ARTICLE pubs.acs.org/JPCA
Matrix Isolation Infrared Spectroscopic and Theoretical Study of the Reactions of Tantalum Oxide Molecules with Methanol Guanjun Wang, Jia Zhuang, and Mingfei Zhou* Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai 200433, People’s Republic of China ABSTRACT: The reactions of tantalum monoxide (TaO) and dioxide (TaO2) molecules with methanol in solid neon were investigated by infrared absorption spectroscopy. The groundstate TaO molecule reacted with CH3OH in forming the CH3OTa(O)H molecule via the hydroxylic hydrogen atom transfer from methanol to the metal center spontaneously on annealing. The observation of the spontaneous reaction is consistent with theoretical predictions that the OH bond activation process is both thermodynamically exothermic and kinetically facile. In contrast, the TaO2 molecule reacted with CH3OH to give primarily the TaO2(CH3OH) complex, which further rearranged to the CH3OTa(O)OH isomer via the hydroxylic hydrogen atom transfer from methanol to one of oxygen atom of metal dioxide upon visible light excitation.
’ INTRODUCTION Methanol is widely used in industry as a fuel, solvent, and raw material for producing chemicals and other materials.13 It is therefore vital to understand the interactions of methanol with metal centers as the latter are often used as catalysts for the chemical transformation of methanol. The reactions of neutral metal atoms with methanol molecules have been studied by matrix isolation spectroscopy,413 a powerful method for delineating reaction mechanisms by facilitating the isolation and characterization of the reactive intermediates.1416 These studies have shown that metal atoms, in general, formed complexes with methanol molecules in solid matrixes. Photoexcitation of the complexes with visible or UV light induced oxidative insertion of the metal atoms into the OH and/or CO bonds of methanol.48 Spontaneous insertion of the metal atoms into the OH bond of methanol has also been reported for some metal systems including Mg, Be, and Sc.911 Recent investigations found that the ground-state early transition metal atoms can react spontaneously with two methanol molecules to form low valent M(OMe)2 methoxide salts with release of dihydrogen.17 Besides the metal atom reactions, the metal oxide reactions have also gained considerable attention. The reactions of CH3OH with some high valent transition metal oxyhalide compounds were previously investigated by matrix isolation infrared spectroscopy.1820 The results showed that the reactions proceed with the initial formation of a hydrogen bonded complex, a species that further reacted to eliminate HCl under photoexcitation in solid matrix. The ion-molecular reactions of methanol with vanadium oxide cations and anions were studied using mass spectrometric methods.2124 Dehydrogenation was observed to be the dominating reaction pathway in the gas phase. In addition, the reactions of neutral vanadium oxide and iron oxide clusters with methanol in a fast flow reactor were investigated by time-of-flight mass r 2011 American Chemical Society
spectrometry. Association and hydrogen abstraction products were observed.25,26 Recent investigations in this group have shown that pulsed laser evaporation of bulk metal oxide target followed by condensation with excess argon or neon is an effective method in preparing isolated transition metal oxide molecules in solid argon or neon matrixes. Thus, the reactions of some transition metal oxide molecules including monoxides and dioxides with small molecules such as N2, H2, CH4, and H2O in solid noble gas matrixes were investigated by infrared spectroscopy.2733 In this paper, we report a combined matrix isolation infrared spectroscopic and theoretical study on the reactions of tantalum monoxide and dioxide molecules with methanol. We will show that the ground state TaO molecule is able to react with CH3OH to form the CH3OTa(O)H molecule spontaneously on annealing, while the TaO2 molecule reacted with CH3OH to give primarily the TaO2(CH3OH) complex, which further rearranged to the CH3OTa(O)OH isomer upon visible light excitation.
’ EXPERIMENTAL AND THEORETICAL METHODS The experimental setup for pulsed laser evaporation and matrix isolation Fourier transform infrared (FTIR) spectroscopic investigation has been described in detail previously.13,34 Briefly, the 1064 nm Nd:YAG laser fundamental (Continuum, Minilite II, 10 Hz repetition rate and 6 ns pulse width) was focused onto a rotating bulk Ta2O5 target through a hole in a CsI window cooled normally to 4 K by means of a closed-cycle helium refrigerator. The laser-evaporated metal oxide species were codeposited with methanol in excess neon onto the CsI window. The bulk Ta2O5 Received: May 10, 2011 Revised: July 8, 2011 Published: July 11, 2011 8623
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Figure 1. Infrared spectra in the 37803600 and 18501700 cm1 regions from codeposition of laser-evaporated tantalum oxides with 0.1% CH3OH in neon: (a) 1 h of sample deposition at 4 K; (b) after 9 K annealing; (c) after 11 K annealing; (d) after 30 min of 400 nm < λ < 580 nm irradiation.
target was prepared from sintered metal oxide powder. The methanol/neon mixtures were prepared in a stainless steel vacuum line using standard manometric technique. Isotopically labeled 13 CH3OH (Isotec, 99%), CH318OH (Isotec, 99%), CH3OD (Isotec, 99%), and mixtures were used in different experiments. Isotopic exchange with water adsorbed on the walls of the vacuum line occurred readily; in the experiments with the CH3OD sample, the CH3OH absorptions were also presented. In general, matrix samples were deposited for 30 min at a rate of approximately 4 mmol/h. After sample deposition, infrared spectra of the resulting samples were recorded on a Bruker IFS 80 V spectrometer at a 0.5 cm1 resolution between 4000 and 450 cm1 using a liquid nitrogen cooled broad band HgCdTe (MCT) detector. Bare mirror backgrounds, recorded prior to sample deposition, were used as references in processing the sample spectra. The spectra were subjected to baseline correction to compensate for infrared light scattering and interference patterns. Samples were annealed to different temperatures and cooled back to 4 K for spectral acquisition. For selected samples, photoexcitations were performed through a quartz window mounted on the assembly. A 250 W high pressure mercury arc lamp with band-pass filters was used and duration of the irradiations varied from 10 to 30 min. Density functional calculations were performed using the Gaussian 03 program.35 The three-parameter hybrid functional according to Becke with additional correlation corrections due to Lee, Yang, and Parr (B3LYP) was utilized.36 The AUG-CC-PVTZ basis set for H, C, and O atoms and the SDD pseudopotential and basis set for Ta were used.37,38 The B3LYP functional is the most popular density functional method and can provide reliable predictions on the structures and vibrational frequencies of early transition metal containing compounds.39,40 The geometries were fully optimized, and the stability of the electronic wave function was tested. The harmonic vibrational frequencies were calculated with analytic second derivatives, and zero point energies (ZPEs) were derived.
’ RESULTS AND DISCUSSION Infrared Spectra. The TaO and TaO2 molecules were prepared by laser evaporation of bulk Ta2O5 target. As has been
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Figure 2. Infrared spectra in the 11701100 and 1025870 cm1 regions from codeposition of laser-evaporated tantalum oxides with 0.1% CH3OH in neon: (a) 30 min of sample deposition at 4 K; (b) after 9 K annealing; (c) after 11 K annealing; (d) after 30 min of 400 nm < λ < 580 nm irradiation.
Table 1. Infrared Absorptions (cm1) from Codeposition of Laser-Evaporated Tantalum Oxides with CH3OH in Solid Neon CH3OH
13
CH3OH
CH318OH
CH3OD
TaO2(CH3OH) 3624.7
3624.7
3613.3
2675.7
959.9
956.2
955.5
965.1
TaO2 sym. stretch
O—H stretch
942.5
928.2
923.8
949.1
C—O str.
897.6
897.3
897.5
897.8
TaO2 antisym. stretch
3740.5 1144.5
3741.0 1129.6
3740.7 1109.0
2757.9 1144.5
O—H stretch C—O stretch
962.6
962.2
962.6
962.6
TadO stretch
699.8
699.8
699.7
694.4
Ta—OH stretch
1776.0
1775.9
1775.9
1278.4
Ta—H stretch
1123.6
1108.9
1089.3
1119.7
C—O stretch
977.6
977.2
976.9
977.5
TadO stretch
CH3OTa(O)OH
CH3OTa(O)H
reported previously, codeposition of laser-evaporated tantalum oxides with pure neon at 4 K formed only the TaO (1020.0 cm1) and TaO2 (ν3, 920.9 cm1; ν1, 979.2 cm1) molecules.32,33 Samples were made by co-condensation of laserevaporated tantalum oxide molecules and CH3OH/Ne mixtures at 4 K, followed by annealing experiments raising progressively the sample temperature and recording IR spectra after different annealing steps, up to 12 K. Next, broad band UVvisible excitation experiments were performed. Experiments were done with different methanol concentrations. The CH3OH/Ne molar ratios were varied from 0.05% to 0.2%. The spectra in selected regions from codeposition of tantalum oxides with a 0.1% CH3OH/Ne sample are shown in Figures 1 and 2, and the frequencies of the newly observed product absorptions are listed in Table 1. After 30 min of sample deposition at 4 K, strong 8624
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Figure 3. Infrared spectra in the 36503600 and 975875 cm1 regions from codeposition of laser-evaporated tantalum oxides with isotopic-labeled methanol in excess neon: (a) 0.1% CH3OH; (b) 0.05% CH3OH + 0.05% 13CH3OH; (c) 0.05% 13CH3OH; (d) 0.05% CH3OH + 0.05% CH318OH; (e) 0.05% CH318OH. Spectra were taken after 30 min of sample deposition followed by 12 K annealing.
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Figure 5. Infrared spectra in the 18001750 and 13001250 cm1 regions from codeposition of laser-evaporated tantalum oxides with 0.1% (CH3OH + CH3OD) in excess neon: (a) after 30 min of sample deposition at 4 K; (b) after 9 K annealing; (c) after 11 K annealing; (d) after 30 min of 400 nm < λ < 580 nm irradiation.
Figure 4. Infrared spectra in the 11501075 cm1 region from codeposition of laser-evaporated tantalum oxides with isotopic-labeled methanol in excess neon: (a) 0.1% CH3OH; (b) 0.05% 13CH3OH + 0.05% CH3OH; (c) 0.05% 13CH3OH; (d) 0.05% CH318OH + 0.05% CH3OH; (e) 0.05% CH318OH. Spectra were taken after 30 min of sample deposition at 4 K followed by 12 K annealing and 20 min of 400 nm < λ < 580 nm irradiation.
absorptions due to TaO, TaO2 and CH3OH were observed. These absorptions decreased on sample annealing. A group of absorptions at 3624.7, 942.5, and 897.6 cm1 is weak on sample deposition, increased markedly upon annealing to 9 and 11 K, and was destroyed with the production of a new group of absorptions at 3740.5, 1144.5, 962.6, and 699.8 cm1 when the sample was subjected to broad-band visible irradiation (400 < λ < 580 nm). A third group of absorptions at 1776.0, 1123.6, and 977.6 cm1 was formed on 9 K annealing, grew strongly on 11 K annealing, and remained almost unchanged upon visible light irradiation (400 < λ < 580 nm).
Figure 6. Optimized structures (bond lengths in angstroms and bond angles in degrees) of the species involved in the TaOx + CH3OH (x = 1, 2) reactions.
Isotopic studies were led with 13CH3OH, CH318OH, CH3OH + CH3OH, CH3OH + CH318OH, and CH3OH + CH3OD mixtures. The resulting spectra in selected frequency regions with different samples are shown in Figures 35, respectively. CH3OTa(O)H. The 1776.0, 1123.6, and 977.6 cm1 absorptions increased together on annealing and are assigned to
13
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Table 2. Calculated Total Energies (in Hartrees, after Zero-Point Energy Corrections), Vibrational Frequencies (cm1), and Intensities (km/mol) of the Species Involved in the TaOx (x = 1, 2) + CH3OH Reactions Etot
frequency (intensity)
CH3OH ( A)
115.72574 282.7 (105), 1034.5 (121), 1074.9 (1), 1169.9 (0), 1364.1 (24), 1476.5 (3), 1498 (3), 1508.4 (5), 2996.4 (63), 3042.4 (53)
TaO (2Δ) TaO2 (2A1)
132.21370 1008.3 (104, σ) 207.54029 332.8 (6, a1),914.6 (193, b2), 66.2 (60, a1)
TaO(CH3OH)
247.97538 57.6 (5), 70.9 (1), 101.4 (1), 163.2 (2), 278.8 (10), 366.5 (46), 944.0 (79), 992.8 (151), 1071.1 (46), 1162.8 (1), 1330.6 (62),
CH3OTa(O)H
248.07492 72.0 (0), 101.8 (9), 161.6 (9), 252.5 (9), 330.7 (17), 515.6 (23) 611.4 (41), 977.4 (177), 1140.7 (323), 1177.6 (1), 1185.1 (29),
CH3Ta(O)OH
248.11018 92.3 (0), 119.1 (1), 162.7 (3), 213.8 (11), 394.1 (148), 400.9 (57), 530.4 (148), 567.4 (35), 594.8 (3), 698.2 (84), 977.2 (153),
CH3OTaOH
248.03643 42.4 (11, a0 ) 43.7 (11, a”) 244.9 (0, a0 ) 245.6 (0, a00 ) 440.0 (72, a”) 440.1 (72, a0 )509.5 (28, a0 ) 723.6 (173, a0 ) 1182.3 (0, a0 ) 1182.4 (0, a”) 1206.2 (378, a0 ) 1479.2 (2, a0 ) 1494.2 (6, a00 ) 1494.3 (6, a0 ) 3106.6 (77, a0 ) 3082.7 (21, a0 ) 3082.8 (21, a”) 3969.7 (281, a0 )
1
1458.2 (8), 1494.2 (7), 1499.7 (11), 3058.8 (26), 3139.1 (8), 3167.7 (3) 3674.3 (94) 1481.2 (4), 1495.7 (5), 1497.1 (10), 1808.9 (252), 3022.0 (62), 3090.7 (21), 3093.6 (18) 1194.4 (18), 1409.9 (4), 1434.8 (5), 2929.8 (4), 3060.7 (4), 3115.6 (3), 3889.1 (190)
TaO2(CH3OH) 323.33406 51.0 (10), 56.6 (1), 77.4 (5), 108.3 (1), 178.8 (2), 273.0 (0), 346.7 (40), 390.7 (46), 907.3 (229), 946.9 (161), 964.0 (57), 1103.0 (32), 1170.5 (1), 1368.1 (34), 1481.6 (1), 1494.1 (6), 1504.3 (15), 3064.1 (16), 3150.2 (5), 3169.9 (2) 3776.0 (91) CH3OTa(O)OH 323.41075 14.2 (0), 59.1 (0), 76.5 (7), 159.9 (2), 199.2 (5), 230.2 (22), 442.2 (143), 450.7 (100), 520.7 (100), 683.0 (117), 963.5 (187), 1161.6 (330), 1180.5 (51), 1181.0 (2), 1480.7 (11), 1495.9 (11), 1497.1 (5), 3015.2 (75), 3081.8 (24), 3082.0 (23), 3896.1 (183) TS1
247.96330 1351.1i (2316), 54.8 (7), 72.9 (5), 179.8 (2), 207.6 (4), 430.1 (131), 767.0 (33), 975.8 (234), 1011.9 (157), 1167.7 (19),
TS2
323.31436 1536i (1736), 59.9 (3), 106.2 (0), 149.7 (21), 181.1 (2), 208.2 (13), 365.0 (90), 520.0 (19), 861.3 (151), 958.9 (226), 1071.2 (176), 1131.1 (92), 1181.4 (4), 1211.5 (9), 1482.9 (1), 1497.6 (4), 1505.1 (8), 1940.9 (52), 3028.3 (54), 3096.4 (19), 3107.1 (18)
TS3
247.95566 750.4i (1847), 62.4 (6), 67.1 (1), 117.4 (1), 182.6 (4), 384.7 (60), 465.6 (3), 742.4 (1), 750.8 (13), 895.7 (10), 993.2 (169),
TS4
247.93517 1681.0i (973), 120.5 (0), 150.9 (3), 180.5 (1), 353.0 (102), 504.3 (15), 881.5 (146), 1023.1 (64), 1080.8 (73), 1171.7 (5),
1177.0 (0), 1465.6 (3), 1491.7 (5), 1501.1 (10), 1727.5 (70), 3042.7 (54), 3118.0 (12), 3131.9 (11)
1075.6 (7), 1433.1 (6), 1438.4 (1), 3110.2 (2), 3262.7 (1), 3268.7 (1), 3796.5 (101) 1200.5 (9), 1472.1 (2), 1497.8 (10), 1499.5 (10), 1830.7 (72), 3019.0 (41), 3086.0 (25), 3096.6 (20)
different vibrational modes of the CH3OTa(O)H molecule (Table 1). The 1776.0 cm1 absorption showed no shift with either 13CH3OH or CH318OH but shifted to 1278.4 cm1 with CH3OD. The band position and H/D isotopic frequency ratio (1.389) indicate that this absorption is due to a TaH stretching vibration. The 1123.6 cm1 absorption shifts to 1108.9 cm1 with 13CH3OH and to 1089.3 cm1 with CH318OH. The isotopic shifts indicate that the 1123.6 cm1 absorption is due to a CO stretching mode. As a reference point, the CH3OMH molecules (M = ScFe) have been produced via the reactions of metal atoms with methanol in solid argon. The CO stretching mode was observed around 1150 cm1 for all of the CH3OMH molecules.13 In the experiments with the CH3OH + 13CH3OH and CH3OH + CH318OH mixed samples (Figure 4), doublets were observed, implying the involvement of only one CO subunit in this mode. The 977.6 cm1 absorption showed very small shifts with isotopic substituted methanol samples (0.4 cm1 shift with 13CH3OH, 0.7 cm1 shift with CH318OH, and 0.1 cm1 shift with CH3OD), suggesting that this absorption is due to a terminal TadO stretching mode. The band position is slightly lower than that of H2TaO (986.9 cm1 in Ar matrix)41 and HTa(O)OH (987.2 cm1 in Ne matrix).33b Density functional calculations predicted the CH3OTa(O)H molecule to have a doublet ground state without any symmetry (Figure 6). As listed in Table 2, the three experimentally observed modes were calculated at 1808.9, 1140.7, and 977.4 cm1 with appreciable IR intensities, in good agreement with the experimental values. The calculated isotopic frequency ratios also match the experimentally observed values (Table 3). TaO2(CH3OH). The 3624.7, 959.9, 942.5, and 897.6 cm1 absorptions are assigned to the TaO2(CH3OH) complex.
Judging by the band position and isotopic shifts (11.4 cm1 CH316OH/CH318OH shift and 949.0 cm1 CH3OH/CH3OD shift), the 3624.7 cm1 absorption corresponds to the OH stretching mode of the complex. Besides the absorptions observed with either CH316OH or CH318OH only, no additional absorption was observed in the experiment with the CH3OH + CH318OH mixture, indicating that only one CH3OH subunit is involved in this complex. The 942.5 cm1 absorption shifted to 928.2 cm1 with 13CH3OH and to 923.8 cm1 with CH318OH. The observed isotopic shifts indicate that this absorption is due to the CO stretching mode of the complex. The 959.9 and 897.6 cm1 absorptions are attributed to the symmetric and antisymmetric TaO2 stretching modes of the complex, which are 19.3 and 23.3 cm1 red-shifted from the corresponding modes of TaO2 in solid neon. The 897.6 cm1 absorption exhibited very small shifts with isotopic substituted methanol samples, indicating that this absorption is due to a pure antisymmetric TaO2 stretching mode. However, the 959.9 cm1 absorption showed several wavenumber shifts with isotopic substituted methanol samples, implying that this absorption is not a pure symmetric TaO2 stretching mode but is coupled with the CO stretching mode. The TaO2(CH3OH) complex was predicted to have a doublet ground state without any symmetry, in which the Ta atom of TaO2 is coordinated to the O atom of CH3OH with a TaO distance of 2.226 Å. The optimized structure is shown in Figure 6. The TadO bond length is elongated by about 0.008 Å upon CH3OH coordination. As listed in Table 2, the four experimentally observed modes were calculated at 3776.0, 964.0, 946.9, and 907.3 cm1. These modes were predicted to have the largest IR intensities. The calculated isotopic frequency ratios are in quite good agreement with the observed values (Table 3) and add 8626
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Table 3. Comparisons between the Calculated and Experimentally Observed Vibrational Frequencies (cm1) and Isotopic Frequency Ratios of the Products 12
frequency mode CH3OTa(O)H
TaO2(CH3OH)
CH3OTa(O)OH
C/13C
16
O/18O
H/D
exptl
calcd
exptl
calcd
exptl
calcd
exptl
calcd 1.4085
Ta—Hstr
1775.0
1808.9
1.0001
1.0000
1.0001
1.0000
1.3892
C—Ostr
1123.6
1140.7
1.0133
1.0140
1.0315
1.0345
1.0035
1.0000
TadOstr O—Hstr
977.6 3624.7
977.4 3776.0
1.0004 1.0000
1.0003 1.0000
1.0007 1.0032
1.0004 1.0033
1.0001 1.3547
1.0018 1.3733
TadOs,str
959.9
964.0
1.0039
1.0065
1.0046
1.0075
0.9946
0.9923
C—Ostr
942.5
946.9
1.0154
1.0133
1.0202
1.0181
0.9930
0.9950
TadOa,str
897.6
907.3
1.0003
1.0003
1.0001
1.0004
0.9998
0.9996
O—Hstr
3740.5
3896.1
0.9999
1.0000
0.9999
1.0000
1.3563
1.3725
C—Ostr
1144.5
1161.6
1.0132
1.0139
1.0320
1.0347
1.0000
1.0000
TadOstr
962.6
963.5
1.0004
1.0002
1.0000
1.0002
1.0000
1.0000
Ta—OHstr
699.8
683.0
1.0000
1.0000
1.0001
1.0000
1.0078
1.0314
additional support to the assignment. The ground state TaO2(CH3OH) complex correlates to the ground state of TaO2 (2A1). The binding energy was predicted to be 42.7 kcal/mol, about twice as large as that of the TaO2(H2O) complex.33b The complexes of TaO2 with other ligands such as CH4, H2, and N2 were reported, and their predicted binding energies (4.9, 1.8, and 5.5 kcal/mol, respectively) are much lower than those of TaO2(CH3OH) and TaO2(H2O).3133 The CH4, H2, and N2 molecules are nonpolar species and are poor donors. CH3OTa(O)OH. Absorptions at 3740.5, 1144.5, 962.6, and 699.8 cm1 were produced upon visible light irradiation (400 < λ < 580 nm) at the expense of the TaO2(CH3OH) complex absorptions, suggesting that the 3740.5, 1144.5, 962.6, and 699.8 cm1 absorptions should be due to different vibrational modes of a structural isomer of TaO2(CH3OH). The 3740.5 cm1 absorption shifted to 2757.9 cm1 in the experiment with CH3OD. The band position and deuterium isotopic shift indicate that this absorption is due to an OH stretching vibration. This absorption exhibited no shift when the CH318OH sample was used, indicating that the O atom of the OH subunit originated from tantalum oxide instead of CH3OH. The 699.8 cm1 absorption is the corresponding TaOH stretching mode. The 1144.5 cm1 absorption shifted to 1129.6 cm1 with 13 CH3OH and to 1109.0 cm1 with CH318OH. The band position and isotopic shifts indicate that the 1144.5 cm1 absorption is due to a CO stretching mode. The 962.6 cm1 absorption is due to a TadO stretching mode, which is about 57.4 cm1 red-shifted from that of TaO in solid neon. As shown in Figure 6, the CH3OTa(O)OH molecule was predicted to have a doublet ground state without any symmetry. The experimentally observed modes were calculated at 3896.1, 1161.5, 963.5, and 683.0 cm1, which matched the experimental values very well. The calculated isotopic frequency ratios also fit the observed values except the TaOH stretching mode (Table 3). The calculated H/D ratio of 1.0314 is much higher than the observed value of 1.0078. The same situation was observed for the TaOH stretching mode of the CH3Ta(O)OH molecule. We assume that this mode is in Fermi resonance with low-lying levels.31a Reaction Mechanism. Laser evaporation of bulk Ta2O5 target followed by co-condensation with neon at 4 K formed the TaO and TaO2 molecules. The infrared spectra shown in Figures 1 and 2 clearly demonstrate that the TaO molecule reacted with
Figure 7. Potential energy profiles of the TaOx + CH3OH (x = 1, 2) reactions calculated at the B3LYP/AUG-CC-PVTZ/SDD level of theory.
CH3OH to form the CH3OTa(O)H molecule, reaction 1. The spontaneous formation of CH3OTa(O)H upon sample annealing implies that negligible activation energy is required for this methanol OH bond activation process. TaOð2 ΔÞ þ CH 3 OH f CH 3 OTaðOÞH
ð1Þ
ΔE ¼ 85:0 kcal=mol The potential energy profile for reaction 1 calculated at the B3LYP/AUG-CC-PVTZ/SDD level of theory is shown in Figure 7. The reaction was predicted to proceed with the initial formation of a TaO(CH3OH) complex intermediate without any barrier. The complex was predicted to be 22.6 kcal/mol lower in energy than the ground state reactants: TaO (2Δ) + CH3OH. This value is significantly lower than the binding energy of TaO2(CH3OH) due to reduced electrostatic interactions. From the TaO(CH3OH) complex intermediate, the hydroxylic H atom transfers from CH3OH to the metal center to form CH3OTa(O)H via a transition state (TS1), which has a threecenter structure. The overall TaO + CH3OH f CH3OTa(O)H reaction was predicted to be exothermic by 85.0 kcal/mol and proceeds via a transition state lying 15.0 kcal/mol lower in energy than the ground state reactants. The high exothermicity of the 8627
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The Journal of Physical Chemistry A overall reaction and the negative energy barrier height as compared to the reactants imply that the formation of CH3OTa(O)H is both thermodynamically and kinetically favorable, which is consistent with the experimental observations. Besides the above-mentioned channel, two other reaction pathways leading to CH3Ta(O)OH via OH transfer and CH3OTaOH via H atom transfer are also considered. In our previous study, the CH3Ta(O)OH molecule was formed and identified as the product from the reaction between TaO2 and CH4.31a Both the CH3Ta(O)OH and CH3OTaOH molecules are structural isomers of CH3OTa(O)H. Neither the CH3Ta(O)OH molecule nor the CH3OTaOH molecule was observed in the present experiments. Theoretical calculations predicted that the CH3Ta(O)OH molecule is 22.1 kcal/mol lower in energy than the CH3OTa(O)H isomer. On the basis of theoretical calculations, the conversion from TaO(CH3OH) to CH3Ta(O)OH proceeded via a transition state (TS3) lying 4.8 kcal/mol higher in energy than TS1; therefore, the formation of CH3Ta(O)OH is kinetically unfavorable here. The reaction from TaO(CH3OH) to CH3OTaOH was predicted to proceed via a transition state (TS4) lying 17.7 kcal/mol above TS1; therefore, the formation of CH3OTaOH is both thermodynamically and kinetically unfavorable. The TaO2 molecule reacted with CH3OH to give primarily the TaO2(CH3OH) complex on annealing. Under visible light (400 < λ < 580 nm) excitation, the CH3OTa(O)OH absorptions appeared with the disappearance of the TaO2(CH3OH) complex absorptions. This observation suggests that the TaO2(CH3OH) complex undergoes a photoinduced isomerization reaction to form CH3OTa(O)OH. Theoretical calculations predicted that the CH3OTa(O)OH molecule is 35.5 kcal/mol more stable than the TaO2 (CH3 OH) complex. The TaO2 (CH3 OH) to CH3OTa(O)OH isomerization reaction was predicted to proceed via a transition state lying 12.4 kcal/mol above the TaO2(CH3OH) complex (Figure 7). Although the reaction of TaO2 with CH3OH to form the TaO2(CH3OH) complex is exothermic by 42.7 kcal/mol, and the barrier to the subsequent isomerization reaction is 12.4 kcal/mol, the complex was stabilized in the matrix, suggesting that the excess energy of the in situ formed TaO2(CH3OH) complex can be very effectively quenched by the matrix. In the case of TaO + CH3OH reaction, the TaO(CH3OH) complex intermediate was not stabilized. Note that the barrier for the TaO(CH3OH) to CH3OTa(O)H isomerization reaction is only 7.6 kcal/mol, lower than that of the TaO2(CH3OH) to CH3OTa(O)OH isomerization reaction. The result may also indicate that the energy relaxing of the TaO(CH3OH) complex is less efficient than that of the TaO2(CH3OH) complex in solid neon matrix as the TaO2(CH3OH) complex has more low-lying levels than the TaO(CH3OH) complex. The reactions reported here are very similar to the previously reported reactions of tantalum oxides with other small hydrogen containing molecules including H2O, CH4, and CH3Cl.31,33 All these reactions were found to proceed with the initial formation of a 1:1 complex, which further rearranged to a more stable hydrogen atom transferred isomer in solid noble gas matrixes. However, the monoxide and dioxide reaction systems exhibit different hydrogen atom transfer mechanisms. In the monoxide reactions, the hydrogen atom of the ligand is transferred to the metal center in forming the oxometal hydride complex, whereas the hydrogen atom is transferred to one of oxygen atoms of metal dioxide to form the metal hydroxide complex in the dioxide reaction systems.
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’ CONCLUSIONS The reactions of tantalum monoxide (TaO) and dioxide (TaO2) molecules with methanol were investigated by matrix isolation infrared absorption spectroscopy as well as density functional theoretical calculations. The TaO and TaO2 reactants were prepared via pulsed laser evaporation of the bulk metal oxide target. It was found that the ground state TaO molecule reacted with CH3OH in forming the CH3OTa(O)H molecule spontaneously on annealing in solid neon. Theoretical calculations predicted that the reaction of TaO and CH3OH in forming CH3OTa(O)H is both thermodynamically exothermic and kinetically facile. The TaO2 molecule reacted with CH3OH to give primarily the TaO2(CH3OH) complex, which further rearranged to the CH3OTa(O)OH isomer via hydrogen atom transfer upon visible light irradiation (400 < λ < 580 nm). ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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