. I Phys. . Chem. 1990, 94, 2810-2814
2810
Ab Initio Calculation of Harmonic Force Fields and Vibrational Spectra for the Arsine Oxides and Sulfides R,AsY (R = H, F; Y = 0, S) and Related Compounds Winfried Schneider, Walter Thiel,* Theoretische Chemie, Bergische Universitat-Gesamthochschule, 0-5600Wuppertal I , West Germany
and Andrew Komornicki Polyatomics Research Institute, 1101 S a n Antonio Rd., Mountain View, California 94043 (Received: July 6 , 1989; In Final Form: October I I, 1989)
Ab initio self-consistent-field calculations using effective core potentials and polarized double-zeta basis sets are reported for the arsenic compounds H,As, H,AsO, H,AsS, F3As, F3As0, F3AsS,cis- and tranr-H2AsOH, and HAsO. The calculated geometries, rotational constants, vibrational frequencies, Coriolis coupling constants, centrifugal distortion constants, infrared band intensities, and force fields are compared with the available experimental data. Good agreement is found in the case of the known molecules, especially H3As and F,As, so that the predictions for the unknown molecules are expected to be realistic. The theoretical results confirm a recent spectroscopic identification of H,AsO, H2AsOH, and HAsO and suggest reassignment of several observed frequencies.
1. Introduction
The harmonic force fields and vibrational spectra of the phosphine oxides and sulfides R3PY (R = H, F, CH,; Y = 0, S) have recently been studied at the ab initio level.' This work has provided predictions of the gas-phase infrared spectra of H 3 P 0 and H3PS, improved force fields for F,PO and F3PS, more detailed assignments for the trimethyl compounds, and explanations for trends in the calculated force constants and dissociation energies for PO and PS bonds. The present paper reports analogous calculations for the arsine oxides and sulfides R,AsY ( R = H, F; Y = 0, S) and the corresponding arsines R3As. Experimentally, high-resolution gasphase rotational and vibrational spectra are available for H3As2-Io and F3As.I1-l4 Empirical force fields have been derived from the experimental data for H , A S ' ~and ~ ' ~F , A S , ~ ~ - ' and ~ , ' ' there have been several ab initio studies of these compounds18-22which were
( 1 ) Schneider, W.; Thiel, W.; Komornicki, A. J. Phys. Chem. 1988, 92, 5611. (2) Sarka, K.; Papousek, D.; Rao, K. N. J. Mol. Spectrosc. 1971, 37, I . (3) Chu, F. Y . ;Oka, T. J. Chem. Phys. 1974,60, 4612. (4) Olson, W. 9.;Maki, A. G.; Sams, R. L. J. Mol. Spectrosc. 1975, 55, 252. (5) Helms, D. A,; Gordy, W. J. Mol. Spectrosc. 1978, 69, 473. (6) Burenin, A. V.; Kazakov, V. P.; Krupnov, A. F.; Melnikov, A. A,; Shapin, S . M. J. Mol. Spectrosc. 1982, 94, 253. (7) Carlotti, M.; DiLonardo, G.; Fusina, L. J. Mol. Spectrosc. 1983, 102, 310. (8) DiLonardo, G.; Fusina, L.; Johns, J. W. C. J. Mol. Spectrosc. 1984, 104, 282. (9) Kazakov, V. P.; Krupnov, A. F.; Saveliev, V. N.; Ulenikov, 0. N. J . Mol. Spectrosc. 1987, 123, 340. (IO) McRae, G. A.; Gerry, M. C. L.; Wong, M.; Ozier. 1.; Cohen, E. A. J . Mol. Spectrosc. 1981, 123, 321. (11) Hoskins, L. C.; Lord, R. C. J . Chem. Phys. 1965, 43, 155. (12) Reichman, S.;Smith, D. F., Jr.; Overend, J. Spectrochim. Acta 1970, 26A, 927. (13) Chikaraishi, T.; Hirota, E. Bull. Chem. SOC.Jpn. 1973, 46, 2314. (14) Smith, J. G. Mol. Phys. 1978, 35. 461. (15) Duncan, J. L. J . Mol. Spectrosc. 1976, 60, 225. (16) Mirri, A. M. J. Chem. Phys. 1967, 47, 2823. (17) Konaka, S. Bull. Chem. Soc. Jpn. 1970, 43, 3107. (18) Marynick, D. S. Chem. Phys. Lett. 1980, 71, 101. (19) Marynick, D. S.;Rosen, D. C.; Liebman, J. F. J . Mol. Struct. 1983, 94, 47. (20) Trinquier, G.; Daudey, J.-P.: Caruana, G.;Madaule, Y. J. Am. Chem. Soc. 1984, 106, 4794.
0022-3654/90/2094-28 10$02.50/0
mainly concerned with their structures and inversion barriers. Comparatively little is known about the arsine oxides and sulfides. F,AsO has been generated in an argon or krypton matrix by photolysis of ozone in the presence of F,As and characterizzd by its vibrational spectrum.23 In a similar manner, H3As0 has been detected very recently in an argon matrix.24 H,AsS and F,AsS are still unknown. In view of this situation the present paper provides a number of theoretical predictions for the arsine oxides and sulfides, especially with regard to their harmonic force fields and gas-phase vibrational spectra. Our theoretical results for H3As0 will be compared with some very recent experimental data24which were published after our calculations had been completed. In order to discuss the suggested assignments for H3As0,24we include the theoretical results for two isomers, cis- and trans-H2AsOH, and for HAsO.
2. Details of the Calculations All quantum-chemical calculations were carried out at the HartreeFock level by using the GRADSCFprogram system25which was modified to handle analytical second derivatives for effective core potentials (ECP).26 Standard ECPs for As and S were taken from the literature,*' and the corresponding ECP basis sets2' were used in a double-{contraction [3/21] augmented by one set of six polarization d functions with the following exponents: As, 0.35; S, 0.65. The standard 6-31 G* basis28,29was employed for H, 0,and F. Molecular geometries were completely optimized within the constraint of C,, symmetry (C, for cis- and trans-H2AsOH and HAsO). The Cartesian force constants and Cartesian dipole moment derivatives were evaluated analytically at these theoretical equilibrium geometries and then converted to spectroscopic constants in the usual (21) Dixon. D. A.; Arduengo, A. J. J. Am. Chem. Soc. 1987, 109, 338. (22) Clotet, A.; Rubio, J.; Illas, F. J. Mol. Struct. 1988, 164, 351. (23) Downs, A. J.; Gaskill, G. P.; Saville, S. 9.Inorg. Chem. 1982, 21. 3385. (24) Andrews, L.; Withnall, R.; Moores, 9.W. J . Phys. Chem. 1989, 93, 1279. (25) Komornicki, A. GRADSCF: An ab Initio Gradient Program System, Version 9.5; Polyatomics Research Institute: Mountain View, CA, 1988. (26) Breidung, J.; Thiel. W.; Komornicki, A. Chem. Phys. Lett. 1988, 153, 76. (27) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985.82, 284. (28) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J . Chem. Phys. 1972, 56, 2257. (29) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (30) Wilson, E. B., Jr.; Decius, J. C.; Cross, P. C. Molecular Vibrations; McGraw-Hill: New York, 1955.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 2811
Arsine Oxides and Sulfides TABLE I: Molecular Geometries and Rotational Constants'
molecule H~As
typeb T
E E H~AsO H~AsS FIAS F~AsO F~AsS
E T
T T E T
T
re, A
1.511 1.5108 (4) 1.5111 1.513 (2) 1.497 1.494 1.684 1.7041 (IO) 1.642 1.651
Re, A
1.597 2.063
1.553 1.990
a,,deg 94.0 92.08(4) 92.07 92.08(7) 101.4
101.1
95.7 95.77(1 2) 99.5 98.3
A,, C,, cm-I
3.4241
Be, cm-I 3.8618
3.5352
3.7998( I )
4 7 IO
3.1 I80 3.1413 0.1423 0.1388(3) 0.1412 0.1422
0.4323 0.1637 0.2014 0.1967 0.2014 0.0764
14
ref
'For notation see text. bT = theoretical, E = experiment; uncertainty of last digit given in parentheses. TABLE 11: Vibrational Frequencies wi and vi (em-') and Infrared Intensities Ai (km/mol) calcdb molecule mode vi description" ai 2357 2118 a1 y1 As-H s-stretch H~As 1034 909 y2 HAsH s-deform e y3 As-H d-stretch 2365 2125 1136 998 y4 HASH d-deform H~AsO a1 V I As-H s-stretch 2452 2203 y2 HAsH s-deform 1 I47 1008 u3 As-0 stretch 1019 934 e y4 As-H d-stretch 2439 2191 "3 HAsH d-deform 1 IO8 974 y6 HAsH d-rock 680 774 H~AsS a1 V I As-H s-stretch 2449 2200 u1 HAsH s-deform 1 IO3 969 u3 As-S stretch 523 523 e y4 As-H d-stretch 2439 2192 HAsH d-deform 1102 969 y6 HAsH d-rock 670 589 F~As a1 VI As-F s-stretch 806 740 380 342 FAsF s-deform y1 765 703 e y3 As-F d-stretch 288 259 y4 FAsF d-deform 1 I42 1045 F~AsO a1 "I As-0 stretch 838 755 PI As-F s-stretch 377 323 y3 FAsF s-deform e y4 As-F d-stretch 859 773 y5 FAsF d-rock 365 312 y6 FAsF d-deform 278 238 F~AsS a1 V I As-F s-stretch 847 781 y2 As-S stretch 578 573 y3 FAsF s-deform 366 331 e y4 As-F d-stretch 840 771 u3 FAsF d-rock 312 280 y6 FAsF d-deform 221 199
Ai
61 41 326 39 63 56 172 308 108 112 1 I7 34 263 192 72 19 82 41 273 16 99 37 51 246 156 1
171 53 58 225 48 15
obsd' ui 2115.17 906.75 2126.42 999.22
footnote d
e
937.9 2173.5,2170.1 983.4,979.8 687.2
740.55s 336.5m 702.2s 262.3m 1045.5w 752.5m 333.4m 774.3s 306.8m
f f g
h
i
i i
i i
'Based on the calculated potential energy distributions. For scale factors see Table 111; intensities refer to the scaled theoretical force fields. Infrared frequencies and relative intensities in standard notation. dReferences 4 and 8,gas phase. CReference24,argon matrix. 'Matrix splittings according to ref 24. #New assignment,see text. *References 1 1 and 12,gas phase. 'Reference 23,argon matrix. jMixture of d-rock and d-deform. the root-mean-square (rms) deviation between the frequencies vi The conventions for defining the internal and symmetry coordinates in the molecules with C3,symmetry were the same as calculated from the scaled theoretical force field and the observed fundamental frequencies vPM. In the case of unknown molecules, before:' The z axis is thc C, axis, and the xz plane is taken as the scale factors were transferred from known related molecules, a plane of symmetry. R denotes an A s 0 or ASS distance, r an e.g., c, and c, from H3As or F3As (see below). ASH or AsF distance, and (Y an HASH or FAsF angle. The symmetry coordinates Si were defined as ~ s ~ a(see 1 Tables ~ ~ * ~ ~ 3. Results and Discussion I and I I of ref I ) . For scaling the theoretical harmonic force fields in symmetry coordinates, we employed the same procedureg5as In this section we present our calculated geometries, rotational in our previous studies:'*3zA scale factor ci is introduced for each constants, vibrational frequencies, infrared intensities, Coriolis diagonal force constant Fii associated with symmetry coordinate coupling constants, centrifugal distortion constants, force fields, Si. The scaled off-diagonal force constants are then defined in and relative energies and compare them with the available exterms of the unscaled ones by F;.SC = ci1/zFi,c.1/2. Since identical perimental data. scale factors apply for qualitatively similar distortions, there are Table I lists the calculated and molecular geonly three scale factors in the molecules with C,, symmetry, Le., ometries and rotational constants. The theoretical results for H3As cR, c,, c,. Their optimum values were obtainedg2by minimizing and F3As agree well with experiment and are of similar accuracy as those for the corresponding phosphines.' The theoretical predictions for H,AsO, H,AsS, F 3 A s 0 , and F3AsS are therefore (31) Califano, S. Vibralional States; Wiley: New York, 1976. (32) Schneider, W.; Thiel, W. J . Chem. Phys. 1987, 86, 923. also expected to be realistic. When going from the arsines R,As (33) Duncan, J. L.; Mills, 1. M . Specrrochim. Acta 1964, 20, 523. to the arsine oxides or sulfides R3AsY, the bond length re(R-As) (34) Aldous, J.; Mills, I . M. Specrrochim. Acfa 1962, 18, 1073. is calculated to decrease while the angle a,(RPR) increases. (35) Pulay. P.;Fogarasi, G.; Pongor, G.;Boggs, J. E.; Vargha, A. J . Am. Fluorine substitution shortens the bond length R,(As-Y) sigChem. SOC.1983, 105, 7037.
2812
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ref
TABLE V: Centrifugal - Distortion Constants (10'" cm-') molecule n type" D, DK DJK H,As 4 T 1.0445 -1.3104 1.0252 E 0.9777 (18) -1.2421 (2) 1.1198 (15) E 0.9762 ( I ) -I ,2422 1.1 189 H~AsO 6 T 0.3449 6.1849 36.5218 H~AsS 6 T 0.0615 1.2884 44.0642 F~As 7 T 1.6980 -2.2478 1.0970 E 1.5448 -2.0600 ( I ) E 1.54 ( 1 ) -2.06 (2) T 3.5414 F~AsO 8 4.8052 -3.6161 F~AsS 8 T 0.8076 7.4883 -3.681 1
2 8, 9
"T = theoretical equilibrium value from scaled force field: E = experimental ground-state value.
TABLE 111: Scale Factors c, and rms Deviations u (cm-I) molecule H3As HlAsO H~AsS F~As F~AsO FjAsS
C,
CR
0.8076 0.8076 0.8076 0.8425 0.8107 0.8425
0.8399 1 .OOOO 0.8399 1 .OOOO
-
C,
U
0.7720 0.7720 0.7720 0.8087 0.7320 0.8087
1.7 2.8 5.0
TABLE IV: Coriolis Constants' molecule H~As H~AsO H~AsS F,As
F~ASO F~AsS
typeb
T E E T T T E T T
{#
-0.0073 -0.018 -0.0460 -0.0355 0.2357 0.2434 0.2271 0.221 8
f"+I -0.4288 -0.4458 -0.4550 -0.2871 -0.3173 -0.5283 -0.5308 -0.2305 -0.2629
L+z
Schneider et al.
ref 6 7
14 16
0.4025 0.3789 14 0.4625 0.3095
= 3 for H,As and F3As: n = 4 otherwise. b T = theoretical, E = experiment. O n
nificantly. All these trends are also found in the corresponding phosphorus compounds, with shifts of similar magnitude.' Table I1 contains the unscaled theoretical harmonic frequencies ai, the scaled theoretical frequencies vi, and the calculated infrared intensities A,, together with the available experimental data.4.8J','2*23s24 Table 111 collects the scale factors ci for each molecule and the rms deviations between the scaled theoretical and observed frequencies. The optimized scale factors for H3As, F3As,and F3As0(see section 2) have been transferred to H3As0, H,AsS, and F3AsS, in a manner that is obvious from the data in Table 111. Due to the lack of suitable reference data, the scale factor for the As-S stretching coordinate in arsine sulfides has been set to cR = I , in analogy to the corresponding P-S scale fact0r.l The scaled theoretical frequencies for H3Asand F3Asare in excellent agreement with the observed frequencies which may be explained as follows. Because of the close correspondence between normal and symmetry coordinates in these molecules, optimization of the scale factors c, and c, is expected to reproduce the degeneracy-weighted average of the frequencies v I , u3 and v2, u4, respectively, which is indeed found (see Table 11). Hence, errors in the scaled theoretical frequencies mainly arise from errors in the calculated splittings between these pairs of frequencies. Obviously, these splittings (and the associated coupling force constants) are predicted quite well since the rms deviations for the frequencies are below 3 cm-I (see Table Ill). The five known fundamental frequencies2) of F3As0are also reproduced well by the scaled theoretical frequencies, with an rms deviation of 5 cm-l. Our prediction for the v6 frequency of 238 cm-I should be more reliable than the previous estimate2) of 286 cm-I. It is not surprising that the v6 band has not yet been observed since it is calculated to be very weak (see Table 11). According to previous e ~ p e r i e n c e , ' , ~ ~the , ~ ~transfer J' of scale factors between related molecules is usually accompanied by errors of typically 20 cm-' in the scaled theoretical frequencies. This can be checked in the case of H 3 A s 0which has been detected e ~ p e r i m e n t a l l yafter ~ ~ our calculations had been completed. The expected accuracy is indeed reached or surpassed for the u,, u4, and u5 bands. In the case of the v6 band, the original assignment (8 17 cm-I) is incompatible with our prediction (680 cm-l) and should be revised, in our opinion. On the basis of the detailed discussion in section 4,we suggest to assign the peak at 687 cm-' in the experimental spectrum24as u6. The theoretical predictions for H3AsSand F3AsS(see Table 11) should also be accurate to about 20 cm-I. Larger deviations ( 3 6 ) Breidung, J.; Thiel, W. J . Phys. Chem. 1988, 92, 5597. (37) Breidung, J.; Thiel, W.; Komornicki, A . J . Phys. Chem. 1988, 92, 5603.
24.x
?xD
2wo
1BM
IEa
luy)
12m
lmo
BM
e a
Figure 1. Predicted infrared spectrum of H,AsO in the gas phase a t 298 K. optical density x = 0.138 atm cm, resolution 3 cm-'.
may occur especially for the As-S stretching vibration due to the use of an unscaled As-S stretching force constant (cR = 1, see above). In analogy to F,AsO the u6 band is predicted to be the weakest band for H3AsSand F3AsS. Tables 1V and V list the Coriolis constants and the centrifugal distortion constants, respectively. The deviations between theory and experiment for H3As and F3As are of the usual magnit ~ d e ,Le.,~ f0.02 ~ ~ for ~ the ~ Coriolis ~ ~ , constants ~ ~ and *IO% for the centrifugal distortion constants. A similar accuracy is expected for the other molecules. I n Table V I the scaled theoretical force fields for H3Asand F3Asagree well with the most recent empirical which lends some credence to the predictions for the other molecules. The trends in the calculated force constants are very similar to those discussed previously for the corresponding phosphorus compounds.' In particular, comparing the pairs H3AsO/F3As0 and H3AsS/F3AsS,fluorine substitution is found to increase all force constants significantly in absolute value, indicating stronger bonds and stiffer angles in the trifluoro compounds (as well as stronger coupling between stretching and deformation modes). The As-0 and As-S stretching force constants F 3 ) increase by 22-26% upon fluorine substitution which reflects the enhanced role of .rr back-bonding due to the introduction of three electronegative fluorine atoms. Comparing the pairs H3AsO/H3AsS and F,AsO/ F3AsS,the calculated force constants are surprisingly similar, with the obvious exception of F3, (see Table VI). The dissociation energies Doof the As0 and ASSbonds may be estimated as fol10ws:l~~~ The reaction energy AE, for the hydrogenation R3AsY H2 R3As+ H2Y is calculated from theoretical total energies and converted first to LEo by including the theoretical zero-point vibrational energies and then to Doby adding the e ~ p e r i m e n t a lheat ~ ~ , of ~ ~the reaction H,Y H2 Y at 0 K. The results in Table VI1 show that the A s 0 bond is considerably stronger than the ASSbond. Fluorine substitution increases the dissociation energy in the arsine oxide by 1 I % but has little effect in the arsine sulfide. Compared with those for the PO and PS bonds in the corresponding phosphorus compounds,' the calculated dissociation energies for the As0 and ASS
+
-
-
+
(38) Wallmeier, H.; Kutzelnigg, W. J . Am. Chem. Soc. 1979, 101, 2804. (39) Stull, D. R.; Prophet, H. J A N A F Thermochemical Tables, 2nd ed.; 6 . S . National Eureau of Standards: Washington, DC, 1971; NSRDS-NBS 37. (40) Chase, M. W., Jr.; Curnutt, J. L.; Downey, J . R.; McDonald, R. A.; Syverud, A . N . ; Valenzuela, E. A . J . Phys. Chem. ReJ Data 1982. 1 1 , 695.
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 2813
Arsine Oxides and Sulfides TABLE VI: Scaled Theoretical and EmDirical Force Fields (mdyn/A)' molecule type F,, Fl2 F22 Fi3 F23 0.120 0.594 H3As T 2.631 E 2.710 (22) 0.123 (6) 0.569 ( 3 ) 0.230 0.612 E 2.845 0.051 0.530 0.185 -0.115 H3As0 T 2.862 0.075 0.488 0.124 -0.129 H3AsS T 2.852 0.123 1.431 F~As T 5.068 E 5.078 ( 7 3 ) 0.11 (12) 1.425 (17) E 4.80 -0.48 I .66 0.95 0.64 E 5.39 F~AsO T 5.750 0.208 0.858 0.156 -0.118 F~AsS T 5.718 0.252 0.948 0.234 -0.175
Fmmb
F33
6.819 3.668
8.348 4.623
2.643 2.731 (22) 2.899 2.805 2.805 4.321 4.314 (30) 4.34 4.42 5.229 5.219
'Angle-bending coordinates have been scaled with a unit bond length of I A. TABLE V11: Reaction Energies AEe and AEo and Dissociation Energies Do (kcal/mol)'
molecule H~AsO H~AsS F~AsO F~AsS
AE, -46.2 -24.0 -39.3 -25.7
AEO
DOb
-42.9 -23.5 -34.9 -24.1
73.2 46.3 81.2 45.7
'See text. Do values for the corresponding phosphorus compounds:, 102.7, 54.1, 128.9, and 64.2 kcal/mol.
I
I , 24w
ZmI
zoo0
1 m
Ism
l
a
1100
1 m
m
600
Figure 2. Predicted infrared spectrum of H,AsS in the gas phase at 298 K, optical density x = 0.059 atm cm, resolution 3 cm-I.
bonds are considerably lower, by 14-37%, indicating a lower thermochemical stability of the arsine oxides and sulfides. To conclude this section, Figures 1 and 2 present plots of the predicted infrared spectra of H 3 A s 0 and H,AsS in the gas phase at 298 K. The simulations of the spectra were carried out by using a modified version of program KIL041 and input data derived from scaled theoretical force fields (see Tables I-V) as described p r e v i ~ u s l y . l -All ~ ~rovibrational lines up to J',,,,, = 100 and K',,, = 50 with intensities greater than a cutoff i= 0.0051,,,) were generated and represented by Gaussian profiles of width w = 3 cm-' to account for the spectral resolution. The resulting intensities were plotted as transmission spectra. It is hoped that these simulations will facilitate the spectroscopic identification of H 3 A s 0 and H3AsS in the gas phase. 4. Assignment of Observed Matrix Spectra A recent experimental reports infrared spectra of arsineozone complexes and of their reaction and photolysis products in solid argon. Codeposition of H3As and O3at high dilution in argon led to the appearance of new infrared bands which were attributed to an H3As-03 complex and to various reaction products labeled 2 and 3. Red photolysis changed the relative intensities of these bands and produced other new bands labeled 1 and 4. Ultraviolet photolysis led to further intensity changes and additional new absorptions. The assignment of the observed bands was largely based on their variations with experimental conditions, on their shifts with isotopic substitution, and on comparisons with and t h e o r e t i ~ a results l ~ ~ for the corresponding (41) Betrencourt-Stirnemann, C.; Graner, G.; Jennings, D. E.; Blass,
E.J . Mol. Spectrosc. 1978, 69, 179.
W.
(42) Withnall, R.; Hawkins, M.; Andrews, L. J . Phys. Chem. 1986, 90, 575. (43) Withnall, R.; Andrews. L. J . Phys. Chem. 1987, 91, 784. (44) Withnall, R . ; Andrews. L. J . Phys. Chem. 1988, 92, 4610.
Fm,b F",b F46 F56 F66 -0.038 0.646 -0.023 (9) 0.668 (4) -0.063 0.682 -0.095 0.599 0.013 -0.159 0.634 -0.079 0.575 -0.006 -0.1 1 I 0.514 -0.075 0.896 -0.064 (7) 0.942 (6) -0.23 0.918 -0.39 0.35 -0.214 0.862 0.168 -0.234 0.696 -0.212 0.961 0.147 -0.224 0.633
b m = 3, n = 4 for H3As and
F3As; otherwise m = 4,
ref
IO 15
14 16 17
n = 5.
phosphorus compounds. The following identifications of the observed absorptions were suggested: 1, H,AsO; 2, cis-H2AsOH; 3, trans-H2AsOH; 4, HAsO. Our present theoretical results support the i d e n t i f i ~ a t i o nof~ ~ H 3 A s 0 through the u3, u4. and u5 bands (see Table 11). To clarify the discrepancies with regard to the V6 band (see section 3), we have studied cis-H2AsOH (2), trans-H2AsOH (3), and H A s O (4) which coexist with HJAsO (1) after the photolysis of the H3As-0, complex.24 The optimized equilibrium geometries and relative energies of 1-4 are given in Table VIII. Although the calculated relative energies should only be regarded as semiquantitative estimates at the present theoretical level, it is obvious that the energy of 1 is considerably higher than that of 2 and that this energy difference is much larger than for the corresponding phosphorus compounds (33 vs 7 k ~ a l / m o at l ~comparable ~ theoretical levels). Table IX lists the predicted vibrational frequencies and intensities for those four isotopomers of 1-4, which have also been investigated e~perimentally.~~ In analogy to a previous theoretical ~ simple uniform scaling study of cis- and t r a n ~ - H ~ P 0 H a, "very has been employed for 2-4 in Table IX, Le., vi = 0 . 9 which ~ ~ implies Fiy = 0.8 1Fij for all i, j . This procedure is less elaborate than that used for 1 (see section 2), but it should be sufficient for assignment purposes. Note that the uniform scale factor of 0.81 for 2-4 is close to the average of the scale factors for 1 (see Table 111). A comparison of the theoretical results in Table IX with the experimental data24suggests several obvious reassignments: The calculated u6 frequencies for the isotopomers of 1 (680.1, 676.9, 5 13.6, 509.3 cm-') should be associated with the absorptions (687.2, 686.5, 516.1, 512.7 cm-I) originally assigned" to a rocking mode in 2. The frequencies originally attributed to Y6 in 1 (81 7. I , 815.4, 576.3, 574.2 cm-I) are close to the calculated v5 frequencies in 2 or 3, but on intensity grounds they probably belong to 2 (calcd 802.2, 801.7, 57 I .2, 571.1 cm-I). The suggested torsional frequencies vg of 2 (297.5, 296.5, -, 223 cm) are quite different from the calculated values for 2 or 3 and may belong to a different species. On the other hand, the observed frequencies (367, 364, -, - cm-l) assigned to 3 would fit nicely with the calculated torsional frequencies of 2 (369.6, 368.1, 270. I , 268.4 cm-I). The observed frequencies (647.9, 619.4, 647.8, 623.5 cm-I) are confirmed to correspond to the A s 4 stretching mode Y6 in H2AsOH, but the first two may well belong to 2 and the last two to 3. In the majority of cases, however, the original assignments of the observed bands24are supported by our theoretical results. This is generally true for the As-H and 0 - H stretching modes but also applies to other characteristic modes. In H 3 A s 0 the observed v3 frequencies (937.9, 894.0, 938.2, 894.9 cm-I) for the As-0 stretching mode are in excellent agreement with our calculated values (933.5, 889.4, 934.4, 890.9 cm-I), and the observed us frequencies (979.8, -, 704.9, 704.8 cm-' for the lower component of the matrix splitting) are reasonably close to our predictions (973.7,973.7,692.6,692.5 cm-I). In cis-H2AsOH, the calculated u3 frequencies (1030.2, 1027.5, 775.3, 763.9 cm-I) agree well with (45) Person, W. B.; Kwiatkowski, J . S.; Bartlett, R. J . J . Mol. Sfruct.1987, 157, 237.
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Schneider et al.
TABLE VIII: Calculated Equilibrium Geometries' and Relative Energiesbfor 1-4 molecule
AS-0 1.597 1.770 1.780 1.585
1
2 3 4
AS-H 1.497 1.519 1.513 1.550
0-H
HAsH
HAsO
0.948 0.947
101.4 92.4 93.3
116.7 99.3 96.9 102.6
AsOH
AE.C
114.0 110.7
32.9 0 1.5 5 1.4e
AE , ~ 31.9 0 1.4 44.9'
"Bond lengths A-B in A, angles ABC in degrees; C3, for 1, C, for 2-4. kcal/mol.
