Photolytic Preparation and Isomerization of HNSO, HOSN, HSNO, and

With this "missing link" the HNSO transformation scheme is completed which shows that starting from HNSO in a low-temperature matrix, the molecules HO...
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J . Phys. Chem. 1987, 91, 5203-5209

5203

Photolytic Preparation and Isomerization of HNSO, HOSN, HSNO, and HONS in an Argon Matrix. An Experimental and Theoretical Study M. Nonella, J. Robert Huber,* Physikalisch- Chemisches Institut der Universitat Zurich, CH-8057 Zurich, Switzerland

and Tae-Kyu Ha Laboratory of Physical Chemistry, Swiss Federal Institute of Technology, E TH- Zentrum, CH-8092 Zurich, Switzerland (Received: March 19, 1987)

The molecule HONS in the trans geometry has been produced by selective photolysis from HNSO and several of its isotopes in an argon matrix at 12 K. The new molecule was characterized by its IR spectrum in conjunction with a normal-coordinate analysis and ab initio calculations. With this "missing link" the HNSO transformation scheme is completed which shows that starting from HNSO in a low-temperature matrix, the molecules HOSN, HSNO, and HONS in the cis and/or trans configuration(s)can be produced by selective photolysis. According to ab initio calculationsthe stability in order of increasing ground-state energy is HNSO > HOSN > HSNO > HONS. In view of a selective cis == trans isomerization cycle, the isomerization barriers were calculated for HSNO (3200 cm-I, trans TS), HONS (4400 cm-I), and HNSO (5000 cm-I). HOSN was predicted to have no stable trans geometry. Furthermore, the lower electronically excited states of the four molecules were calculated by using the MRCI as well as the MCSCF-CI methods.

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1. Introduction At room temperature thionyl imide, HNSO, is of limited stability. Starting from this molecule isolated in a low-temperature matrix, energy-selective photolysis permits the transformation to thiocyanic acid, HOSN, and thionitrous acid, HSNO, with the former emerging in the cis conformation and the latter in the trans and cis conformation^.'-^ Beside HNSO, HOSN, and H S N O also HONS is now shown to be produced when appropriate photolysis conditions are utilized. Hence, from the chemists' point of view all reasonable permutations of the four atoms give rise to molecules which are stable under matrix-isolated conditions and thus can be investigated by spectroscopic methods. A particularly interesting property of these molecules is the cis e trans is~merization.~ For HSNO, light-induced isomerization can selectively be performed when excitation with two different colors is applied. Albeit this conformer switching is not yet as perfect as in the case of C H 3 S N 0 , 6the cis trans transformation of H S N O is even induced by IR light which prevents photochemical destruction. In this context it is reminded that reversible isomerizations involving a low torsional barrier could become an attractive process for digital information storage, if cis (hv,) trans (hv,) cis switching is achieved with short pulses of different IR color^.^^^-^ The aim of the present study was twofold. First, we completed the calculation of the ground-state geometries, the force constants, and vibrational frequencies of the cis and trans conformers of HNSO, HOSN, HSNO, and HONS, and we report experimental data of the latter species. In view of an IR-induced isomerization, the isomerization barriers of all four compounds were evaluated by a b initio calculations. Second, the lowest electronic states of the more stable conformer of each compound were calculated by the multireference configuration interaction (MRCI) and the MCSCF-CI methods in order to estimate transition energies

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( I ) Muller, R. P.; Huber, J. R. Rev. Chem. Intermed. 1984, 5 , 423. (2) Perrutz, R. N. Chem. Rev. 1985, 85, 97. (3) Tchir, P. 0.;Spratley, R. D. Can. J. Chem. 1975, 53, 2311. (4) Tchir, P. 0.;Spratley, R. D. Can. J. Chem. 1975, 53, 2318. (5) Muller, R. P.; Nonella, M.; Russegger, P.; Huber, J. R. Chem. Phys. 1984, 87, 35 1. ( 6 ) Muller, R. P.; Huber, J. R. J. Phys. Chem. 1984, 88, 1605. (7) Hall, R. T.; Pimentel, G. C. J . Chem. Phys. 1963, 38, 1889. (8) McDonald, D. A.; Shirk, J. S. J. Chem. Phys. 1982, 77, 2355. (9) Shirk, A. E.; Shirk, J. S. Chem. Phys. Lett. 1983, 97, 549.

0022-3654/87/2091-5203$01.50/0

relevant to the photolysis and isomerization processes observed for these molecule^.^-^

2. Experimental Section The infrared spectra were recorded between 170 and 4000 cm-I using a Perkin-Elmer 580B spectrometer in conjunction with a closed cycle low-temperature cryostat (Displex CS-202 Air Products). The details of the apparatus as well as the matrix preparation procedure have been described elsewhere.1° Photolysis at 250 nm was conducted with a 500-W high-pressure mercury lamp (Osram HBO) using a water filter and either a monochromator, a suitable cutoff filter, or a interference reflection filter (Schott UV-R-250, band-pass 20 nm). H N S O was prepared in the gas phase by the method of Schenk." A bulb was filled with -1 Torr of SOC1, (>99% purity, Fluka), another bulb with a mixture of .=6 Torr of N H , (>99.998% purity, Matheson) and 1 atm of argon. The isotopic purity of I5NH3, ND3, and I5ND3 exceeded 99%. Half of the NH,/Ar (I5NH3/Ar, ND3/Ar, I5ND3/Ar) mixture was transferred into the reaction bulb with SOCl, where the educts reacted spontaneously to H N S O and NH4CI. (HI5NSO and I5NH4CI, D N S O and ND,Cl, DI5NSO and I5ND4C1). Following the reaction, the sample/argon mixture was further diluted with argon to finally provide a M:A ratio > 1000. An impurity always present in noticeable amounts was SO2. Since H N S O polymerizes upon condensation," this impurity could not be removed by this procedure. 3. Method of Calculation 3.1. Ground State. The calculations were performed with various basis sets and programs. The following basis sets were employed in the S C F calculations: basis set (a) (10s,6p,ld)/ [6s,4p,ld] for the S atom, (7~,3p,ld)/[4~,2p,ld] for 0 and N atoms, and (3s,lp)/[2s,lp] for the H atom (standard basisI2);the orbital exponents of the polarization functions were as = 0.7, .a = 1.25, aN = 0.8, and aH = 0.7; basis set (b) (lls,7p,ld)/ [6s,4p,ld] for S, ( 9 ~ , 5 p , l d ) / [ 5 ~ , 3 p , l dfor ] N and 0, and (5s,lp)/[3s,lp] for H (extended basis'*), with as = 0.55, cyo = ( I O ) Muller, R. P.; Hollenstein, H.; Huber, J. R. J . Mol. Spectrosc. 1983, 100, 95. ( 1 1 ) Schenk, P. W. Ber. Abt. E 1942, 7 5 , 94. ( 1 2) Huzinaga, S. Approximate Atomic Functions I; University of Alberta:

Edmonton Canada, 1971.

0 1987 American Chemical Society

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The Journal of Physical Chemistry, Vol. 91, No. 20, 1987

Nonella et a1 2

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3000 2000 cm” 1000 600 200 Figure 1. Infrared spectrum of matrix-isolated HNSO after irradiation with X = 250 nm and X = 365 nm ( T = 12 K). The fundamentals of frans-HONS are numbered and the assignments are given in Table 111 4000

1.25, cyN = 1.0, and cyH = 1 .O. Additional calculations were performed with the GAUSSIANBZprogramI3 using a D95** Huzinaga basis with Dunning contraction denoted as basis set (c) and a 6-31G** basis, basis set (d). The CI optimizations of the ground states and the transition states of the cis-trans isomerization of H S N O and HONS were carried out with the COLUMBUS C I program applying the basis set (a). 3.2. Excited States. The molecular geometries chosen for these calculations were obtained by the CI optimization in the case of trans-HSN05 and trans-HONS, and from experimental data for cis-HNS03 and c ~ s - H O S N .The ~ excited electronic states were calculated with two different methods and two basis sets. In conjunction with the MRCI method we used a Huzinaga basis set with a Dunning contraction: (1 ls,7p,ld)/[6~,4p,ld]for S, (lOs,5p,ld)/[4~,2p,ld]for 0 and N , and (5s,lp)/[3s,lp] for H, with the exponents of the polarization functions as = 0.6, a. = 0.85, cyN = 0.8, and aH = 1.2. The configuration interaction (CI) calculations were carried out within the frozen-core restriction including single and double excitations from some selected reference configurations. The four lowest lying MO’s were taken as frozen and 52 lower lying valence shell orbitals were adopted as orbitals of variable occupancy in the CI calculations. The configurations were selected by using the second-order perturbation theory with an energy au. As reference the following configurations threshold of 2 X were selected for the A‘ state: [...(13a’)2(3a”)2], [...(13a’)*(3a”) I( 14a’) I], [...( 1 3a’) I ( 14a’)’ ( 3a”)2], and [...( 13a’) ( 1 4a’) I (3a”)’(4a”)’]; and for the A” state [...(13a’)1(4a’’)1], [... ( 1 2a’) I( 1 3a’)*(4a’’)’], [.. .(1 3a’) I( 14a’)2(“’’)1], and [ ...(1 2a’)’( 1 3a’) 2( 2a”) I(4a”) 1. For the lower lying A” states the S C F calculations preceding the CI study were carried out by an open-shell restricted Hartree-Fock method.’8 The MELD program packagei9was employed for the C I calculations. The second series of calculations was based on the MCSCF-CI method. All calculations were performed with the COLUMBUS program ~ y s t e m ’ ~ - ’ ’using . ~ ~ a standard Huzinaga basis set: (13) Pople, J . A,; Binkley, J. S.: Frisch, M. J.; DeFrees, D. J.; Raghavachari, K.; Whiteside, R. A.; Schlegel, H. B.; Fluder, E. M.; Seeger, R. GAUSSIAN82 Computer Program; Carnegie-Mellon University: Pittsburgh, PA, 1983. (14) (a) Dupuis, M.; Rys, J.: King, H. F. J . Chem. Phys. 1976.65, 1 1 1. (b) Rys, J.: Dupuis, M.: King, H. F. J . Comput. Chem. 1983, 4, 154. ( I 5) Pitzer, R. M. J . Chem. Phys. 1973, 58, 31 11. (16) Lischka, H.; Shepard, R.: Brown, F. B.; Shavitt, I. f n t . J . Quantum Chem. Symp. 1981, 15. 91. (17) Shavitt, I. Annual Report to NASA Ames Research Center, June, 1979. (18) Davidson, E. R.; Stenkamp, L. Z. fnr. J . Quantum Chem. S y m p . 1976. I O . 21 (19) McMurchie. L.; Elbert, S. T.; Langhoff, S. R.; Davidson, E. R . MELD Program Package: University of Washington: Seattle, WA, 1978. (20) Shepard. R.: Shavitt, 1.; Simons, J. J . Chem. Phys. 1982, 76. 543.

(12~,8p,ld)/[Ss,4p,ld]for S, (lOs,Sp,ld)/[4~,2p,ld]for 0 and N, and (4s,lp)/[2s,lp] for H with as = 0.7, a. = 1.25, aN= 0.8, and cyH = 0.7. The reference function of the CI calculation was, for each level, the corresponding MCSCF wave function. The computer capacity forced us to restrict the dimension of the CI matrix in such a way that six core and six virtual orbitals (5a’ and la”) had to be frozen. For the calculation of the A‘ states we considered the following eight reference configurations: [ . ..( 4a”)2], [...(3a”) 2( 13a’)2], [. ..(3a”)*( 1 3a’) I ( 14a’) ‘1, [ . . .(3a”) ( 13a’)2(4a”) , [...(3a”) ( 1 3a’) (4a”) ( 14a’) I], [... (3a”)’ ( 13a’) (4a”) I ( 14a’) I], [...( 1 3a’) 2(4a”)2], and [ ...( 1 3a’) I (4a”)2(14a’)1]. For the calculation of the A” states, six reference configurations were considered: [...(3a”)2(13a’)1(4a’’)1], [... ( 3a”)2(4a”) ( 14a’) ‘1, [...(3a”)’ ( 13a’)’ ( [...(3a”) (4a”)*( 14a’) I], [ ...(3a”) ( 13a’)2(14a’) I], and [...(3a”)’ ( 13a’) ( 14a’) *I. 4. Results 4.1, IR Spectra and Normal-Coordinate Analysis. An argon matrix containing H N S O (M/A =lOOO) was irradiated with A,, = 250 nm21 until the IR absorptions of cis-HNSO almost disa ~ p e a r e d .The ~ matrix contained now mainly cis-HOSN and cisand trans-HSNO. This mixture was further irradiated with 365 nm. In the region of this excitation wavelength an electronic transition of HSNO was expected since the stable C H 3 S N 0 shows there a broad a b s o r p t i ~ n . ~During ~ . ~ ~ several hours of irradiation, the IR absorptions of H S N O and HOSN steadily decreased and the known absorption bands of cis-HNSO appeared as well as additional absorptions. According to an earlier analysis,24some of these new bands were readily assigned to trans-HNSO. The remaining six absorption bands could then be assigned to the new, up to now not observed, species H O N S in the trans conformation. The analysis of its IR spectrum shown in Figure 1 was facilitated by the results of the calculations which revealed that trans-HOSN is likely not to exist (vide infra). The absorption at 3501 cm-I is characteristic of an OHstretching vibration. A comparison with the OH frequency of H O N O (cis, 3424 cm-I; trans, 3588 cm-’)*’ and with that predicted by the SCF calculation (cis, 3753 cm-I; trans, 3921 cm-’) supports the assignment to the trans rotamer of HONS. The I5N species showed a shift of 18.2 cm-I on the 968.5-cm-’ band and of 14.1 cm-I on the 842.1-cm-I band which is consistent with shifts expected for the NS stretching and the N O stretching vibrations, respectively. The results of the experiment and the calculation (21) Allegretti, J. M.; Merer, A . J. Can. J . Phys. 1972, 50, 404. (22) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. J . Phys. Chem. 1983, 87, 7 . ( 2 3 ) Christensen, D. H.; Rastrup-Andersen, N.; Jones, D.; Klaboe, P.: Lippincott, E. R. Spectrochim. Acta, Part A 1968, 24A, 1581. (24) Tchir, P.0.; Spratley, R. D. Can. J . Chem. 1975, 53, 2331. (25) McGraw, G. E.; Bernitt, D. L.; Hisatsune, 1. C. J . Chem. Phys. 1966, 45. 1392.

The Journal of Physical Chemistry, Vol. 91, No. 20, 1987

Preparation of HNSO, HOSN, HSNO, and H O N S

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TABLE I: Force Constants’ of cis- and trans-HONS

CI TVFF,~ trans trans Diagonal Force Constants OH str

6.9454 (0.0470) 7.1415 (0.0658) 7.0396 (0.5293) 0.8045 (0.0324) 3.5827 (0.3 130) 1.7494 (0.5081) 0.1280 (0.0040)

OD str SN str HON bend NO str SNO bend tors

SCF

cis

trans

cis

7.600

6.7936

8.5963

7.891 1

7.0100

6.7407

8.0546

7.4397

1.0562

1.0780

1.1622

1.1940

4.7854

5.1777

6.2597

6.6177

1.809 1

1.9930

1.9215

2.0671

0.1744

0.21 19

0.1677

0.2152

-0.0265 0.1021 0.1183 0.1236 0.0865 1.0072

0.0763 0.1351 0.4686 -0.0618 -0.0453 0.8889

-0.0872 0.0634 0.1526 0.0989 0.05 1 1 1.5069

0.0031 0.121 1 0.4561 -0.08 16 -0.0165 1.5334

0.5577

0.6846

0.5182

0.7010

0.5593

0.4729

0.6203

0.4885

0.0479

-0.1081

0.1927

-0.0789

0.8254

0.6689

Interaction Force Constants OH str

SN str HON bend NO str SNO bend HON bend NO str

SN str

SNO bend

HON bend

NO str SNO bend

NO str

SNO bend

1.4710 (0.4105) 1.7306 (0.7110) 0.2189 (0.1873) 0.1882 (0.1528) 0.4181 (0.2738)

Units are in mdyn.A-’, mdyn-rad-’, or mdyn&ad-*.

0.6648

0.5742

bThe number in parentheses represents the standard deviation.

TABLE 11: Infrared Ar Matrix Spectra. Assignments, and Calculated (TVFF) Fundamental Frwuencies (cm-’) of trans -HONS fundam HONS HO15NS DONS DOI5NS svm no. Xobsd) re1 abs % Ncalcd) AXobsd) At(calcd) A;(obsd) ANcalcd) AXobsd) AiXcalcd) a’

1 2 3 4 5 6

a”

3528.0 1363.3 969.5 842.1 476.5 53 1.3

50 46 60 67 13 100

3528.0 1359.3 968.5 841.7 478.3 531.2

0.0 5.8 18.2 14.1 2.7 0.3

0.0 5.5 19.4 12.5 5.0 0.7

920.0 260.3 17.7 59.1 11.0

a

920.0 256.3 18.6 57.3 15.1 138.0

921.0 272.6 33.3 60.3 14.5

a

920 265.7 35.1 63.7 21.9 139.0

’Not observed. TABLE III: Calculated Frequencies (cm-I) and Potential Energy Distribution of trans-HONSg (TVFFI (CI) (SCF) OH HON SN NO 3528.0 1359.3 968.5 841.7 478.3 53 1.2

3685.1 1482.4 1071.4 947.4 512.0 600.0

3920.8 1616.2 1146.4 1071.4 538.2 607.7

SNO

tors

100 (100; 100) 90 (93; 90) 7 (9; 8)

0 (6; I O ) 79 (70; 46) 19 (9; 44)

11 (20; 31) 74 (78; 55) 13 (0; 13)

0 13 12 76

(0; 4) (0; 3) (12; 0) (97; 93) 100 (100; 100)

‘The potential energy distribution (in 76) determined with the three methods are given as follows: TVFF(CI; SCF). for all six modes of trans-HONS are summarized in Tables 1-111. It should be noted that while in HSNO, HOSN,26 and nitramide26*27 the a b initio S C F calculations predicted very well the frequency of the torsional mode, this was not the case for trans-HONS. However, since the torsion is expected to exhibit only a small isotopic shift upon ISN substitution, the vibration at 531.1 cm-’, showing a I5N shift of merely 0.3 cm-I, was assigned as the torsional mode. For the isotopomer DONS, five of the six fundamentals were observed and assigned as given in Table 11. The a’‘ torsional mode, which was expected to be of lower intensity in the D species according to our finding in HSNO, could not be ~

~~~

(26) Nonella, M. Ph.D. Thesis, Universitat Zurich, Switzerland, 1986. (27) Nonella, M.; Muller, R. P.; Huber,J. R. J . Mol. Specrrosc. 1985, 112, 124.

detected. Moreover, the absorption of 951.3 cm-I, previously attributed to the S N stretch of t r ~ n s - D N S 0 ,was ~ ~ assigned to the S N stretching vibration of trans-DONS. Thus, the present work provides a complete set of fundamentals for the new molecule trans-HONS. The assignments were based on a normal-coordinate analysis (NCA) and on a b initio calculations. The NCA was performed in conjunction with the transferable valence force field (TVFF) method.28 Following the same procedure as before5sZ6the harmonic force field of transH O N S was constructed by starting from the S C F optimized geometry. The internal coordinates were chosen to be SI, OH stretch (a’); S2, HON bend (a’); S3,S N stretch (a’); S4, O N S (28) Hollenstein, H.; Gunthard, Hs. H. J . Mol. Specrrosc. 1980, 84, 457.

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The Journal of Physical Chemistry, Vol. 91, No. 20, 1987

Nonella et al.

TABLE IV: Ab Initio Calculated and Observed Vibrational Frequencies (cm-') of the Molecules HNSO, HOSN, HSNO, and HONS"

molecule cis-HNSO trans-HNSO cis-HOSN CLS-HSNO trans-HSNO Irons- HONS

1 (a')

I(obsdj ?(calcd) ?(obsd) ;(calcd) i;(obsd) ;(calcd) i;(obsd) ;(calcd) t(obsd) ?(calcd) ?(obsd) t(calcd)

2(a') 1248.7 1469.8 1382.0 1484.5 1320.8 1396.9 1570.0 1872.1 1596.0 1894.6 1363.3 1616.2

3308.5 3633.0 h 363 I .3 3519.8 39 12.9 2566.0 2819 2 2613.0 2891 .O 3528.0 3920.8

4b') 900.4 1086.5 881.0 1099.1 673.9 832.1 503.0 682.5 543.5 649.7 842.1 1071.4

3b') 1082.7 1282.7 986.0 1248.4 991.8 1206.1 858.5 935.7 877.5 1082.9 969.5 1146.4

6 (a")

5(a') 447.4 508.0 h 561.5 374.0 445.6 307.0 416.2 297.0 47 1.8 476.5 538.2

754.7 900.5 65 1 .O 810.1 417.5 398.4 406.5 407.1 386.5 375.9 531.3 607.7

"Observed frequencies of HNSO and HOSN, see ref 3, 4; observed frequencies of HSNO, see ref 5. *Not observed. TABLE V: Calculated Energies of the Cis and Trans Conformations and the Saddle Points of the Cis-Trans Isomerization of the Molecules HNSO. HOSN. HSNO, and HONS"

cis-HNSO

trans-HNSO

cis-HOSN

rrans-HOSN

cis-HSNO

rrans-HSNO

cis-HONS

trans-HONS

0.00 0.00 0.00 0.00

23.05 20.42 19.24 21.49

35.76 42.08 45.69 45.24

57.42 60.27 63.32 64.20

35.03 56.57 70.41 68.39

32.66 55.74 69.23 66.54

73.65 86.63 92.37 95.37

73.23 86.50 91.88 93.76

E(rel), kJ/mol

basis (a) basis (b) basis (c) basis (d) geometry of the TS basis (a) basis (b) basis (c) basis (d)

planar

nonplanar

nonplanar

nonplanar

5143

Barriers for Cis-Trans Isomerizations (Trans to TS), cm-' none 2540

4585

5180 4844

3139 3323 3188

4359 4532

124 82

"The absolute energies of sis-HNSO are calculated to be -527.04854 hartree (basis a), -527.17943 hartree (basis bj, -527.351 36 hartree (basis c), and -527.32393 hartree (basis d j . bend (a'); and S6, torsion (a"). In accord with the TVFF criteria, we disregarded all interaction constants involving the OH stretch (SI)as well as the interaction constant between the S N stretch (S,) and H O N bend (S2),thus fitting 12 force constants to 1 1 frequencies and 11 isotope shifts. The quality of the final fit, expressed in terms of rms values, was 2.6 cm-' for frequencies and 2.2 cm-' for isotope shifts. The fitted force constants are listed in Table I and the calculated frequencies are given in Tables I1 and 111. The discarded elements corresponded to the smallest elements in the S C F force field. The rest of the interaction force constants of the NCA are in satisfactory agreement with the SCF force field. As for HONO,*j we were also not able in the case of HONS to fit the OH and the OD stretching vibration with one force constant. A comparison of the fitted diagonal elements with force constants of similar molecules is instructive. The H N O bending force constant (0.804 mdyn 8, rad-2) compares well with the corresponding constant in HONO (0.765 mdyn 8, rad-* *j), as does the N - 0 stretching force constant which is 3.084 mdyn ikl.The value in H O N S was found to be 3.583 mdyn A-l. The S N bond in HONS as well as in HNSO has double-bond character. The force constant of this vibration yielded a value of 7.04 mdyn 8,-' for HONS and 6.47 for H N S 0 . 3 As expected, this value lies in between that of an S N single bond (1.544 mdyn in HSNO') and an S N triple bond (9.90 mdyn 8,-'in HOSN4). Furthermore, the force constant of the S N O bending vibration in HONS (1.749 mdyn A rad-*) compares favorably with that in HSNO (1.420 mdyn 8, rad-25) and the torsional mode of the former (0.128 mdyn A rad-*) agrees well with that in H O N O (0.1295*5)and HSNO (0.12285). 4.2. Ground-State Calculations. The geometries of the cis and trans conformations of the molecules HONS, HNSO, HOSN, and HSNO were optimized and their energies calculated at the S C F level by using various basis sets. The previously employed basis set (a) was found too small for a determination of relative energies among these molecules although the geometries and cis-trans isomerization data agree well with those calculated by using basis sets (b)-(d). W e will not consider these results (a) any further in the discussion but merely list them in order to relate the earlier data to those of the basis sets (b)-(d).

C-HNSO 365

11

250

C-HOSN

C-HSNO

61 0

1

It

365

t-HONS Figure 2. Summary of the various light-induced transformation steps of

matrix-isolated HNSO after energy-selectiveirradiation. The irradiation wavelengths are given in nanometers. The initial substance of the light-induced transformation processes is cis-HNSO (Figure 2). It shows the lowest ground-state energy among the species considered and is thus taken as reference. The highest ground-state energy-about 91 kJ/mol above HNSO-was calculated for cis- and trans-HONS. While basis sets (b)-(d) provided very similar results for HNSO, HOSN, and HONS, an energy spread of about i 10% was obtained for HSNO. Although the agreement is still satisfactory, this particular configuration of the four atoms appears to be most sensitive to the choice of the basis set. The barrier height of the cis-trans isomerization process, given in Table V in terms of the energy difference between the trans form and the transition state (TS), was calculated at the SCF level to be 5000 cm-l for HNSO, 3200 cm-' for HSNO, and 4400 cm-I for HONS. Figure 3 depicts the molecular energy vs. the torsional angle or the relevant bending angle. In the case of HOSN, essentially no energy minimum was

Preparation of HNSO, HOSN, HSNO, and H O N S

The Journal of Physical Chemistry, Vol. 91, No. 20, 1987 5207

E [Hartreel

- 526.990

E [Hartreel

AE IkJIMoll

1

AE IkJIMolI

-527.026

7

1

HONS

- 20

75

-

-527.028

- 15 - 527.030 -10 -527.032-5 -527.034

-0

0

Torsion

cis

E lktreel

trans AElkJIMoll

- 527.020 -

I

I

1

I

I

30

60

90

120

150

trans

E IHartree 1

A€ IkJlMoll

HNSO

-527.010-

HSNO

180

Torsion

CIS

-100

- 40

-527.023-30

- 75

-527.020-

-527.026-

-527.029-

-20

-527.032-

-10

-527.030

- 50

-

- 25

- 527.040 -527.035-0

-0 -527.038

0

I

I

I

I

I

30

60

90

120

150

180

TABLE VI: Ab Initio Optimized Geometry of the Molecules HNSO, HOSN, HSNO, and HONS and the Transitions State (TS) (Basis Set c)" HXYZ HX XY YZ H X Y X Y Z torsion c~s-HNSO

TS (SCF) trans-HNSO c~s-HOSN

TS (SCF) trans-HOSN cis-HSNO

TS (SCF) trans-HSNO cis-HONS

TS (SCF) trans-HONS

1.008 0.987 1.007 0.952 0.949 0.950 1.337 1.334 1.331 0.957 0.950 0.949

1.497 1.456 1.503 1.636 1.649 1.646 1.752 1.853 1.763 1.310 1.377 1.329

1.443 1.438 1.436 1.453 1.459 1.451 1.170 1.157 1.168

1.562 1.542 1.552

117.0 170.5 109.6 112.3 112.8 111.5 97.6 92.7 93.4 109.9 107.3 105.3

120.1 118.0 114.8 114.2 112.6 111.8 116.3 113.4

0.0 0.0 180.0 0.0 117.0 180.0 0.0 86.5

113.7

180.0

118.1 115.2 115.2

0.0 85.9 180.0

-527.050

i

llOo

1

I

1400 1700

I

I

I

Zoo0 2300 260°

In order to assess the influence of electron correlation, the barrier height of H O N S was recalculated at the CI level by optimizing the geometries of the cis and trans conformers and of the transition state. The small energy correction obtained, 4618 cm-' (SCF) vs. 4681 cm-' (CI), permits us to neglect this effect on the barrier height for the present molecules. The vibrational frequencies of all molecules and configurations (except of trans-HOSN) were determined by using the force constants and geometries obtained by the SCF calculations. The results are summarized in Table IV. As expected for this method, the calculated frequencies are higher than the experimental However, there seems to exist a trend within this group of molecules which allows for a correction. If the highest frequencies are reduced by lo%, the calculated and the experimental values agree well. The force constants, frequencies, and potential energy

'Bond lengths in angstroms and bond angles in degrees (29) Pulay, P. In Application of Electronic Structure Theory; Schaefer

found for the trans configuration, the barrier lying below the zero-point vibrational energy. Thus, the calculation predicts that this conformer is not stable.

111, H . F., Ed.; Plenum: New York, 1977; p 170. (30) Fogarasi, G.; Pulay, P. In Annual Reoiew of Physical Chemistry; Rabinovitch, B. S . , Ed.: Annual Reviews Inc.: Palo Alto, CA 1984: Val. 35, p 191.

5208

Nonella et al.

The Journal of Physical Chemistry, Vol. 91, No. 20, 1987

TABLE VII: Excitation Energies (eV) and Oscillator Strengths“ (in Parentheses) of the iMolecules HNSO, HOSN, HSNO, and HONS cis-HNSO

state SI S?

s3

cis-HOSN

MRCI(SD) MCSCF-CI(SD) 4.14 ( n r * ) (0.0053) 6.79 (m*) (0.01 1 I ) 7.28 (a**) (0.302)

4.31 (A”)

3.64 (nr*) 3.78 (aa*) 5.53 ( n a * )

3.66 (A’) 3.94 (A”)

MRCI(SD)

9.36 (A”)

trans-HONS

MRCI(SD)

MCSCF-CI(SD)

2.09 ( n a * ) (0.0016) 4.00 ( a x * ) (0.723) 7.35 (na*) (3.4 x 10-4)

2.23 (A”)

1.70 ( n r * ) (1.6 x 10-4) 6.02 ( n a * ) (1.12 x 10-3) 6.78 ( a x * ) (0.371)

1.77 (A”)

1.08 (nr*) 3.06 (.a*) 6.98 (na*)

1.38 (A”) 3.35 (A’)

3.09 (n.*) (0.0036) 6.37 ( n r * ) (1.78 x 10-4) 7.78 (.a*) (0.195)

6.84 (A’)

trans-HSNO

MCSCF-CI(SD) MRCI(SD) MCSCF-CI(SD) Singlet 3.29 (A”) 6.73 (A’) 9.39 (A”)

3.86 (A’) 5.35 (A”)

5.95 (A’) 6.75 (A”)

Triplet TI

T? TI

“Calculated according tof

= 2AEltI2/3,

1.65 ( n a * ) 4.08 (*a*) 5.10 ( n a * )

2.46 (A”) 4.41 (A‘)

1.37 (na*) 2.91 (TT*) 4.45 (na*)

1.36 (A”) 2.99 (A’)

where t is the transition moment.

TABLE VIII: Ab Initio C1 Results (in au) of HNSO, HOSN, HSNO, and HONS

state

cis-HNSO MRCI(SD) MCSCF-CI(SD) SCF MCSCF

-527.66463 (-527.35239) ’A“(na*) -527.5 1266 (-527.23401) ’A‘(?riT*) -527.39692 “4‘(!3s)

‘A“(na*)

-527.41 494

-527.52964 (-527.245 18) ’A‘(HT*) -527.52566 (-527.27083) -527.46 I53 ’A”(na*) -527.36366 ]A‘(TK*) ’A”( na*)

cis-HOSN MRCI(SD) MCSCF-CI(SD) SCF MCSCF

-527.57013 (-527.02523) -527.41 161 (-527.02523) -527.31885 (-526.92748) -527.22625 (-526.8 1642)

-527.63837 (-527.33111) -527.52477 (-527.241 82) -527.35249

-527.425 19 (-527.07275) -527.43560 (-527.03592)

-527.57773 (-527.27559) -527.48851 (-527.21 165) -527.45103 -527.34137

-527.40443

trans-HSNO MRCI(SD) MCSCF-CI(SD) SCF MCSCF

-527.54739 (-527.1 7643) -527.42655 (-527.03105) -527.30028 (-526.9 1824) -527.20225 (-526.79308)

-527.64455 (-527.32010) -527.56774 (-527.25729) -527.49742

-527.45683 (-527.06986) -527.38326 (-527.01 191)

-527.60495 (-527.28666) -527.53202 (-527.23126) -527.38803 -527.34870

-527.37443

trans-HONS MRCI(SD) MCSCF-CI(SD) SCF MCSCF

-527.49651 (-527.17738) -527.41455 (-527.053 12) -527.35477 (-526.99452) -527.29993 (-526.9 1820)

-527.62530 (-527.3 1098) -527.56282 (-527.28302) -527.37620

-527.44577 (-527.08156) -527.37352 (-527.0271 1)

-527.57498 (-527.28302) -527.5 I836 (-527.25015) -527.461 89 -527.32367

-527.404 16

-527.53815 (-527.17401) -527.47327 (-527.08193) -527.3 1937 (-526.92669) -527.29007 (-526.85430) -527.48834 (-527.09454) -527.42831 (-527.06178)

TABLE IX: Dipole Moments (D) for the Lower Lying States of HNSO, HOSN, HSNO, and HONS Calculated with the MRCI Wave Function cis-HNSO cis-HOSN trans-HSNO trans-HONS 1 ‘A’(gs) I’A”(na*) Z’A’(an*)

13A”(nx*) 13A’(aa*)

0.779 1.248 1 . 1 12 2.162 3.023

1.439 1.665 2.867 2.932 4.325

1.336 1.903 0.867 2.019 3.429

1.516 1.420 3.226 2.869 4.254

distribution of HONS and its investigated isotopomers H 0 I 5 N S , DONS, and DOI5NS are given in Tables 1-111. These data were essential for the identification of the molecule as well as the H S N O isomassignment of its vibrations. For the H O N S erization, which in contrast to the other molecules involves not a torsional motion but a 1,3-H shift, the geometry of the planar transition state predicted by the S C F calculation is reproduced in Figure 4. 4.3. Calculations of Excited States. The lowest electronically excited states of the more stable conformers, namely cis-HNSO, trans-HSNO, cis-HOSN, and trans-HONS, were calculated by using methods and basis sets described in section 3. The results in terms of excitation energies and oscillator strengths are given in Table VI11 while the calculated dipole moments of the lower electronic states are summarized in Table IX. The energies obtained by the MRCI and MCSCF-CI methods agree well for the SI, T i , and T2 states; the agreement between the higher electronic states is less satisfactory. Among these molecules, HSNO and HONS are predicted to possess the lowest energy So SI transition. (It is noted that cis and trans conformers are not expected to differ greatly with respect to the electronic transition energy). The S , state of trans-HSNO and trans-HONS are of nr* character and lie 2.09 (2.23) and 1.70 (1.77) eV above the ground state, respectively. This theoretical result is consistent with the absorption behavior found in the photolysis experiments (cf. Figure 2). Irradiation of H S N O using light of 585 nm (2.12 eV) changes its cis-trans +

-

\

0

--

Figure 4. Optimized geometry (angstroms and degrees) of the transition state (TS) of the reaction cis-HONS cis-HSNO. The calculated TS (150 kJ/mol) is in good barrier of activation from cis-HONS agreement with calculations of similar 1,2-H shift^.^^,-'^ The calculations were performed with basis set (a).

-

ratio, and with light of X > 610 nm (250 nm (C5 eV). The theoretical result indicates in these energy regions the So SI (na*) and the So S2 (a**) transitions. For HONS, only the long wavelength absorption mentioned above has been experimentally evident; it agrees well with the calculation.

-

-

5. Discussion A remarkable property of the four atoms H, 0, N, and S is the fact that they arrange to the four molecules HNSO, HOSN, HSNO, and HONS. Starting from HNSO in a low-temperature matrix, selective photolysis permits the formation of the other molecules either in both the cis and trans conformations or in the more favorable conformer under these preparation conditions. Figure 2 summarizes the various light-induced, unimolecular transformation steps involved in the formation process. When a freshly prepared sample of cis-HNSO in an argon matrix is irradiated with X = 250 nm, a mixture of cis-HSNO and trans-HSNO is formed via cis-HOSN. A second photolysis step with X = 365 nm gives rise to the disappearance of cis- and trans-HSNO and to the production of trans-HONS. Simultaneously, a portion of the initial cis-HNSO is re-formed. If this sample is then irradiated with X > 610 nm, HONS is transformed back into HSNO but without the formation of HOSN. A further irradiation process at 365 nm produces HONS, a fact that establishes the HSNO molecule as the precursor of HONS. In separate processes the cis trans cycle of HSNO can selectively be induced by using either light in the I R region or of 585 nm. Table X summarizes the pertinent results of the calculations. With basis sets (b), (c), and (d), the ground-state energies in both the cis and trans configurations agree well among these basis sets for HNSO, HOSN, and HONS. Moreover, our HNSO and HOSN results are in good agreement with previously reported data by Turner3I and Ehrhardt and A h l r i ~ h s . ) Even ~ though the

energy values of HSNO show a spread of = I 4 kJ/mol among the three basis sets, this result can still be considered satisfactory. Thus, based on the calculated ground-state energies the following order of stability emerges: HNSO > HOSN > HSNO > HONS. The cis geometry is either the more stable one (HNSO and HOSN) or cis and trans show essentially the same energy (HSNO and HONS). The cis F= trans isomerization process in HOSN, HSNO, and HONS involves a rotational motion while in HNSO a H shift in the molecular plane is operative. The calculations of the barrier height AE(trans-TS) with the three different basis sets (b, c, d) resulted in a very consistent picture. The lowest barrier of only 3200 cm-I is predicted for HSNO followed by HONS with 4400 cm-I. The H-shift mechanism of HNSO requires a slightly higher activation energy of 4 0 0 0 cm-I. A different behavior is revealed in the case of HOSN. This molecule is predicted to possess no stable trans conformer and therefore cis F= trans isomerization is not possible. The low activation energies predicted for isomerization in HSNO, HONS, and HNSO is promising for IR-induced isomerizations. Although the calculated value of the barrier might be subject to a substantial error and although the matrix will influence the barrier height, one can safely conclude that isomerization of HSNO, HONS, and HNSO can be achieved with IR light. In the case of HSNO embedded in an argon matrix, IR-induced isomerization has previously been o b s e r ~ e d .With ~ an appropriate I R laser an investigation of the selectivity with respect to the cis trans or trans cis process seems worthwhile. To this end, irradiation into the S-H stretching vibration (5 = 3100 cm-I) or into the first overtone of the N=O stretch (=2 X 1600 cm-') might be sufficient to reach the transition state. For all four components the lowest electronic transitions, SI So, is of na* character and thus out of plane. It is calculated to be at 580 and 720 nm for H S N O and HONS, respectively. At considerably higher energy is this transition calculated for H O S N (390 nm) and HNSO (300 nm). The second transition S, So is predicted around 200 nm except for HSNO where it is calculated at 320 nm. Where photolysis experiments allowed a location of an electronic transition, the theoretical result is, as shown above, consistent with the experimental information.

-

-

+

+

Acknowledgment. Support of this work by the Schweizerischer Nationalfonds zur Forderung der wissenschaftlichen Forschung is gratefully acknowledged. We thank Mr. Rolf Pfister for synthesizing DISNSO and Dr. R. P. Muller for valuable discussions. Computer time has been provided by the Computing Center of the University of Zurich, the Computer Center of the ETH Zurich, and the Centre de Calcul of the EPFL Lausanne. Registry No. cis-HNSO, 40908-38-1; trans-HNSO, 5697 1-20-1; HOSN, 56971-18-7; HSNO, 29335-37-3; HONS, 29335-37-3. ~~

(31) Turner, A. G.Inorg. Chim. Acta 1984, 84, 85. (32) Ehrhardt, C.; Ahlrichs, R. Chem. Phys. 1986, 108, 417

~~

(33) Meyer, W. J . Chem. Phys. 1976, 64, 2901. (34) Dykstra, C. E.; Schaefer 111, H. F.; Meyer, W. J . Chem. Phys. 1976, 65, 2740.