Reactions of Selenium and Oxygen. Matrix Infrared Spectra and

The OSeOO species is probably a planar cis chain isomer (Cs symmetry), which is ... Sulfur dioxide is the major player, occurring all too often in the...
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J. Phys. Chem. 1996, 100, 16487-16494

16487

Reactions of Selenium and Oxygen. Matrix Infrared Spectra and Density Functional Calculations of Novel SexOy Molecules G. Dana Brabson and Lester Andrews* Department of Chemistry, UniVersity of Virginia, CharlottesVille, Virginia 22901

Colin J. Marsden IRSAMC, UniVersite´ Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex, France ReceiVed: May 14, 1996; In Final Form: July 17, 1996X

Reactions of selenium and oxygen were performed using five different methods, and the products were trapped for matrix infrared spectroscopic study. Based on different chemistry, photolysis and annealing behavior, selenium and oxygen isotopic substitution, and DFT structure and frequency calculations, several new selenium oxides are characterized. In addition to SeO, SeO2, and SeO3, and the (Se2)(O2) complex, the new molecules SeSeO, SeOO, Se2O2, and OSeOO are identified. Several isomers of Se2O2 are accidentally almost degenerate; the species identified, which contains a three-membered ring with an exocyclic SedO unit, is one of the most stable. The OSeOO species is probably a planar cis chain isomer (Cs symmetry), which is calculated to lie about 100 kJ/mol above the D3h structure of SeO3.

Introduction Mixed chalcogenides are most commonly represented by sulfur oxides. Sulfur dioxide is the major player, occurring all too often in the environment; however, the more reactive SO, S2O, OSSO, and SO3 molecules1-7 and (S2)(O2) complex8 are also known. The analogous selenium oxides SeOx (x ) 1, 2, 3) have been characterized.9-14 In addition, selenium is expected to form larger molecules that may be more stable than the sulfur analogs. We report here the matrix synthesis of new selenium oxides using five different experimental approaches: (1) photolysis of the (Se2)(O2) complex,8 (2) reaction of Se2 with O3, (3) microwave discharge of Ar/O2/Sex mixtures, (4) reaction of Se with O2, and (5) photolysis of SeO2 with and without O3 present. In addition, a systematic density functional theory study of possible product structures and vibrational frequencies was done to complement the matrix infrared spectroscopic investigation. Experimental Section The matrix isolation apparatus and selenium sources have been described previously.15-18 A closed-cycle refrigerator (CTI Cryogenics Model 22) cooled the CsI sample window to 11 ( 1 K, and spectra were recorded on a Perkin-Elmer 983 spectrometer at resolution 1 cm-1 and accuracy (0.2 cm-1. Diatomic Se2 was generated by superheating selenium vapor at 900 °C using a selenium reservoir at 120 °C.17 The amount of Se2 in the matrix was monitored from the Se2 fine-structure band at 513.6 cm-1; best results were obtained when this band and the 370 cm-1 band for Se4 were weak.17 Superheated selenium vapor was codeposited with Ar/O2 mixtures (typically 1% and 0.5%) for 8-10 h. Selenium dioxide was evaporated from a similar quartz heater at 117 °C and codeposited with argon at 2-3 mmol/h. Certain experiments involved codeposition of SeO2 with Ar/O3 samples using ozone prepared as described previously.19 Selenium atoms and Se2 were also generated by microwave discharge using a 220 °C reservoir as described earlier.17 The discharge was kept well forward of the reservoir. X

Abstract published in AdVance ACS Abstracts, September 1, 1996.

S0022-3654(96)01388-3 CCC: $12.00

In some experiments 1% O2 was added to the argon dischargecarrier gas. Photolysis was performed using filtered light from a high-pressure mercury arc (BH-6) with a cooled water cell to absorb near-infrared radiation for 30 min periods. Ultraviolet transmitting (235-360 nm, Corning No. 9863 plus saturated cobalt acetate) and long-wavelength pass glass filters were employed. Annealing of samples was done while monitoring window mount temperature with a diode indicator. Results Infrared spectra of new selenium oxides will be presented using five different experimental methods, and DFT calculations of product structures and frequencies will be given. Photolysis of (Se2)(O2). The (Se2)(O2) complex was prepared by codepositing superheated selenium vapor containing mostly Se2 with Ar/O2 mixtures and characterized by very strong 1404.5 cm-1 and weak 391 cm-1 absorptions; the weak spin-orbit Se2 band was observed at 513.6 cm-1.8,17 The spectrum of a typical experiment is shown in Figure 1a. The sample was exposed to near-UV radiation; the (Se2)(O2) bands decreased 15%, and a strong new band was observed at 875.9 cm-1 with weak bands at 966.2 and 883 cm-1, as shown in Figure 1b,c. Photolysis with visible radiation reduced the (Se2)(O2) bands another 50% and the 875.9 cm-1 band by 60% and produced strong 966.2 and 964.0 cm-1 bands and weaker 1061.0, 921.0, 636.5, 457.0, and 366 cm-1 bands as illustrated in Figure 1d. Continued photolysis with visible and near-UV light (Figure 1e) reduced (Se2)(O2) another 70%, destroyed the 1061.0 cm-1 band, increased the 966.2, 636.5, and 457.0 cm-1 bands together, and markedly increased the 875.9 cm-1 band. Further photolysis with the full arc (Figure 1f) continued these trends, produced a very weak 1141.5 cm-1 band, decreased the Se2 absorption, and increased the Se4 band. Annealing (Figure 1g) increased the weak 1141.5 cm-1 band, sharpened the 966.2, 636.5, and 457.0 cm-1 bands (labeled Se2O2), and increased the 875.9 cm-1 band. These photochemical results were reproduced in several experiments. When the photochemical cycle was reversed and visible (λ > 490 nm) radiation employed first, the band growth was essentially the same as in Figure 1d. Continued photolysis (λ > 380 nm) increased the product bands while adding the UV © 1996 American Chemical Society

16488 J. Phys. Chem., Vol. 100, No. 41, 1996

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Figure 1. Infrared spectra in the 1450-250 cm-1 region for Ar/O2 ) 200/1 sample codeposited at 11 ( 1 K with Se2 from superheater: (a) 8 h sample deposit, (b) after 265 < λ < 360 nm photolysis, (c) after 235 < λ < 360 nm photolysis, (d) after λ > 490 nm photolysis, (e) after λ > 290 nm photolysis, (f) after λ > 220 nm photolysis, and (g) after 25 K annealing.

(λ > 290 nm, λ > 220 nm) slightly reduced the band intensities. Under best resolution conditions, the 966.2 cm-1 band became a 1/2/4/1 relative intensity quartet at 970.3, 968.2, 966.2, and 964.1 cm-1. Similar superheater experiments were performed with 18O2, and isotopic counterparts are listed in Table 1. With 16,18O2 only two band systems gave intermediate components: the (Se2)(O2) triplet and the 959.5, 948.0, 917.5 cm-1 triplet and 888 cm-1 central component for the symmetric stretching mode of perturbed SeO2. All of the major product band systems gave pure isotopic doublets. Even the strong 966.2 cm-1 band system showed no evidence of splittings with 16,18O2. The oxygen-18 counterpart gave a 1/2/4/1 quartet at 925.0, 923.1, 920.9, and 918.7 cm-1. Reaction of Se2 + O3. The reaction of Se2 and O3 was investigated by codepositing superheated Sex vapor with Ar/O3 samples. The stronger (Se2)(O2) complex band was observed at 1404.5 cm-1, and the major reaction products gave a broad absorption at 904 cm-1 and a sharp absorption at 883.2 cm-1. Figure 2 shows spectra from an experiment with Se2 and 16,18O3; the initial deposit contained broad 904, 862 cm-1 and sharp 883.2, 841.8 cm-1 doublets (Figure 2a). Photolysis with visible light produced the 966.2, 920.9 cm-1 doublet and the 959.4, 948.0, 917.5 cm-1 triplet, increased the broad 904, 862 cm-1 doublet and the sharp 883.2, 841.8 cm-1 doublet, and produced a new weak 875, 834 cm-1 doublet (Figure 2b,c). Continued photolysis with UV and visible radiation reduced the original reaction products in favor of the photolysis products (Figure 2d,e). A final annealing increased the 875, 834 cm-1 doublet and decreased the 966.2, 920.9 cm-1 doublet (Figure 2f). Reaction of Discharged Reagents. Both selenium vapor and oxygen gas were discharged in argon carrier gas and condensed

TABLE 1: Product Absorptions (cm-1) in Solid Argon Observed from Reactions of Selenium and Oxygen 16

16,18O 2

18O

1404.5 391 1141.5 952.0

1365.4 391 1140.8b 1111.7 1109.2 1078.5 1032.2 1031.2

1325.2 391 1077.5

O2

1061.0 997 w 968.8c 964.8c,d 966.2c 959.4 926.0 922.1 920.6 909.1e 907.5e 902.7e 883.2e 882 she 875.4e 636.5 513.6 457 366 370

954.4 952.3 948.0 887.5 887

356

2

1002.2

921.1 920.9b 917.5 875.2 875 866.3e 863.4e 860.3e 841.8e 840 she 834.2e 606.0 513.6 436 350 370

resultsa

identification

1 1 3, 5 3, 5 3, 5 3, 5 3, 4, 5

(Se2)(O2) complex (Se2)(O2) complex cis-OSeOO cis-OSeOO cis-OSeOO cis-OSeOO SeOO SeOO SeO3 (O)(SeO2) complex SeO2 Se2O2 exocycle (SeO2)(Se) complex (O)(SeO2) complex SeO2 (SeO2)(Se) complex SeO SeO site (SeO)(complex) SeSeO (O)(SeSeO) complex (SeSeO)(O) complex Se2O2 exocycle Se2 Se2O2 exocycle SeO2 Se4

3, 5 5 3 1 1 5 3 1 3 2, 3 2, 3 2, 3 2, 3 1, 4 1 1, 2, 4 1 3 1, 2, 4

a Bands favored in numbered results subsections. b Bands from 18O 3 + SeO2 experiment, section 5. c Natural selenium isotopic splittings resolved. d Bands at 964.4, 921.2, and 365.4 cm-1 from SeO2 codeposition. e Bands from 80Se experiment, section 3.

at 11 ( 1 K. The spectrum revealed strong 964.8, 366.0 cm-1 and weak 921.2 cm-1 isolated SeO2 bands, ozone at 1039.6

Reactions of Selenium and Oxygen

J. Phys. Chem., Vol. 100, No. 41, 1996 16489

Figure 2. Infrared spectra in the 980-800 cm-1 region for Ar/16,18O3 ) 200/1 sample codeposited at 11 ( 1 K with Se2 from superheater: (a) 8 h sample deposit, (b) after λ > 490 nm photolysis, (c) after λ > 380 nm photolysis, (d) after λ > 290 nm photolysis, (e) after λ > 220 nm photolysis, and (f) after 35 K annealing.

cm-1, weak NSe2 absorption at 1019.7 cm-1, and a weak 1404.5 cm-1 (Se2)(O2) band (Figure 3a).8,18,19 The major new product bands were 1141.5, 1061.1, 909.1, 883.2, and 875.9 cm-1. Red light photolysis (λ > 590 nm) had no effect, but green light (λ > 490 nm) markedly decreased the 875.9 cm-1 band, as before, and a stepwise UV sequence (λ > 380, λ > 290 nm, full arc) reduced and destroyed the 1061.0 cm-1 band and decreased the 1405.5 and 1141.5 cm-1 bands. A similar study with 18O2 gave the spectrum in Figure 3b, and the isotopic frequencies are listed in Table 1. The isotopic enrichment is revealed by the weak Se16,18O2 band at 952.3 cm-1 above Se18O2 at 921.1 cm-1. This experiment was repeated several times with 16,18O2, and one of these employed 80Se reagent. The lower 940-830 cm-1 region (Figure 4a) shows a family of doublets marked SeO and SeSeO. Photolysis at λ > 490 nm (Figure 4b) markedly decreased the 875.4, 834.2 cm-1 doublet, but irradiation at λ > 220 nm (Figure 4c) decreased the 883.2, 841.8 cm-1 doublet and increased the former doublet, an effect also found for the 882, 840 cm-1 shoulder doublet. Annealing continued the latter trend (Figure 4d). The upper region reveals bands at 1141.1, 1111.3, 1108.8, 1078.3, 1077.5 cm-1 and at 1061.0, 1032.2, 1031.2, 1002.2 cm-1. Full arc photolysis decreased the former bands by 30% and the latter quartet by 80%. Reaction of Se with O2. Selenium atoms from a microwave discharged argon stream seeded with selenium vapor were codeposited with a separate Ar/O2 mixture using a range of selenium reservoir temperatures, again keeping selenium cluster absorptions17 very weak. The major differences from the experiments passing both selenium and oxygen through the discharge are as follows: (1) 1405.5 cm-1 is again the strongest band, (2) 1061.0 cm-1 is stronger than SeO2, (3) the 1141.5 cm-1 band is weak, (4) the 875.9 cm-1 band is much stronger than the 883.2 cm-1 band, and (5) the broad 655 cm-1 polymer band is absent.

Selenium Dioxide. Selenium dioxide was evaporated into condensing argon, and the strong 964.4, 365.4 cm-1 and weaker 921.2 cm-1 fundamentals were observed as reported;12,13 a resolved 1/2/4/1 selenium isotopic quartet was observed at 969.8, 967.0, 964.4, 962.8 cm-1. When the condensing argon was subjected to microwave discharge to irradiate the condensing Ar/SeO2 sample, a sharp new absorption was observed at 1061.0 cm-1 without other products. Likewise, seeding the argon discharge with selenium gave the same 1061.0 cm-1 band with no other product species. When the argon discharge was seeded with oxygen, the 1141.5 and 1061.0 cm-1 bands were both observed, and no 966.2, 636.5 cm-1 absorption was detected. Selenium dioxide was codeposited with Ar/O3 sample, and no reaction occurred. After photolysis with UV light (235 < λ < 360 nm), a strong 1141.5 cm-1 band (A ) absorbance ) 0.08) and weak 997 cm-1 band (A ) 0.01) appeared along with new blue shoulders at 968.8 and 926.1 cm-1 on the SeO2 band. Continued full arc photolysis gave resolved selenium isotopic splittings at 974.2, 971.4, 968.8 cm-1, which increased as the 964.4 cm-1 SeO2 absorption decreased. The 926.1 cm-1 band increased as the 921.2 cm-1 SeO2 absorption decreased, until the SeO2 product and reagent bands were almost equal. It is important to note that the 1061.0 cm-1 absorption was absent. This photochemical investigation was repeated with 18O3, and the same 968.8 cm-1 shoulder was produced along with weaker new bands at 956.4, 954.4, 952.0, and 926.0 cm-1, weak 1140.8 cm-1, and stronger 1111.7, 1109.2, and 1078.5 cm-1 bands. A weak 16-16-18 ozone band was produced at 1025.5 cm-1.19 No evidence was found for Se18O3, which unfortunately falls under the Se16O18O absorption. Calculations Preliminary SCF-level calculations were done at the San Diego Supercomputer Center, using the Wadt-and-Hay valence

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Figure 3. Infrared spectra in the 1500-200 cm-1 region for selenium vapor and 1% oxygen in argon passed through microwave discharge and condensed at 11 ( 1 K for 10 h: (a) 16O2 and (b) 18O2.

Figure 4. Infrared spectra in the 940-830 cm-1 region for 80Se vapor and 1% 16,18O2 in argon microwave discharged and condensed at 11 ( 1 K: (a) 10 h sample deposit, (b) after λ > 490 nm photolysis, (c) after λ > 220 nm photolysis, and (d) after 35 K annealing.

double-zeta bases (“LANL1DZ”) and pseudopotentials20 incorporated into the Gaussian 92 program.21 Although these

calculations gave information on the more stable structures, calculated vibrational frequencies were generally too high to

Reactions of Selenium and Oxygen be of diagnostic value, especially since the extent of the error was rather variable. Therefore, higher-level structure and vibrational frequency calculations were performed, taking the major effects of electron correlation into account. This set of calculations was undertaken in Toulouse, again using Gaussian 92. A double-zeta valence basis for Se was adopted, together with the pseudopotential due to Barthelat and Durand,22 augmented with diffuse s and p functions and a set of d-type polarization functions (exponent 0.34). A standard all-electron double-zeta basis was adopted for oxygen,23 also augmented both with diffuse s and p functions and a set of d-type polarization functions (exponent 1.0). This basis is referred to hereafter as DZP+. Since many different molecular systems were to be investigated, some of which had several possible isomers, it was important for the theoretical method adopted to be computationally efficient. At the same time, good performance was obviously desired for the calculation of vibrational frequencies. Tests of several different methods were therefore undertaken to see how accurately the vibrational frequencies could be calculated for a selection of well-characterized small molecules containing selenium and oxygen. Details of these calibration studies will be provided elsewhere, as will the complete set of results obtained.24 Here, it suffices to say that a variety of density functional theory usually known as “B3LYP” (where B3 indicates a hybrid, parametrized treatment of exchange proposed by Becke25 and LYP indicates that the correlation functional used was that proposed by Lee, Yang, and Parr,26 which includes both local and nonlocal terms) was found to have the best overall performance in a cost-effective sense, being much more reliable than second-order perturbation theory. The B3LYP level of theory has gained widespread recognition recently, especially when the emphasis is on the calculation of vibrational frequencies.27 In a few cases, however, when systems with high multireference character were studied, CASSCF calculations were also performed. Technical restrictions limited our active space to no more than 10 electrons in eight orbitals. Discussion As preliminary consideration, bands clearly above 1000 cm-1 are unlikely to be due to Se-O or SedO bonds, as the highestfrequency motion assigned to an SedO unit is found at 997 cm-1, due to monomeric SeO3.14 A typical frequency for an O-O single bond is 877 cm-1 for hydrogen peroxide,28 while the (harmonic) frequency for O2 is 1580 cm-1,29 so bands above 1000 cm-1 are due to compounds with substantial O-O multiple-bond character. The individual new selenium-oxygen species will be assigned with the help of isotopic substitution and DFT calculations. SeO. The sharp band at 909.1 cm-1 is just above the 906.2 cm-1 gas-phase fundamental9-11 for 80Se16O and it is assigned accordingly. The 2.9 cm-1 blue matrix shift is near the 6.8 cm-1 value for SeN.18 The observed 16/18 (1.04941) and 76/ 80 (1.00429)18 isotopic ratios are slightly lower than calculated harmonic isotopic ratios for the diatomic oscillator (1.049 81, 1.004 39) in strong support for this matrix assignment. Finally, the DFT prediction of the harmonic SeO fundamental at 912.4 cm-1 is in excellent agreement with the observed values. SeO2. The 964.4, 921.2, and 365.4 cm-1 absorptions observed for SeO2 are in agreement with previous workers.12,13 The experiments with 16,18O2 show asymmetric triplets for the two upper bands as given in Table 1 for the isotopic SeO2 species prepared by microwave discharge. The 16/18 ratios for the ν3 and ν1 stretching modes, 1.0474 and 1.0524, respectively,

J. Phys. Chem., Vol. 100, No. 41, 1996 16491 bracket the diatomic value as expected. The bands that appear at 959.4 and 920.6 cm-1 on photolysis of (Se2)(O2) are analogous to the above isolated SeO2 absorptions. These bands are assigned to a perturbed SeO2 species, most likely (SeO2)(Se). New bands appear at 968.8 and 926.1 cm-1 only after ozone is photolyzed in the presence of SeO2. These bands are assigned to the photolysis product atomic oxygen complex (O)(SeO2), which does not react straightaway to give SeO3. Finally, the DFT calculations predict fundamentals at 981, 944, and 357 cm-1 for SeO2, which are in good agreement with the experimental values. Note that the Se-O stretching modes are calculated too high by only 1.8% and 2.4%. SeO3. A weak band at 997 cm-1 in the discharge experiments is probably due to SeO3 (D3h) based on agreement with earlier workers.14 A similar weak band appears at 997 cm-1 on photolysis of SeO2 and O3 mixtures. SeSeO. Two bands are major products of the Se2 + O3 reaction: the broad 904 cm-1 band is due to SeO and complexes, and the sharp 883.2 cm-1 band has identical isotopic characteristics. Both are doublets with 16,18O2 denoting vibrations of a single O atom, and the 16/18 (1.04918) and 76/80 (1.00419) ratios for the latter band are in near agreement with the above diatomic SeO values. Clearly, the 883.2 cm-1 band is also due to a stable selenium oxide product as both bands are observed in the microwave discharge of Ar/Sex/O2 mixtures. However, the chemistry of the Se2 + O3 reaction suggests that SeSeO should be a major product, and the 883.2 cm-1 band is assigned to SeSeO.

Se2 + O3 f [Se-Se-O-O2]* f Se + SeO + O2 f SeSeO + O2

(1a) (1b)

DFT calculations predict the most stable Se2O isomer to be a bent SeSeO molecule, like SSO, with a strong Se-O fundamental at 908.7 cm-1, below SeO diatomic, and a weak Se-Se fundamental at 363.1 cm-1, below Se2 diatomic. Assignment of the 883.2 cm-1 band to SeSeO is made on the basis of the above chemistry, isotopic shifts, and DFT calculations. The Se-Se mode is too weak to be observed here, particularly in the presence of weak Se4 absorptions. Ultraviolet photolysis of the (Se2)(O2) complex gives a strong 875.4 cm-1 band with identical isotopic characteristicssa doublet with 16,18O2 and diatomic 16/18 ratio (1.0494). The 875.4 cm-1 band is also a candidate for SeSeO, but the latter band grows on annealing at the expense of the former band in discharge and superheater experiments and is most likely due to perturbed SeSeO. The ready formation on 235-360 nm photolysis of (Se2)(O2) complex suggests ground state O as the perturbing species, a proposal supported by the efficient production of Se2O2 on further λ > 490 nm photolysis. (Se2)(O2)

UV hν

SeSeO---O

(2a)

O SeSeO---O

VIS hν

SeSeO

(2b)

There are two other possible Se2O isomers, both with C2V symmetry. Both can be rejected as candidates for the 883 cm-1 band, as neither has sufficiently high frequencies. The highestfrequency band predicted for the three-membered ring “closed” isomer is at 663 cm-1, while for the “open” or “ozone-like” SeOSe isomer it is 718 cm-1. These high-energy species are less stable than SeSeO by 100 or 165 kJ/mol, respectively, at the DZP+/B3LYP level of theory.

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TABLE 2: Frequencies (cm-1), Intensities (km/mol), and Isotopic Shifts (cm-1) Calculated for the Three Strongest Absorptions of SexOy Using DFT with the B3LYP Functional molecules

freq (int)

freq (int)

freq (int)

Se2 SeO SeO2 18-80-16 18-80-18 O3 singlet SeOO 80-16-18 80-18-16 80-18-18 singlet SeOO (CAS) 80-16-18 80-18-16 80-18-18 triplet SeOO 80-16-18 80-18-16 80-18-18 SeSeO 80-80-18 SeO3 exocycle Se2O2 ring 0-18 exo 0-18 both 0-18 planar SeOSeO 80-16-80-18 80-18-80-16 80-18-80-18 cis-OSeOO 16-80-18-16 16-80-16-18 16-80-18-18 18-80-16-16 18-80-18-16 18-80-16-18 18-80-18-18 trans-planar SeOSeO trans-planar OOSeO

379 (0) 912.4 (12) 981.2 (b2, 127) -11.3a -43.5 1235 (b2, 225) 1197.7 (266) -35.0 -32.4 -68.6 1086.6 (156)

943.6 (a1, 13) -34.5a -48.0 1261 (a1, 0) 652.4 (23) -2.2 -31.2 -33.4 525.8 (13)

357.1 (a1, 27) -8.3a -16.8 725 (a1, 6) 382.2 (0) -12.7 -4.8 -17.2 275.8 (13)

-33.8 -27.3 -62.1 1087 (55) -33.0 -27.8 -61.9 908.7 (109) -43.2 980 (e′, 170)b 966 (134) -0.0 -45.8 -45.8 947.6 (108) -45.2 -0.5 -45.4 1158 (230) -30.2 -33.5 -64.6 -0.9 -31.3 -34.7 -66.2 951.8 (180) 1117 (262)

-6.2 -28.9 -29.3 532 (2) -4.7 -23.8 -28.9 363.1 (18) -0.5 335 (e′, 49) 645 (38) -30.7 -0.2 -30.9 633.4 (58) -0.2 -31.8 -32.0 962 (107) -0.4 -0.7 -1.5 -44.8 -45.1 -45.3 -45.7 628.6 (66) 958 (140)

-6.5 -5.3 -11.3 276 (4) -7.7 -4.3 -11.6 236.7 (7) -9.4 261 (a2′′, 26) 463 (11) -21.0 -0.1 -21.1 518.5 (22) -1.0 -24.1 -25.2 564 (21) -25.8 -2.9 -29.0 -1.8 -27.8 -4.6 -30.9 527.9 (14) 584 (9)

b

a Shifts given as -cm-1 below reference molecule using all O-16. Also 875 cm-1 (a1′, 0).

Se2O2. There are at least 14 possible isomers with the stoichiometry Se2O2. We studied a subset of these, considering only those which seemed likely to be the most important in light of results already obtained in an earlier study of S2O2.30 The most stable species found by our DFT calculations has only C1 symmetry; it is based on a three-membered SeSeO ring and contains an exocyclic SedO unit. However, several other isomers are so close in energy that the present calculations cannot determine the true global minimum for Se2O2 with certainty. Planar trans- and cis-OSeSeO isomers, whose symmetries are C2h or C2V, respectively, are only 1 or 3 kJ/mol higher than the C1 species, while planar cis- or trans-SeOSeO chain structures, both with Cs symmetry, are 39 or 59 kJ/mol above it. Predicted IR frequencies are summarized in Table 2. While the two strongest bands calculated for the C1, C2h, and C2V isomers are all compatible with the bands observed at 966 and 636 cm-1, the third band observed at 457 cm-1 is clearly in better agreement with the 463 cm-1 band predicted for the former than the 518 cm-1 band predicted for the latter. Furthermore, mixed isotopic data require two inequivalent O atoms, which rules out OSeSeO structures. The Cs SeSeOO isomer can be eliminated from further consideration, since its predicted O-O stretching frequency is far too high, at 1236 cm-1, and although an SO3-like isomer, with C2V symmetry,

would have an intense band at 963 cm-1, it would not have any counterpart for the 636 or 457 cm-1 features. The 966.2 cm-1 band is sharp, and it exhibits natural selenium isotopic splittings appropriate for the vibration of a single Se atom. Furthermore, the observation of a sharp doublet with 16,18O indicates the involvement of a single O atom with 16/ 2 18 ratio 1.04919; analogous characteristics are found for the 636.5 and 457 cm-1 bands (16/18 ratios 1.0503 and 1.0482). The calculated isotopic shifts for single- and double-18O substitution (Table 2) show doublets for uncoupled motions of inequivalent oxygen atoms as required by the observed spectra. DFT calculations predict the exocyclic terminal SedO vibration in Se2O2 to be 44 cm-1 above that for diatomic SeO, in good agreement with the strongest observed band at 966 cm-1. Furthermore, the 636 and 457 cm-1 bands are indicative of bridging Se-O-Se vibrations predicted at 645 and 463 cm-1. Finally, DFT calculations predict 16/18 ratios 1.0498, 1.0503, and 1.0477, respectively, for the above bands, in excellent agreement with the observed ratios. Thus, the most stable Se2O2 isomer is identified as the exocyclic ring by comparison with DFT calculated isotopic frequencies. We note that one of the stable S4 structures has been identified as the exocyclic ring,31 and the only S2O2 isomer observed thus far is cis-S2O2.5 In the ultraviolet region, a broad absorption extending from 235 to 310 nm was found with maxima at 256 and 293 nm in addition to the Se2 band system between 310 and 377 nm.17 This is, of course, in accord with the hypothesis that this longerwavelength feature can be assigned to an electronic transition dominated by the Se2 chromophore and the shorter-wavelength feature is dominated by the O2 chromophore. SeOO and OSeOO. Sharp new absorptions in the O-O stretching region at 1141.5 and 1061.0 cm-1 exhibit similar isotopic characteristics but different chemistries. Both form quartets in 16,18O2 reactions indicating two inequivalent oxygen atoms, and the 1141.5/1077.5 ) 1.0594 and 1061.0/1002.2 ) 1.0587 ratios identify O-O stretching modes. The 1141.5 cm-1 band is favored when both Sex and O2 are microwave discharged, whereas the 1061.0 cm-1 band dominates when discharged Ar/Sex is codeposited with Ar/O2. The 1061.0 cm-1 band is produced exclusively when SeO2 is subjected to vacuumultraviolet photolysis, whereas photolysis of O3 to generate O atoms for reaction with SeO2 gives only the 1141.5 cm-1 band, and codeposition of discharged Ar/O2 with SeO2 gives both bands. Both increase slightly on annealing. In a Sex/16,18O2

Se + O2 f SeOO

(3a)

OSe + O2 f OSeOO

(3b)

VUV hν

OSeO 98 SeOO UV hν

OSeO + O3 98 OSeOO + O2 f SeO3 + O2

(4) (5a) (5b)

discharge experiment, the 1061.0 and 1002.2 cm-1 bands were sharp (1.0 cm-1 fwhm) and the 1032.2, 1031.2 cm-1 central component was partially resolved; however, the 1141.1 and 1078.3-1077.5 cm-1 bands were broader (2-3 cm-1 fwhm), and the intermediate component with resolved 1111.3, 1108.8 cm-1 maxima was even broader (5 cm-1 fwhm). It is clear that differences between per-16 and per-18 components at 1141.5, 1077.5 cm-1 and the above bands at 1141.1 and 1078.3 cm-1 are due to interaction of the O-O stretching mode with a third inequiValent oxygen atom. Also, agreement of the 1061.0

Reactions of Selenium and Oxygen and 1002.2 cm-1 bands in 16O2, 16,18O2, and 18O2 experiments, respectively, shows that the latter bands are due to the O-O stretching mode of inequivalent oxygen atoms without coupling to another oxygen. The 1061.0 cm-1 band is thus assigned to the asymmetric SeOO isomer calculated by DFT to be some 300 kJ/mol above the normal SeO2 isomer. Although the O-O stretching frequency calculated for the singlet state is at 1197.7 cm-1, 13% higher than the observed band, the calculated isotopic shifts are in good agreement. Moreover, it should be noted that the analogous B3LYP frequency calculated for singlet ozone is also too high, by a similar amount. It is well-known that ozone is a very “difficult” molecule to study by standard quantum chemical methods,32 as it is not well described by a single determinant; in an MCSCF treatment, the coefficient for the Hartree-Fock reference configuration is 0.90 while for the second configuration, which corresponds to the HOMO f LUMO double excitation, it is 0.33. Since SeOO is valenceshell isoelectronic with ozone, similar multireference character might be anticipated. In fact, we find even greater multireference character for SeOO, as the analogous coefficients are 0.87 and 0.42, respectively. It is therefore asking a lot for any correlated treatment to correct the severe deficiencies inherent in a single-reference treatment of SeOO, and the DFT frequencies calculated for such molecules are not likely to be quantitatively reliable. CASSCF calculations (10 electrons in eight orbitals, the largest active space that we could feasibly study) were undertaken for SeOO, and the vibrational frequencies are reported in Table 2. The O-O stretching motion is now found at 1087 cm-1, in pleasing agreement with the 1061 cm-1 experimental value; the calculated isotopic shifts are, however, in rather poorer accord with the measured values. While this agreement provides strong support for the assignment to singlet SeOO, we must not overlook other possibilities. In particular, the HOMO-LUMO gap is small for SeOO (only 0.31 au at the SCF level), and triplet states must therefore also be investigated. Since the a′′ second-HOMO and a′ HOMO are almost degenerate, there are two triplet states which are very close in energy and in fact almost degenerate with the singlet at the B3LYP level of theory. Remarkably, the O-O stretching frequency predicted for the 3A′ state, which is 6 kJ/mol above the singlet, is 1087 cm-1 that is effectively indistinguishable from the CASSCF value for the singlet. The 3A′′ state is 0.7 kJ/mol above the singlet; its predicted O-O stretching frequency is 1160 cm-1. While the B3LYP prediction therefore provides some support for the assignment of the 1061 cm-1 band to 3A′ SeOO, the CASSCF value of the O-O stretching frequency for that state is only 948 cm-1. When DFT and CASSCF vibrational frequencies differ considerably, we feel that the latter are more reliable, and so singlet SeOO is our preferred assignment for the 1061 cm-1 band, though we cannot be firm about that preference at this stage. It is unfortunate that the experimental observations do not allow state determination. The analogous SOO isomer is predicted to be some 460 kJ/ mol higher than the normal isomer.33 An intense band at 1074 cm-1 in similar tellurium experiments has been identified as TeOO, the analogous asymmetric isomer.34 This illustrates increasing stability of the asymmetric AOO isomer going down the periodic table. Photolysis of 16O3 in the presence of Se16O2 gives further evidence for identification of the 1141.5 cm-1 absorption: three sets of new bands are observed, weak SeO3 at 997 cm-1, strong 1141.5 and weak 952.0 cm-1 bands, and the selenium isotopic multiplet at 968.8 cm-1 on the blue side of the ν3 SeO2 band at 964.4 cm-1 with a weaker 926.0 cm-1 band on the blue side of

J. Phys. Chem., Vol. 100, No. 41, 1996 16493 the ν1 SeO2 band at 921.2 cm-1. Clearly, photolysis of ozone in the presence of SeO2 leads to a new perturbed SeO2 species and photochemical reaction to give little SeO3 and more of a new product absorbing at 1141.5 and 952.0 cm-1. Observation of the 968.8 and 926.0 cm-1 bands unshifted with 18O3 photolysis leads to identification of the perturbed species as (O)(SeO2) where the O is only interacting physically. It is likely that this is ground state oxygen, and excited oxygen is required for reaction 5. However, the 18O3 experiment also produces a selenium isotopic multiplet at 956.4, 954.4, 952.0 cm-1 for the analogous perturbed Se16O18O vibration and demonstrates that 18O is exchanging with Se16O . In addition, new bands are 2 observed at 1140.8, 1111.7, 1109.2, and 1078.5 cm-1, which are isotopic components of the major photolysis product. This molecule is identified here as the cis-OSeOO species on the basis of the formation chemistry, isotopic shifts, and DFT calculations of the strongest band at 1158 cm-1 and weaker band at 962 cm-1 (Table 2). Note that the 18O3 + Se16O2 experiments cannot form the per-16 and per-18 isotopic modifications, but the three mixed 16-16-18 modifications are produced. The 18O18OSe16O molecule observed at 1078.5 cm-1 requires reaction of a second 18O atom. Note that ozone 16-16-18 is produced by photolysis, thus verifying oxygen atom exchange in this experiment! 18

UV

O3 + Se16O2 98 (18O)(Se16O2) f (16O)(Se16O18O) UV

98

OSe16O16O + 16OSe16O18O + 1140.8 cm-1 1109.2 cm-1 18

16

OSe18O16O 1111.7 cm-1 The calculated harmonic oxygen isotopic shifts are in excellent agreement and strongly support the identification of cis-OSeOO. The calculated shift (from 16-16-Se-16) for 16-16-Se-18 is 0.9 cm-1 (0.7 cm-1 observed); the calculated shifts for 16-18-Se-16 and 18-16-Se-16 are 30.2 and 33.5 cm-1 (observed 29.8 and 32.3 cm-1); the calculated shift for 18-18-Se-16 is 64.6 cm-1 (observed 63.0 cm-1); the calculated shift for 18-18-Se-18 is 66.2 cm-1 (observed 64.0 cm-1). The present identification of cis-OSeOO is supported by the recent identification of cis-OSOO from the ultraviolet photolysis of SO3.35 B3LYP calculations imply that the trans-planar OSeOO isomer is a little less stable than the cis, by 16 kJ/mol. More importantly, it highest vibrational frequency is predicted at 1117 cm-1, rather lower than the observed band at 1141 cm-1, whereas the assignment to the cis isomer would imply a slight overestimation, of 1.5%, quite typical of the errors found throughout this work with the B3LYP method. However, this evidence is not strong enough to exclude the trans isomer definitively. Another possible isomer of OSeOO is the threemembered SeO2 ring and an exocyclic SedO double bond with Cs symmetry; although this species lies some 53 kJ/mol below the cis-planar OSeOO chain isomer at the DZP+/B3LYP level of theory, its highest vibrational frequency is predicted at 999 cm-1, and so it cannot be a contender for the 1141 cm-1 band. Acknowledgment. We gratefully acknowledge financial support from NSF Grant CHE 91-22556 and computer time from the IDRIS-Toulouse and San Diego Supercomputer Centers. References and Notes (1) Allavena, M.; Rysnik, R.; White, D.; Calder, V.; Mann, D. E. J. Chem. Phys. 1969, 50, 3399.

16494 J. Phys. Chem., Vol. 100, No. 41, 1996 (2) Hopkins, A. G.; Brown, C. W. J. Chem. Phys. 1975, 62, 2511. (3) Tang, S.-Y.; Brown, C. W. Inorg. Chem. 1975, 14, 2856. (4) Hopkins, A. G.; Daly, F. P.; Brown, C. W. J. Phys. Chem. 1975, 79, 1849. (5) Lovas, F. J.; Tiemann, E.; Johnson, D. R. J. Chem. Phys. 1974, 60, 5005. (6) Tang, S.-Y.; Brown, C. W. J. Raman Spectrosc. 1975, 3, 387. (7) Bondybey, V. E.; English, J. H. J. Mol. Spectrosc. 1985, 109, 221. (8) Brabson, G. D.; Citra, A.; Andrews, L.; Davy, R. D.; Neurock, M. J. Am. Chem. Soc. 1996, 118, 5469. (9) Barrow, R. F.; Deutsch, E. Proc. Phys. Soc. 1963, 82, 548. (10) Barrow, R. F.; Lemanczyk, R. Z. Can. J. Phys. 1975, 53, 553. (11) Varma, K. K.; Reddy, S. P. J. Mol. Spectrosc. 1977, 67, 360. (12) Hastie, J. W.; Hauge, R. H.; Margrave, J. L. J. Inorg. Nucl. Chem. 1969, 31, 281. (13) Cesaro, S. N.; Spoliti, M.; Hinchcliffe, A. J.; Ogden, J. S. J. Chem. Phys. 1971, 55, 5834. (14) Brisdon, A. K.; Ogden, J. S. J. Mol. Struct. 1987, 157, 141. (15) Mielke, Z.; Brabson, G. D.; Andrews, L. J. Phys. Chem. 1991, 95, 75. (16) Brabson, G. D.; Mielke, Z.; Andrews, L. J. Phys. Chem. 1991, 95, 79. (17) Brabson, G. D.; Andrews, L. J. Phys. Chem. 1992, 96, 9172. (18) Andrews, L.; Hassanzadeh, P. J. Chem. Soc., Chem. Commun. 1994, 1523. The 76Se16O and 80Se16O molecules were observed at 912.9 and 909.0 cm-1 and the 76Se2O and 80Se2O molecules at 886.7 and 883.0 cm-1 in solid argon from microwave discharge as measured by FTIR to (0.1 cm-1. (19) Andrews, L.; Spiker, R. C., Jr. J. Phys. Chem. 1972, 76, 3208. (20) Wadt, W. R.; Hay, P. J. Chem. Phys. 1985, 82, 284. (21) Gaussian 92-DFT. Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H.

Brabson et al. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andrews, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; DeFrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian, Inc., Pittsburgh, PA, 1992. (22) Barthelat, J.-C.; Durand, P. Gazz. Chim. Ital. 1978, 108, 225. (23) Dunning, T. H. J. Chem. Phys. 1970, 53, 2823. (24) Marsden, C. J. Manuscript in preparation. (25) Becke, A. D. J. Chem. Phys. 1993, 98, 1372, 5648. (26) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (27) See, for example: Hertwig, R. H.; Koch, W. J. Comput. Chem. 1995, 16, 576. Ricca, A.; Bauschlicher, C. W. Theor. Chim. Acta 1995, 92, 123. Ventura, O. N.; Kieninger, M. Chem. Phys. Lett. 1995, 245, 488. (28) Chase, M. W.; Curnutt, J. L.; Hu, A. T.; Prophet, H.; Syverud, A. N.; Walker, L. C. J. Phys. Chem. Ref. Data 1974, 3, 311. (29) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure. Constants of Diatomic Molecules, Van Nostrand-Reinhold: New York, 1979. (30) Marsden, C. J.; Smith, B. J. Chem. Phys. 1990, 141, 335. (31) Hassanzadeh, P.; Andrews, L. J. Phys. Chem. 1992, 96, 6579. (32) Lee, T. J.; Scuseria, G. E. J. Chem. Phys. 1990, 93, 489 and references therein. (33) Kellogg, C. B.; Schaeffer, H. F., III J. Chem. Phys. 1995, 102, 4177. (34) Thompson, C. A.; Andrews, L. To be published. (35) Jou, S.-H.; Shen, M.; Yu, C.; Lee, Y.-P. J. Chem. Phys. 1996, 104, 5745.

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