Reactions of Late Lanthanide Metal Atoms and Methanol in Solid

ARTICLE pubs.acs.org/JPCA

Reactions of Late Lanthanide Metal Atoms and Methanol in Solid Argon: A Matrix Isolation Infrared Spectroscopic and Theoretical Study Yu Gong,† Lester Andrews,*,† Mingyang Chen,‡ and David A. Dixon*,‡ † ‡

Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904-4319, United States Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487-0336, United States

bS Supporting Information ABSTRACT: The reactions of laser-ablated late lanthanide atoms and methanol were studied using matrix isolation infrared spectroscopy and electronic structure calculations at the density functional theory level. Both terbium and lutetium atoms react with methanol spontaneously to form the CH3OTbH and CH3OLuH insertion products, which react further with another methanol molecule to give the Tb(OCH3)2 and Lu(OCH3)2 products as found previously for uranium. The reactions of Dy through Yb and methanol first produce Ln(CH3OH) complexes during sample annealing, which isomerize to the CH3OLnH insertion products on visible irradiation. The Ln
H stretching frequencies of the CH3OLnH molecules exhibit a unique trend from Tb to Lu, which is also reproduced by theoretical calculations at the B3LYP level of theory. Although the CH3LnOH molecules are predicted to be more stable than the O
H bond insertion products, formation of the C
O bond insertion isomers is kinetically prohibited as revealed by calculated potential energy surfaces.

’ INTRODUCTION Reactions involving lanthanide metal atoms exhibit unique trends across the whole lanthanide series because of the presence of electrons in 4f orbitals. Although these 4f electrons are in the valence space, they are less involved in the bonding to form molecules. The reaction mechanisms, as well as the product structures, are determined by a range of properties of the free lanthanide atoms and their oxidation state/electronic structure in the product molecules. Gas phase studies using mass spectrometry have provided a number of examples on the reactivity of lanthanide cations toward both organic and inorganic molecules; the electronic states in the cations were found to play important roles in the reactions.1
3 For the reaction products of neutral lanthanide atoms, most experimental determinations come from infrared spectroscopic studies in cryogenic matrices.4
13 Reactions involving small diatomic and polyatomic molecules have been systematically investigated. Whereas the structural characters of most lanthanide reaction products are almost the same, exceptions are often found for Eu, Yb, and other lanthanide atoms with divalent oxidation states.5,10
13 The reactions of methanol, the simplest alcohol molecule, with lanthanide cations have been systematically studied in the gas phase. By using a time-of-flight mass spectrometer, the LnOCH3+ and LnOCH2+ cations were produced from dehydrogenation of LnCH3OH+ complexes for the Ln cations La+
Nd+, Gd+, Tb+, Ho+, Er+, and Lu+, while no product was found for the remaining lanthanide cations, which were unreactive unless they were solvated by a number of methanol molecules.14 In a following study employing Fourier transform ion cyclotron resonance (FTICR) mass spectrometry, the LnO+ and LnOH+ cations were identified as the major products for the reactions of most r 2011 American Chemical Society

lanthanide cations with CH3OH except for Tm+ and Yb+, where no reaction was observed with methanol.15 Despite the efforts in understanding the reactions of lanthanide cations with methanol in the gas phase, no experimental study has been reported on the reactions of neutral lanthanide metal atoms with methanol. Previous matrix isolation infrared spectroscopic studies of metal atom reactions with methanol have focused on transition metals,16
19 main group elements,20
23 and uranium.24 In this paper, we provide a combined matrix isolation infrared spectroscopic and theoretical electronic structure study of the reactions of the late lanthanide atoms with methanol in a solid argon matrix. We find that Tb and Lu spontaneously insert into the OH bond of methanol to form the CH3OTbH and CH3OLuH molecules, which can react further with another methanol molecule to give the dimethoxyl lanthanide complexes, Ln(OCH3)2. In contrast, Ln(CH3OH) complexes are initially formed for the other late lanthanide atoms, and the insertion products are only produced upon visible irradiation.

’ EXPERIMENTAL AND THEORETICAL METHODS The apparatus and procedure for investigating laser-ablated lanthanide atom reactions during condensation in excess argon at 4 K have been described previously.5,7,25 The Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate with 10 ns pulse width) was focused onto a freshly cleaned lanthanide metal target (Johnson Matthey) mounted on a rotating rod. Laser-ablated lanthanide atoms were codeposited with 3
4 mmol of argon Received: September 21, 2011 Revised: November 2, 2011 Published: November 06, 2011 14581

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(Matheson, research) containing 0.5% CH3OH onto a CsI cryogenic window for 60 min. CH3OH (Omni Solv, 99.99%), 13 CH3OH (CIL), CH318OH (CIL), and CH3OD (CIL) samples were cooled to 77 K using liquid N2 and evacuated to remove

Figure 1. Infrared spectra of laser-ablated Yb atom and CH3OH reaction products in solid argon at 4 K: (a) Yb + 0.5% CH3OH deposition for 60 min; (b) after annealing to 30 K; (c) after λ > 470 nm irradiation; (d) after annealing to 30 K; (e) after λ >220 nm irradiation; (f) after annealing to 30 K. The asterisks denote the site absorptions of the CH3OYbH molecule.

residual air before use. FTIR spectra were recorded at 0.5 cm
1 resolution on a Nicolet 750 FTIR instrument with a HgCdTe range B detector. Matrix samples were annealed to different temperatures and cooled back to 4 K for spectral acquisition. Selected samples were subjected to broadband photolysis using medium-pressure mercury arc street lamp (Philips, 175W) radiation with the outer globe removed directed through glass filters. Density functional theory (DFT)26 calculations were performed using the Gaussian 03/09 program systems.27 The geometries of the products of the Ln + CH3OH reactions were optimized and second derivatives were calculated to predict the vibrational frequencies for all lanthanide metals. The energies for the reaction CH3OH + Ln f CH3OLnH, the complex (CH3OH)Ln, and CH3LnOH for multiple spin states were calculated to predict the ground spin state for each reaction product. The DFT calculations were performed with the B3LYP hybrid exchange correlation functional28 with the DZVP229 basis set on H, C, and F, and the segmented small core relativistic effective core potential (ECP)30 from the Stuttgart group (ECP28MWB)31 with the corresponding segmented basis set (ECP28MWB_SEG) on the lanthanide elements.32 There are 28 electrons subsumed in the ECP leaving the 4s, 4p, 4d, 5s, 5p, 6s, and 4f (or 5d) electrons in the valence space for the Ln. The Ln basis set is of the form (14s,13p,10d,8f,6 g/10s,8p,5d,4f,2g), and we denote this as the Stuttgart basis set. This combination of basis sets and exchange-correlation functional has been found to work well for our analysis of similar reactions of Ln with CH2F2 CH3F, and CHF3 .9 For selected molecules, additional

Table 1. Infrared Absorptions (cm
1) Observed (and Calculated) for Five Ln(CH3OH) Complexes in Solid Argon with Methanol Absorptions for Comparison moleculea

assignment

CH3OH

Dy(CH3OH)

CH3 deform. COH bendb

1437.0 1310.4

CH3 rockc C
O str.

7

4

Ho(CH3OH)

3

Er(CH3OH)

2

Tm(CH3OH)

1

Yb(CH3OH)

CH3OH

13

CH3OH + 13CH3OH

CH318OH

1431.7 1303.8

1437.1, 1431.7 1310.4, 1303.7

1436.5 1303.7

1436.7

1437.2 858.3

1456 (4) 1297 (212)

1063.5

1061.1

1063.6, 1061.1

1063.6

1063.5

1211.1

1047 (181)

978.3

959.9

978.3, 959.9

953.2

978.4, 953.2

979.1

CH3 deform.

1437.1

1431.9

1436.6

1437.4, 1436.7

1437.2

1456 (8)

COH bendb

1311.2

1303.2

858.5

1340 (45)

CH3 rock.c

1063.3

1060.2

1063.4, 1060.2

1063.8

1063.5

1211.2

1067 (47)

C
O str.

978.8

960.4

978.8, 960.4

953.6

978.8, 953.6

978.5

990 (76)

CH3 deform. COH bendb

1437.4 1310.9

1431.7 1304.5

1436.7 1303.0

1437.5, 1436.6

1436.7 858.1

1456 (7) 1343 (55)

CH3 rock.c

1063.1

1059.9

1063.1, 1059.9

1063.6

1063.2

1212.0

1068 (50)

C
O str.

978.5

960.1

978.5, 960.1

953.2

CH3 deform.

1437.1

1432.0

CH3OH

CH3OH + CH318OH

1304.6

1436.5

978.5, 953.2

CH3OD

978.2

CH3OH DFT (Int, km/mol)

991 (105)

992 (81)

1436.8

1437.0

1456 (8)

858.2

1339 (45)

1063.3

1212.4

1065 (48)

COH bendb

1311.0

1304.6

CH3 rock.c

1063.1

1059.6

1063.2, 1059.6

1063.6

C
O str.

978.9

960.4

978.8, 960.4

953.7

978.9, 953.7

978.9

990 (75)

CH3 deform. COH bendb

1437.9 1311.4

1432.5 1304.8

1437.3 1303.6

1438.0, 1437.3

1437.7 858.4

1456 (7) 1342 (46)

CH3 rock.c

1062.8

1059.2

1062.8, 1059.1

1063.3

1062.9

1211.6

1067 (47)

C
O str.

981.3

962.9

981.2, 962.8

956.0

CH3 deform.

1451.8

1446.0

1451.4

COH bend

1331.8

1324.4

1324.9

864.7

1358 (28)

CH3 rock

1077.7

1070.9

1075.3

1226.8

1075 (3)

C
O str.

1033.7

1018.5

1008.1

1042.3

1048 (130)

1304.7

981.3, 956.1

980.7 1451.4

991 (77) 1473 (2)

a

Spin state (in superscript) assigned from the DFT (B3LYP/DZVP2/Stuttgart) calculations. See text and Supporting Information. b This band in the 13 CH3OH and CH318OH experiments overlaps with the 1305.6 cm
1 methane absorption, and its uncertainty is about 0.5 cm
1. c CH3 rocking mode is mixed with COH bending mode. 14582

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Table 2. Infrared Absorptions (cm
1) Observed (and Calculated) for the Late CH3OLnH Insertion Products in Solid Ar.a moleculeb 8

CH3OTbH

assignment

CH3OH

13

CH3OH

CH3OH + 13CH3OH

CH318OH

CH3OH + CH318OH

Tb
H str.

1385.9

1385.8

1385.8

1385.1

1385.5

C
O str.

1141.1

1123.4

1141.1, 1123.4

1105.0

1141.1, 1105.0

7

Dy
H str.

1391.2

1391.2

1391.2

1391.2

1391.2

C
O str.

1136.4

1119.0

1136.4, 1119.0

1100.4

1136.4, 1100.4

6

Ho
H str.

1399.6

1399.6

1399.6

1399.6

1399.6

C
O str.

1141.9

1124.8

1141.9, 1124.8

1105.5

1141.9, 1105.5

5

Er
H str.

1403.3

1403.3

1403.3

1403.3

1403.3

C
O str. Tm
H str.

1145.8 1236.3

1128.5 1236.1

1145.8, 1128.5 1236.1

1108.6 1235.4

CH3ODyH CH3OHoH CH3OErH

2

CH3OTmH

CH3OD

CH3OH DFT (Int, km/mol)

986.2

1439 (320)

1146.5

1154 (408)

983.0

1384 (226)

1142.2

1151 (389)

987.8

1411 (266)

1147.6

1165 (355)

995.7

1432 (248)

1145.8, 1108.6 1235.7

1150.5 881.7

1169 (247) 1283 (261)

1146.1

1157 (413)

882.8

1284 (169)

C
O str.

1142.4

1123.9

1142.4, 1123.9

1108.1

1142.4, 1108.1

1

Yb
H str.

1237.7

1237.7

1237.7

1237.1

1237.1

C
O str.

1142.4

1123.8

1142.4, 1123.8

1108.1

1142.4, 1108.1

1144.8

1161 (349)

2

Lu
H str.

1422.3

1422.3

1422.3

1421.3

1421.8

1014.8

1447 (296)

C
O str.

1163.8

1146.7

1163.8, 1146.7

1126.0

1163.8, 1126.0

1166.6

1168 (359)

CH3OYbH CH3OLuH

a

Only the major site absorptions for the C
O and Ln-H stretching modes are listed. b Spin state (in superscript) assigned from the DFT (B3LYP/ DZVP2/Stuttgart) calculations. See text and Supporting Information.

calculations were performed with the larger 6-311++G(d,p) basis set for C, H, and O.

’ EXPERIMENTAL RESULTS Infrared Spectra. In the reactions of late lanthanide metal atoms and methanol, metal independent absorptions for molecules, such as HCO,33 CH2OH,34 H2CO, CH4, and the solvated proton complex (ArnH+),35 were observed after sample deposition, similar to the results from laser-ablated uranium plus CH3OH experiments.24 The first three absorption sets decreased when the sample was annealed or photolyzed while the methane absorption at 1305.6 cm
1 increased slightly upon UV
visible irradiation, and the proton complex was destroyed. Weak absorptions due to the methanol dimer also increased when the sample was annealed to 30
40 K.36 In addition to these metal independent bands, lanthanide hydride absorptions were produced on sample annealing.7 Figure 1 shows infrared spectra from the reactions of laserablated Yb atoms and methanol in solid argon. No product absorptions were observed after sample deposition at 4 K except for two weak bands at 1151.8 and 1145.5 cm
1. New product absorptions were produced simultaneously at 1437.9, 1311.4, 1062.8, and 981.3 cm
1, and the 1151.8 cm
1 peak increased when the sample was annealed to 30 K (Figure 1, trace b). The former four absorptions were completely destroyed during subsequent irradiation through the λ > 470 nm filter, during which another two new bands at 1235.1 and 1139.3 cm
1 were produced. These two absorptions sharpened and slightly blueshifted to 1237.7 and 1142.4 cm
1 upon further sample annealing (Figure 1, trace d). When the sample was exposed to broad band irradiation (λ > 220 nm), the absorptions at 1237.7 and 1142.4 cm
1 almost disappeared while the diatomic YbO absorption at 660.0 cm
1 increased (not shown).5b Infrared spectra of reaction products of Dy, Ho, Er, and Tm with methanol are similar with those for Yb, and the product absorptions observed in these experiments are listed in Tables 1 and 2. In the reactions of laser-ablated lutetium atoms and methanol, no metal dependent product absorptions were observed right

Figure 2. Infrared spectra of laser-ablated Lu atom and CH3OH reaction products in solid argon at 4 K: (a) Lu + 0.5% CH3OH deposition for 60 min; (b) after annealing to 30 K; (c) after annealing to 40 K; (d) after λ >220 nm irradiation; (e) after annealing to 35 K.

after sample deposition at 4 K, as shown in Figure 2, trace a. The lutetium dihydride absorption at 1426.4 cm
17 as well as new product absorptions at 1422.3, 1163.8, 1147.9, and 1181.2 cm
1 appeared when the sample was annealed to 30 K. The dihydride band remained unchanged on annealing to 40 K while the intensities of the absorptions at 1147.9 and 1181.2 cm
1 doubled. At the same time, the absorptions at 1422.3 and 1163.8 cm
1 also increased slightly. These new bands all decreased on broadband irradiation. Similar product absorptions were observed in the reactions of Tb and methanol. The absorptions at 1385.9 and 1141.1 cm
1 grew on annealing to 30 K. Two new bands at 1159.8 and 1130.8 cm
1 increased when the sample was further annealed to 40 K, during which the intensities of the absorptions at 1159.8 and 1130.8 cm
1 slightly decreased. To help identify the newly observed reaction products, experiments with 13CH3OH, CH3OH + 13CH3OH, CH318OH, 14583

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Figure 3. Infrared spectra of laser-ablated Yb atom and isotopically substituted CH3OH reaction products in solid argon at 4 K after annealing to 30 K: (a) Yb +0.5% CH3OH; (b) Yb + 0.5% 13CH3OH; (c) Yb + 0.5% CH3OH + 0.5% 13CH3OH; (d) Yb + 0.5% CH318OH; (e) Yb + 0.5% CH3OH + 0.5% CH318OH; (f) Yb +0.5% CH3OD.

Figure 4. Infrared spectra of laser-ablated Lu atom and isotopically substituted CH3OH reaction products in solid argon at 4 K after annealing to 40 K: (a) Lu +0.5% CH3OH; (b) Lu + 0.5% 13CH3OH; (c) Lu + 0.5% CH3OH + 0.5% 13CH3OH; (d) Lu + 0.5% CH318OH; (e) Lu + 0.5% CH3OH + 0.5% CH318OH. The asterisk denotes a methanol dimer absorption.

and CH3OH + CH318OH samples were also carried out. In addition, the reactions of lanthanide atoms and methanol were also repeated by using the CH3OD sample so as to characterize possible Ln-H stretching modes of the O
H bond insertion products. Typical infrared spectra from the reactions of Yb and Lu with different isotopically substituted methanol samples are shown in Figures 3
5, and the experimental vibrational frequencies are listed in Tables 1
3 as well.

’ COMPUTATIONAL RESULTS Geometry optimizations and frequency calculations were carried out on the Ln(CH3OH), CH3OLnH, CH3LnOH, and Ln(OCH3)2 molecules for all of the late lanthanide atoms from Tb to Lu at the B3LYP level of theory, which are related to the

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Figure 5. Infrared spectra of laser-ablated Yb atom and isotopically substituted CH3OH reaction products in solid argon at 4 K after λ > 470 nm irradiation followed by annealing to 30 K: (a) Yb +0.5% CH3OH; (b) Yb + 0.5% 13CH3OH; (c) Yb + 0.5% CH3OH + 0.5% 13 CH3OH; (d) Yb + 0.5% CH318OH; (e) Yb + 0.5% CH3OH + 0.5% CH318OH. The asterisks denote the site absorptions of CH3OYbH and its isotopomers.

species observed in our experiments. The calculated vibrational frequencies are given in Tables 1
3 for the reaction products with CH3OH. The structural parameters and the frequencies with various isotopic substitutions are given in Supporting Information for the complex Ln(CH3OH), CH3OLnH, CH3LnOH and Ln(OCH3)2. The reaction energies show that an initial complex can be formed between Ln and CH3OH in an exoergic process for Tb to Lu. The complexation energies of 5 to 15 kcal/mol are consistent with the formation of a Lewis acid
base (Ln)
(CH3OH) complex. For Tb and Dy, we note that DFT with the B3LYP functional predicts the wrong ground state for the atoms as the calculated 4f 85d16s2 (octet) ground state of Tb is 61.9 kcal/mol lower than the expected 4f96s2 (sextet) state and calculated 4f 95d16s2 (septet) ground state of Dy is 23.7 kcal/mol lower than the expected 4f106s2 (quintet) state. B3LYP appears to favor splitting the f9 and f10 into f8d1 and f9d1. The initial atomic state would be expected to be retained in the complex considering the Lewis acid
base type bonding. However, the Tb(CH3OH) complex could be an exception since the energy of f to d promotion is extremely low.37 The representative optimized Yb(CH3OH) structure is shown in Figure 6 with the calculated frequencies listed in Table 1. The Ln
O bond distances in the complex are all roughly 2.5 Å except for Ln = Lu for which the Ln
O bond distance is about 0.1 Å shorter, similar with the Ln
O distances of the Ln(H2O) complexes identified previously.11 Calculations on the CH3OLnH molecules were performed with different geometries and electronic states. The staggered structures are the most favorable conformation for most products while the CH3OTbH molecule prefers the eclipsed structure. On the basis of the predicted relative stabilities, all the O
H bond insertion products have the same spin states as the corresponding lanthanide atoms except for CH3OTbH. However, comparisons between calculated and experimental frequencies led us to conclude that the higher spin states for the CH3OLnH molecules with Dy, Ho, and Er give much better frequencies, suggesting the relative stabilities of different spin states for these three 14584

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Table 3. Infrared Absorptions (cm
1) Observed (and Calculated) for Two Ln(OCH3)2 Complexes in Solid Ar moleculea

assignment

Tb(OCH3)2 antisym. C
O str.b

6

sym. C
O str Lu(OCH3)2 antisym. C
O str.b

2

sym. C
O str

CH3OH

13

CH3OH CH3OH + 13CH3OH CH318OH CH3OH + CH318OH CH3OD CH3OH DFT (Int, km/mol)

1130.8

1111.8

1130.8, 1119.1, 1111.8

1096.5

1130.8, ...,c 1096.5

1130.5

1140 (581)

1159.8

1144.8

1159.8, 1153.5, 1144.8

1125.2

1159.8, 1151.6, 1125.2

1159.8

1157 (254)

1147.9

1129.5

1147.9, 1136.4, 1129.5

1111.9

1147.9, 1121.1, 1111.9

1147.9

1162 (485)

1181.2

1166.3

1181.2, 1175.5, 1166.3

1139.2

1181.1, 1172.0, 1139.2

1180.7

1194 (186)

a Spin state(in superscript) assigned from the DFT (B3LYP/DZVP2/Stuttgart) calculations. Octet is also a possible ground state for Tb(OCH3)2. See text and Supporting Information. b Major site absorption. c Absorptions not observed due to band overlap.

Figure 6. Calculated structures of Yb(CH3OH), CH3OYbH, CH3OLuH, and Lu(OCH3)2 molecules at the B3LYP level with the DZVP2/Stuttgart (first) and/6-311++G(d,p)/Stuttgart basis sets (second). Bond lengths in Å and bond angles in degrees.

CH3OLnH molecules at B3LYP level are not reliable enough. As shown in the Supporting Information, the Ln
O and Ln
H bond lengths are strongly influenced by the change of spin states while all of the C
O bond lengths remain around 1.41 Å. The CH3OLnH molecules with high spin states are predicted to have shorter Ln
O and Ln
H bond lengths, which are also consistent with the corresponding stretching frequencies. In addition to the O
H bond insertion products, the geometries and vibrational frequencies of the experimentally unobserved CH3LnOH isomers, which result from C
O bond insertion, are also provided in Supporting Information. All the CH3LnOH molecules are predicted to be more stable than the corresponding O
H bond insertion products. Most C
O bond insertion products prefer lower spin states while the ground state for CH3DyOH is septet instead of quintet. However the energy difference between high and low spin states is not large enough for Tb through Er. Generally, the Ln
C and Ln
O bond lengths in the CH3LnOH molecules with high spin states are shorter than those with low spin states, similar to the trend calculated for the O
H bond insertion isomers. Frequency calculations reveal that the CH3LnOH molecules exhibit a strong absorption around 600 cm
1 and a weak absorption around 1100 cm
1 due to Ln-O stretching and CH3 deformation modes, respectively. The calculated results for the Ln(OCH3)2 molecules are listed in Table 3 and Supporting Information, and the structure of Lu(OCH3)2 is shown as an example in Figure 6. All of the

Ln(OCH3)2 molecules are predicted to have approximately C2v symmetry with two characteristic absorptions due to antisymmetric and symmetric C
O stretching vibrations. Both the C
O bond lengths and the corresponding frequencies are close to the values of the CH3OLnH molecules. Although B3LYP calculations suggest that either high spin or low spin states should be lowest in energy for different metals, the energy difference is not large enough to predict reliably the real ground state of the Ln(OCH3)2 molecules in most cases.

’ DISCUSSION Three new lanthanide molecular species can be identified from the matrix infrared spectra and density functional calculations. Ln(CH3OH). The four absorptions at 1437.9, 1311.4, 1062.8, and 981.3 cm
1 produced in the reactions of Yb and methanol are due to different vibrational modes of the same new species on the basis of the their identical behaviors throughout the experiments. The absorption at 981.3 cm
1 shifted to 962.9 and 956.0 cm
1 in the experiments with 13CH3OH and CH318OH samples. The isotopic 12C/13C and 16O/18O frequency ratios are similar to those of the C
O stretching mode of the methanol molecule observed at 1033.7 cm
1 in our experiments (frequencies given in Table 1). A small deuterium shift was found for this band when the CH3OD sample was used. Mixed isotopic samples revealed a doublet with the band positions almost the same as those in pure 13CH3OH and CH318OH experiments, suggesting 14585

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The Journal of Physical Chemistry A that only one C
O mode is involved in the product (Figure 3). The weak absorption at 1437.9 cm
1 shifted to 1432.5 cm
1 with 13 CH3OH isotopic sample, suggesting that this band is due to the CH3 deformation band of the methyl group, as attested by the smaller shift to 1437.3 cm
1 with 18O substitution. The absorptions at 1311.4 and 1062.8 cm
1 exhibit small 13C and 18O isotopic shifts while larger 453 cm
1 red and 148.8 cm
1 blue shifts were found in the CH3OD experiment. The deuterium shift for the 1062.8 cm
1 absorption is about the same as for the CH3 rocking mode of methanol at 1077.7 cm
1. The absorption at 1311.4 cm
1 is probably a vibrational mode involving a methyl group with less participation of the hydrogen atom in the OH group, which corresponds to the weak 1331.8 cm
1 COH bending mode for methanol. Since all of the absorptions at 1437.9, 1311.4, 1062.8, and 981.3 cm
1 are related to vibrational modes of methanol (Table 1), we assign these four absorptions to the Yb(CH3OH) complex. Similarly, the complexes Dy(CH 3 OH), Ho(CH 3 OH), Er(CH3OH), and Tm(CH3OH) were identified in the reactions of Dy through Tm and methanol following the Yb(CH3OH) example. Each complex is characterized by four infrared absorptions listed in Table 1. The observed frequencies for all the Ln(CH3OH) complexes are almost the same, and the major vibrational modes and isotopic shifts of methanol are preserved upon metal coordination, suggesting that the electronic and geometric structures of methanol in the Ln(CH3OH) complex are similar to those in the isolated CH3OH. Note that no Ln(CH3OH) complexes were observed for Tb and Lu as their C
O insertion reactions are spontaneous. The calculated vibrational frequencies (Table 1 and Supporting Information) for the Ln(CH3OH) complexes are in good agreement with the values observed in solid argon. The vibrational frequencies for the C
O stretching modes in Ln(CH3OH) are calculated to be at 990
995 cm
1 for Ln = Dy to Yb, consistent with the observed absorptions near 980 cm
1. The C
O stretch for Lu(CH3OH) is calculated to be lower at 955 cm
1 as compared to the same mode for the others, consistent with the shorter Lu
O bond distance. This suggests a stronger interaction between Lu and methanol than that in other Ln(CH3OH) complexes. The fact that the complexation energy for the Lu(CH3OH) complex is not as large as that of some other lanthanide-methanol complexes with longer Ln
O bond distances suggests that a number of factors go into the complexation energy. The CH3 deformation frequencies are predicted to be about 20 cm
1 higher than the experimental values around 1437 cm
1. The calculated frequencies for Ln = Ho, Er, Tm and Yb are within 30 cm
1 of the C
O
H bending and 5 cm
1 of the CH3 rocking frequencies, with the calculated values being larger as expected. Experimentally, the absorptions due to COH bending and CH3 rocking modes are observed around 1311 cm
1 and 1063 cm
1 respectively. For Dy(CH3OH), the Ln occupancy appears to be f9d1s2 from our calculations. The f10s2 quintet molecule cannot be optimized because the B3LYP wave function does not converge. The — C
O
Dy for the septet structure is about 10 larger than that in the other structures except for Ln = Tb and the predicted frequencies for the C
O
H bending and the CH3 rocking modes are slightly smaller than the observed values by less than 20 cm
1. It would be likely for the Tb(CH3OH) molecule to show the same difference if produced (Tb(CH3OH) was not produced) as found for Dy(CH3OH) since there is a similar issue with the atomic occupancies on Tb and the frequencies and geometries of

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Tb(CH3OH) and Dy(CH3OH) are comparable. The assignments of the Ln(CH3OH) complexes are further confirmed by comparisons between calculated and observed isotopic shifts in Supporting Information. CH3OLnH. In the Lu + CH3OH reaction, new absorptions at 1422.3 and 1163.8 cm
1 were produced during sample annealing. Almost no isotopic shift was found for the absorptions at 1422.3 cm
1 in the experiments with 13CH3OH and CH318OH samples, while the absorption shifted to 1014.8 cm
1 when CH3OD was the reactant. The large deuterium shift of 407.5 cm
1 and H/ D isotopic frequency ratio 1.4016 require assignment to an Lu
H stretching mode.7 A similar large H/D ratio was observed for the CH3OUH molecule.24 The other band at 1163.8 cm
1 shifted to 1146.7 cm
1 with 13CH3OH and to 1126.0 cm
1 using CH318OH (Figure 4, traces b and d). Both the 12C/13C and 16O/18O ratios, 1.0166 and 1.0310, are in the range of values for a C
O stretching vibration, but the higher 16O/18O frequency ratio for CH3OLuH suggests that this mode has antisymmetric C
O
Lu stretching character, as found for the uranium analog.24 No intermediate absorption was produced in the experiments with mixed isotopic samples, indicating the involvement of only one C
O mode in this molecule (Figure 4, traces c and e). Observation of new C
O and Lu
H stretching vibrations leads us to assign the absorptions at 1422.3 and 1163.8 cm
1 to the CH3OLuH molecule. The CH3OLnH molecules were produced in all the reactions of late lanthanide atoms and methanol, and they are identified by the characteristic C
O and Ln
H stretching vibrations. However, in the Tb, Dy, Ho, Er, Tm, and Yb cases, the spectra were complicated by matrix environmental site absorptions. Using Yb as an example, the absorptions at 1151.8 and 1145.5 cm
1 were produced after sample deposition. Experiments with isotopically substituted methanol samples revealed that both bands are due to C
O stretching vibrations, and only one C
O moiety is involved for each band (Figure 5). These two bands are assigned to matrix environmental site absorptions of the CH3OYbH product, which is produced during sample deposition. Similarly, the 1235.1 and 1139.3 cm
1 absorptions are assigned to less stable matrix site bands produced after visible irradiation, which converted into 1237.7 and 1142.4 cm
1 absorptions when the sample was annealed. In order to support our assignments, frequency calculations were performed on the CH3OLnH molecules listed in Table 2 (see also the Supporting Information). Four major absorptions due to Ln
H stretching, C
O stretching, CH3 deformation and Ln-O stretching modes are predicted above 400 cm
1. Since the infrared intensities of the latter two bands are much lower than the other two, we will only focus on the former two bands, which are also observed in our experiments. The C
O stretching modes from the DFT calculations are 10
20 cm
1 higher than the observed absorptions. However, frequency calculations on this mode for different spin states of the same molecule give almost the same results, from which it is not possible to predict the actual ground state of the O
H bond insertion products. Diagnostic assignments can be made from the comparison between experimental and theoretical Ln
H frequencies because of the obviously lower values for CH3OTmH and CH3OYbH molecules compared with the others. As shown in Table 2 and the Supporting Information, the high spin structures have higher Ln
H stretching frequencies in the range of 1350
1450 cm
1 while the frequencies for the low spin molecules are around 1300 cm
1. 14586

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The Journal of Physical Chemistry A As a result, a direct comparison leads us to conclude that the octet for Tb, septet for Dy, sextet for Ho, quintet for Er, doublet for Tm, singlet for Yb, and doublet for Lu provide the most reasonable Ln
H stretching frequencies, which are about 50 cm
1 higher than the experimental measurements without anharmonic correction, although many of these structures are not the lowest in energy. Generally, the energy difference between different spin states is within 2 to 3 kcal/mol while that of quintet and septet CH3ODyH molecules differ by about 12 kcal/mol. A similar trend in Ln
H stretching frequencies has been observed in the HLnOH molecules produced in the reactions of lanthanide atoms and water.11 Frequency calculations on the 13CH3OLnH, CH318OLnH and CH3OLnD isotopomers (Supporting Information) also provide similar isotopic shifts with the experimentally observed values, which also support the assignments of O
H bond insertion products. The CH3OLnH molecules can be divided into two groups based on the difference in Ln
H stretching frequencies, which essentially result from the difference in electronic configurations. The first group of CH3OLnH, including Ln = Tb, Dy, Ho, Er (f ns2), and Lu (f14d1s2), has an electron configuration of fn
1dα or f n‑1dβ (f14dα or f14dβ for CH3OLuH, α/β means spin up/ down) after losing two e
to form mostly ionic bonds with H and OCH3. Therefore, one f electron of the metal needs to be promoted into the metal d orbitals to form CH3OLnH except for CH3OLuH, where the Lu atom has a f14d1s2 ground state. The promotion into the d orbital is due to the tightness of the f orbitals and the fact that they do not overlap as well with the ligand orbtials as do the d orbitals and, so do not form strong bonds. For the remaining two molecules, CH3OTmH and CH3OYbH, there are no d electrons on these two metals. It is not favorable for their f electrons to transfer into d orbitals, simply because the f orbital energies become more stable as electrons fill the orbitals to form a full (f14) or nearly full shell (f13). This is consistent with the fact that Yb and Tm have much a greater energy gap between the f ns2 ground state and the f n‑1d1s2 excited state than do Tb, Dy, Ho, and Er.37 For Yb, the f n
1d1s2 state is even higher than its f ns1p1 excited state. As a result, the higher spin states of CH3OTmH and CH3OYbH are higher in energy by more than 15 kcal/mol than their ground spin states, whereas the energy differences for the other CH3OLnH molecules are smaller. The energy differences between high and low spin states were also found to play important roles in the HLnOH systems.11 As listed in Table 2, the Ln-H frequencies for the O
H bond insertion products generally increase from Tb to Lu, which is the result of the lanthanide contraction.38 Consistent with this notion, the calculated Ln
H bond lengths slightly decrease from Tb to Lu except for Tm and Yb, where the bond lengths are longer than the others (Supporting Information). The bonding in the Yb
H molecular orbital is mostly the H 1s orbital with Yb contributing d orbital character. A similar bonding interaction is found in the Lu
H bond but with a more negative molecular orbital energy as compared to the Yb
H bond. In addition, the unpaired electron in the hybridized sdn orbital on Lu has an additional interaction with H. Thus, all of the results point to the Lu
H bond being stronger than the Yb
H bond. Similarly, the Lu
O bond is also calculated to be stronger than the Yb
O bond based on the bond lengths and Ln
O stretching frequencies. Ln(OCH3)2. In the lutetium experiments, new absorptions at 1181.2 and 1147.9 cm
1 increased when the sample was annealed to higher matrix temperatures. The absorptions at 1181.2 and

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1147.9 cm
1 shifted to 1166.3 and 1129.5 cm
1 in the 13CH3OH experiment with isotopic frequency ratios of 1.0128 and 1.0163, which are on the lower side of the values for C
O stretching vibrations. In the mixed CH3OH + 13CH3OH experiment, two triplet patterns were observed at 1181.2, 1175.5, 1166.3 cm
1 and 1147.9, 1136.4, 1129.5 cm
1 respectively (Figure 4, trace c). The observation of the two intermediate absorptions indicated that two equivalent C
O moieties are involved in both modes. Consistent with the 13CH3OH experiment, reaction with CH318OH sample also revealed that the absorptions at 1181.2 and 1147.9 cm
1 are due to C
O stretching modes with shifts to 1139.2 and 1111.9 cm
1 and ratios of 1.0369 and 1.0324, which are on the higher side of the values for C
O stretching vibrations. Additional intermediate absorptions were observed at 1172.0 and 1121.1 cm
1 in the mixed CH3OH + CH318OH experiment (Figure 4, trace e). No other absorption was found to track the absorptions at 1181.2 and 1147.9 cm
1. Hence, we assign the two new bands at 1181.2 and 1147.9 cm
1 to symmetric and antisymmetric C
O stretching vibrations of the Lu(OCH3)2 molecule with clear participation of the Lu metal atom. The absorptions at 1159.8 and 1130.8 cm
1 produced in the reaction Tb + CH3OH were found to have similar isotopic ratios and splittings to the absorptions at 1181.2 and 1147.9 cm
1 observed in the Lu experiment (Table 3). Accordingly, the absorptions at 1159.8 and 1130.8 cm
1 are assigned to symmetric and antisymmetric C
O stretching vibrations of the Tb(OCH3)2 molecule. The assignments of these bands to those of the Tb(OCH3)2 and Lu(OCH3)2 molecules are strongly supported by our theoretical calculations. Frequency calculations reveal that both the antisymmetric and symmetric C
O stretching vibrations for Lu(OCH3)2 are about 5 cm
1 higher than the experimental values. For the antisymmetric mode observed at 1147.9 cm
1, the 12C/13C and 16O/18O ratios of 1.0163 and 1.0324 are in excellent agreement with the calculated values of 1.016 and 1.033 at the B3LYP/DZVP2+Stuttgart level and of 1.0165 and 1.0332 with the larger 6-311++G(d,p) basis set. The 12C/13C and 16 O/18O frequency ratios for the symmetric C
O mode are calculated to be 1.014 and 1.037 at the B3LYP/DZVP2+Stuttgart level and 1.0125 and 1.0380 with the larger 6-311++G(d,p) basis set, in agreement with the experimental ratios of 1.0128 and 1.0369. A similar correlation between calculated and experimental frequencies and isotopic frequency ratios is also found for the Tb(OCH3)2 molecule. Comparable calculated isotopic ratios are obtained for the unobserved Ln(OCH3)2 (Ln = Dy
Yb) molecules, where the 12C/13C and 16O/18O ratios of the C
O antisymmetric stretching vibrational frequencies are predicted to be about 1.017 and 1.032, and the 12C/13C and 16O/18O ratios of the C
O symmetric modes about 1.017 and 1.036 (Supporting Information). Reaction Mechanisms. The reactions of late lanthanide atoms and methanol in solid argon can be summarized as follows:

Reactions of Dy, Ho, Er, Tm, and Yb atoms with methanol in solid argon produce the Ln(CH3OH) complexes first, which 14587

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Figure 7. Calculated potential energy surface for the reaction of Yb + CH3OH. Energies are in kcal/mol.

isomerize to the more stable CH3OLnH product on visible irradiation. These complexes are computed to be bound by 5
14 kcal/mol (Supporting Information). The stabilization of the lanthanide metal
methanol complex suggests that the energy barrier for the O
H bond insertion reaction is higher than the energy released with the formation of the Ln(CH3OH) complex, which prohibits the spontaneous production of the CH3OLnH molecule for these five lanthanide metals. However, visible irradiation activates the insertion reaction. In contrast, both terbium and lutetium atoms react with methanol molecules to form the inserted CH3OTbH and CH3OLuH products spontaneously during sample annealing without observation of the Tb(CH3OH) and Lu(CH3OH) complexes, although both of them are predicted to be stable. This is similar with the mechanism proposed for the reactions of methanol with scandium and uranium.18,24 To understand further the difference in reaction mechanisms, representative potential energy surfaces for the reactions of Yb and Lu were obtained at B3LYP level of theory for comparison. The calculated potential energy surface for the Yb + CH3OH reaction is given in Figure 7. The Yb(CH3OH) COMPLEX is initially formed with a complexation energy of
6.0 kcal/mol. The reaction can then proceed in two directions from this complex, leading to the CH3OYbH or CH3YbOH insertion products. The transition state (TS1) leading to the CH3OYbH product has a geometry qualitatively like that of the Yb(CH3OH) complex; the main change is an increase in the C
O
Yb bond angle from 135.0 in the complex to 179.3 in the transition state and a decrease in the Yb
O bond distance from 2.503 Å to 2.117 Å. The barrier height for the reaction proceeding through TS1 is calculated to be 21.7 kcal/mol. No direct transition state to the CH3YbOH product was found by us, similar to the reaction mechanism for the reaction of methanol with scandium.18 The formation of the C
O bond insertion product is quite exothermic. It is also possible that the barrier to exchange the CH3 between the O and Yb is higher than the barrier leading to a dissociative reaction. Our calculation shows that Yb(CH3OH) can split into the CH3 and YbOH radical fragments via a transition state (TS2) with a barrier height of 24.4 kcal/mol. The C
O bond distance

of 2.037 Å in the TS2 is 0.58 Å longer than that in the Yb(CH3OH) complex and the C
Yb
O angle of 94.6 is smaller than that of 135.0 in the Yb(CH3OH) complex. TS1 has a barrier height slightly lower than the barrier height of TS2 by 2.7 kcal/mol, and TS1 is the transition state directly connected to the CH3OYbH product. TS2 leads to CH3 + YbOH instead of the CH3YbOH, and the generated radicals can undergo various reactions that lead to other products. Thus, the absence of the product CH3YbOH in the matrix, which should be formed by a very exothermic reaction, is likely the result of kinetic control. The CH3YbOH molecule is computed to have a strong band around 600 cm
1 due to the Yb
O stretch (Supporting Information). However, no new absorption was observed in that region, which is also the case for other lanthanide metals (Dy
Tm). The calculated PES of the Lu + CH3OH reaction (Figure 8) differs significantly from the that of the Yb + CH3OH reaction. The transition state TS1 leading to CH3OLuH on the Lu + CH3OH PES is predicted to be 3.3 kcal/mol higher than the energy of the COMPLEX, without a zero point energy correction (ZPE). The inclusion of the substantial ZPE correction leads to the energy of TS1 being below that of the complex by 1.2 kcal/mol suggesting that the potential energy surface is quite anharmonic in this region. This is in contrast to the Yb PES which shows a substantial barrier of 21.7 kcal/mol starting from the complex and passing through TS1. The Lu
O bond distance of 2.152 Å in TS1 is 0.2 Å shorter than the Lu
O bond distance in the COMPLEX and 0.2 Å greater than the Lu
O bond distance in the CH3OLuH insertion product. The — C
O
Lu bond angle in TS1 also falls between the — C
O
Lu of 138.6 in the COMPLEX and 173.7 in the CH3OLuH product. Another difference between the Lu + CH3OH PES and the Yb + CH3OH PES is that TS2 in the Lu PES is the transition state between CH3OLuH and CH3LuOH whereas in the Yb PES such a transition state does not exist, and instead, CH3YbOH is formed via a two-step reaction from the Yb(CH3OH) complex with radical intermediates involved. In fact TS2 for Yb correlates with the COMPLEX rather than CH3OYbH. The Lu, O, and C atoms in TS2 on the Lu + CH3OH PES form a triangle with 14588

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Figure 8. Calculated potential energy surface for the reaction of Lu + CH3OH and Lu + 2CH3OH. Energies are in kcal/mol.

r(Lu
O) = 1.888 Å, r(C
O) = 1.971 Å, and r(Lu
C) = 2.576 Å. In contrast, TS2 in the Yb + CH3OH PES has r(Yb
O) = 2.131 Å, r(C
O) = 2.048 Å, and r(Yb
C) = 3.092 Å. The activation energy to reach TS2 from CH3OLuH is very high, 68.8 kcal/mol, so even though CH3LuOH is more stable by 21.5 kcal/mol than CH3OLuH, it cannot be readily accessed and the observed product is CH3OLuH in the matrix. Atomic Lu has a f14d1s2 electron configuration as compared to the closed shell f14s2 configuration for atomic Yb. The fact that the Lu atom is an open shell radical with an available d electron is most likely the reason why Lu has lower barriers to H transfer than does Yb. According to the experimental observations, the potential energy surfaces of Dy through Tm reactions should be similar with that of Yb since promotions from their fns2 ground states to the f n‑1d1s2 electron configurations require about 7 kcal/mol for Dy, Ho, Er and more than 13 kcal/mol for Tm and Yb.37 Different from these metals, the f8d1s2 state of Tb is only 0.8 kcal/mol above the ground state,37 which essentially makes Tb behave the same as Lu during the reactions with methanol. In Figure 8, we also show the PES for the addition of the second CH3OH to the initial COMPLEX Lu(CH3OH) and to CH3OLuH. The first complexation reaction is exothermic by ∼
7 kcal/mol. The complexation reaction of CH3OH with CH3OLuH is more exothermic,
14.2 kcal/mol. The initial complex CH3OLnH(CH3OH) undergoes a change in structure to form a more stable CH3OLnH(CH3OH) complex with the O
Lu
O bond angle close to 180. This allows the hydridic H bonded to the Lu to form an intramolecular H 3 3 3 H bond with the protic H on the O with a short H 3 3 3 H bond distance of 1.68 Å. The transition state for loss of H2, TS3, is higher in electronic energy than the complex by 1.0 kcal/mol. An intrinsic reaction coordinate (IRC) path39 was calculated to show that TS3 connects to the correct reactants and products. As found for TS1, the imaginary frequency in TS3 is not small, ∼ 680i cm
1, so the zero point energy corrected value falls below the energy of the complex. Again, the results suggest that this region of the potential energy

surface is quite anharmonic. The results show that the addition of the second CH3OH to CH3OLuH will rapidly lead to formation of the dimer with loss of H2. The adiabatic energy difference between CH3LnOH and CH3OLnH ranges from 17 to 21 kcal/mol with the former always being more stable. To better understand the energy difference between the two isomers, we calculated the Ln
R and Ln
OR0 bond dissociation energies as given in Table 4. The results show that the CH3Ln
OH BDEs are larger than the CH3O
LnH BDEs in the complex by 5 to 10 kcal/mol. The CH3
LnOH BDEs are smaller than the CH3OLn-H BDEs by a comparable amount. Another possible interpretation is to assume that Ln(II) is formed in the reaction and that the two ligands are anions. Thus one can dissociate the CH3OH molecule into CH3O + H or CH3 + OH. The CH3O
H BDE40 is 105.1 kcal/mol at 298 K and the CH3O and H electron affinities are 1.570 ( 0.02241 eV and 0.7541942 eV respectively. Thus the processes to form CH3O
and H
together are about 52 kcal/mol endothermic. The CH 3
OH BDE 40 is 92.0 kcal/mol at 298 K and the electron affinities of CH3 and OH are