Matrix Isolation Spectroscopic and Theoretical Study of Water

ARTICLE pubs.acs.org/JPCA

Matrix Isolation Spectroscopic and Theoretical Study of Water Adsorption and Hydrolysis on Molecular Tantalum and Niobium Oxides Mingfei Zhou,* Jia Zhuang, Guanjun Wang, and Mohua Chen Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai 200433, China ABSTRACT: The reactions of molecular tantalum and niobium monoxides and dioxides with water were investigated by matrix isolation infrared spectroscopy. In solid neon, the metal monoxide and dioxide molecules reacted with water to form the MO(H2O) and MO2(H2O) (M = Ta, Nb) complexes spontaneously on annealing. The MO(H2O) complexes photochemically rearranged to the more stable HMO(OH) isomers via one hydrogen atom transfer from water to the metal center under visible light excitation. In contrast, the MO2(H2O) complexes isomerized to the more stable MO(OH)2 molecules via a hydrogen atom transfer from water to one of the oxygen atoms of metal dioxide upon visible light irradiation. The aforementioned species were identified by isotopic-substituted experiments as well as density functional calculations.

’ INTRODUCTION The interaction of water with transition-metal centers is an important subject in a wide range of fields, such as catalysis, corrosion science, and environmental chemistry. The reactions of transition-metal atoms and ions with water have been the subject of considerable experimental and theoretical studies.1-14 It was found that early transition-metal atoms and ions reacted with water to form the metal monoxide neutrals and cations in the gas phase.1-7 Matrix isolation infrared spectroscopic studies showed that early transition-metal atoms reacted with water molecules to initially form the HMOH insertion intermediates spontaneously upon annealing.8,9 The insertion molecules could either photochemically isomerize to the high-valent H2MO isomers or decompose to the metal monoxide and H2. It was found that the actinide metal thorium and uranium atoms are able to directly insert into the O-H bond of water to form the H2ThO and H2UO molecules in low-temperature noble gas matrices.10,11 The later transition-metal atoms as well as the lanthanide metal atoms reacted with water to give primarily the M(H2O) complexes, which rearranged to the insertion products under UV-visible light excitation.12-14 Compared with the extensive studies on the reactions of ionic and neutral transition-metal atoms with water, the reactions of transition-metal oxides with water have gained much less attention. Water adsorption on the metal oxide surfaces has been studied by several groups using FTIR spectroscopy and atomic force microscopy as well as theoretical calculations.15 Mass spectrometric studies provided information on the reaction kinetics and mechanism regarding the chemistry of some cationic oxides and water in the gas phase.16,17 Water adsorption and hydrolysis on molecular transition-metal oxides (TiO2 and CrO3) and oxyhydroxides (ScO(OH), VO2(OH), and r 2011 American Chemical Society

MnO3(OH)) neutrals has been studied by means of quantum chemistry.18 In the investigated reactions, each reaction step comprised the breaking of one MdO bond and the formation of two OH groups. The reactions were found to involve stable water complexes as intermediates. Matrix isolation spectroscopic studies in this laboratory indicate that neutral metal dioxide molecules such as TiO2 and BO2 reacted spontaneously with water to form the oxyhydroxide molecules,19,20 whereas only water complexes were observed in the reaction of PtO2 and H2O in solid argon.21 In the present study, we report a matrix isolation infrared spectroscopic and density functional theoretical study on the reactions of molecular niobium and tantalum oxides with water.

’ EXPERIMENTAL AND THEORETICAL METHODS The experimental setup for pulsed laser evaporation and matrix isolation infrared spectroscopic investigation has been described in detail previously.22 Briefly, the 1064 nm fundamental of a Nd:YAG laser (Continuum, Minilite II, 10 Hz repetition rate and 6 ns pulse width) was focused onto a rotating metal oxide 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 H2O/Ne mixtures onto the CsI window. In general, matrix samples were deposited for 30 min at a rate of approximately 4 mmol/h. The bulk Ta2O5 and Nb2O5 targets were prepared from sintered metal oxide powder. The H2O/Ne mixtures were prepared in a stainless steel vacuum line using standard Received: January 6, 2011 Revised: February 10, 2011 Published: February 28, 2011 2238

dx.doi.org/10.1021/jp200143y | J. Phys. Chem. A 2011, 115, 2238–2246

The Journal of Physical Chemistry A manometric technique. Distilled water was cooled to 77 K using liquid N2 and evacuated to remove volatile impurities. Isotopically labeled D2O and H218O (Cambridge Isotopic Laboratories, 96%) were used without further purification. Isotopic exchange with water adsorbed on the walls of the vacuum line occurred readily; in the experiments with the D2O sample, the HDO and H2O absorptions were also presented. The infrared absorption spectra of the resulting samples were recorded on a Bruker IFS 80 V spectrometer at 0.5 cm-1 resolution between 4000 and 450 cm-1 using a liquid-nitrogen-cooled HgCdTe (MCT) detector. Samples were annealed to different temperatures and cooled back to 4 K for spectral acquisition, and selected samples were subjected to broad-band irradiation using a high-pressure mercury arc lamp with glass filters. Quantum chemical calculations were performed to determine the molecular structures and to help the assignment of vibrational frequencies of the observed reaction products. The calculations were performed at the level of density functional theory (DFT) with the B3LYP method, where Becke’s threeparameter hybrid functional and the Lee-Yang-Parr correlation functional were used.23 The AUG-CC-PVTZ basis set was used for the H and O atoms, and the SDD pseudo-potential and basis set were used for the Nb and Ta atoms.24,25 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.26 The geometries of various reactants, intermediates, and products were fully optimized, and the harmonic vibrational frequencies were calculated with analytic second derivatives. The zero-point energies (ZPE) were derived. Transition state optimizations were done with the synchronous transit-guided quasi-Newton (STQN) method and were verified through intrinsic reaction coordinate (IRC) calculations.27 All these calculations were performed by using the Gaussian 03 program.28

’ RESULTS AND DISCUSSION Previous investigations in this group have shown that pulsed laser evaporation of bulk metal oxide target followed by condensation with excess argon is an effective method in preparing transition-metal oxide species for their reactivity study in solid argon matrix.29-31 It was found that some transition-metal oxide molecules such as group V metal dioxides trapped in solid argon are coordinated by one or more argon atoms and should be regarded as transition-metal-oxide-argon complexes.32,33 In order to minimize the matrix effect, the more inert neon is used as matrix in the present study. TaOx þ H2O. Pulsed laser evaporation of bulk Ta2O5 target under controlled laser energy followed by condensation with pure neon formed only the TaO (1020.0 cm-1) and TaO2 (ν3, 920.9 cm-1; ν1, 979.2 cm-1) molecules.34 No higher oxide species were observed. The metal dioxide molecules are favored with relatively low evaporation laser energy, whereas the metal monoxide molecules are favored with relatively high laser energy. Experiments were performed using the H2O/Ne sample as reagent gas. The spectra in selected regions from codeposition of laser-evaporated tantalum monoxide and dioxide with 0.05% H2O in neon are shown in Figures 1 and 2, respectively, and the product absorptions are listed in Table 1. Sample deposition at 4 K revealed strong water absorptions, weak water dimer absorptions, and strong TaO and TaO2 absorptions (Figures 1 and 2, trace a). When the sample was annealed to 12 K, the TaO and

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Figure 1. Infrared spectra in the 1030-890 cm-1 region from codeposition of laser-evaporated tantalum oxides with 0.05% H2O in neon. (a) 30 min of sample deposition at 4 K, (b) after 12 K annealing, (c) after 15 min of visible light (500 < λ < 580 nm) irradiation, and (d) after 15 min of visible light (400 < λ < 580 nm) irradiation.

Figure 2. Infrared spectra in the 3820-3580 and 1820-1520 cm-1 regions from codeposition of laser-evaporated tantalum oxides with 0.05% H2O in neon. (a) 30 min of sample deposition at 4 K, (b) after 12 K annealing, (c) after 15 min of visible light (500 < λ < 580 nm) irradiation, and (d) after 15 min of visible light (400 < λ < 580 nm) irradiation.

TaO2 absorptions decreased, while new product absorptions at 984.0, 957.8, and 903.8 cm-1 in the TadO stretching frequency region and at 3674.2, 3595.1, 1602.6, and 1566.6 cm-1 in the OH stretching and bending vibrational frequency regions were produced (Figures 1 and 2, trace b). The 984.0 and 1566.6 cm-1 absorptions together with two weak absorptions at 3670.7 and 3589.1 cm-1 (overlapped by the strong 3674.2 cm-1 and (H2O)2 absorptions, but can be resolved from difference spectrum) were totally destroyed when the sample was subjected to visible light irradiation using the high-pressure mercury lamp with a 500 nm long wavelength pass filter (500 < λ < 580 nm), while new absorptions at 3729.7, 1793.2 (with site absorptions at 2239

dx.doi.org/10.1021/jp200143y |J. Phys. Chem. A 2011, 115, 2238–2246

The Journal of Physical Chemistry A

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Table 1. Infrared Absorptions (in cm-1) from Codeposition of Laser-Evaporated Tantalum Oxides with Water in Excess Neon H2O

H218O

HDO

D2O

assignment

3670.7 3656.0 3616.3

2728.2 TaO(H2O) antisym. O-H str.

3589.1 3580.6 2665.1

2619.9 TaO(H2O) sym. O-H str.

1566.6 1559.6 1382.6

1162.2 TaO(H2O) O-H bend.

984.0

983.9 TaO(H2O) TadO str.

984.0

3674.2 3660.5 3635.8 3595.1 3588.0 2675.2

2732.3 TaO2(H2O) antisym. O-H str. 2627.1 TaO2(H2O) sym. O-H str.

1602.6 1595.7 1415.9

1186.5 TaO2(H2O) O-H bend.

957.8

957.8

903.8

903.7

957.8 TaO2(H2O) sym. TadO str. 903.4 TaO2(H2O) antisym. TadO str.

3729.7 3717.8

2751.1 HTaO(OH) O-H str.

1793.2 1793.2

1284.4 HTaO(OH) Ta-H str.

1785.0 1785.0

1282.6 HTaO(OH) Ta-H str. site

1783.7 1783.7 1778.0 1778.0

1277.9 HTaO(OH) Ta-H str. site 1274.8 HTaO(OH) Ta-H str. site

987.2

987.0

987.0 HTaO(OH) TadO str.

727.3

699.4

712.8 HTaO(OH) Ta-OH str.

492.9

HTaO(OH) TaOH bend.

3741.0 3730.5 3742.5, 2759.7 2758.2 TaO(OH)2 antisym. O-H str. 973.4

973.3

708.1

676.0

973.5 TaO(OH)2 TadO str. 691.2

Figure 4. Difference IR spectra in the 3800-3550 and 28002600 cm-1 regions from codeposition of laser-evaporated tantalum oxides with water in excess neon (spectrum taken after 15 min of visible light (400 < λ < 580 nm) irradiation minus spectrum taken after 15 min of visible light (500 < λ < 580 nm) irradiation). (a) 0.05% H2O/Ne, (b) 0.06% H216O þ 0.04% H218O/Ne, and (c) 0.04% H2O þ 0.08% HDO þ 0.04% D2O/Ne.

689.6 TaO(OH)2 antisym. Ta-OH str.

Figure 3. Difference IR spectra in the 1850-1100 cm-1 region from codeposition of laser-evaporated tantalum oxides with water in excess neon (spectrum taken after 15 min of visible light (500 < λ < 580 nm) irradiation minus spectrum taken after 12 K annealing). (a) 0.05% H2O/ Ne, (b) 0.06% H216O þ 0.04% H218O/Ne, and (c) 0.04% H2O þ 0.08% HDO þ 0.04% D2O/Ne.

1785.0, 1783.7, and 1778.0 cm-1, which evolved to one absorption at 1793.2 cm-1 on annealing), and 987.2 cm-1 (Figures 1 and 2, trace c) together with 727.3 and 492.9 cm-1 (not shown) were produced in concert. Subsequent visible light irradiation using the high-pressure mercury lamp with a 400 nm long wavelength pass filter (400 < λ < 580 nm) destroyed the 3674.2, 3595.1, 1602.6, 957.8, and 903.8 cm-1 absorptions and produced new absorptions at 3741.0 and 973.4 cm-1 (Figures 1 and 2, trace d) as well as 708.1 cm-1 (not shown).

Figure 5. Difference IR spectra in the 1650-1120 and 750-650 cm-1 regions from codeposition of laser-evaporated tantalum oxides with water in excess neon (spectrum taken after 15 min of visible light (400 < λ < 580 nm) irradiation minus spectrum taken after 15 min of visible light (500 < λ < 580 nm) irradiation). (a) 0.05% H2O/Ne, (b) 0.06% H216O þ 0.04% H218O/Ne, and (c) 0.04% H2O þ 0.08% HDO þ 0.04% D2O/Ne.

Experiments were also repeated using the isotopic-labeled H218O and H2O þ HDO þ D2O mixtures. The spectra in selected regions with different isotopic samples are shown in Figures 3-5, respectively. The corresponding absorptions are also summarized in Table 1. NbOx þ H2O. Experiments were also performed using a bulk Nb2O5 target. The spectra in selected regions from codeposition of laser-evaporated niobium monoxide and dioxide molecules with 0.05% H2O in neon are shown in Figures 6 and 7, 2240



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Table 2. Infrared Absorptions (in cm-1) from Codeposition of Laser-Evaporated Niobium Oxides with Water in Excess Neon H2O H218O

HDO

1583.6 1576.8 1397.6 945.1

945.1

943.4

943.4

Figure 6. Infrared spectra in the 1000-880 and 740-680 cm regions from codeposition of laser-evaporated niobium oxides with 0.05% H2O in neon. (a) 30 min of sample deposition at 4 K, (b) after 12 K annealing, (c) after 15 min of visible light (500 < λ < 580 nm) irradiation, (d) after 15 min of visible light (400 < λ < 580 nm) irradiation, and (e) after 12 K annealing.

Figure 7. Infrared spectra in the 3820-3600 and 1740-1560 cm-1 regions from codeposition of laser-evaporated niobium oxides with 0.05% H2O in neon. (a) 30 min of sample deposition at 4 K, (b) after 12 K annealing, (c) after 15 min of visible light (500 < λ < 580 nm) irradiation, (d) after 15 min of visible light (400 < λ < 580 nm) irradiation, and (e) after 12 K annealing.

respectively, and the product absorptions are listed in Table 2. Besides the water absorptions, the absorptions due to NbO (978.5 cm-1) and NbO2 (ν3, 907.8 cm-1; ν1, 957.2 cm-1) dominated the spectrum after sample deposition at 4 K (Figures 6 and 7, trace a).34 The NbO and NbO2 absorptions decreased and new absorptions at 1583.6, 945.1/943.4, 3693.9, 3615.3, 1607.6, 940.7, and 894.2 cm-1 appeared on annealing (Figures 6 and 7, trace b). When the sample was subjected to visible light irradiation in the wavelength range of 500-580 nm (Figures 6 and 7, trace c), the 1583.6 and 945.1/943.4 cm-1 absorptions disappeared with the production of new absorptions

assignment

1173.2 NbO(H2O) O-H bend. 944.8 NbO(H2O) NbdO str. 943.3 NbO(H2O) NbdO str. site

3693.9 3679.9 3655.5

2745.2 NbO2(H2O) antisym. O-H str.

3615.3 3608.1 2688.9 1607.6 1600.5 1404.5

2641.5 NbO2(H2O) sym. O-H str. 1179.1 NbO2(H2O) O-H bend.

940.7

940.7

894.2

894.2

940.6 NbO2(H2O) sym. NbdO str. 893.7 NbO2(H2O) antisym. NbdO str.

3727.6 3713.7

-1

D2O

2747.6 HNbO(OH) O-H str.

1725.7 1725.6

1239.2 HNbO(OH) Nb-H str.

1718.6 1718.5

1234.5 HNbO(OH) Nb-H str. site

1713.5 1713.4

1231.0 HNbO(OH) Nb-H str. site

1702.3 1702.3 980.9 980.8

1223.1 HNbO(OH) Nb-H str. site 980.8 HNbO(OH) NbdO str.

715.9

695.4

704.5 HNbO(OH) Nb-OH str.

3738.6 3728.1 3740.0, 2757.2 2755.6 NbO(OH)2 antisym. O-H str. 968.0 NbO(OH)2 NbdO str.

968.1

968.0

726.3

716.1

718.3

706.7 NbO(OH)2 antisym. Nb-OH str.

683.7

663.4

671.4

663.2 NbO(OH)2 sym. Nb-OH str.

at 3727.6, 1725.7, 1718.6, 1713.5, 1702.3, 980.9, and 715.9 cm-1. The 1718.6, 1713.5, and 1702.3 cm-1 absorptions disappeared while the 1725.7 cm-1 absorption increased on annealing, suggesting that these absorptions are due to site absorptions. The 3693.9, 3615.3, 1607.6, 940.7, and 894.2 cm-1 absorptions decreased under 500< λ < 580 nm irradiation and were completely destroyed upon additional visible light irradiation in the wavelength range of 400-580 nm (Figures 6 and 7, trace d). Additional absorptions at 3738.6, 968.1, 726.3, and 683.7 cm-1 were produced under broad-band 400< λ < 580 nm irradiation. The spectra in selected regions from similar experiments with the isotopic-labeled H218O and H2O þ HDO þ D2O samples are illustrated in Figures 8-10, respectively. TaO(H2O). The 984.0, 1566.6, 3589.1, and 3670.7 cm-1 absorptions are assigned to different vibrational modes of the TaO(H2O) complex. The 984.0 cm-1 absorption is due to the TadO stretching mode of the complex, which is 36.0 cm-1 redshifted from the diatomic TaO absorption in solid neon. The 1566.6 cm-1 absorption is due to the H2O bending mode of the TaO(H2O) complex. The corresponding mode of TaO(D2O) and TaO(HDO) was observed at 1162.2 and 1382.6 cm-1, respectively. The experiments with mixtures of H216O þ H218O and H2O þ HDO þ D2O (Figure 3) confirmed the involvement of only one H2O subunit in this complex. The much weaker 3670.7 and 3589.1 cm-1 absorptions are assigned to the antisymmetric and symmetric OH stretching modes of the complex. The assignment is supported by DFT calculations. As shown in Figure 11, the TaO(H2O) complex was computed to have a doublet ground state with nonplanar Cs symmetry, in which the O atom of the H2O fragment is coordinated to the metal center. The diatomic TaO molecule was determined to have a 2Δ ground state.35 The electron configuration of the 2A00 ground state TaO(H2O) complex correlates to the ground state of TaO. The calculated harmonic vibrational frequencies and intensities are listed in Table 3. The H2O bending and TadO stretching 2241



Figure 8. Difference IR spectra in the 1750-1100 cm-1 region from codeposition of laser-evaporated niobium oxides with water in excess neon (spectrum taken after 15 min of visible light (500 < λ < 580 nm) irradiation minus spectrum taken after 12 K annealing). (a) 0.05% H2O/ Ne, (b) 0.05% H216O þ 0.05% H218O/Ne, and (c) 0.04% H2O þ 0.08% HDO þ 0.04% D2O/Ne.

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Figure 10. Difference IR spectra in the 750-660 cm-1 region from codeposition of laser-evaporated niobium oxides with water in excess neon (spectrum taken after 15 min of visible light (400 < λ < 580 nm) irradiation minus spectrum taken after 15 min of visible light (500 < λ < 580 nm) irradiation). (a) 0.05% H2O/Ne, (b) 0.05% H216O þ 0.05% H218O/Ne, and (c) 0.04% H2O þ 0.08% HDO þ 0.04% D2O/Ne.

Figure 9. Difference IR spectra in the 3800-3520 and 28002600 cm-1 regions from codeposition of laser-evaporated niobium oxides with water in excess neon (spectrum taken after 15 min of visible light (400 < λ < 580 nm) irradiation minus spectrum taken after 15 min of visible light (500 < λ < 580 nm) irradiation). (a) 0.05% H2O/Ne, (b) 0.05% H216O þ 0.05% H218O/Ne, and (c) 0.04% H2O þ 0.08% HDO þ 0.04% D2O/Ne.

modes were predicted to have the largest IR intensities with their frequencies and isotopic frequency shifts in good agreement with the experimental values. The antisymmetric and symmetric OH stretching modes also were predicted to have appreciable intensities. Apparently, the relative intensities of these two modes are slightly overestimated. DFT calculations occasionally overestimate the relative intensities of hydrogen-atom-involved vibrational modes.29a,36 NbO(H2O). Similar absorptions at 1583.6 and 945.1/ 943.4 cm-1 in the niobium experiments are assigned to the NbO(H2O) complex. The 945.1 and 943.4 cm-1 absorptions are

Figure 11. Optimized structures (bond lengths in angstroms and bond angles in degrees) of the species involved in the TaOx þ H2O reactions (x = 1, 2).

due to the NbdO stretching mode at two trapping sites. The band position of the upper site absorption is red-shifted about 33.4 cm-1 from the NbO absorption in solid neon. The 1583.6 cm-1 absorption is attributed to the H2O bending mode, but the two O-H stretching modes were not observed due to 2242



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Table 3. Calculated Total Energies (in Hartree, after Zero-Point Energy Corrections), Vibrational Frequencies (cm-1), and Intensities (km/mol) of the Species Involved in the MOx þ H2O Reactions (M = Ta, Nb, x = 1, 2) energy

frequency (intensity)

H2O (1A1) TaO (2Δ) TaO2 (2A1) TaO(H2O) (2A00 ) TaO(D2O) TaO(H218O) HTaO(OH) (2A) DTaO(OD) HTaO(18OH) TaO2(H2O) (2A0 ) TaO2(D2O) TaO2(H218O) TaO(OH)2 (2A1) TaO(OD)2 TaO(OH)(18OH)

-76.44496 -132.22733 -207.56872 -208.69164

TS1 TS2

-208.67818 -284.02991

NbO (4Σ) NbO2 (2A1) NbO(H2O) (4A) HNbO(OH) (2A) NbO2(H2O) (2A0 ) NbO(OH)2 (2A1)

-132.19410 -207.52227 -208.65899 -208.73844 -284.00200

1029.0(106) 988.1(63), 937.3(198), 334.1(6) 3742.7(84), 3638.4(82), 1582.1(154), 992.8(184), 400.2(9), 294.7 (77), 265.4(77), 103.0(10), 51.7(4) 2745.1(37), 2620.4(37), 1162.3(85), 992.8(183), 292.6(7), 264.5(2), 214.5(79), 77.8(5), 50.4(4) 3727.0(85), 3631.2(83), 1575.0(153), 992.8(184), 398.6(9), 290.6(97), 254.7(55), 102.7(11), 50.8(4) 3885.2(195), 1826.0(230), 987.7(151), 716.9(93), 585.9(15), 537.0(144), 410.9(98), 309.4(18), 184.3(21) 2830.7(127), 1295.9(116), 986.2(144), 685.3(123), 430.9(2), 417.1(69), 319.5(52), 229.6(9), 168.4(21) 3872.2(189), 1825.9(230), 987.6(152), 690.3(74), 582.1(18), 528.7(146), 407.1(98), 309.0(17), 181.4(21) 3819.6(173), 3726.5(22), 1631.9(101), 959.1(93), 914.3(259), 530.3(7), 350.1(9), 300.6(156), 266.0(2), 168.2(12), 62.2(1), 60.7(11) 2802.4(88), 2684.6(15), 1198.2(58), 959.1(92), 913.9(256), 397.7(2), 337.8(10), 261.6(2), 234.6(75), 120.4(6), 59.1(0), 58.0(11) 3803.5(173), 3719.0(21), 1624.7(99), 959.1(93), 914.3(260), 526.7(8), 341.6(10), 297.5(154), 259.8(4), 168.2(12), 61.1(0), 60.1(11) 3891.0(0), 3888.5(354), 973.2(155), 697.3(149), 684.4(43), 472.0(190), 462.3(94), 447.4(0), 439.0(209), 213.2(22), 169.9(2), 47.7(0) 2835.3(0), 2832.8(235), 973.0(155), 673.5(185), 667.4(70), 372.2(30), 360.7(108), 336.7(0), 332.5(103), 196.7(24), 158.7(4), 44.5(0) 3889.9(144), 3876.7(204), 973.1(157), 692.6(118), 658.0(55), 470.4(189), 458.7(98), 445.6(1), 436.7(208), 212.3(22), 166.1(2), 47.2(0) 3732.8(81), 1668.1(49), 1010.3(121), 989.8(152), 527.3(117), 484.0(126), 420.8(37), 102.6(14), 1343.7i(1857) 3857.9(120), 1898.7(79), 1228.0(22), 966.0(176), 866.3(146), 644.4(29), 602.6(88), 468.7(87), 379.0(134), 194.7(22), 90.9(3), 1479.2i(1432) 1010.4(165) 997.6(128), 953.8(245), 349.7(1) 3802.7(100), 3703.4(42), 1608.7(112), 973.5(249), 382.5(11), 290.0(149), 249.0(26), 75.1(12), 67.6(6) 3877.7(196), 1777.2(283), 1008.0(187), 715.5(129), 591.2(22), 531.6(129), 404.3(103), 333.2(50), 203.5(18) 3834.9(174), 3746.4(53), 1635.0(103), 964.3(129), 928.0(313), 531.7(6), 366.6(17), 323.7(147), 273.8(3), 139.4(20), 83.1(16), 74.6(1)

-284.07029

3879.1(0), 3876.2(348), 991.3(199), 719.3(185), 665.5(50), 488.0(190), 459.0(88), 456.0(0), 441.0(184), 220.3(18), 178.2(2), 13.4(0)

-208.78860 -284.04696

-284.12498

Figure 12. Optimized structures (bond lengths in angstroms and bond angles in degrees) of the species involved in the NbOx þ H2O reactions (x = 1, 2).

weakness. As shown in Figure 12, the NbO(H2O) complex was predicted to have a quartet ground state without symmetry, in which the O atom of H2O is coordinated to the Nb atom of NbO with a Nb-OH2 distance of 2.400 Å. The NbdO bond length is elongated by 0.018 Å upon H2O coordination. The ground state of NbO(H2O) complex correlates to the ground state of NbO, which was determined to be a 4Σ state.37 HTaO(OH). The 3729.7, 1793.2, 987.2, and 727.3 cm-1 absorptions appeared together under visible light irradiation at the expense of the TaO(H2O) complex absorptions. These absorptions are assigned to different vibrational modes of

HTaO(OH), a structural isomer of the TaO(H2O) complex. The 1793.2 cm-1 absorption exhibited no shift with H218O, but shifted to 1284.4 cm-1 upon deuterium substitution. The band position and isotopic H/D frequency ratio of 1.396 indicate that this absorption is due to a Ta-H stretching mode. The 987.2 cm-1 absorption exhibited very small shifts with isotopic-labeled water samples and is assigned to the TadO stretching mode, which is 3.2 cm-1 blue-shifted from that of TaO(H2O). The 3729.7 and 727.3 cm-1 absorptions are due to the O-H and Ta-OH stretching vibrations. The HTaO(OH) molecule has been reported from the reaction between TaO2 and H2 in solid argon. The O-H, Ta-H, TadO, and Ta-OH stretching modes were observed at 3715.0, 1780.5, 972.1, and 701.1 cm-1 in solid argon, respectively.31a The argon matrix values are slightly red-shifted from those of neon matrix values as expected. The HTaO(OH) molecule was predicted to have a doublet ground state without symmetry. The structure optimized at the B3LYP/ AUG-CC-PVTZ/SDD level of theory is shown in Figure 11. HNbO(OH). Similar absorptions at 3727.6, 1725.7, 980.9, and 715.9 cm-1 in the niobium experiments are assigned to the O-H, Nb-H, NbdO, and Nb-OH stretching vibrations of the HNbO(OH) molecule in solid neon (Table 2). The same modes were observed at 3714.6, 1702.0, 966.3, and 696.9 cm-1 in solid argon.38 TaO2(H2O). The 3674.2, 3595.1, 1602.6, 957.8, and 903.8 cm-1 absorptions appeared together on annealing at the expense of the TaO2 absorptions and are assigned to the TaO2(H2O) complex. The 957.8 and 903.8 cm-1 absorptions are attributed to the symmetric and antisymmetric OTaO stretching vibrations, which are 21.4 and 17.1 cm-1 red-shifted from the corresponding modes of TaO2 isolated in solid neon. The 3674.2 and 3595.1 cm-1 absorptions shifted to 3660.5 and 3588.0 cm-1 with H218O. The upper band exhibited larger O-18 shift than that of the low band and is assigned to the antisymmetric O-H stretching mode. The low band is due to the symmetric 2243


The Journal of Physical Chemistry A stretching mode. In the experiment with the H2O þ HDO þ D2O mixture, six absorptions at 3674.2, 3635.8, 3595.1, 2732.3, 2675.2, and 2627.1 cm-1 were presented (Figure 4, trace c). The 2732.3 and 2627.1 cm-1 absorptions are due to the antisymmetric and symmetric O-D stretching modes of TaO2(D2O), and the 3635.8 and 2675.2 cm-1 absorptions are attributed to the O-H and O-D stretching modes of TaO2(HDO). The 1602.6 cm-1 absorption is due to the H2O bending mode. The same mode for TaO2(HDO) and TaO2(D2O) was observed at 1415.9 and 1186.5 cm-1. The TaO2(H2O) complex was predicted to have a doublet ground state with Cs symmetry, in which the O atom of H2O is coordinated to the metal center of TaO2 (Figure 11). The TaOH2 bond distance was predicted to be 2.270 Å, ca. 0.026 Å shorter than that of TaO(H2O). Upon H2O coordination, the bond length of TaO2 is elongated by 0.017 Å. As listed in Table 3, the calculated vibrational frequencies fit the observed values. The calculated isotopic shifts also match the experimental values and add additional support on the assignment. NbO2(H2O). The absorptions at 3693.9, 3615.3, 1607.6, 940.7, and 894.2 cm-1 are assigned to the NbO2(H2O) complex (Table 2). DFT calculation predicted the NbO2(H2O) complex to have a 2A0 ground state with nonplanar Cs symmetry, which can be viewed as being formed via the interaction of the ground state NbO2 (2A1) and H2O. TaO(OH)2. The absorptions at 3741.0, 973.4, and 708.1 cm-1 are assigned to the TaO(OH)2 molecule (Table 1). These absorptions appeared upon visible light (400-580 nm) irradiation at the expense of the TaO2(H2O) complex absorptions. The 973.4 cm-1 absorption showed very small shifts with isotopiclabeled water samples and is attributed to the TadO stretching mode. The 3741.0 cm-1 absorption shifted to 3730.5 cm-1 with H218O, and four absorptions at 3742.5, 3741.0, 2759.7, and 2758.2 cm-1 were observed in the experiment with the H2O þ HDO þ D2O mixed sample. The 2758.2 cm-1 absorption is due to TaO(OD)2, while the 3742.5 and 2759.7 cm-1 absorptions are assigned to the O-H and O-D stretching vibrations of TaO(OH)(OD). The 708.1 cm-1 absorption is assigned to the antisymmetric Ta-OH stretching mode. The TaO(OH)2 molecule was calculated to have a 2A1 ground state with planar C2v symmetry, in which the two OH groups are in the cis-cis form (Figure 11). Similar structure has been reported for the TiO(OH)2 molecule.19 In contrast, the BO(OH)2 molecule was characterized to have a planar Cs structure with the OH groups in the cis-trans form. The calculated vibrational frequencies and isotopic shifts are in quite good agreement with the experimental values (Table 3). NbO(OH)2. In the niobium experiments, absorptions at 3738.6, 968.1, 726.3, and 683.7 cm-1 were produced at the expense of the NbO2(H2O) absorptions and are assigned to the NbO(OH)2 molecule following the example of TaO(OH)2. Only one O-H(D) stretching mode was observed for the NbO(OH)2 and NbO(OD)2 molecules, while both the O-H and O-D stretching modes of NbO(OH)(OD) were observed at 3740.0 and 2757.2 cm-1. The 726.3 and 683.7 cm-1 absorptions are attributed to the antisymmetric and symmetric NbOH stretching modes. Analogous to TaO(OH)2, the NbO(OH)2 molecule also was characterized to have a planar C2v symmetry, as shown in Figure 12. Reaction Mechanism. Pulsed laser evaporation of bulk Ta2O5 and Nb2O5 targets produced metal monoxide and dioxide molecules as the major products. The metal monoxide and

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dioxide molecules trapped in solid neon reacted with water to form the 1:1 metal oxide-water complexes, reactions 1-4. The spontaneous formation of these metal oxide-water complexes upon sample annealing implies that negligible activation energy is required for these addition reactions. No higher complexes with two or more water molecules are observed in the present experiments. TaO ð2 ΔÞ þ H2 O ð1 A 1 Þ f TaOðH2 OÞ ð2 A 00 Þ ΔE ¼ - 12:1 kcal=mol

ð1Þ 2 0

TaO2 ð A 1 Þ þ H2 O ð A 1 Þ f TaO2 ðH2 OÞ ð A Þ 2

1

ΔE ¼ - 20:9 kcal=mol

ð2Þ

NbO ð4 ΣÞ þ H2 O ð1 A 1 Þ f NbOðH2 OÞ ð4 AÞ

ð3Þ

ΔE ¼ - 12:5 kcal=mol NbO2 ð2 A 1 Þ þ H2 O ð1 A 1 Þ f NbO2 ðH2 OÞ ð A0 Þ 2

ΔE ¼ - 21:8 kcal=mol

ð4Þ

The binding energies of the TaO(H2O) and NbO(H2O) complexes with respect to TaO þ H2O and NbO þ H2O were predicted to be 12.1 and 12.5 kcal/mol, which are larger than the corresponding values of the metal monoxide-methane and chloromethane complexes.31 The binding energies of the TaO2(H2O) and NbO2(H2O) complexes with respect to TaO2 þ H2O and NbO2 þ H2O were predicted to be 20.9 and 21.8 kcal/ mol, which are about twice as large as the corresponding values of the monoxide complexes due to increased electrostatic interactions. The TaO(H2O) and NbO(H2O) complexes rearranged to the HTaO(OH) and HNbO(OH) isomers under visible light irradiation with the wavelength range of 500-580 nm. According to DFT calculations, the HTaO(OH) and HNbO(OH) molecules are about 60.9 and 49.9 kcal/mol lower in energy than the TaO(H2O) and NbO(H2O) complexes, respectively. The potential energy profile for the TaO þ H2O reaction calculated at the B3LYP/AUG-CC-PVTZ level of theory is shown in Figure 13. The initial interaction between the 2Δ ground state TaO and water is the formation of the 1:1 complex without any barrier. From the complex, the reaction proceeds by a hydrogen atom migration from O to the metal center to form HTaO(OH) via a transition state (TS1). This hydrogen atom transfer process was predicted to be exothermic by 60.9 kcal/mol with a barrier of only 8.4 kcal/mol. From the TaO(H2O) complex, another possible reaction path is one hydrogen atom transfer from water to the oxygen atom of metal monoxide to form the Ta(OH)2 molecule. However, the Ta(OH)2 molecule was not observed in the experiments. DFT calculations predicted that the Ta(OH)2 molecule has a quartet ground state with linear geometry, which is 25.3 kcal/mol less stable than the HTaO(OH) isomer. The TaO2(H2O) and NbO2(H2O) complexes rearranged to the TaO(OH)2 and NbO(OH)2 isomers via one hydrogen atom transfer from water to one of the oxygen atoms of metal dioxide under visible light irradiation with the wavelength range of 400580 nm. The TaO(OH)2 and NbO(OH)2 molecules are 48.9 and 42.9 kcal/mol more stable than the TaO2(H2O) and NbO2(H2O) complexes based upon DFT calculations. As shown in Figure 13, the reaction from the TaO2(H2O) complex to 2244



Figure 13. Potential energy profiles of the TaOx þ H2O (x = 1, 2) reactions calculated at the B3LYP/AUG-CC-PVTZ/SDD level of theory.

TaO(OH)2 proceeds via a transition state TS2, which lies about 10.7 kcal/mol above the TaO2(H2O) complex.

’ CONCLUSIONS Water adsorption and hydrolysis on molecular tantalum and niobium monoxides and dioxides have been investigated by matrix isolation infrared spectroscopy as well as density functional theoretical calculations. The metal monoxide and dioxide molecules prepared from pulse laser evaporation of bulk metal oxide targets reacted with water to form the MO(H2O) and MO2(H2O) (M = Ta, Nb) complexes spontaneously on annealing. These 1:1 complexes were characterized to involve bonding interactions between the O atom of water and the metal center. The MO(H2O) complexes photochemically rearranged to the more stable HMO(OH) isomers via one hydrogen atom transfer from water to the metal center under visible light excitation with the wavelength range of 500-580 nm. In contrast, the MO2(H2O) complexes isomerized to the more stable MO(OH)2 molecules via a hydrogen atom transfer from water to one of oxygen atom of metal dioxide upon visible light irradiation in the wavelength range of 400-580 nm. ’ AUTHOR INFORMATION Corresponding Author

*E-mail [email protected].

’ ACKNOWLEDGMENT We gratefully acknowledge financial support from National Natural Science Foundation (20933003) and National Basic Research Program of China (2007CB815203 and 2010CB732306). ’ REFERENCES (1) Liu, K.; Parson, J. M. J. Chem. Phys. 1978, 68, 1794. (2) Tilson, J. L.; Harrison, J. F. J. Phys. Chem. 1991, 95, 5097. (3) Guo, B. C.; Kerns, K. P.; Castleman, A. W. J. Phys. Chem. 1992, 96, 4879. (4) (a) Clemmer, D. E.; Aristov, N.; Armentrout, P. B. J. Phys. Chem. 1994, 98, 6522. (b) Chen, Y. M.; Clemmer, D. E.; Armentrout, P. B. J. Phys. Chem. 1994, 98, 11490.

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Matrix Isolation Spectroscopic and Theoretical Study of Water

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