Isocyanate Formation from Reactions of Early Lanthanide Metal Atoms

Article pubs.acs.org/JPCA

Isocyanate Formation from Reactions of Early Lanthanide Metal Atoms with NO and CO in Solid Argon Published as part of The Journal of Physical Chemistry virtual special issue “W. Lester S. Andrews Festschrift”. Jiwen Jian, Qingnan Zhang, Xuan Wu, and Mingfei Zhou* Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: The reactions of early lanthanide metal atoms (Ce, Pr, and Nd) with carbon monoxide and nitric oxide mixtures are studied by infrared absorption spectroscopy in solid argon. The reaction intermediates and products are identified via isotopic substitution as well as theoretical frequency calculations. The results show that the reactions proceed with the initial formation of inserted NLnO molecules, which subsequently react with CO to form the NLnO(CO) complexes on annealing. The NLnO(CO) complexes further isomerize to the more stable isocyanate OLnNCO species under UV light excitation.

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INTRODUCTION Transition-metal catalytic reduction of NO and CO, which are two of the main pollutants in exhaust gases from gasoline engines, has been the topic of extensive theoretical and experimental studies.1−3 Previous investigations show that the noble metals have the most pronounced catalytic activities for the conversion of carbon monoxide and nitric oxide to harmless N2 and acceptable CO2.1−3 Since the first detection of isocyanate (NCO) surface complex in the NO + CO reaction on supported noble metal catalysts by infrared spectroscopy,4−6 the formation mechanism and the role of the catalytic reduction of NO and CO have been extensively studied. It has been shown that the isocyanate species acts as an important intermediate in the production of N2O or N2.7−11 Nanosecond time-resolved in situ Fourier transform infrared spectroscopic study found that the key intermediate step in the reaction between carbon monoxide and nitric oxide is the flip of a cyanide group from a silver nanoparticle to the alumina support.12 However, the formation mechanism of isocyanate species is still debated. Previous studies reached similar conclusions that the initial step may be the dissociation of the NO molecule on the surface, and this may be followed by attack of a CO molecule to form the surface isocyanate species.13−21 But more recent studies indicate that the breaking of the NO bond may be directly assisted by the attack of CO molecule to form a surface nitride species, which then reacts with CO to form the isocyanate NCO species.8,12,22,23 The reactions of metal atoms with NO and CO serve as the simplest model in understanding the intrinsic mechanism of catalytic NO and CO reduction processes. Previous matrix isolation infrared spectroscopic studies on the reactions of transition-metal atoms with CO and NO mixtures show that some metal atoms react with NO and CO to form unsaturated metal carbonyl nitrosyl complexes.24−28 The end-on bonded © XXXX American Chemical Society

M(CO)(NO) complexes isomerize to the side-on bonded M(CO)(η2-NO) complexes, which serve as precursors for the insertion reaction that leads to the reduction of NO and the formation of isocyanate OMNCO species.24−26 Here we report a combined matrix isolation infrared spectroscopic and theoretical study on the reactions of early lanthanide metal atoms with NO and CO mixtures in solid argon. We will show that the reactions proceed with the initial formation of inserted NLnO species, which subsequently react with CO to form the NLnO(CO) complexes. The isocyanate OLnNCO species are formed from the NLnO(CO) complexes under UV light excitation. We note that early lanthanide metals and oxides are widely used as catalyst dopants and/or supports for selective catalytic reduction of NO and CO.29−31 The species reported here may act as both structural and functional models for surface-bound intermediates in lanthanide-related catalytic reduction processes.

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EXPERIMENTAL AND COMPUTATIONAL METHODS The experimental setup for matrix isolation infrared spectroscopy has been described in detail previously.32 Briefly, the 1064 nm fundamental output of a Nd:YAG laser (Continuum, Minilite II; 10 Hz repetition rate) was used to ablate a rotating lanthanide metal target to produce lanthanide metal atoms. The laser-evaporated metal atoms were codeposited with premixed CO + NO reagent gas in excess argon onto a cryogenic CsI window, which was maintained at 4 K by means of a closedcycle helium refrigerator. The CO + NO/Ar mixtures were prepared in a stainless steel vacuum line using standard Received: August 28, 2017 Revised: September 19, 2017 Published: October 3, 2017 A

DOI: 10.1021/acs.jpca.7b08586 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

to the inserted NCeO molecule from reaction with nitric oxide.43 The CeO and CeO2 absorptions at 808.3 and 736.9 cm−1 were also observed.44−46 Cerium carbonyl complexes were barely observed.47 Stepwise annealing the sample to 25 and 30 K increased the NCeO absorptions and produced new absorptions at 2169.3 and 747.5 cm−1. The 2169.3 and 747.5 cm−1 absorptions were almost destroyed when the sample was subjected to broadband irradiation using a mercury arc lamp with a 280 nm long-wavelength pass filter, during which a group of new absorptions at 2189.5, 767.6, 643.9, and 640.5 cm−1 was produced. The spectra from codeposition of laser-evaporated Pr atoms with 0.05% NO + 0.1% CO in argon are shown in Figure 2.

manometric technique. The CO (Arkonic Gases & Chemical Inc., >99.99%), NO (Dalian DT, >99.9%), and isotopic-labeled 13 CO (ISOTEC, 99%), C18O (ISOTEC, 95%), and 15NO (Cambridge isotope laboratories Inc., 98%) were used without further purification. After 1 h of sample deposition at 4 K, infrared absorption spectrum in the mid-infrared region (4000−450 cm−1) was recorded using a Bruker Vertex 80 V spectrometer at 0.5 cm−1 resolution using a liquid nitrogen cooled HgCdTe (MCT) detector. Selected samples were subjected to annealing and photolysis experiments to initiate diffuse and photoinduced reactions. Quantum chemical calculations were performed to determine the molecular structures and to support the assignment of vibrational frequencies of the observed species. The threeparameter hybrid functional according to Becke with additional correlation corrections due to Lee, Yang, and Parr (B3LYP) implemented in Gaussian 09 program package was used to optimize the geometry structures and to calculate the harmonic vibrational frequencies.33−35 Considering the strong relativistic effects on lanthanides, the quasi-relativistic pseudopotentials with 28-core electrons and the associated valence basis sets (ECP28MWB_SEG) for the Ce, Pr, and Nd atoms36,37 and Dunning’s correlation consistent basis set with polarized tripleζ plus diffuse functions (aug-cc-pVTZ) for the C, N, and O atoms were used.38 Transition states were optimized applying the synchronous transit-guided quasi-Newton (STQN) method and were verified by intrinsic reaction coordinate (IRC) calculations.39

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RESULTS AND DISCUSSION Infrared Spectra. The infrared spectra in selected regions from codeposition of laser-evaporated Ce atoms with 0.05% NO + 0.05% CO mixture in argon are illustrated in Figure 1. Besides the strong CO, NO, and (NO)2 absorptions, metalindependent absorptions for NxOy, (NO)2+, (NO)2−, OC-H2O, and (CO)2−, which are common for laser-ablated metal atom reactions with nitric oxide and carbon monoxide, were observed.40−42 After sample deposition, cerium-dependent absorptions appeared at 757.2 and 690.9 cm−1, which are due

Figure 2. Infrared spectra in the 2300−2100 and 920−700 cm−1 regions from codeposition of Pr atoms with 0.05% NO + 0.1% CO in argon (a) after 1 h of sample deposition at 4 K, (b) after annealing to 20 K, (c) after annealing to 30 K, and (d) after 15 min of λ > 280 nm UV light irradiation.

Metal-dependent absorptions of NPrO at 900.8 and 742.0 cm−1 and PrO at 817.0 cm −1 were produced on sample deposition.43−46 The NPrO absorptions increased markedly on annealing. Three new absorptions at 2170.9, 886.1, and 734.9 cm−1 were also produced on sample annealing, which were almost destroyed when the sample was subjected to λ > 280 nm broadband UV light irradiation. With the disappearance of the 2170.9, 886.1, and 734.9 cm−1 absorptions, a new group of absorptions at 2189.0, 773.4, 643.4, and 640.7 cm−1 was produced under λ > 280 nm broadband UV light irradiation. Figure 3 shows the spectra from similar experiments using a Nd target. After sample deposition, two absorptions at 768.7 and 661.4 cm−1 were presented. These absorptions were previously observed in the reaction of Nd atoms with NO in solid argon and have been assigned to the inserted NNdO molecule.43 Two additional groups of new product absorptions were produced under subsequent annealing and photolysis. The first group involves three absorptions at 2170.1, 756.7, and 653.1 cm−1, which was produced on sample annealing but was destroyed under λ > 280 nm broadband UV light irradiation. In contrast, the second group with four absorptions at 2189.9, 789.7, 644.6, and 640.7 cm−1 was produced at the expense of the first group of absorptions upon λ > 280 nm broadband UV light irradiation.

Figure 1. Infrared spectra in the 2210−2100 and 820−600 cm−1 regions from codeposition of cerium atoms with 0.05% NO + 0.05% CO in argon (a) after 1 h of sample deposition at 4 K, (b) after annealing to 20 K, (c) after annealing to 30 K, and (d) after 15 min of λ > 280 nm UV light irradiation. B


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lanthanide metal atoms with NO in solid argon.43 On sample annealing, new product absorptions were observed for all the metal systems studied here, namely, at 2169.3 and 747.5 cm−1 for Ce, at 2170.9, 886.1, and 734.9 cm−1 for Pr, and at 2170.1, 756.7, and 653.1 cm−1 for Nd. These absorptions were only produced in the mixed NO + CO/Ar experiments, implying that both NO and CO are involved. The absorptions near the 2170 cm−1 region show no shift with 15NO but quite large shifts with 13CO and C18O. The isotopic 12CO/13CO ratios (1.0226, 1.0224, and 1.0227) and the C16O/C18O ratios (1.0242, 1.0243, and 1.0242) indicate that these absorptions are due to terminally bonded carbonyl stretching vibrations. The doublet isotopic structures in the mixed NO + 12CO + 13CO and NO + C16O + C18O experiments confirm that only one CO fragment is involved. These CO stretching vibrational frequencies are blue-shifted from that of free CO in solid argon (2138.2 cm−1), suggesting that the CO fragment is coordinated on a positively charged metal center, which causes a blue shift of the CO stretching frequency due to electrostatic interactions.48,49 In the Ln−N and Ln−O stretching frequency region, the absorptions show isotopic shifts with 15NO but zero isotopic shifts with the 13CO and C18O samples, indicating that the N and O atoms involved in these Ln−N and Ln−O stretching modes are originated from NO. No intermediate absorptions were observed in the experiments with mixed 14NO + 15NO + CO samples, confirming that the observed species are reaction products from one NO. Accordingly, the observed absorptions are assigned to the NLnO(CO) complexes (Ln = Ce, Pr, and Nd). In the case of Ce, only one absorption at 747.5 cm−1 was observed to track with the upper CO stretching mode at 2169.3 cm−1. This absorption is only 9.7 cm−1 redshifted from the Ce−O stretching mode of NCeO with quite similar isotopic N-15 shift (3.6 cm−1 versus 5.3 cm−1). Thus, the 747.5 cm−1 absorption is attributed to the Ce−O stretching mode of the NCeO(CO) complex. The Ce−N stretching mode was not observed due to weakness. (This mode was predicted to have very low IR intensity as will be discussed below.) The 886.1 and 734.9 cm−1 absorptions in the Pr + CO + NO reaction are assigned to the Pr−N and Pr−O stretching modes of the NPrO(CO) complex. Upon CO coordination, the Pr−N and Pr−O stretching modes are red-shifted by 14.7 and 7.1 cm−1, respectively. The 756.7, and 653.1 cm−1 absorptions in the Nd system are attributed to the Nd−O and Nd−N stretching modes of the NNdO(CO) complex, which are 12.0 and 8.3 cm−1 red-shifted from those of NNdO in solid argon. Density functional theoretical calculations were performed to support the assignment and to elucidate the structure and bonding of the observed NLnO(CO) complexes. The optimized structural parameters are shown in Figure 5. Calculations were first performed on the bare NLnO molecules. The NCeO molecule has a linear 2Σ ground state with an electronic configuration of (core) (1π)4 (1σ)2(2π)4 (2σ)1. The nondegenerate 1σ and the doubly degenerate 1π molecular orbitals are composed of Ce 5d and O and N 2p atomic orbitals. The nondegenerate 2σ and the doubly degenerate 2π molecular orbitals are formed by the interactions of Ce 4f orbitals with the O and N 2p atomic orbitals. All of these molecular orbitals are both Ce−N and Ce−O bonding in character. Therefore, the Ce center in NCeO has an oxidation state of +IV. The NPrO molecule was determined to have a closed-shell 1Σ singlet ground electronic state with an electronic configuration of (core) (1π)4 (1σ)2(2π)4 (2σ)2. The Pr center has an (f0d0) configuration and an oxidation state of +V as its

Figure 3. Infrared spectra in the 2250−2100 and 800−600 cm−1 regions from codeposition of neodymium atoms with 0.05% NO + 0.05% CO in argon (a) after 1 h of sample deposition at 4 K, (b) after annealing to 25 K, (c) after annealing to 30 K, and (d) after 15 min of λ > 280 nm UV light irradiation.

Experiments were repeated using the isotopic-labeled mixture samples including 15NO + CO, 14NO + 15NO + CO, NO + 13CO, NO + 12CO + 13CO, NO + C18O, and NO + C16O + C18O. The infrared spectra in selected regions with different isotopic samples are shown in Figure 4 for Ce. The spectra for Pr and Nd are shown in Figures S1 and S2 of Supporting Information. The band positions of the observed species are summarized in Table 1. NLnO(CO). The ground-state Ce, Pr, and Nd atoms react readily with NO in forming the inserted NCeO (757.2 and 690.9 cm−1), NPrO (900.8 and 742.0 cm−1), and NNdO (768.7 and 661.4 cm−1) molecules in solid argon. These inserted molecules were previously identified in the reactions of

Figure 4. Difference infrared spectra in the 2220−2075 and 790−700 cm−1 regions from codeposition of cerium atoms with isotopic-labeled NO and CO in excess argon (spectrum recorded after 15 min of UV light irradiation minus spectrum recorded after 30 K annealing). (a) 0.05% NO + 0.05% CO, (b) 0.05% 15NO + 0.05% CO, (c) 0.05% NO + 0.05% 15NO + 0.05% CO, (d) 0.05% NO + 0.05% 13CO, and (e) 0.05% NO + 0.05% 12CO + 0.05% 13CO. C


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Table 1. Infrared Absorptions (cm−1) from Reactions of Early Lanthanide Metal Atoms with NO + CO Mixtures in Solid Argon NO + CO NCeO NCeO(CO) OCeNCO

NPrO NPrO(CO)

OPrNCO

NNdO NNdO(CO)

ONdNCO

757.2 690.9 2169.3 747.5 2189.5 767.6 643.9 640.5 900.8 742.0 2170.9 886.1 734.9 2189.0 773.4 643.4 640.7 768.9 661.5 2170.1 756.7 653.1 2189.9 789.7 644.6 640.7

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NO + CO 751.9 686.6 2169.3 743.9 2177.2 767.6 640.7 637.5 879.1 737.0 2170.9 865.2 729.6 2176.8 773.4 640.2 636.9 760.0 648.6 2170.1 748.8 639.8 2177.5 789.7 641.2 637.4

NO + 13CO

NO + C18O

mode

757.2 690.9 2121.3 747.5 2128.9 767.6 ? 622.7 900.8 742.0 2122.8 886.1 734.9 2128.4 773.4 625.5 622.8 768.9 661.5 2121.9 756.7 653.1 2129.0 789.7 627.2 622.7

757.2 690.9 2118.1 747.5 2176.3 767.6 639.4 635.9 900.8 742.0 2119.6 886.1 734.9 2176.1 773.4 638.7 635.4 768.9 661.5 2118.9 756.7 653.1 2176.8 789.7 640.5 635.9

CeO str CeN str CO str CeO str NCO asym str CeO str NCO bend NCO bend PrN str PrO str CO str PrN str PrO str NCO asym str PrO str NCO bend NCO bend NdO str NdN str CO str NdO str NdN str NCO asym str NdO str NCO bend NCO bend

OLnNCO. New product absorptions at 2189.5, 767.6, 643.9, and 640.5 cm−1 for Ce, at 2189.0, 773.4, 643.4, and 640.7 cm−1 for Pr, and at 2189.9, 789.7, 644.6, and 640.7 cm−1 for Nd were produced under λ > 280 nm broadband UV light irradiation at the expense of the NLnO(CO) complex absorptions. These absorptions are assigned to vibrational fundamentals of the OLnNCO molecules, a structural isomer of NLnO(CO). Taking the Ce system as an example, the upper absorption at 2189.5 cm−1 exhibits 12.3 cm−1 red shift with 15NO, 60.6 cm−1 red shift with 13CO, and 13.2 cm−1 red shift with C18O. The band position and isotopic frequency shifts are appropriate for an antisymmetric NCO stretching vibration, which is ∼266 cm−1 blue-shifted from that of free NCO in solid argon.52 The band position falls into the range of the isocyanate species in solid noble metal catalyst surfaces, which were observed in the 2160−2270 cm−1 region.4−9 The doublet isotopic structures in the mixed 14NO + 15NO + CO, NO + 12CO + 13CO, and NO + C16O + C18O experiments confirm that only one NCO subunit is involved in this mode. The 766.5 cm−1 absorption shows no isotopic shifts with 14NO, 13CO, and C18O, suggesting that this absorption is a terminal Ce−O stretching vibration with the O atom coming from the NO reactant. The 643.9 and 640.5 cm−1 absorptions exhibit shifts with 15NO, 13CO, and C18O substitutions and can be attributed to the in-plane and outof-plane NCO bending vibrations. The assignments are strongly supported by the DFT/B3LYP calculations. The OLnNCO molecules were predicted to possess 2A″, 3A″, and 4A′ electronic ground states for Ce, Pr, and Nd, respectively (Figure 5). All three of these molecules have very similar geometric parameters. The Ln−O and Ln−N bond lengths decrease slightly from 1.826 and 2.351 Å for Ce, to 1.816 and 2.320 Å for Pr, and to 1.813 and 2.317 Å for Nd due mainly to the lanthanide contraction. The predicted bond

isoelectronic PrO2+ cation.50,51 The Pr−N and Pr−O are tripleand double-bonded with bond lengths of 1.677 and 1.765 Å, respectively, at the B3LYP level. The NNdO molecule was predicted to have a linear quartet (4Σ) ground state, which has an electronic configuration of (core) (1π) 4 (1σ) 2 (2π) 4 (2σ)1(1δ)2. The unpaired electrons are located on the 1δ and 2σ molecular orbitals. The doubly degenerate 1δ orbitals are nonbonding Nd-based 4f atomic orbitals. Thus, the Nd center in NNdO has an (f2) electronic configuration and an oxidation state of +IV following the general rules for the determination of formal oxidation states. The NLnO(CO) complexes retain the same electronic ground states as that of the NLnO molecules. The CO ligand is terminally coordinated to the metal centers with quite long Ln−C bond distances (2.822 Å for Ce, 2.829 Å for Pr, and 2.790 Å for Nd), suggesting weak interaction between NLnO and CO. Upon CO coordination, the Ln−O bond of the NLnO moiety is only slightly elongated, but the Ln−N bond is elongated by 0.021 Å for Ce, 0.014 Å for Pr, and 0.027 Å for Nd. The NLnO subunit becomes slightly bent with a bond angle of 174.4° for Ce, 175.3° for Pr, and 176.9° for Nd. The C−O bond lengths (1.124 or 1.125 Å) in NLnO(CO) are slightly shorter with respect to free CO (1.126 Å) calculated at the same level. The calculated C−O, Ln−O, and Ln−N stretching frequencies and isotopic frequency ratios are compared with the experimental values in Table 2. Both the vibrational frequencies and isotopic frequency ratios are in good agreement with the experimental values. In the case of NCeO(CO), the Ce−N stretching mode was predicted to have very low IR intensity (3 km/mol) and was not observed experimentally. Because of weak interactions, the bonding of NLnO in the NLnO(CO) complexes is essentially the same as that of free NLnO. D


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Figure 5. Optimized structures (bond lengths in angstrom and bond angles in degrees) of NLnO, NLnO(CO) and OLnNCO (Ln = Ce, Pr, Nd).

annealing, implying that the metal insertion reactions as well as the subsequent CO addition reactions require negligible activation energy. The calculated reaction energies listed in Table 3 indicate that the metal atom insertion reactions in forming NLnO are highly exothermic, but the exothermicity of Nd is much lower than those of Ce and Pr due to low bonding efficiency. Note that NNdO has the lowest Nd−N stretching frequency among the three NLnO molecules. The CO addition reactions in forming the NLnO(CO) complexes are also exothermic for all these three systems. The NLnO(CO) complexes isomerize to the isocyanate OLnNCO isomers under UV light excitation. These isomerization reactions were predicted to be highly exothermic. The reactions proceed only under UV light irradiation, implying that these isomerization reactions require activation. As shown in Figure 6, the NCeO(CO) → OCeNCO reaction was predicted to proceed via a three-centered transition state lying ∼8.1 kcal/mol higher in energy than the NCeO(CO) complex on the doublet potential energy surface. The NPrO(CO) → OPrNCO isomerization reaction requires spin crossing, as the NPrO(CO) complex has a singlet ground state, whereas the

distances suggest that the Ln−O bonds are triple bonds, while the Ln−N bonds are single bonds. The Ln−O bond lengths are close to the sum of the triple-bond covalent radii of Ln and O atoms, while the Ln−N bond distances are approximately the same as the sum of the single-bond covalent radii of Ln and N as reported by Pyykkö et al.53,54 The OLnN bond angles are quite acute ranging from 116.3° to 115.4°, while the NCO subunit is almost linear. Population analyses indicate that the unpaired electrons are mainly located on the metal centers with electronic configuration of f1 for Ce, f2 for Pr, and f3 for Nd. Therefore, all of the metal centers have the most common oxidation state of +III. As listed in Table 2, the calculated vibrational frequencies and the isotopic frequency ratios are in quite good agreement with the experimental values, which strongly support the experimental assignments. Reaction Mechanisms. The experimental observations clearly indicate that the ground-state lanthanide metal atoms react with nitric oxide to form the inserted NLnO molecules, which further interact with CO to form the NLnO(CO) complexes on annealing. Both the inserted NLnO molecules and the NLnO(CO) complexes are formed spontaneously on E


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Table 2. Comparison Between the Calculated and Observed Vibrational Frequencies (cm−1) and Isotopic Frequency Ratios of the Observed NLnO(CO) and OLnNCO Molecules calcd NCeO(CO)

OCeNCO

NPrO(CO)

OPrNCO

NNdO(CO)

ONdNCO

a

obsd

mode

ν

R14/15

R12/13

R16/18

ν

R14/15

R12/13

R16/18

CO str CeO str CeN str NCO asym str CeO str NCO bend NCO bend CO str PrN str PrO str NCO asym str PrO str NCO bend NCO bend CO str NdO str NdN str NCO asym str NdO str NCO bend NCO bend

2203.3 (252) 790.6 (359) 722.5 (3) 2261.2 (1727) 807.1 (327) 655.0 (21) 647.6 (18) 2219.5(196) 987.9(386) 819.5(178) 2262.6 (1721) 810.0 (323) 653.0 (21) 649.1 (18) 2209.1 (229) 817.9 (303) 711.2 (35) 2267.6 (1639) 808.9 (317) 653.9 (22) 647.1 (18)

1.0000 1.0051 1.0268 1.0056 1.0000 1.0052 1.0050 1.0000 1.0264 1.0051 1.0056 1.0000 1.0052 1.0051 1.0000 1.0125 1.0192 1.0056 1.0000 1.0052 1.0051

1.0229 1.0000 1.0000 1.0286 1.0000 1.0286 1.0286 1.0228 1.0000 1.0000 1.0286 1.0001 1.0287 1.0285 1.0229 1.0000 1.0000 1.0286 1.0000 1.0285 1.0286

1.0246 1.0000 1.0000 1.0064 1.0000 1.0075 1.0076 1.0246 1.0000 1.0000 1.0064 1.0001 1.0076 1.0078 1.0246 1.0000 1.0000 1.0064 1.0000 1.0075 1.0077

2169.3 747.5

1.0000 1.0048

1.0226 1.0000

1.0242 1.0000

2189.5 767.6 643.9 640.5 2170.9 886.1 734.9 2189.0 773.4 643.4 640.7 2170.1 756.7 653.1 2189.9 789.7 644.6 640.7

1.0056 1.0000 1.0050 1.0047 1.0000 1.0241 1.0073 1.0056 1.0000 1.0050 1.0059 1.0000 1.0118 1.0207 1.0057 1.0000 1.0053 1.0052

1.0284 1.0000 ? 1.0286 1.0227 1.0000 1.0000 1.0285 1.0000 1.0286 1.0287 1.0227 1.0000 1.0000 1.0286 1.0000 1.0277 1.0289

1.0061 1.0000 1.0070 1.0072 1.0242 1.0000 1.0000 1.0059 1.0000 1.0073 1.0082 1.0242 1.0000 1.0000 1.0059 1.0000 1.0064 1.0075

a

The calculated IR intensities are listed in parentheses (in km/mol).

and proceeds via a transition state lying 11.9 kcal/mol above the NNdO(CO) complex.

Table 3. Calculated Reaction Energies (kcal/mol) for the Ln + CO/NO Reactions at the B3LYP level Ce + NO → NCeO Pr + NO → NPrO Nd + NO → NNdO NCeO + CO → NCeO(CO) NPrO + CO → NPrO(CO) NNdO + CO → NNdO(CO) NCeO(CO) → OCeNCO NPrO(CO) → OPrNCO NNdO(CO) → ONdNCO

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−116.1 −95.0 −43.2 −6.5 −6.1 −24.5 −81.9 −96.9 −94.3

CONCLUSIONS The reactions of early lanthanide metal atoms (Ce, Pr, and Nd) with carbon monoxide and nitric oxide mixtures have been investigated using matrix isolation infrared absorption spectroscopy. The results show that the reactions proceed with the initial formation of the previously reported linear inserted NLnO molecules spontaneously on annealing in solid argon. The NLnO molecules subsequently react with CO in forming the NLnO(CO) complexes, which show blue-shifted CO stretching frequencies. The NLnO(CO) complexes further isomerize to the more stable OLnNCO isomers under UV light irradiation, which are characterized to have planar structures with the metal centers in the most common +III oxidation state. These isocyanate species show antisymmetric NCO stretching frequencies around 2189 cm−1. The species characterized here may help in understanding the mechanisms of isocyanate species formation in the catalytic treatment of automobile exhaust.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b08586. Complete ref 35 and isotopic IR spectra (PDF)

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Figure 6. Potential energy profile of the Ce + NO + CO → OCeNCO reaction calculated at the B3LYP level.

AUTHOR INFORMATION

Corresponding Author

*Phone: (+86) 21-6564-3532. E-mail: [email protected].

OPrNCO isomer has a triplet ground state. In the case of Nd, both the NNdO(CO) and ONdNCO molecules have quartet ground states, and the isomerization reaction conserves spin

ORCID

Mingfei Zhou: 0000-0002-1915-6203 F


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The Journal of Physical Chemistry A Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant Nos. 21688102 and 21433005).

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Isocyanate Formation from Reactions of Early Lanthanide Metal Atoms

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