infrared spectra for laser ablated Ln metal reactions with H2O2 and with D2O2 are discussed first followed by photochemical reactions of Ln metal atoms with H2 and O2 and their isotopic mixtures. Frequencies observed for new product molecules are then compared with frequencies from electronic structure calculations. Frequencies are grouped by product going across the lanthanide row in the Periodic Table. We did not study the reactions of Pm due to its radioactivity. We summarize the general findings and then examine the individual Ln in more detail. Argon matrix spectra of Ln atom reaction products with H2O2 and D2O2. Figures 1 and 2 summarize spectra in the terminal O-H and O-D stretching regions for the major reaction products of La and the Ce-Lu lanthanide series with H2O2, La with D2 + O2, and Ce-Lu with D2O2. Infrared spectra for the lanthanide metal atom reaction products in the corresponding Ln-

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OH and Ln-OD stretching regions are shown in Figures 3 and 4. Tables 1 and 2 collect the observed frequencies for the major products, and Table 3 does likewise for the minor products. Figure 5 illustrates the product Ln=O stretching region for these spectra. The most striking feature of the spectra in Figures 1 and 2 is the almost linear relationship of the highest frequency product absorptions (dark arrows for Ln(OH)3 and Ln(OD)3) and the lowest series of absorption pairs (double arrows for Ln(OH)2+ and Ln(OD)2+ ) on going across the lanthanide series; a small (~ 5 cm-1) deviation is observed for Pr and Nd will be discussed below. The highest frequency product absorptions for La (3740.5 cm-1) and Ce (3742.7cm-1) have already been identified as the antisymmetric O-H stretching modes for the trihydroxides La(OH)3 and Ce(OH)3, and their other major product absorptions in Table 1 have also been assigned previously.20,27 The dihydroxide cations were the only products to exhibit resolved anti- and symmetric O-H and O-D stretching modes (3-5 cm-1, double arrows). The two O-H stretching frequencies for Ln(OH)2 (open arrows) were not resolved. This middle absorbing series showed more deviations from a linear plot than the other two, and these assignments were sometimes more difficult to make. UV irradiation of these samples increased the Ln(OH)2,3 and Ln(OH)2+ product absorptions, which helped in the identification of these major products. The Ln-OH and Ln-OD stretching modes for the dihydroxides were strongest in intensity and lowest in frequency for the Sm, Eu and Yb species (Figures 3, 4); Sm, Eu and Yb are known for the stability of metal +II oxidation state species.1 Notice that the dihydroxide cation product (+III oxidation state) absorptions have the highest frequency in this region, and they exhibit a steadily increasing trend going across the lanthanide metal row. Following these periodic trends and correlations with the computed frequencies assures that the assignments are made correctly. The antisymmetric Ln-OH and Ln-OD stretching modes for the trihydroxides (dark arrows) also

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Figure 1. Infrared spectra in the O-H stretching region for laser ablated La and Ln metal and hydrogen peroxide reaction products condensed in excess argon at 4 K. Spectra recorded after deposition, annealing, and λ > 220 nm photolysis. UV irradiation increases these major product absorptions. The dark arrowheads on the left denote the highest frequency Ln(OH)3 molecules, the middle series with open arrowheads indicate Ln(OH)2 products, and the double arrows on the right mark absorptions for the Ln(OH)2+ cations. Open squares indicate the tetrahydroxide. W identifies water, and C denotes the H2O-O complex formed by in situ photodecomposition of hydrogen peroxide, Ref.54.

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Figure 2. Infrared spectra in the O-D stretching region for laser ablated La and D2 + O2 or Ln and D2O2 reaction products condensed in excess argon at 4 K. Spectra recorded after deposition, annealing, and λ > 220 nm photolysis for Ln metals and La. The dark arrowheads on the left denote the highest frequency Ln(OD)3 molecules, the middle series with open arrowheads indicate Ln(OD)2 products, and the double arrows on the right mark absorptions for the Ln(OD)2+ cations. The open square denotes a tetrahydroxide. C denotes the D2O-O complex formed by in situ photodecomposition of hydrogen peroxide, Ref.54.

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Figure 3. Infrared spectra in the Ln-OH stretching region for laser ablated Ln metal and hydrogen peroxide reaction products condensed in excess argon at 4 K. Spectra recorded after deposition and full arc photolysis. UV irradiation increases these major product absorptions. The double arrowheads on the left denote the highest frequency Ln(OH)2+ cations, the open squares denote tetrahydroxides, the open arrowheads indicate Ln(OH)2 products, vertical lines show LnOH species, and the dark arrows in the middle mark absorptions for the Ln(OH)3 molecules. The vertical marks represent LnOH molecules, and the circles containing a minus sign label the Tm(OH)2− anion.

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Figure 4. Infrared spectra in the Ln-OD stretching region for laser ablated Ln metal and deuterium peroxide reaction products condensed in excess argon at 4 K. Spectra recorded after deposition and full arc photolysis. UV irradiation increases these major product absorptions. The double arrowheads on the left denote the highest frequency Ln(OD)2+ cations, the open squares denote tetrahydroxides, the open arrowheads indicate Ln(OD)2 product bands, vertical marks show LnOD species, and the dark arrows in the middle mark absorptions for the Ln(OD)3 molecules. The circles containing a minus sign denote Ln(OH)2− anions.

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Figure 5. Infrared spectra in the Ln=O stretching region for laser ablated lanthanide metal atom and H2O2 reaction products condensed in excess argon at 4 K. Spectra recorded after sample deposition for 40 to 60 min, annealing to 20 K and λ > 220 nm irradiation for 15 to 20 min. The hp indicates H2O2, * denotes HLnO, and the bold arrows mark OLnOH species.

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Table 1. Frequencies (cm-1) Observed in the O-H and O-D Stretching Regions for Lanthanide Hydroxide Species in Solid Argon and their H/D Frequency Ratios. metal La

Ln(OH)3 3740.6

Ln(OD)3 2758.7

H/D ratio 1.3560

Ln(OH)2 3730.3

Ln(OD)2 2753.3

H/D ratio 1.3548

Ln(OH)2+ 3691.0, 3693.4

Ln(OD)2+ 2722.0, 2725.7

H/D ratio 1.3560

Ce

3742.7

2760.5

1.3560

3719.9

2744.0

1.3556

3691.0, 3693.7

2722.3, 2726.1

1.3558

Pr

3745.5

2762.5

1.3558

3722.7

2746.9

1.3552

3691.3, 3694.1

2722.6, 2726.7

1.3558

Nd

3750.7

2770.2

1.3539

3739.3

2766.3

1.3517

3696.1, 3699.0

2726.0, 2730.2

1.3540

Sm

3759.3

2773.2

1.3556

3752.7

2769.6

1.3550

3704.4, 3707.6

2732.6, 2737.1

1.3556

Eu

3763.7

2776.8

1.3554

3751.0

2770.2

1.3541

3713.6, 3716.1

2739.8, 2739.1

1.3554

Gd

3770.2

2781.8

1.3553

3764.7

2776.9

1.3557

3719.2, 3722.2

2744.3, 2748.3

1.3552

Tb

3774.1

2784.7

1.3553

3758.9

2774.5

1.3548

3723.5, 3726.4

2747.7, 2751.9

1.3551

Dy

3778.1

2787.9

1.3552

3769.7

2770.1

1.3609

3726.5, 3729.7

2751.0, 2755.1

1.3546

Ho

3782.1

2791.0

1.3551

3771.0

2782.9

1.3549

3732.0, 3734.7

2754.1, 2758.2

1.3540

Er

3786.5

2794.5

1.3550

3772.4

2784.3

1.3549

3735.3, 3737.9

2756.9, 2760.6

1.3549

Tm

3791.3

2798.2

1.3549

3777.0

2787.6

1.3549

3740.7, 3743.4

2761.0, 2766.6

1.3549

Yb

3795.6

2801.6

1.3548

3768.9

2780.4

1.3554

3745.3, 3748.0

2766.5, 2768.8

1.3538

Lu

3798.1

2803.6

1.3547

3778.5

2790.5

1.3541

3750.1, 3752.7

2767.9, 2771.5

1.3548

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Table 2. Frequencies (cm-1) Observed in the Ln-OH and Ln-OD Stretching Regions for Lanthanide Hydroxide Species in Solid Argon.

a

metal

Ln(OH)3

Ln(OD)3

Ln(OH)2

Ln(OD)2

Ln(OH)2+

Ln(OD)2+

La

480.8

471.1

491.4, 519.5

488.6, 517.3

580.5, 624.8

568.8, 610.4

Ce

510.3

496.5

518.7, 553.9

505.8, 537.9

590.7, 632.1

578.9, 617.5

Pr

484.1

478.3

507.1, 555.2

495.5, 542.1

591.4, 636.3

580.2, 622.0

Nd

518.2

508.2

493.8, n.o.

483.3, 521.6

604.4, 644.2

593.2, 628.3

Sm

531.8

521.4

461.7, 487.5

451.8, 477.8

612.4, 652.0

Eu

552.4, 527.9

525.3

461.4, 487.0

451.3, 477.6

Gd

548.6

530.0

557.6, 575.7

Tb

560.4

538.1

Ln(OH)

Ln(OD)

600.4, 637.6

505.8

494.2

615.2, 653.5

609.6, 649.3

506.3

494.4

538.9, 569.4

621.6, 627.9

609.4, 644.6

584.1

569.0

572.2, 589.6

558.7, 575.3

630.8, 664.3

618.3, 651.2

526.8

514.6

a

Dy

563.2

544.8

526.8, 542.2

513.8

642.4, 667.8

630.1, 654.6

Ho

564.3

548.6b

(592)c

568.2,602.2b,d

641.0, 671.0

629.1b, 658.2

532.7

518.7

Er

568.1

552.8e

f

571.7, 606.3e,g

644.4, 675.0

632.4, 662.0

546.2

531.4

Tm

574.4

559.5

(600)h

576.0, 610.1i

650.5, 680.2

637.9, 667.7

555.8

535.2,596.3

Yb

581.1

565.2

490.3,503.2

483.2, 495.7

651.4, 681.1

639.2, 670.0

550.1

535.3

Lu

580.5

572.3

562.2,587.3

546.0, 575.9

655.8, 679.3

643.7, 670.3

j

654.8

Additional peaks at 562.0, 583.0 cm-1. b Additional peaks at 571.7, 555.1, and at 564.2, 598.2 and at 621.3 cm-1 (Italic bands for same

metal are matrix site splittings). c Additional peaks at 532.7, 548.8 cm-1. d Additional peak at 518.7 cm-1.

e

Additional peaks at 562.4,

and at 567.9, 602.5 cm-1. f Additional peak at 546.4 cm-1. g Additional peak at 531.4 cm-1. h Additional peak at 555.1 cm-1 . I Additional peak at 535.2 cm-1. j Obscured by CO2 peak.

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Table 3. Frequencies (cm-1) Observed in the Ln-H, Ln=O, Ln-OH, and Ln-OD Stretching Regions for Lanthanide Hydroxide Species in Solid Argon.a metal HLnO

a

DLnO

LnOH

LnOD

OLn(OH)

OLn(OD)

Ce

1286.5, 796.1

925.1, 793.5

750.0, 487.2

749.5, 477.6

Pr

n.o., 778.2

n.o., 772.4

758.6, 498.4

758.2, 488.8

Nd

n.o., 777.4

n.o., 772.7

759.8

759.4

Sm

n.o.

n.o.

760.8

760.4

Eu

n. o.

n.o.

746.3

745.7

Gd

784.6

782.2

764.2, 521.8

763.9, 508.8

Tb

1309.6, 791.9 940.0, 791.5

526.8

514.6

Dy

1317.9, 799.7 n.o.

773.8

772.0

Ho

n.o., 800.3

780.3

780.3

Er

n.o., 801.3

780.4

780.4

Tm

1376.5, 806.5 986.5, 805.6

784.9

785.1

Yb

n.o.

Lu

1417.8, 808.2 1009.4, 807.5

506.3

532.7

n.o.

550.1

494.4

518.7

535.3

unidentified 691.7, 690.9 788.8, 778.2

491.9, 483.6 778.7, 778.1

785.9, n.o.

n. o. = not observed

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785.8, n.o.

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exhibit consistent periodic trends, which helped us to assign the strong 538.1 cm-1 band to Tb(OD)3 instead of the strong 597.1 cm-1 band, which will be identified below with the help of calculations for Tb(OD)4. Deuterium shifts in the antisymmetric Ln-OH and Ln-OD stretching modes for the Ln(OH)3 and Ln(OD)3 species mostly range between 10 and 22 cm-1, and these shifts are close to the differences in the computed frequencies. However, deuterium shifts for the Pr, Eu and Lu trihydroxides are smaller, and this is attributed to vibronic interactions, which red shift the Ln-OH counterparts. Electronic structure calculations at the level of density functional theory with the B3LYP exchange-correlation functional and at the correlated molecular orbital MP2 level were done for these three major products of the above Ln reactions, and comparison of calculated and observed frequencies serve to validate the computational methods (see Tables 4-6). We focus on the MP2 results below and the DFT results are given in the Supporting Information. First, the calculated Ln(OH)3 antisymmetric O-H and Ln-OH stretching frequencies increase like the observed values, and the calculated Ln-O bond lengths decrease smoothly going across the Ln row. Second, the Ln(OH)2+ cations have the lowest O-H and the highest Ln-OH stretching frequencies for both observed and calculated frequencies, and the calculated separations of the anti- and symmetric O-H and O-D stretching modes are comparable to the observed splittings of 3-5 cm-1 (Tables 15). Third, the trihydroxides have the highest O-H or O-D stretching modes for all Ln, but have the lowest Ln-OH and Ln-OD stretching frequencies for a few of the Ln (Figures 1, 2, 3, and 4). (4) The computed O-H stretching frequencies are higher for all Ln(OH)3 than for the corresponding Ln(OH)2 at the MP2 level.

15



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Table 4. Calculated Ln(OH)3 Harmonic Frequencies (cm-1) at the MP2 Level.a Ln

Sym.

O–H asym Str

O–H sym Str

Ln–O sym Str

Ln–O asym Str.

La

C3v

3916.0 (e) (210)

3918.0 (a1) (68)

560.5 (a1) (13)

514.6 (e) (614)

Ce

Cs

3993.4 (a') (88)

566.5 (a') (35)

Pr

C1

3895.6 (58)

580.2 (19)

Nd Pm

C3v D3h

3897.62 (a1) (39) 3904.6 (a'1) (0)

584.9 (a1) (23) 584.55 (a'1) (0)

Sm

Cs

3911.6 (a') (138) 3911.4 (a") (138)

3912.4 (a') (14)

572.6 (a') (8)

538.7 (a') (275) 541.1 (a") (261)

Eu

D3h

3914.6 (e') (334)

3914.3 (a'1) (0)

599.1 (a'1) (0)

562.2 (e") (564)

Gd

C3v

3917.5 (a1) (11)

603.2 (a1) (6)

Tb

C2v

3924.4 (a1) (4)

607.8 (a1) (0)

Dy

C2v

3924.5 (a1) (124)

610.2 (a1) (0)

Ho

D3h

3917.7 (e) (330) 3924.5 (a1) (166) 3924.3 (b2) (164) 3922.6 (a1) (66) 3922.7 (b2) (152) 3928.1 (e') (340)

3927.9 (a'1) (40)

617.1 (a'1) (0)

565.3 (e) (514) 573.7 (b2) (248) 536.7 (a1) (127) 580.8 (b2) (217) 581.5 (a1) (260) 587.8 (e") (472)

Er

D3h

3935.8 (e') (266)

3936.1 (a'1) (96)

621.1 (a'1) (0)

593.9 (e") (460)

Tm

D3h

3939.4 (e') (380)

3939.4 (a'1) (10)

626.6 (a'1) (0)

600.0 (e") (442)

Yb

Cs

3945.1 (a') (181) 3945.1 (a") (182)

3945.11 (a') (3)

629.4 (a') (0)

605.3 (a') (207) 605.4 (a") (205)

638.2 (a1) (0)

612.6 (a1) (204) 612.7 (b2) (204)

3900.7 (a') (101) 3900.1 (a") (106) 3897.6 (111) 3895.5 (135) 3897.3 (e) (171) 3905.0 (e') (318)

3966.7 (a1) (85) 3969.4 (a1) (85) 3967.2 (b2) (123) a Infrared Intensities in parentheses in km/mol. Lu

C2v

16


512.8 (a') (274) 513.0 (a")(292) 525.6 (303) 527.1 (297) 539.3 (e) (584) 547.28 (e") (602)

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Table 5. Calculated Ln(OH)2 Harmonic Frequencies (cm-1) at the MP2 Level.a

a

Ln

spin

O–H sym. Stretch a1

O–H asym. Str b2

Ln–O sym. Str a1

Ln–O asym. Str b2

La

2

3887.5(136)

3886.6(134)

582.5 (111)

538.4 (219)

Ce

3

3889.1(135)

3888.7(151)

587.5 (96)

546.2 (233)

Pr

4

3884.5(130)

3884.9(162)

598.7 (100)

547.0 (229)

Nd

5

3889.5(140)

3889.7(165)

593.6 (84)

540.3 (283)

Pm

6

3890.0(123)

3890.3(159)

606.5 (97)

576.0 (137)

Sm

7

3886.3 (36)

3885.9 (46)

510.1 (89)

479.4 (224)

Eu

8

3893.8 (37)

3894.3 (44)

513.6 (69)

485.5 (229)

Gd

9

3902.7(132)

3902.6(133)

613.9 (68)

573.6 (184)

Tb

8

3905.0(113)

3905.2(145)

618.1 (71)

492.5 (249)

Dy

7

3913.7 (82)

3914.0(161)

617.1 (33)

551.7 (198)

Ho

6

3914.9(102)

3916.3(145)

629.5 (42)

595.3 (162)

Er

5

3919.1(109)

3919.3(135)

633.9 (39)

604.5 (143)

Tm

4

3924.6 (96)

3925.3(148)

639.6 (42)

607.0 (149)

Yb

1

3441.2 (38)

3941.4 (66)

540.6 (44)

528.2 (172)

Lu

2

3926.2 (98)

3926.5(139)

644.9 (34)

613.8 (154)

Infrared Intensities in parentheses in km/mol.

17



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Table 6. Calculated Ln(OH)2+ Harmonic Frequencies (cm-1) at the MP2 Level.a

a

Ln

Spin

O–H sym. Stretch a1

O–H asym. Stretch b2

Ln–O sym. Stretch a1

Ln–O asym. Stretch b2

La

1

3824.1 (265)

3820.6 (357)

661.5 (98)

611.9 (307)

Ce

2

3818.6 (323)

3814.4 (316)

675.6 (104)

609.8 (269)

Pr

3

3815.9 (310)

3811.7 (361)

684.1 (96)

625.4 (294)

Nd

4

3817.3 (335)

3812.8 (327)

692.6 (103)

640.7 (255)

Pm

5

3822.5 (316)

3818.3 (374)

697.0 (92)

648.5 (228)

Sm

6

3825.1 (315)

3820.9 (382)

702.8 (87)

645.7 (333)

Eu

7

3828.4 (316)

3822.1 (379)

705.0 (80)

658.2 (228)

Gd

8

3832.0 (332)

3827.3 (360)

709.9 (76)

664.4 (229)

Tb

7

3838.0 (341)

3833.0 (334)

716.8 (74)

663.4 (216)

Dy

6

3846.0 (257)

3841.3 (471)

724.0 (51)

685.6 (251)

Ho

5

3846.4 (297)

3841.6 (400)

726.8 (60)

687.8 (220)

Er

4

3850.0(308)

3844.4 (379)

728.2 (56)

691.1 (256)

Tm

3

3857.4 (282)

3851.6 (423)

734.4 (47)

695.7 (231)

Yb

2

3863.7 (258)

3859.2 (450)

739.2 (45)

711.1 (180.0)

Lu

1

3857.3 (297)

3852.7 (407)

745.8 (51)

713.2(189)

Infrared Intensities in parentheses in km/mol.

Reaction energetics. The first reaction is the insertion reaction (1), which is highly exothermic Ln + H2O2 → Ln(OH)2

(1)

for all Ln metal atoms (Table 7). All of the discussion of the reaction energies will use the CCSD(T) values given in the Table. The DFT/B3LYP and MP2 values are given in the Supporting Information. The most exothermic reaction at -221 kcal/mol is for Ho while Tm and Yb are the least exothermic. Reaction (2) is that of Ln(OH)2 with another H2O2 molecule to form the trihydroxide and an OH radical, and is much less exothermic. Ln(OH)2 + H2O2 → Ln(OH)3 + OH

(2)

18


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Table 7 Calculated Reaction Energies at the R/UCCSD(T) level for Reactions 1 – 5, 7, 8', 9', 12, and 13 in kcal/mol and the Ionization Potential (IP, Reaction 6) and Electron Affinity (EA) of Ln(OH)2 in eV. 1

2

3

4

5

IP 6(eV)

EA (eV)

7

8'

9'

12

12 TSa

13b

La

-197.9

-75.0

-82.6

-190.3

-72.1

4.96

-0.46

-83.5

-98.1

-147.5

-15.5

29.4

-2.9

Ce

-194.5

-75.9

-81.1

-189.4

-161.4

5.04

-149.3

-19.2

27.4

85.5

Pr

-187.6

-69.6

-63.8

-193.4

-114.8

5.17

-138.2

-23.3

15.9

45.2

Nd

-152.8

-93.9

-55.3

-191.3

-102.8

4.36

0.31

-52.3

-60.4

-109.1

-5.1

34.0

8.9

Pm

-163.7

-76.1

-56.9

-183.0

-85.8

5.16

-0.12

-44.8

-63.6

-113.3

-6.7

30.2

9.7

Sm

-151.1

-58.9

-53.3

-156.7

-69.4

6.01

0.24

-12.6

-32.9

-81.7

20.3

54.2

10.5

Eu

-146.8

-34.5

-50.6

-130.7

-41.4

7.18

0.35

18.7

-3.4

-45.0

47.2

74.5

6.9

Gd

-184.1

-69.5

-75.7

-177.8

-82.1

5.75

0.52

-51.5

-69.5

-116.3

6.2

34.9

12.6

Tb

-209.6

-62.9

-111.2

-161.3

-142.3

6.08

0.60

-69.4

-90.0

-135.2

25.5

44.2

79.4

Dy

-195.3

-71.1

-86.8

-179.9

-109.5

6.17

0.84

-53.0

-78.8

-113.1

8.0

30.8

38.4

Ho

-220.7

-71.1

-89.0

-182.3

-67.7

5.85

0.48

-65.4

-70.4

-124.2

8.3

12.5

-3.4

Er

-172.6

-78.7

-68.4

-183.0

-80.9

5.79

0.79

-39.2

-52.0

-99.8

16.3

42.1

2.2

Tm

-141.8

-76.6

-37.0

-181.4

-84.9

5.90

-64.4

44.7

41.6

8.3

Yb

-138.0

-53.3

-41.7

-149.6

-44.6

6.96

-21.9

69.6

76.8

-8.7

Lu

-197.7

-74.7

-97.9

-174.5

-90.8

6.14

-116.0

29.1

50.2

16.1

Ln

-0.42 -0.41

0.89 0.42 0.84

a

Barrier energy for reaction (12): LnOH →HLnO.

b

OH bond dissociation energy for the reaction Ln(OH)4 → Ln(OH)3 + OH.

19

-78.4 -68.4

-5.7 22.5 -56.2


-100.3 -87.1

7.8 24.2 -68.8


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Page 20 of 67

The Nd reaction is the most exothermic at -94 kcal/mol and the Eu reaction is the least exothermic at -34 kcal/mol. Reaction (3) is similar to reaction (1) and leads to the monohydroxide, which provides another route to the trihydroxide, reaction (4). Ln + H2O2 → LnOH + OH

(3)

The exothermicities of reaction (3) are similar in magnitude to that of reaction (2) with the most exothermic being the reaction with Tb at -111 kcal/mol and the least exothermic for Tm of -37 kcal/mol. Reaction (4), as might be expected, is highly exothermic like reaction (1). There are a number of highly exothermic reactions near -190 kcal/mol and the least exothermic reaction is for Eu at -131 kcal/mol. A minor product, the tetrahydroxide, is formed via reaction (5), LnOH + H2O2 → Ln(OH)3

(4)

Ln(OH)2 + H2O2 → Ln(OH)4

(5)

The reaction energies are exothermic with the most exothermic values for Ce, Pr, Tb, and Dy and the least exothermic for Eu and Yb. Thus, it is no surprise that the Ce, Pr and Tb tetrahydroxides are the ones observed here. For thorium reactions, the tetrahydroxide is the major product.28 The stability for the +IV oxidation states have been discussed previously for these lanthanide metals.1,17 Considering the similarities in the bond energies of the weak F-F bond in F2 and the weak O-O bond in H2O2, it is not surprising that some of the Ln(OH)4 were observed. The tetrafluorides CeF4, PrF4 and TbF4 have been observed17 just as have the tetrahydroxides of the same lanthanides in this study. Although NdF4 and DyF4 have also been observed,17 it is likely based on the Ln(OH)3-OH bond dissociation energies (BDEs) that only Dy(OH)4 could possibly be observed as discussed in more detail below as the Ln(OH)3-OH BDEs are weaker than the LnF3-F BDEs17 by on the order of 20 kcal/mol.

20


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Experimental isotopic frequency ratios. The O-H/O-D isotopic frequency ratios were computed from the three major hydroxide product experimental frequencies and are listed in Table 1. These ratios were close to 1.355 for each trihydroxide and cation product with variation between 1.356 and 1.354. A slightly larger 1.352 to 1.361 range was found for the dihydroxides. The most striking observation is the regularity of the decreasing ratios for Ln(OH)3/Ln(OD)3. These ratios decrease steadily from 1.3560 for La and Ce to 1.3548 for Yb and 1.3547 for Lu; Nd at 1.3539 is the only metal not in this linear progression. Harmonic ratios computed for Ce(OH)3 and Ce(OD)3 were 1.3742 using MP2, so we see immediately that the lower observed ratios for the observed argon matrix frequencies derive mostly from anharmonicity. There is a small matrix shift in these frequencies, probably about 10 cm-1 and proportional to the frequency itself, so the H/D isotopic frequency ratios should not change significantly from the gas to the matrix. The computed Ln-O bond lengths decrease from 2.175 Å for La(OH)3 to 1.998 Å for Lu(OH)3 at the MP2 level, demonstrating the lanthanide contraction. This contraction will increase the asymmetric perturbation in the O-H potential function, and thus the anharmonicity in the O-H stretching potential function and frequency, which will reduce the OH/OD frequency ratio more for the heavier lanthanide metals. Photolysis. Several product species were derived from photolysis of H2O2 by the laser ablation plume, namely the H2O-O complex, the HO2 and OH radicals, and matrix solvated cations ArnH+ and ArnD+ at 903.8 and 643.5 cm-1, respectively, which are independent of the metal used in the ablation process. 54,55,56,57 Observation of the latter cations demonstrates the presence of vacuum ultraviolet radiation in the laser ablation plume. Accordingly, some of the major cation products can arise from such photoionization, but many follow from direct autoionization, reactions (6)

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Page 22 of 67

and (7). These ionization reactions will sustain a red matrix shift owing to a stronger interaction of the cation with the argon matrix than of the neutrals. Ln(OH)2 + hν → Ln(OH)2+ + e−

(6)

Ln + H2O2 → Ln(OH)2+ + e−

(7)

The calculated ionization potentials for the dihydroxides (Table 7) range from 4.36 eV for Nd to 7.18 eV for Eu. These values are broader than the range of atomic first ionization potentials which range from 5.46 eV (Pr) to 6.25 eV for Yb 58 and smaller than the range of ionization potentials of the +II oxidation states 59 of the lanthanides which range from 19.18 eV for La to 24.92 eV for Eu and 25.05 eV for Yb. The high ionization potential for Ln(OH)2 for Eu and Yb means that the chemi-ionization reaction (7) is unlikely to be observed for Eu and Yb and possibly not from Dy and Lu as well. Lanthanum. Although La is not officially a lanthanide metal as no f orbitals are occupied in the atom, many of its product frequencies fall in line with those of the lanthanide metals. The La(OH)2+ assignments made previously are reaffirmed here on this basis,21 and these benefit from the lowest O-H and Ln-OH stretching mode positions in the cation family (Tables 1 and 2). The strongest antisymmetric La-OH(D) modes for the H2 and D2 reactions with

16

O2 produced

sharp absorptions at 580.5 and 568.8 cm-1, and the reaction with HD and 16O2 produced only an intermediate band for La(OH)(OD)+ at 574.1 cm-1. These observations show that the cation is generated from a single hydrogen isotopic molecule. The O-H and O-D symmetric and antisymmetric stretching modes are resolved for the cations, and the reaction with HD and O2 accordingly gave intermediate bands, at 3692.5 and 2723.9 cm-1. The H2 reaction with statistically scrambled 16,18O2 gave a triplet absorption at 580.7, 563.3, 553.7 cm-1, which verifies that the two O atoms are equivalent in La(OH)2+. La with a relatively low ionization energy58 of

22


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5.5770 ± 0.0006 eV enjoyed a particularly high yield of the La(OH)2+ cation relative to the neutral dihydride La(OH)2 in these experiments. Finally, the 519.4 and 491.2 cm-1 bands assigned by the Zhou group7 to La(OH)2 along with our 3730.3 cm-1 band are due to La(OH)2. The D2 + O2 reaction for La gave 2753.3 and 489.2 cm-1 bands, and substitution of HD produced an intermediate band at 490.8 cm-1, which is appropriate for La(OH)(OD). New evidence is now be presented for the La(OH)3 species. The 3730.3 cm-1 band from the H2O2 reaction increases markedly on λ > 220 nm irradiation, decreases in favor of the 3740.5 cm-1 absorption on annealing to 20 K, then increases again on λ > 220 nm irradiation, and a lower 480.8 cm-1 peak tracks the latter band. The H2 + O2 reaction follows a different mechanism as the dihydroxide bands increase on annealing to 20 K, and the 2758.7 and 471.1 cm-1 bands from the D2 + O2 reaction for La(OD)3 behave likewise. The substitution of

18

O2 resulted in

457.8 and 449.4 cm-1 bands for the H and D trihydroxide species, which are in very good agreement with the positions and isotopic shifts computed for the C3v species at the MP2 level (Supporting Information). Cerium. The product absorptions observed for Ce27 are given in the Supporting Information. In addition, extensive spectra of the cerium system also revealed absorptions for Ce(OH)4 (3714.8, 559.8 cm-1),27 which is the first lanthanide tetra-hydroxide observed here.1 Our MP2 calculations predict this diagnostic triply degenerate Ce-O stretching frequency to be higher, 574.1 cm-1, than experiment27 as expected for comparing harmonic and anharmonic frequencies. In contrast the B3LYP value of 544.7 cm-1 is below the experimental value.27 The lower frequency region for the cerium reaction products is illustrated in Figure 6. Notice a reversal in the order of the Ce-OH stretching frequencies from that for the O-H stretching frequencies. The Ce-OH stretching frequencies for the cation are the highest and both

23



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Figure 6. Infrared spectra in the Ce-OH stretching region for laser ablated Ce and hydrogen peroxide reaction products condensed in excess argon at 4 K for two identical experiments in terms of laser ablation conditions and H2O2 concentrations, but the argon for the bottom five spectra contained 0.1% CCl4 to serve as an electron trap. Spectra recorded after (a, f) sample deposition with H2O2 for 60 min, (b, g) after 240-380 nm irradiation, (c, h) after λ > 220 nm irradiation, (d, i) after annealing to 20 K, and (e, j) after annealing to 30 K.

24


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anti- and symmetric stretching frequencies are observed at 590.7 and 632.1 cm-1. The bottom set of spectra in Figure 6 were recorded from an argon/H2O2 sample containing 0.1% CCl4 added to serve as an electron scavenger. Stable chloride anions are spontaneously formed, which assists in the survival of cations isolated in the matrix that might have been neutralized by ablated electrons. In this case the relative cation/neutral band intensities were 3-fold higher with CCl4 added. This procedure is a diagnostic test for cation absorptions.2,3,20,21,60,61 More than half of these Ln/H2O2 experiments have been done with CCl4 doping and, in all cases, the bands assigned to the Ln(OH)2+ cations increased 2- to 5-fold in intensity relative to the corresponding Ln(OH)2 absorptions. The middle Ln=O stretching region for the 13 stable lanthanides reacting with H2O2 is shown in Figure 5. The bands at 796.1, 780.7, and 750.0 cm-1 have been assigned respectively to HCeO, OCe(OH)2 and OCeOH.27 Therefore, other reactions that take part during sample deposition and on UV irradiation of the cold sample (in situ) can occur. Due to the weak O-O bond in hydrogen peroxide (50 kcal/mol),25 reaction (3) is exothermic. Although the HLnO isomer is more stable than LnOH for La, Ce and Pr, there is a substantial energy barrier between the triatomic product isomers as discussed in more detail below. When an energetic reaction such as that initiated by laser ablation takes place and the products are relaxed in an argon matrix, the higher energy isomer can be trapped, reaction (8) because of the energy barrier between the Ln* + H2O2 → LnOH* + OH → [(Ln)OH]*+ OH → HLnO + OH

(8)

isomers. The predicted energetics for reaction 8' are given in Table 7. Reaction 8' is exothermic Ln+ H2O2 → HLnO + OH

(8')

for most of the Ln except for Tm and Yb with Eu being near thermoneutral. Another reaction that can occur with energetic Ln atoms (or in some cases ground state atoms) is the partial

25



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Page 26 of 67

decomposition reaction (9). The exothermicity of reaction (1) is large enough that an O-H bond can break as given in Table 7 for reaction 9'. Ln* + H2O2 → [Ln(OH)2]** → OLnOH + H

(9)

Ln+ H2O2 → OLnOH + H

(9')

Reaction 9' is predicted to be exothermic for all Ln with the smallest exothermicity of -22 kcal/mol for Yb. The OLnOH molecule will have a strong diagnostic Ln=O stretching absorption with a very small deuterium shift. Praseodymium. The results for Pr follow those for cerium with slightly higher frequencies for its major products, which are given in Tables 1 and 2 and illustrated in Figures 1 to 5, 7, and 8. The Pr-OH anti- and symmetric stretching frequencies are 591.4 and 636.3 cm-1 for Pr(OH)2+, which are just 0.7 and 4.2 cm-1 higher than the above Ce(OH)2+ values, and the O-H stretching frequencies are 3691.3 and 3694.1 cm-1, only 0.3 and 0.4 cm-1 higher than observed for Ce(OH)2+. These cation absorptions increased with UV irradiation for both lanthanides. Figure 7 also shows the apparent Pr-OH stretching modes for Pr(OH)2 at 555.2 and 507.2 cm-1, with the former mode broader and more than twice as strong as the latter peak which increases slightly more on UV irradiation than the former. Our MP2 calculations for the quartet state, in contrast, find the higher symmetric stretch to have almost half of the intensity of the lower antisymmetric stretching mode. The Pr(OD)2 counterparts at 542.1 and 495.4 cm-1 have a relative intensity ratio of about 0.5 and our calculations predict 0.35 relative intensities. The anomalous intensity behavior for Pr(OH)2 may arise from the broad underlying 550 cm-1 band. The Pr-OH stretching frequencies (Figure 7) at 507.2 and 555.2 cm-1 for Pr(OH)2 are separated by 48.0 cm-1; the MP2 frequencies are separated by 52 cm-1 and are about 50 cm-1 too large. The ground state of Pr(OH)2 is predicted to be the quartet at the CCSD(T) level by 26 kcal/mol.

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Figure 7. Infrared spectra in the Pr-OH or Pr-OD stretching region for laser ablated Pr and hydrogen or deuterium peroxide reaction products condensed in excess argon at 4 K. Spectra (a,e) recorded after deposition for 60 min, (b,f) recorded after irradiation at 240-380 nm, (c,g) recorded after λ > 220nm irradiation for 15 min, and (d,h) after annealing to 20 K.

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Although the O-H stretching frequency of Pr(OH)3 is 2.8 cm-1 higher than that for Ce(OH)3, in the direction expected, the Ln-O stretching mode of Pr(OH)3 at 484.1 cm-1 is 15.9 cm-1 lower than this mode for Ce(OH)3, and our MP2 calculation predicted it to be 12.8 cm-1 higher. The Ce(OD)4 molecule was observed at 496.5 cm-1, and MP2 calculations predict Pr(OD)4 to be 13.5 cm-1 higher. A weak band in this region in Figure 7 at 526.8 cm-1 in the reaction with D2O2 is destroyed by 240-380 nm irradiation and is appropriate for assignment to Pr(OD)4. A weak band at 535.4 cm-1 is assigned tentatively to Pr(OH)4. A supporting comparison can be made with PrF4 and PrF3 where the former absorbs 94 cm-1 above the latter, and decreases at the favor of the latter on UV photolysis.17 Here we find the antisymmetric Pr-OD stretching mode for Pr(OD)4 to be 48.5 cm-1 above that for Pr(OD)3. Our H2 and O2 experiment produced almost the same 3691.4 and 3694.2 cm-1 bands that shifted to 3679.8 and 3682.3 cm-1 with 18O2 for Pr(OH)2+ (Figure 8). The shifts of 11.6 and 11.9 cm-1 are almost the same as the 11.6 and 11.5 cm-1 shifts found for Ce(OH)2+. The deuterium enriched sample gave a weak band at 3693.4 cm-1 for Pr(OH)(OD)+ from the reaction with HDO2 in the D2O2 sample; notice that this frequency is intermediate between the O-H stretching modes for Pr(OH)2+. The O-18 shifts of 16.9 and 17.1 cm-1were larger for the PrO-D stretching modes of Pr(OD)2+; these shifts for Pr(OD)2+ are very close to the 16.7 and 17.1 cm-1 shifts for the two O-D stretching modes of Ce(OD)2+. It is interesting that Pr(OH)2+ cations are not produced in the reaction of laser ablated Pr atoms with H2 and O2 in argon, but the cations appear when this sample is exposed to UV irradiation (Figure 8). This suggests that ionization of PrO2, a major product in this reaction,2 occurs as attested by a weak 914 cm-1 band of PrO2+. This weak band is destroyed in favor of Pr(OH)2+ upon UV photolysis, likely by reaction with H2, reaction (10). PrO2+ + H2 → Pr(OH)2+

(10)

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II

I

Pr(18 OD)3 18 Pr( OD)2 18 Pr( OD)2

W

(f)

Pr(OD)3

Pr(OD)2

Absorbance

Pr(OD)2

(e)

(d)

18

18 Pr( OH)3 Pr( OH)2 18 Pr( OH)2

(c)

Pr(OH)3 Pr(OH)2 Pr(OH)2 0.02

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3780

(b)

W

(a)

3740

3700 3660 2780 Wavenumbers (cm-1 )

29


2740

2700


III

Pr(18 OD)2

Pr(18 OD)2

(f)

Pr(OD)2

Pr(OD)2 (e)

Absorbance

Pr(OD)3 (d)

Pr(18 OH)2

Pr(18 OH)2

Pr(18 OH)3 (c)

Pr(OH)2

Pr(OH)2

(b)

0.03

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Pr(OH)3 640

600

560

520

(a)

480

Figure 8. Infrared spectra in the O-H (I), O-D(II), and Pr-O (III) stretching regions for laser ablated Pr metal atom and hydrogen peroxide reaction compared with hydrogen (0.6%)-oxygen (0.4%) mixture reaction products condensed in excess argon at 4 K. Spectra recorded after (a) sample deposition with H2O2 followed by 240-380 nm irradiation, (b) sample deposition with H2 and O2 after λ > 220 nm irradiation, (c) sample deposition with H2 and

18

O2 after > 220 nm

irradiation, and (d) after sample deposition with D2O2 followed by 240-380 nm irradiation, (e) sample deposition with D2 and O2 after λ > 220 nm irradiation, and (f) sample deposition with D2 and O2 after λ > 220 nm irradiation and annealing to 20 K. W denotes water.

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The PrO2+ is likely generated by reaction (11).2,62 The Pr-OH stretching modes for the cation at Pr + O2 + hν → PrO2+ + e−

(11)

591.4 and 636.3 cm-1 are slightly higher than the cerium counterparts. The Pr-OD stretching modes for the cation shift lower to 580.2 and 622.0 cm-1 as H and D are spectator weights for this mostly Pr-O stretching mode. The antisymmetric stretching mode for Pr(OD)2+ at 580.2 cm-1 exhibits a 584.8 cm-1 satellite, which is almost half way to the 591.4 cm-1 band for Pr(OH)2+. The 584.8 cm-1 band arises from reaction with HDO2 in the D2O2 sample and can be assigned to the antisymmetric Pr-O stretching mode of Pr(OH)(OD)+. The H2 and O2 experiment gave a slightly higher antisymmetric O-H stretching mode for Pr(OH)3 at 3746.6 cm-1 which shifted to 3734.2 cm-1 with oxygen-18, a shift of 12.2 cm-1. The H2 and O2 reaction gave the same 3722.7 cm-1 band for Pr(OH)2 as H2O2, which shifted 10.8 cm1

using oxygen-18. A new band in the Pr=O stretching region at 758.7 cm-1 shifted to 720.4 cm-1

with almost the same 16/18 frequency ratio, 1.0532, as PrO itself, 1.0538.2 This band is most likely due to OPrOH as supported by our frequency calculations which give 774.0 and 734.1 cm1

, with a calculated isotope ratio of 1.0543. MP2 calculations for Pr(OH)2+ predicted the Pr-OH stretching modes to be 15.3 and 8.5

cm-1 higher than experiment and the two PrO-H stretching modes to be 2.7 cm-1 lower than those for Ce(OH)2+ instead of higher. These differences are small compared to the absolute accuracy of the frequency calculations. The PrO-H(D) and Pr-O(D) stretching modes fit the lanthanide periodic relationship very well for the cation (Figures 1-4). The B3LYP calculations predicted the O-H stretching mode to be just 4 cm-1 lower for Pr(OH)2 than for Pr(OH)3, and this mode is at least 30 cm-1 lower from our spectra.

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Although the PrO-H(D) stretching modes for the trihydroxides fit the periodic trend very well, the Pr-OH or Pr-OD stretching modes for Pr(OH)3 or Pr(OD)3 are approximately 30 cm-1 lower than would be suggested by a linear plot of Ln-OH or Ln-OD stretching modes (as in Figures 3 and 4). We also notice that the Pr(OH)3 band at 484.1 cm-1 has a 487.8 cm-1 satellite (all measured after λ > 220 nm photolysis), but the Pr(OD)3 counterpart is sharper and shifted only 5.8 cm-1 to 478.3 cm-1, which is about half of the H-D shift found for the Pr(OH)2 and Pr(OH)2+ modes and predicted by MP2 calculations for Pr(OH)3. Our experiment with Pr, gave the major product at 488.0 cm-1 after

16

O2 and H2

λ > 220 nm photolysis, which shows that this is a

different matrix site from the strongest band at 484.1 cm-1 from the H2O2 reaction with Pr. The analogous experiment with

18

O2 and H2 gave the major product at 466.4 cm-1 with a 21.6 cm-1

isotopic shift. This can be compared with the 23.3 cm-1 shift for the analogous 538.1 cm-1 band of Tb(OD)3. These O2/H2 experiments gave 507.2 and 495.9 cm-1 peaks for the antisymmetric Pr-OH and Pr-OD stretching modes for Pr(OH)2 and Pr(OD)2 with 18.8 and 22.3 cm-1 oxygen isotopic shifts. Such observations show that the antisymmetric Pr-OH stretching mode for Pr(OH)3 sustains an interaction which red shifts this mode slightly more than that for the Pr(OD)3 counterpart, which in effect halves the D shift. We suggest the possibility of a vibronic interaction with a low nearby excited electronic state, which may be responsible for the broad 550 cm-1 band described above. A similar anomaly is observed in the matrix vibrational spectra of PrF3, which was explained by the presence of a degenerate ground electronic state with the antisymmetric stretching mode at 458 cm-1 in solid argon involved in a resonance interaction with another electronic state. 63 The small deuterium shift for this degenerate mode and its lower frequency also suggest a resonance interaction with a degenerate or very low-lying excited electronic state. Thus, our matrix absorptions for the antisymmetric Pr-OH stretching modes of

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Pr(OH)3 and Pr(OD)3 have some additional vibronic character due to the presence of a low-lying electronic state. To better understand the potential for low-lying electronic states, we performed a CASSCF (2e, 7o) calculation for Pr(OH)3 (C3v) as a triplet with f2 occupancy. (The CASSCF calculations were done with MOLPRO.) The results in Table 8 and the Supporting Information show that there are 4 low-lying states within 1.1 kcal/mol of the ground state. These states are clearly composed of different f2 orbital occupancies. The energetic results are consistent with the above hypothesis of the presence of additional low-lying electronic states perturbing the vibrational modes. Two bands derived from Pr reactions in Figure 5 remain to be identified. The band marked with an asterisk (*) at 778.2 cm-1 has an intermediate deuterium shift of 5.8 cm-1 to 772.4 cm-1. Our MP2 calculations predict the Pr=O stretching mode for HPrO to be at 868.2 cm-1 with the DPrO value at 862.6 cm-1. Unlike the major products which increase on UV photolysis, the intensity of these bands are reduced by half. The Pr-H and Pr-D stretching modes are probably masked by strong H2O2 and D2O2 absorptions. Next the band indicated by a bold arrow at 758.6 cm-1 increases on UV photolysis and shifts only 0.4 cm-1 on deuterium substitution, a property shared by the MP2 calculated value for the Pr=O stretching mode for OPrOH at 774.0 cm-1. A weaker band at 498.4 cm-1 in Figure 7 also increases on UV irradiation and shows a larger deuterium shift to 488.2 cm-1, which is characteristic of a Ln-OH stretching vibration. Our MP2 calculations predict this mode at 512.5 cm-1 with a slightly larger deuterium shift. The weaker OH stretching mode is not observed. The O=PrOH product arises from the energetic primary insertion reaction (1) followed by breaking of an OH bond (Reaction (9') for a ground state Pr). The calculated exothermicity of this reaction of -138 kcal/mol is more than an O-H bond dissociation energy.25

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Table 8 CASSCF Results for the Selected Lanthanide Hydroxides (Energies in kcal/mol with 1 kcal/mol = 349.75 cm-1) Molecule /CASSCF

Statea 3

3

7

Pr(OH)3 (C3v) CAS(2, 7)

Eu(OH)3 (D3h) CAS(6, 7)

5

Nd(OH)2 (C2v) CAS(4, 7)

4

2

Nd(OH)3 (Cs) CAS(3, 7)

Yb(OH)3 (D3h) CAS(13, 7)

6

Ho(OH)2 (C2v) CAS(11, 8) c

A'' A' 3 2 A'' 7 B2 7 A2 5 A2 5 B2 5 A1 4 A' 4 2 A' 34A' 2 B1 2 A1 3

6

B1

0.00

A1 B1 5 A1

0.57 0.00 0.40

4

0.00

6 5

4

Er(OH)2 (C2v) CAS(12, 8) c

Tm(OH)2 (C2v) CAS(12, 8) c

5

a

Yb(OH)2+ (C2v) CAS(13, 7)

A2

4

B1

0.15

B2 d

0.22

2

0.00 0.92

4 2

Relative Energy 0.00 0.15 0.43 0.00 0.01 0.00 0.02 0.63 0.00 0.13 0.31 0.00 0.09

2

B2 A1

Open shell Orbitals b

Orbital Symmetry

fyz2 fx(x2-3y2) fz3 fz(x2-y2) fxz2 fyz2 fz3 fxz2 fyz2 fz(x2-y2) fxyz fy(3x2-y2) fz3 fz(x2-y2) fyz2 fxz2 fx(x2-3y2) fxyz fz3 fxz2 fz(x2-y2) fy(3x2-y2) fxz2 fyz2 fz(x2-y2) fx(x2-3y2) fz3 fxz2 fz(x2-y2) fx(x2-3y2) fxz2 fyz2 fy(3x2-y2) fz3 fx(x2-3y2) fy(3x2-y2) fz3 fxz2 fx(x2-3y2) fxz2 fz3 1.4 fz(x2-y2) 1.1fz3 1.9 fx(x2-3y2) 1.6 fxz2 1.7 fyz2 1.0 fy(3x2-y2) 1.3 fxyz fz3 fxz2 fy(3x2-y2) fxyz fz3 fz(x2-y2) fx(x2-3y2) fz(x2-y2) fyz2 fy(3x2-y2) 1.5 fz3 1.5 fx(x2-3y2) 1.5 fyz2 1.5 fxyz 1.5 fz3 1.5 fx(x2-3y2) 1.5 fyz2 1.5 fxyz 1.3 fz(x2-y2) 1.7 fx(x2-3y2) 1.3 fy(3x2-y2) 1.7 fxyz fyz2 fz3

A1 + A2 2A1 A1 + A2 A1′+E′+E˝+ A2˝ A1′+E′+E˝+ A2′ 2A1+ B1+B2 A1+ 2B1+B2 2A1+ 2B1 2A′ +A˝ 2A′ +A˝ A′ +2A˝ A2˝ A1′ A1, B1, B2, A2 A1+B1+B2+A2 2A1+B1 A1+2B2 A1, B1, B2, A2 A1, B1, B2, A2 A1, B1, B2, A2 B2 A1

C3v symmetry reduces to Cs symmetry and D3h reduces to C2v symmetry for Abelian point

groups in MOLPRO. b The occupancy of the open shell orbitals for a specific state were obtained by the optimization of the specific state alone. c The inclusion of the active 6s electrons does not change the states for each space symmetry for Ho(OH)2 , Er(OH)2 and Tm(OH)2. d The optimization of state 1.3 did not converge, so the assignment of the open shell orbitals is only approximate.

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Neodymium. The major products exhibit 5-20 cm-1 higher O-H stretching frequencies than the Ce counterparts. The Nd(OH)3/Nd(OD)3 frequency ratio of 1.3539 would have been 1.3557 if it were in line with the other ratios, and the Nd(OD)3 frequency needed to be 3.6 cm-1 lower to attain this 1.3557 value, which is far outside of the experimental frequency accuracy (within 0.2 cm-1). The Nd(OH)2/Nd(OD)2 and Nd(OH)2+/Nd(OD)2+ cation frequency ratios are also the lowest in those ranges. This shows that the Nd(OH)x stretching modes may be the most anharmonic of the Ln metal hydroxides observed here, or, more likely, that there is a different vibronic interaction with the H and D bearing isotopic molecules. An anomaly in the matrix infrared spectrum of the similar NdF3 may have a common explanation involving low-lying electronic states of these species.63,64 Although the Ln-OH stretching modes for Ce(OH)3 and Pr(OH)3 are lower than for their Ln(OH)2 counterparts, a crossover is found for Nd, and both Sm and Eu exhibit much lower Ln-OH modes for the dihydroxides (see Figures 3, 4, and 9). The computed antisymmetric O-H mode is 4 cm-1 lower for Nd(OH)3 than for Pr(OH)3 and we find it to be 5 cm-1 higher, which is within the range of the accuracy of the calculated frequencies, especially given the potential for vibronic coupling. An experiment with H2O2 doped with CCl4, was performed, and as reported above for Ce, the Nd(OH)2+ cation absorptions were 3-fold stronger relative to the Nd(OH)2 band intensities. To better understand the potential for the presence of low-lying electronic states, we performed a CASSCF (3e, 7o) calculation for Nd(OH)3 (Cs) as a quartet with f3 occupancy. The results in Table 8 and the Supporting Information show that there are 4 low-lying states within 0.8 kcal/mol of the ground state and these are different f3 sets of occupancies. These results are consistent with the above hypothesis. In addition, CASSCF (4e, 7o) calculations for the Nd(OH)2 quintet state with a f4 configuration show the presence of 6 states (Table 8 and the Supporting

35



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Page 36 of 67

Information) within 1 kcal/mol of the ground state. The low lying configurations are dominated by the occupation of 4 different f orbitals. Additional bands at 778.0 and 772.7 cm-1 are in accord with previous assignments to the Nd=O stretching modes for HNdO and DNdO.7 Although the unobserved NdOH molecule is 5.1 kcal/mol lower in energy than the HNdO isomer, an energy barrier of 34 kcal/mol separates the two isomers. We believe that the two lowest frequency bands from the earlier work7 are better reassigned to Nd(OH)2. Another pair of absorptions at 759.8 and 759.4 cm-1 are assigned to the strongest absorptions of ONdOH and ONdOD, which are computed by MP2 to be at 775.4 and 775.0 cm-1. Samarium and Europium. The trihydroxides of these lanthanides fit beautifully into the linear LnO-H(D) correlations (leftmost dark arrows in Figures 1 and 2) and Ln-OH(D) plots (middle Figures 3 and 4) even though these metals are known for also having stable +II oxidation states.1 Accordingly, the major product bands for these metals are the Ln-OH(D) stretching modes on the low frequency sides of Figures 3 and 4 for Sm(OH)2 and Eu(OH)2 . The antisymmetric O-H stretching frequencies for Sm(OH)2 and Eu(OH)2 reverse order by 1.7 cm-1 (Table 1). In addition, the Eu(OH)2+ cation exhibits a 9.2 cm-1 higher O-H stretching mode than does Sm(OH)2+; these cation absorptions are relatively weak compared to their neutral species. The strong Sm(OH)2 absorption at 461.7 cm-1 is 10-fold stronger than the Sm(OH)2+ cation absorption after sample deposition, and λ > 220 nm irradiation had little effect on either. A similar H2O2 experiment for Sm and argon doped with CCl4 increased the cation absorptions 5-fold such that they were half of the neutral species absorption intensities on sample deposition, and λ > 220 nm photolysis decreased the neutral by 30% and increased the cation by 60%. Figure 9 shows the low frequency region for the Eu experiments with H2O2 and D2O2. It is clear that the relative

36


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Figure 9. Infrared spectra in the Eu-OH and Eu-OD stretching region for laser ablated Eu and hydrogen or deuterium peroxide reaction products condensed in excess argon at 4 K. The bottom five spectra are from the D2O2 reaction, the middle three involved a mixture of H2O2 and D2O2, and the top 5 spectra are from the H2O2 reaction. Spectra (a,f, i) recorded after deposition for 60 min, (b,g,j) recorded after irradiation at 240-380 nm, (c,h,k) recorded after λ > 220nm irradiation for 15 min, (d,l) after annealing to 20 K, and (e,m) after annealing to 30 K.

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Page 38 of 67

intensities of the Eu(OH)2+ and Eu(OD)2+ cation absorptions to the Eu(OH)2 and Eu(OD)2 bands are the least of any Ln metal, and one reason for this may be the fact that the autoionization reaction (7) with H2O2 is endothermic for Eu, but it is exothermic for all of other Ln metals except for Yb (Table 7). Doping with CCl4 for the Eu reactions again increased the relative cation to neutral absorbance yield 3-fold in spectra recorded after 240-380 nm irradiation, which confirms identification of the dihydroxide cation + III oxidation state absorptions in these matrix isolation experiments. The cations will sustain red matrix shifts due to the greater matrix interactions with the cation than the neutral. Our Sm-OH, Sm-OD, Eu-OH and Eu-OD stretching frequencies from the H2O2 and D2O2 reactions (Reaction (1)) agree within experimental error with those reported by the Fudan group for the Sm and Eu dihydroxides using the water reaction.7 Samarium has product absorptions in the Ln=O region (Figure 5) at 760.8(760.4) cm-1 for H and (D) isotopic products. These bands are probably due to OSmOH and OSmOD based on our calculated values of 790.0(789.7) cm-1. Note the almost linear relationship for the bands marked with bold arrows in Figure 5 which are assigned to OLnOH molecules based on their D and O-18 shifts, which confirm that bands in this region are terminal Ln=O stretching modes, a point reinforced by the calculated frequencies and the appearance of eight LnO molecules in this region (TmO is observed at 832.0 cm-1, LuO is extremely weak at 829.2 cm-1 and EuO and YbO are observed at 667.8 and 660.0 cm-1).2,3 The sharp band at 525.3 cm-1 in the reaction of Eu with D2O2 increases on UV irradiation with the 2776.8 cm-1 band for the O-D stretching mode of Eu(OD)3, like the two Ln(OD)3 bands do in other experiments, so this assignment to the Eu-OD stretching mode for Eu(OD)3 is straightforward, and the dark arrows line up very well from Nd to Lu (Figure 4). Our MP2

38


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calculations predict this mode at 548.3 cm-1 for Eu(OD)3 with a 13.9 cm-1 H/D shift, which places this band at 539.2 (525.3 + 13.9) cm-1 and very near the 540.2 cm-1 median for the two H2O2 product bands observed at 552.4 and 527.9 cm-1. The latter bands increase on UV irradiation like the above Eu(OD)3 bands and the corresponding O-H stretching mode at 3763.7 cm-1 (Figure 1). The middle three spectra for a H2O2 rich mixture with D2O2 cm-1 give precisely the same frequencies for Eu(OH)2 or Eu(OD)2 and their cations that were observed with the pure isotopic precursors, since these molecules involve a single isotopic precursor molecule. However, the band of interest is now at 525.5 cm-1 with a higher frequency shoulder absorption and sharp peaks at 538.1 and 531.3 cm-1; the same weaker peaks are also observed in the bottom spectra using the D2O2 sample (Figure 9), which contains 10% HDO2. The mixed isotopic products Eu(OH)2(OD) and Eu(OH)(OD) 2 are responsible for these intermediate peaks, and it is expected that the Eu-OH absorption for Eu(OH)2(OD) at 538.1 cm-1 is near the unperturbed value for Eu(OH)3. Notice also that the 552.4 and 527.9 cm-1 bands are broader (6.2 and 2.5 cm-1 full width at half maximum) than the 525.3 cm-1 band (1.7 cm-1), which suggests that the broader 552.4 cm-1 band may have a substantial electronic transition character. In contrast the vibrational absorptions for Eu(OH)2 and its cation are sharper (1.5 to 2.0 cm-1). Thus, we are led to the conclusion that the Eu-OH stretching mode for Eu(OH)3 at 527.9 cm-1 has been red shifted due to a vibronic interaction with a low lying electronic state which gives rise to the broader band of comparable intensity at 552.4 cm-1. Both of the 552.4 and 527.9 cm-1 bands are marked with dark arrows for the antisymmetric Eu-OH stretching mode of Eu(OH)3

in Figure 3. To better

understand the potential for low-lying electronic states, we performed a CASSCF (6e, 7o) calculation for Eu(OH)3 (D3h) as a septet with f6 occupancy. The results in Table 8 and the Supporting Information show that there are 6 states within 6.0 kcal/mol of the degenerate ground

39



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state. The next lowest lying state is at 4.4 kcal/mol (1540 cm-1). In order to exactly match the spectral changes, the perturbing electronic state needs to be about 540 cm-1 above ground electronic state. The calculated state energy is too high but incorporation of additional correlation corrections could lower this energy. These states are again generated by moving the hole in the 7 f orbitals. New product bands are observed in Figures 3 and 4 and marked with vertical lines at 505.8, 494.2 cm-1 for Sm and 506.8 and 494.4 cm-1 for Eu. These band pairs show deuterium shifts of 11.6 and 11.4 cm-1, which are appropriate for Ln-OH(D) stretching modes and in good agreement with the calculations (13.6 and 13.8 cm-1) for the strongest bands (520.4 and 525.7 cm-1) of the SmOH and EuOH products of reaction (3). These bands increase on UV irradiation as do the major products (see Figure 9), and they are above the Sm(OH)2 and Eu(OH)2 counterparts. Analogous bands are observed across the lanthanide row up to Yb and the frequencies are listed in Table 2. Gadolinium. The trihydroxide of Gd dominates the upper frequency region at 3770.2 cm-1, and the dihydroxide cation absorptions at 3719.2 and 3722.2 cm-1 are among the strongest of any cation species observed here. Doping with CCl4 again increased the intensity of the cation band 3-fold relative to its neutral counterpart observed at 3764.7 cm-1 in this region. The CCSD(T) calculated ground state is clearly the nonet for Gd(OH)2. Our MP2 calculations predict that the antisymmetric Gd-OH stretching frequency for the octet ground state of Gd(OH)3 is 10.2 cm-1 lower than that for 9Gd(OH)2, as compared to the experimental lowering of 9.0 cm-1 for the dihydroxide. It is predicted to be 3.1 cm-1 higher than for Eu(OH)3 as compared to the experimental increase of 3.8 cm-1 higher. These comparisons show that the MP2 frequency

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calculations are in good agreement with experiment, which should sustain an argon matrix shift of at least 10 cm-1. The Gd-O stretching region, Figure 10, is even more complicated owing to the presence of many product species. The diatomics GdO+ and GdO are observed at 844.6 and 812.7 cm-1 with both H2O2 and D2O2 in excellent agreement with our earlier work using O2 as the reagent.2 The HGdO and DGdO species were observed at 782.8 and 780.8 cm-1 in agreement with the Zhou group,9 and these two bands had identical contours. The strongest band in the upper region at 764.4 or 763.9 cm-1 with H2O2 or D2O2 increases by 10% with 240-380 nm irradiation and on UV photolysis in experiments with O2 and H2 or D2. It shifts 39.6 cm-1 with 18O2 and H2 or 39.5 cm-1 with 18O2 and D2, which are appropriate for a terminal Gd=O stretching mode (GdO shifts 41.8 cm-1 ) in the OGdOH or OGdOD molecules. The highest absorptions in the lower Gd-OH stretching region at 621.6 and 657.9 cm-1 are due to Gd(OH)2+, and they increase on UV irradiation and 3-fold when CCl4 is added to the H2O2 sample. These bands shift to 609.4 and 644.6 cm-1 with D2O2 as shown in Figure 10. The UV irradiation that increases Gd(OH)2+ at the expense of Gd(OH)2 also increases the GdO+ absorption at the expense of GdO. Employing the MP2 calculated frequencies, sharp weak bands at 584.1(569.0) cm-1 are assigned to GdOH(GdOD), and absorptions at 764.2,521.8(763.9,508.8) are assigned to OGdOH(OGdOD) (Table 3). Terbium. Tb gives a straightforward major reaction product spectrum, as shown in Figures 1 and 2, and listed in Tables 1 and 2. The antisymmetric O-H stretching frequency for Tb(OH)3 is predicted to be 6.9 cm-1 higher than that for Gd(OH)3 and it is 3.9 cm-1 higher in the matrix. The antisymmetric Tb-OH stretching mode is split into three bands at 564.6, 560.4, 556.2 cm-1 and the 19.9 cm-1 deuterium shift for the central component is near the 22.3 cm-1 MP2 predicted

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Figure 10. Infrared spectra in the Gd-OH and Gd-OD stretching region for laser ablated Gd and hydrogen or deuterium peroxide reaction products condensed in excess argon at 4 K.

Spectra

(a,f) recorded after deposition for 60 min, (b,g) recorded after irradiation at 240-380 nm, (c,h) recorded after λ > 220nm irradiation for 15 min, (d,i) after annealing to 20 K, and (e,j) after annealing to 26 K.

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value. A strong absorption in the Ln-OH stretching region at 614.9 cm-1 is assigned to the antisymmetric stretching mode for Tb(OH)4, which increases on UV irradiation along with the Tb(OH)2+ bands at 630.8 and 664.3 cm-1. This Tb-OH stretching absorption for Tb(OH)4 is predicted to be 54 cm-1 higher than the analogous mode for the trihydroxide, consistent with the experimental observation of it being 54.5 cm-1 higher. Two bands at 572.2 and 589.6 cm-1 are due to the Tb-OH stretching modes of Tb(OH)2. Lower bands at 526.8 and 514.6 cm-1 in the H2O2 and D2O2 experiments are appropriately assigned to TbOH and TbOD. Absorptions at 1309.6 and 791.9 cm-1 and at 940.0 and 791.5 cm-1 are assigned to HTbO and DTbO in agreement with the Fudan group.7 The H/D ratio 1309.6/940.0 = 1.3932 is as expected for these Tb-H and Tb-D stretching modes.4 Here we are able to trap both HTbO and TbOH even though they differ in energy by 25 kcal/mol because of the presence of an energy barrier of 44 kcal/mol from the more stable TbOH. Figures 3 and 4 show vertical line marks for LnOH(D) counterparts increasing steadily up to YbOH(D), and Figure 5 shows HLnO counterparts marked by an asterisk (*) increasing on going up from Gd to Lu (except for Yb). The Tb reaction produces additional absorptions at 614.9(597.1) cm-1 in Figures 3(4) which show an appropriate D shift for another Tb-OH stretching mode. Our MP2 calculations predict the strong antisymmetric Tb-OH(OD) stretching fundamentals at 627.8(608.7) cm-1 for Tb(OH)4(Tb(OD)4), which are in excellent agreement with the argon matrix absorptions. In addition these calculations predict this mode for Tb(OH)4 to be 53.7 cm-1 higher than the analogous mode for Ce(OH)4, and we observe it to be 55.1 cm-1 higher, again in excellent agreement. We also find the Tb-OH mode for the tetrahydroxide to be 54.5 cm-1 higher than that

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for its trihydroxide, and in the Ce-OH case this difference is 49.5 cm-1. The 614.9 cm-1 band increased slightly upon UV irradiation along with a 3729.9 cm-1 feature at the expense of the Tb(OH)3 bands. Our MP2 calculations predicted the antisymmetric O-H stretching fundamental for Tb(OH)4 at 3902.7 cm-1, 12.7 cm-1 higher than for Ce(OH)4, and we observe it to absorb 15.1 cm-1 higher. A similar comparison can be made for TbF4 and TbF3 where the tetrafluoride Tb-F stretching mode is 80 cm-1 higher than for the trifluoride, and the analogous comparison for Ce gives 62 cm-1.17 Again the tetrafluoride is more stable on photolysis with fluorine present where TbF4 increases at the expense of TbF3. Dysprosium. The assignments for the major products follow the pattern set by Tb in the first two figures, and in the Dy-OD stretching region Dy(OD)2 is lower than Dy(OD)3 but DyOH is lower in the experiment as well as in the calculations. Two additional bands at 799.2 and 773.8 cm-1 in the terminal Ln=O stretching region can be assigned to HDyO and to ODyOH following the computed frequencies: an associated band at 1317.9 cm-1 is due to the Dy-H mode of the former, which is in agreement with the Fudan group.7 Holmium, Erbium, and Thulium. The trihydoxide absorptions for these lanthanides fall in line as shown in Figures 1-4, but another product gives IR spectra that exhibit a unique major difference from the earlier metals. The two Tb-OD stretching modes for Tb(OD)2 lead up to the two Dy-OD modes for Dy(OD)2 and then to the unique matrix split band pairs for Ho, Er and Tm(OD)2 (Figure 4). Spectra for Ho reacting with D2O2 are illustrated for the Ho-OD stretching region in Figure 11 where the unique feature is a sharp doublet at 568.2, 564.3 cm-1, which is associated with weaker bands at 602.2, 598.0 cm-1 in the D2O2 spectra, but the only possible H2O2 counterpart for this species is a weak band at 592 cm-1. Notice that photolysis decreases the higher band in each D2O2 pair and annealing decreases all of these features but the higher

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Figure 11. Infrared spectra in the Ho-OD stretching region for laser ablated Ho and hydrogen or deuterium peroxide reaction products condensed in excess argon at 4 K. Spectrum (a) recorded after deposition for 60 min, (b) recorded after λ > 220nm irradiation for 15 min, (c) recorded after annealing to 18 K, (d) after annealing to 25 K, and (e) after annealing to 30 K.

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component is favored. Another experiment behaved similarly except that the upper components were even more favored. These bands appear to be due to the stronger antisymmetric and weaker symmetric stretching modes of two different matrix sites of a major product molecule. These band pairs fit the MP2 calculations for Ho(OD)2 (Supporting Information) very well in position, separation, and relative intensities. The two modes computed for Ho(OD)2 are 578.3 cm-1 (179 km/mol), 611.4 cm-1 (33 km/mol) with a 33.1 cm-1 difference and a 5.4:1 intensity ratio, in comparison with the observed 568.2 and 602.2 cm-1 with a 5:1 relative intensity and 34.0 cm-1 separation (Table 2). The MP2 frequencies increase 7 and 4 cm-1 for these modes of Er(OD)2 and 4 and 5 cm-1 more for Tm(OD)2, which compare to experimental increases of 4 cm-1 for each mode between each metal. We could not expect better agreement in vibrational spectroscopic detail between the MP2 calculations and the argon matrix frequencies for lanthanide dihydroxides in spite of the fact that the detail observed for these three Ho, Er, Tm(OD)2 species is not found for the OH analogs. The above differences for the three Ho, Er, Tm(OH)2 species most likely arise from different low electronic states and zero point energies, which affect the overall state energy. We carried out CASSCF calculations for 6Ho(OH)2 (11e, 8o), 5Er(OH)2 (12e, 8o), and 4Tm(OH)2 (13e, 8o) with the single s and seven f orbitals active in terms of the valence space. In all cases, the s remains singly occupied. There are 4 states within 1.1 kcal/mol of the ground state for Ho and Er and 3 states within 0.8 kcal/mol of the ground state for Tm. These low-lying states have the f electron orbitals partially occupied with between 1 and 2 electrons for some of the states. Again , the presence of low-lying electronic states due to different f orbital occupancies could perturb the vibrational spectra as discussed above.

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Following established trends, the highest 641.0, 671.0 and 629.1, 658.2 cm-1 pairs are due to the Ho(OH)2+ and Ho(OD)2+ cations, which are characterized by their marked growth on UV irradiation. Resolved O-H and O-D stretching modes for these species are given in Table 1. The sharp satellite at 633.5 cm-1 shows the same behavior and falls between the antisymmetric stretching modes of the Ho(OH)2+ and Ho(OD)2+ cations, which is appropriate for the Ho(OH)(OD)+ cation. The broader bands at 548.6 and 564.3 cm-1 are the lowest frequencies of the major products and are thus due to the trihydroxide species. The satellite features at 555.0 and 571.7 cm-1 are probably due to matrix site splittings. Their O-H and O-D stretching modes observed at 3782.1 and 2791.0 cm-1 in Figures 1, 2, S1A, and S2 are the highest of the Ho product species and due to Ho(OH)3 and Ho(OD)3. The lowest band at 532.7 with a deuterium counterpart at 518.5 cm-1 is consistent with our MP2 calculations which predict HoOH to be at 553.8cm-1 and HoOD at 538.8cm-1. (See Figure S1B as well.) Infrared spectra from a Ho experiment with almost equal amounts of H2O2 and D2O2 are illustrated in Figure S3. Note that the absorptions for Ho(OH)2+, Ho(OD)2+, HoOH and HoOD, which derive from the reaction of a single H2O2 reagent molecule, are sharp as before, and one component of the Ho(OD)2 doublet is observed at 568.2 cm-1. The major absorptions for Ho(OH)3 and Ho(OD)3 were observed as before, and a median feature with peaks at 554.7 and 552.7 cm-1

is characteristic of the

degenerate stretching mode of a trigonal species with a C3 axis. Given the above results, we are left with Ho(OD)2 as the most probable carrier for the pairs of doublets in Figure 11. We turn now to the O-D stretching region in Figure S2 and observe that the second band is at 2782.9 cm-1 with a weaker shoulder at 2784.2 cm-1. Note that these two features follow the doublet components from the same D2O2 experiment featured in Figure 11 on UV photolysis and annealing. A weaker O-H counterpart is observed at 3771.0 cm-1

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in Figure S2. We hypothesize that Ho(OH)2 is more reactive because of higher zero point energy than is Ho(OD)2, and its analogous Ho-OH stretching mode is broadened into the spectral background in the 580 cm-1 region. The infrared spectra for the major Er products reveal absorptions ~4 cm-1 higher than their Ho counterparts as shown in Tables 1 and 2 and Figures 1 and 2, and the pairs of doublets at 606.4, 602.5 cm-1 and at 571.7, 567.9 cm-1 in Figure S4 (bottom), which are assigned to Er(OD)2 and a site splitting, along with the weaker shoulder at 2795.8 cm-1 on the 2794.5 cm-1 O-D stretching band. Again, we observe a weak O-H stretching mode for Er(OH)2 but no Er-OH stretching band. The lower frequency bands for ErOH and ErOD are 13 and 12 cm-1 higher than their HoOH and HoOD counterparts, but lower than the TmOH and TmOD counterparts at 555.8 and 535.2 cm-1 which are predicted by MP2 calculations at 543.7 with deuterium counterpart at 529.2 cm-1. Figure S4 shows spectra analogous to Figure S3 including satellite features 572.5 and 562.4 cm-1 on the major bands for Er(OH)3 and Er(OD)3 with an intermediate band at 557.7 cm-1 in the mixed isotopic experiment. The Tm + H2O2 spectrum in Figure 3 shows two lower frequency bands at 492.0 and 506.9 cm-1 with a deuterium counterpart for the former at 483.6 cm-1. Based on the above identification of Tm(OD)2 at 576.0 and 610.1 cm-1, these new bands are too low for Tm(OH)2, but they are just a few wavenumbers above the two absorptions for Yb(OH)2 (Table 3). Figure S5 (Supporting information) compares two experiments using Tm + H2O2, and the top set are from an experiment with 0.1 % CCl4 added to the argon stream as in Figure 6. One effect of the extra CCl4 is to compete for the laser ablated Tm atoms, and the yield for Tm(OH)3 at 574.4 cm-1 is less than half that in the top set of spectra.

Unfortunately, we do not observe a clear

absorption for Tm(OH)2, but we can compare the relative intensities of the 650.5 cm-1 band for

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Tm(OH)2+ and the new band at 492.0 cm-1. Notice that the cation band is double the intensity and the new band at 492.0 cm-1 is reduced to 0.25 intensity as CCl4 captures electrons favoring the yield of cations and reducing the intensity of absorptions due to anions. MP2 calculations predict the absorptions for 3Tm(OH)2– at 557.1 and 517.8 cm-1, substantially lower than for the neutral Tm(OH)2 dihydroxide at 639.6 and 607.0 cm-1, but much closer to the isoelectronic Yb(OH)2 molecule at 540.6 and 528.2 cm-1. Accordingly, the bands at 492.0 and 506.9 cm-1 are assigned to the Tm(OH)2– anion with the stronger band for Tm(OD)2– at 483.6 cm-1. Figure 4 shows a continuation of anion absorptions at 471.5 and 467.9 cm-1 for the Er and Ho(OD)2– anion counterparts. The corresponding MP2 values are 461.1 cm-1 for Er and 452.4 cm-1 for Ho. The early lanthanides such as La, Ce, Pr, and Pm for Ln(OH)2 are not predicted to bind an electron. For the remaining Ln, the predicted electron affinities are all less than 1.0 eV. The electron affinities for Ln(OH)2 are around 0.80 eV for Dy, Er, Tm and Lu. Gd(OH)2 and Ho(OH)2 are predicted to have the electron affinities of 0.50 eV and the electron affinities for the other lanthanide dihyroxides are less than 0.50 eV. Table 9 presents comparisons for five related species of Tm and the next metal Yb. Note for the Tm dihydroxide anion, neutral, and cation comparison that the antisymmetric Tm-O stretching frequencies increase and the Tm-O bond lengths decrease with increasing positive charge per valence electron, which also applies to the Yb neutral and cation species. Also notice that the isoelectronic Tm(OH)2– anion and Yb(OH)2 neutral species have nearly the same frequencies and computed Ln-O bond lengths. Ytterbium. Yb, like Eu, is also known for its stable +II oxidation state,1 and our spectra in Figure 12 attest to this point in that the intensities of the Yb(OH)2 and Yb(OD)2 absorptions are about an order of magnitude stronger than their cation counterparts, an observation in common

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only with the other two +II oxidation state metals Sm and Eu (Figures 3 and 4). Notice that Yb(OH)2 and Yb(OD)2 also have the lowest frequency product absorptions for Yb, and the deuterium shifts observed for these two modes at 490.3, 503.2 and at 483.2, 495.7 cm-1 are 7.1 and 7.5 cm-1. The MP2 calculated frequencies predict deuterium shifts twice this at 15.5 and 16.0 cm-1 (The observed/calculated ratios for the frequency pairs are 0.928, 0.942 and 0.959, 0.945, respectively, so these assignments are correct). A weaker band observed at 512.5 cm-1 could be the source of a vibronic interaction that red shifts the two Yb(OH)2 bands and thus decreases the apparent deuterium shifts. We predict Yb(OH)2 to have a closed shell ground state with an f14 occupation and a formal +II oxidation state. Recall that we found similar behavior for Pr(OH)3 and Eu(OH)3. However, the trihydroxide Yb-OH mode shifts down 15.9 cm-1 and the computed shift decreases the same 15.9 cm-1 on deuterium substitution.

Table 9. Comparison of Calculated and Observed Properties of Thulium and Ytterbium Hydroxide Species. Molecule

MP2 ν (Ln-O) asy Observed in Ar

Difference, %

MP2 r(Ln-O)

3

Tm(OH)2+

695.7 cm-1

650.5 cm-1

45.1, 7.0

1.920 Å

4

Tm(OH)2

607.0

(593.1)a

13.9, 2.3

1.976

3

Tm(OH)2–

517.8

492.0

25.8, 5.2

2.045

1

Yb(OH)2

528.2

490.3

37.9, 7.7

2.054

2

Yb(OH)2+

711.1

651.4

59.7, 9.2

1.908

a

Observed for Tm(OD)2 and corrected for H shift.

A larger deuterium shift for the Yb-O(H) stretches is observed for the +III f13 species Yb(OH)2+ to Yb(OD)2+. The cation Yb-OH stretching modes red shift 12.2 and 11.1 cm-1 in our experiments and 19.7 and 21.4 cm-1 in our MP2 calculations. This observation suggests a

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Figure 12. Infrared spectra in the Yb-OH and Yb-OD stretching region for laser ablated Yb and hydrogen or deuterium peroxide reaction products condensed in excess argon at 4 K. Spectra (a,e) recorded after deposition for 60 min, (b,f) recorded after irradiation at 240-380 nm, (c,g) recorded after λ > 220 nm irradiation for 15 min, and (d,h) after annealing to 20 K.

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vibronic interaction due to an excited f hole state where the Yb-OH stretching modes are red shifted with respect to the Yb-OD stretching modes and thus decrease the observed deuterium shifts from the computed values. The autoionization reaction (7) for Yb has the largest of the present computed endothermicities. Note that UV photolysis (λ > 220 nm) decreases the intensities of the neutral Yb(OH)2 bands, increases the Yb(OH)2+ cation absorptions, and increases the Yb(OH)3 bands. The 6.25416 eV (198 nm) ionization energy58 of atomic Yb is not reached by our UV lamp, but laser ablation can certainly produce Yb+ cations for reaction on deposition. However the increase on in situ UV photolysis [220 nm photons = 130 kcal/mol] cannot come from direct ionization of the neutral Yb(OH)2 molecules, reaction (6), which we compute to be 160 kcal/mol (6.96 eV). However, the autoionization reaction (7) requires an addition of only 22 kcal/mol for Yb and H2O2, which can occur on UV photolysis. The 565.2 cm-1 band for Yb(OD)3 exhibits higher energy satellites at 571.6 and 578.9 cm-1, which are appropriate for O-D stretching modes for two mixed isotopic components, ie. Yb(OH)2OD and YbOH(OD)2. Finally, our harmonic MP2 O-H stretching frequencies for the three major products are 3945 (Yb(OH)3), 3941 (Yb(OH)2) and 3859 (Yb(OH)2+)cm-1, which are aligned like the observed frequencies and are 3.9, 4.6 and 3.0 % higher than the observed argon matrix values; this difference is due mostly to the lack of inclusion of anharmonicity in the calculated O-H frequencies. A weak band at 550.1 cm-1 with H2O2 also increases slightly on UV photolysis as does its deuterium counterpart at 535.3 cm-1. Our MP2 calculations predict the Yb-O stretching mode for YbOH or YbOD at 543.9 or 529.4 cm-1 in the region between this mode for Yb(OH)3 and Yb(OH)2, and the calculated deuterium shift, 14.5 cm-1, matches the observed difference, 14.8 cm-1 quite well. A possibility for the 550.1 cm-1 band would be OYbOH, which has a predicted

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Yb=O band at 547.2 cm-1 with the Yb-O(H) band predicted at 451.1 cm-1. Our spectrum is clean down to 420 cm-1so the molecule OYbOH remains to be detected. To better understand the potential for low-lying electronic states, we performed a CASSCF (13e, 7o) calculation for Yb(OH)3 (D3h) as a doublet with f13 occupancy and a CASSCF (13e,7o) calculation for the f13 occupancy for 2Yb(OH)2+ (C2v). The results in Table 8 and the Supporting Information show that the 6 excited states for the different f orbital occupancies are within 6.0 kcal/mol of the ground state for 2Yb(OH)3 and within 6.6 kcal/mol for 2

Yb(OH)2+.

Lutetium. Lu, the final member of the lanthanide metal series, has the highest MP2 calculated harmonic (3967.2 cm-1) and observed anharmonic (3798.1 cm-1) antisymmetric O-H stretching frequency for the group of Ln(OH)3 species. Again, the difference in the calculated vs. experimental values is due to the lack of an anharmonicity correction in the calculations. The blue shifts in the observed antisymmetric Ln-OH and LnO-H stretching modes on going from Ln = Ce to Lu are 70.2 and 55.4 cm-1, respectively, for the trihydroxides, and the calculated shifts are 99.7 and 67.1 cm-1. We note that the deuterium shift for the 580.5 cm-1 Lu-OH stretching mode of Lu(OH)3 is 8.2 cm-1, a factor of two lower than the MP2 computed value of 16.2 cm-1. The Lu product spectra also have the highest frequencies observed (3778.5) for a Ln(OH)2 O-H stretching mode and the highest (3750.1, 3752.7 cm-1) for the two stretching modes of a Lu(OH)2+ cation, which are associated with the highest pair at 655.8 and 679.3 cm-1 for the Lu-OH stretching modes of the cation and a lower pair at 562.2 and 587.3 cm-1 for the neutral dihydroxide. The antisymmetric Lu-OH stretch for Lu(OH)2 has observed and calculated MP2 deuterium shifts of 16.2 and 17.8 cm-1, respectively.

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The new band at 654.8 cm-1 in Figure 4 is appropriate for the Lu-OD stretching mode of LuOD, which is computed at 642.5 cm-1 (MP2), but the LuOH counterpart is masked by CO2. It is interesting to note that our calculations predict LuOH to be 116 cm-1 higher than YbOH, and we observe the Lu-OD mode to be 113 cm-1 higher than the Yb-OD mode Two additional bands were observed in the Lu=O stretching region, with the first marked in Figure 5 by an asterisk (*) at 808.2 cm-1 with a deuterium counterpart at 807.5 cm-1. These bands are associated with a Lu-H stretching mode at 1417.8 cm-1 and Lu-D stretching mode at 1009.4 cm-1 with an H:D ratio of 1.4046. The latter bands can be compared with the antisymmetric stretching mode of LuH2(D2) at 1426.4(1021.5) cm-1.4 The above Lu=O stretching mode is slightly higher than the 806.5 cm-1 value for HTmO, and these new bands are assigned to HLuO. Again the hydride isomer is higher energy (29 kcal/mol) than the monohydroxide, but a 50 kcal/mol barrier separates the two isomers. The second band is at785.8 cm-1 with a deuterium counterpart at785.7 cm-1 (Table 3). The latter bands are assigned to OLuOH and OLuOD on the basis of MP2 calculations which predict these strongest bands at 807.4 and 807.1 cm-1. HLnO vs. LnOH. The reaction energies and energy barriers for the following reaction are given LnOH → HLnO

(12)

in Table 7. (See Supporting Information, Figure S6) The formation of LnOH molecules reaction (3) is straightforward as hydrogen peroxide is a pseudo halogen, and the abstraction of one OH group by the laser ablated Ln atom in addition to the more exothermic insertion reaction (1) are both energetically favorable. Two series of bands are marked with vertical lines in Figure 3 from Sm to Yb for LnOH products and in Figure 4 for the LnOD isotopomers. These bands follow the MP2 frequency calculations given in the Supporting Information. The HLnO isomers are more

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stable than LnOH for the early lanthanides La, Ce, Pr, Nd, and Pm, and we observe the HCeO and HPrO species but not HLaO although the HYbO analog was detected in analogous experiments.21 Although the HLnO isomers are higher energy (less stable) from Nd to Lu, we have observed all of these except HYbO as marked with an asterisk (*) in Figure 5 and listed in Table 2. However, once these higher energy isomers are formed a significant energy barrier prevents their rearrangement to the lower energy LnOH forms. The most probable mechanism relies on the considerable excess kinetic energy of the laser ablated lanthanide metal atoms with UV and vac-UV radiation in the ablation plume, which can activate endothermic reactions (8) after initiation by reaction (3). Ln(OH)4 We have previously shown the observed LnF4 compounds have significant Ln-F BDEs for the loss of F from LnF4.17 The only LnF4 which were observed were Ln = Ce, Pr, Tb, Nd, and Dy. Except for Nd, all of the Ln-F BDES for the observed compounds were > 50 kcal/mol. The Nd-F BDE was ~ 30 kcal/mol and the only other compound with a comparable Ln-F BDE was HoF4, which was not observed. We predicted the Ln-OH BDEs for Ln(OH)4 (reaction (13)) as shown in Table 7 to predict which Ln(OH)4 compounds Ln(OH)4 → Ln(OH)3 + OH

(13)

could be observed. Consistent with the LnF4, substantial Ln-OH BDEs were only predicted for Ln = Ce, Pr, Tb, and Dy with the remaining Ln-OH BDEs all < 20 kcal/mol. Thus we would not expect to observe as many Ln(OH)4 as LnF4. Population Analysis. The natural population analysis is given in the Supporting Information. Except for Sm, Eu, and Yb, all of the Ln(OH)2 have ~0.8 e in the 6s and have an s1fn electron configuration. For Sm, the configuration is f6, for Eu it is f7, and for Yb it is f14. There are ~ 0.5 e

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in the 5d and these electrons are spin paired except for Sm, Eu, and Yb where the 5d population is ~0.2 e. The Ln are best described as being in the +II oxidation state for Ln(OH)2. For Ln(OH)2+, the Ln is in the +III oxidation state with the 6s electrons removed and then removal of an f electron. For Ce, this yields a 5d1 configuration. For the rest of the Ln, there is 0.4 to 0.5 e in the d which is spin paired. Except for Ce with 5.39 e in the O 2p, there are 5.47 to 5.51 e in the O 2p, similar to what is found for Ln(OH)2. The Ln(OH)3 have the Ln in the +III oxidation state due to removal of the two valence s orbitals and an f orbital, except for Ce where the d is removed. There are about 0.5 e in the 5d on the Ln that is spin paired except for Ce and Sm which have ~0.7 in the d. Just like LnF4, the Ln(OH)4 are mostly in the +IV oxidation state except for La(OH)4, 8

Eu(OH)4,

9

Gd(OH)4, and Lu(OH)4 which are in the +III oxidation state as were the

corresponding LnF4 for La, Eu, Gd, and Lu. For the +IV oxidation state, there is about 1 electron spin paired in the 5d due to backbonding which we do not include in the description of the formal oxidation state. For the Ln(III)(OH)4, there is less population in the 5d, about 0.5 to 0.6 e. For the +III oxidation states, there is significant spin density of ~ 0.55 e on two of the OH groups. This is similar to what was predicted for the LnF4.17 The geometries of the Ln(IV)(OH)4 are all pseudotetrahedral with 4 approximately equal Ln-OH bond distances. The Ln(III)(OH)4 are highly distorted with two long Ln-O bonds to the OH groups with significant spin and two short Ln-O bonds, just as found for the Ln(III)F4.17 For the two high-spin Ln(III)(OH)4 molecules for Eu and Gd where it is possible to reach a half-filled shell (Gd) or close to a half-filled shell (Eu), the tetrahydorxide prefers the +III oxidation state just as for the tetrafluorides. For La(OH)4, only three valence electrons are available to be ionized so the +III oxidation state is found. In a similar

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way, for Lu(OH)4 removal of the two 6s and one 5d electrons leaves a f14 core so the Lu is also in the +III oxidation state. This follows from the same arguments made for the LnF4. 17 The Ln(OH)2- are in the +I state. The typical configuration is s1fn-1 for these anions for Pr to Gd with smaller amounts of d orbital backbonding than found for the other hydroxides with only 0.2 e in the 5d in many examples. For La, the occupancy is s1d1 and for Ce it is s1d1f1, following the atomic configurations. For Tb to Tm, the 6s is almost doubly occupied so the spin is all in the f electrons. For Yb, the spin is in the 6s with an s1 configuration and Lu is closed shell. The molecules OLnOH and HLnO are in the +III oxidation state except for Yb as discussed previously.13 The LnOH are in the +I oxidation state in the s1fn-1 configuration except for Gd which as the f7 configuration and Yb with a s1 configuration. There is significantly less d backbonding character for LnOH than for many of the other hydroxides. Conclusions We report the observation of the products of the reactions of laser-ablated lanthanide metal atoms with hydrogen peroxide or hydrogen and oxygen mixtures diluted in argon and condensed at 4 K. The major products of are the Ln(OH)3 and Ln(OH)2 molecules and Ln(OH)2+ cations, which are consistent with the high exothermicities for the formation of those lanthanide hydroxides calculated at the CCSD(T) level. Almost linear relationships in the LnO-H(D) and Ln-OH(D) the stretching modes for the major product absorptions are observed on going across the lanthanide row in the periodic. The infrared spectra of lanthanide hydroxides agree well with the calculated MP2 frequencies. Several cases of abnormalities in deuterium shifts for Ln-OH(D) stretching modes are attributed to vibronic interactions with low energy electronic states in the Ln(OH)3 and Ln(OH)2 species. CASSCF calculations show that there are multiple low-lying

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states due to the distribution of the holes in the seven 4f orbitals. The corresponding anions are observed for Tm(OH)2– and for the Tm, Er and Ho(OD)2– counterparts. Absorptions were also assigned to a number of HLnO, LnOH and OLnOH molecules. These reactive intermediates were identified through comparison of matrix infrared spectra using D2O2, HD, D2, 16,18O2 and

18

O2 isotopic substitution with frequencies calculated by the MP2

method, and following the trends observed in frequencies going through six lanthanide metal hydroxide series across the periodic table. We also observed tetrahydroxides for Ce, Pr, and Tb and found that their antisymmetric Ln-OH stretching modes are 50 cm-1 higher, and their MP2 computed Ln-O bond lengths are 0.06-0.07 Å shorter than those of their trihydroxides. The other tetrahydroxides were not observed because of their small OH bond dissociation energies to form the trihydroxides, similar to what was observed for the lanthanide tetrafluorides. For the dominant products, the lanthanides are in the +II oxidation state for Ln(OH)2 and the +III oxidation state for Ln(OH)3 and Ln(OH)2+. The +III oxidation state is preferred over +IV oxidation state for La, Eu, Gd, and Lu in Ln(OH)4, which is consistent with what was predicted for the LnF4.17 The remaining lanthanides in Ln(OH)4 are in +IV oxidation state. For the HLnO and OLnOH molecules, +III oxidation state is dominant for the Ln except for Yb, which is in the +II oxidation state. Not surprisingly, all of the lanthanides in LnOH and Ln(OH)2are in +I oxidation state. The natural population analysis shows that there is no backbonding to the 4f orbital and there are small amounts of d orbital backbonding for the dominant product hydroxides. The 6s electrons are singly occupied for most dihydroxides and monohydroxides. Supporting Information: Complete citations for references 35 and 53. Figures: Supplementary infrared spectra for reactions Ho+H(D)2O2, Er+ H(D)2O2, and Tm+H(D)2O2. Tables: Calculated reaction energies for all studied reactions at the B3LYP, MP2, and CCSD(T) levels. Details of

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the CASSCF calculations for selected lanthanide hydroxides. Calculated Ln(OH)4, Ln(OH)3, Ln(OH)2, Ln(OH)2+, Ln(OH) 2-, LnOH, HLnO, OLn(OH), OLn(OH)2 vibrational frequencies and isotopic frequencies at the MP2 level. Optimized geometries and NBO analysis for all studied lanthanide species at the MP2 level. This material is available free of charge via the internet at http://pubs.acs.org. Acknowledgement. D.A.D acknowledges the Department of Energy, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences, heavy element program under a subcontract from Argonne National Laboratory. D.A.D also thanks the Robert Ramsay Chair Endowment, The University of Alabama, for support. L. A. thanks TIAA for retirement funds. X.-F.W. is grateful for support from NSFC Grant (21173158). References 1

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