3042
J . Phys. Chem. 1984, 88, 3042-3047
8 X 10-l2s, which is about an order of magnitude too large for rotational motion but approximately the value expected for collisional motion with the exposed faces of the CdS6 octahedra. The temperature dependence of the dielectric constant of pyridine-intercalated Cd2P2S6has been used to investigate the
dynamical behavior of the pyridine intercalate. The activation energy for pyridine motion was determined to be 3240 crn-l. The motion is associated with changes in the direction of the pyridine dipole moment perpendicular to the Cd2P2S6stacking axis and is not necessarily that observed in the ESR experiment.
The Photochemistry of Argon Matrices Containing Nitric Oxide and Carbonyl Sulfide. 2. The Photoproducts SNO and SN,O," Michael HawkinsIb and Anthony J. Downs* Inorganic Chemistry Laboratory, University of Oxford, Oxford OX1 3QR, England (Received: October 24, 1983)
The reaction of sulfur atoms, generated by ultraviolet photolysis of carbonyl sulfide, with monomeric and dimeric nitric oxide isolated in argon matrices at 13-20 K leads to the formation of products with the molecular formulas SNO and SN202. Assignments based on the infrared spectra of the three isotopomersS14N160,Sl5NI60,and S14N1s0place the N-0 stretching and N-S stretching fundamentals of SI4Nl6Oat 1554.9 and 790.0 cm-I, respectively, after allowance for Fermi resonance between uNo and 2vNS. The third fundamental, the bending mode, could not be detected with certainty but is estimated to occur at ca. 567 cm-'. SN,02 appears to exist in two isomeric forms, probably S(O)N.NO and ONSNO, the reversible interconversion of which is promoted by photolysis: S(O)N.NO P ONSNO. Three vibrational fundamentals of each isomer and S5N202. have been identified for the two isotopomers SJ4Nz02
Introduction Although there is a wealth of information about the nitrogen oxide molecules N O , (x = 1-3) and N20y0,= 1-5), few of the analogous (acyclic) nitrogen sulfides are known with any certainty. It is true that the gaseous thiazyl radical, NS, has been characterized by its m i c r o w a ~ evisible , ~ ~ ~ emission,2 electron spin resonance,2 photoelectron,4 and high-resolution infrared5 spectra, but no more than tentative claims touch upon the identification of nitrogen disulfide, NS2,6and dinitrogen sulfide, N2S.7 Mixed oxide sulfides of nitrogen like SNO or SN202have fared no better. Tchir and Spratley have reported that two infrared absorptions attributable to nitrogen oxide sulfide, SNO, develop after ultraviolet irradiation of argon matrices containing cis-thionyl imide, "SO.* A single infrared absorption which appeared after vacuum-UV irradiation of matrix-isolated cis-"SO has also been ascribed by the same workers to a second isomer, viz. N S 0 . 9 Evidence for the formation of a species with the molecular formula SNO has also been derived from two studies involving the flash photolysis of gaseous carbonyl sulfide in the presence of nitric oxide. The firstlo revealed a visible emission in the region 350-430 nm exhibiting complex vibrational fine structure, although no attempt was made to identify its source. The second" established (1) (a) Presented in part at the 3rd International Meeting oh Matrix Isolation, Nottingham, England, July 1981. When our investigations were already well advanced, we learned of an independent matrix-isolation study of the photochemically induced interaction of OCS with NO, this was reported by: Fredin, L.; Pimentel, G. C. "Abstracts of Papers", 178th National Meeting of the American Chemical Society, Washington, D.C., Sept 1979; American Chemical Society: Washington, D.C , 1979; PHYS 191. (b) Present address: Sevenoaks School, Sevenoaks, Kent TN13 lHU, U.K. (2) fIeal, H. G. Adu. Inorg. Chem. Radiochem. 1972,15, 375 and references cited therein. (3) Lovas, F. J.; Suenram, R. D J . Mol. Spectrosc. 1982, 93, 416. (4) Dyke, J. M.; Morris, A.; Trickle, I. R. J. Chem. SOC.,Faraday Trans. 2 1977, 73, 147. ( 5 ) Matsumura, K.; Kawaguchi, K.; Nagai, K.; Yamada, C.; Hirota, E. J . Mol. Spectrosc. 1980, 84, 68. (6) Pannetier, G.; Dessaux, 0.; Arditi, I.; Goudmand, P. C. R . Hebd. Seances Acad. Sci. 1964, 259, 2198. (7) Powell, F. X. Chem. Phys. Lett. 1975, 33, 393. (8) Tchir, P. 0.;Spratley, R. D. Can. J . Chem. 1975, 53, 2318. (9) Tchir, P. 0.;Spratley, R. D. Can. J. Chem. 1975, 53, 2331. (10) Basco, N.; Pearson, A. E. Trans. Faraday SOC.1967, 63, 2684. (1 1) Van Roodselaar, A.; Obi, K.; Strausz, 0.P. Int. J. Chem. Kinet. 1978, 10, 31.
0022-3654/84/2088-3042$01.50/0
that sulfur atoms decay at a rate which can be rationalized in terms of the formation of a complex with the composition SNO. Encouraged by the capacity of matrix-isolation methods to bring to light species short-lived under normal conditions, we have investigated the reactions which take place in solid argon matrices between sulfur atoms and either monomeric nitric oxide or the dimer cis-N,02. This paper describes the infrared spectra of the matrices which bear witness to the formation of the molecules SNO and S N , 0 2 ; it also presents evidence that SN202,like NZ03,12 exists in different isomeric forms which are susceptible to photochemical interconversion.
Experimental Section Apparatus. Matrices were deposited on a CsI window cooled to 13-20 K by means of a Displex refrigerator (Air Products, Model CS 202). Temperatures were measured with a chrome1 vs. iron-doped gold thermocouple or with a hydrogen-vapor bulb and were varied by adjusting the voltage applied to a 20-W heater wound around the second stage of the refrigerator. Surrounding the cold station of the refrigerator was a shroud which was evacuated to a pressure less than torr. Infrared spectra were recorded with either a Perkin-Elmer Model 225 or a Perkin-Elmer Model 580 spectrophotometer. With the internal calibration available on the Model 580 instrument, wavenumbers were reproducible to better than 0.4 cm-I, and the resolution was invariably better than 0.5 cm-' in measurements involving precise wavenumber determination. Sample photolysis was effected with either a Philips HPK 125-W medium-pressure mercury arc or a Philips 070 T 25-W cadmium arc. A 2- or 4-cm water filter reduced the amount of infrared radiation incident upon the matrix when the mercury arc was in use, and a Balzer absorption filter served, where necessary, to remove UV light with wavelengths shorter than 375 nm. Whereas the primary UV emission of a mercury arc is in the region of 254 nm, that of the cadmium arc is at 228 nm.I3 Chemicals. Argon (grade "X", 99.995%) was used as received from the British Oxygen Co. Carbonyl sulfide and nitric oxide, (12) Varetti, E. L.; Pimentel, G. C. J . Chem. Phys. 1971, 55, 3813. (13) Meggers, W. F.; Corliss, C. H.; Scribner, B. F. "Tables of Spectral-Line Intensities, Part I", 2nd ed.; National Bureau of Standards, Monograph 145: Washington, D.C., 1975.
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 3043
Photoproducts S N O and SNz02
,d,
26CC
22M
"
OM
7CC
>OW
Ka
VCC
Wo
iW
5W
3m
375 nm for 20 min. Bands marked are associated with photoproduct B; bands marked are associated with photoproduct C.
'1
322.6 wd
i 1 t t
809'6 vw, sh 806.5 w
319.0 w, brd heated by an internal element). Three of the bands, associated with product B, were doublets at 1827.2/1823.5, 1499.0/1495.0, and 825.6/822.5 cm-I; these exhibited average I5N isotopic shifts of 32.0, 27.9, and 16.0 cm-', respectively (Figure 3b). Associated with product C were two absorptions at 1669.2 and 1567.3 cm-', which showed large I5N isotopic shifts of 27.3 and 26.8 cm-I, respectively (Figure 3b), together with a third at 322.6 cm-' (not illustrated in Figure 3) characterized by a 15N shift of only 3.6 cm-'. When the matrix responsible for spectrum b in Figure 3 was exposed to the emission from the spectrophotometer source for 130 min, the bands labeled B were seen to decay while those labeled C, in common with the band at 319.0 cm-I, were seen simultaneously to grow (trace c). When this sample was further exposed to the emission from the mercury arc filtered to give only radiation with X > 375 nm, the pattern was reversed (Figure 3d): those absorptions which had formerly increased in intensity now decreased and vice versa. Table I1 summarizes the behavior of these bands on photolysis, together with their wavenumbers and relative intensities.
Discussion Discussion of the results concerns the identification of the molecular species A, B, and C. Divided into two sections, the discussion will turn first to the assignment of the absorptions associated with product A. The primary photoprocess occurring during UV irradiation of a matrix including carbonyl sulfide is dissociation to carbon monoxide and sulfur atoms (reaction a).16
By analogy with the products NOz and N2O3 formed in argon matrices when oxygen atoms react with monomeric and dimeric nitric oxide, re~pectively,'~ one may anticipate the formation of the species S N O and S N 2 0 2following the photogeneration of sulfur atoms in argon matrices containing nitric oxide. (i) Species A . The two intense absorptions between 1470 and 1600 cm-' associated with a given isotopomer of photoproduct A both exhibit the large I5N and l 8 0 isotopic shifts characteristic of N=O stretching vibrations. Each of these bands gave way to a doublet whenever the matrix contained two different isotopomers of nitric oxide. This behavior seems to point to a molecule containing two inequivalent and vibrationally uncoupled N=O bonds, criteria which are unlikely to apply concurrently to any species, say, with the molecular formula SN20,. Furthermore,
11
11
4
1
B
0I4N-l4NSO
B C C C
C
015N-'5NS0 0I4NSl4NO 015NS15N0 014NS14N0 0I5NSl5NO
B
014N-14NS0
B
0l5N-I5NSO
B
0I4N-l4NSO
B
015N-15NSO
C C
014NS14N0 015NS15N0
'
'Error limits 10.5 cm-l. br = broad, m = medium, sh = shoulder, v = very, w = weak. cIndicates the change in intensity on irradiation (i) with the emission from the infrared spectrometer source (vis-IR) or (ii) with visible light having A > 375 nm (1, increases; 1, decreases). Error limits i 1.O cm-I. TABLE III: Observed and Corrected Wavenumbers and Intensities of the Components of the Fermi Doublet in Nitrogen Oxide Sulfide
obsd, cm-I isotopomer Si4Ni60 SI5Ni6O Si4Ni80
cor, cm-I
wI
2v2
Pa
VI
2v2
1522.6' 1497.7' 1493.7'
1596.V 1568.2d
1.292 1.483 2.05
1554.9 1526.7 1520.5
1564.4 1539.2 1548.5
1575.3d
"Intensity ratio, p = 1(ul)/1(2v2). "Error limits 10.25 cm-l. Each value is an average for the two components of a doublet (see Table I). "Error limits f l . O cm-'. dError limits 10.5 cm-'. the relative intensities of the two absorptions at (i) 1596.8 and 1523.3/1521.9 cm-I, (ii) 1568.2 and 1498.4/1496.9 cm-', and (iii) 1575.3 and 1494.4/1493.0 cm-I in the spectra of matrices containing (i) I4Nl6O, (ii) 15N160,and (iii) l4NI8O,respectively, are not constant but vary from one isotopomer to another, an observation inconsistent with the assignment of both bands to the N=O stretching fundamentals of a single molecule. At the same time we note that photoproduct A is relatively more abundant in matrices rich in monomeric nitric oxide. The appearance of two intense bands in this region of the spectrum we interpret in terms of Fermi resonance arising from the near-degeneracy of a fundamental between 1493 and 1524 cm-' and the first overtone of the fundamental responsible for the weak absorption between 777 and 790 cm-'. The band near 1500 cm-I is associated with the N=O stretching vibration of the triatomic molecule SNO, in agreement with the conclusions of Tchir and Spratley.8 According to Overend," the unperturbed frequency, vcor, of each component of a Fermi doublet is given by VI
vcor
+ y2
VI
f=2
uz(
,"; ;)
-
where v1 and vz are the observed vibrational frequencies in wavenumbers and p is the ratio of the intensities. Table I11 lists the values of p and both the observed and unperturbed frequencies for the components of the Fermi doublet exhibited by the molecules ~~~
(16) Cook,G.; Krogh, 0.D.J . Chem. Phys. 1981, 74,841 and references cited therein.
11 11
origin code likely identity
~
(17) Overend, J. In "Infrared Spectroscopy and Molecular Structure"; Davies, M., Ed.; Elsevier: Amsterdam, 1963; p 345.
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 3045
Photoproducts S N O and S N 2 0 2
SI4Nl6O,SI5Nl6O, and S 1 4 N 1 8 0 . The reason for the comparative weakness of the band at 1575.3 cm-I associated with SI4N1*O(Figure 2b) is now clear. The vibrational transitions in resonance in this isotopomer are separated by 28.0 cm-l while the corresponding energy difference in SI4Nl6O and SI5Nl6Ois much smaller, namely 9.5 and 12.5 cm-l, respectively. The reduced degree of resonance in S14N1s0is also reflected in the intensity of the absorption at 1494.4/1493.0 cm-I when compared with the corresponding feature of SI4Nl6Oat 1523.3/1521.9 cm-I (Figure 2b); more intensity is borrowed from the latter to give an intensity pattern apparently belying the relative proportions of I4Nl6O and I4Nl8O present in the matrix (14N160:14N180 = 1.28:l). Once corrected for Fermi resonance, the frequencies of the overtone absorptions are all 15.6-17.7 cm-l lower than expected on the basis of the fundamental located at 777.6-790.0 cm-’. There is therefore a large anharmonicity term associated with this fundamental, a finding which is not altogether surprising since it is the anharmonicity of molecular vibrations which admits the possibility of Fermi resonance between different vibrational transitions. Assignment of the bands between 777 and 790 cm-’ to a particular normal mode is less straightforward than assignment ofthe bands at higher frequencies. The relatively large mass of the terminal sulfur atom will induce a high degree of mechanical coupling between the N==S stretching and S-N-0 deformation vibrations of the SNO molecule, particularly if the S - N - 0 bond angle is similar to the 0-N-0 bond angle of NOz (134’). Consequently, the two remaining fundamentals are likely both to be characterized by significant I5N and ‘‘0 isotopic shifts. That vibration which involveS predominantly S-N-0 deformation would not be expected to lie above 752 cm-I, corresponding to the location of the deformation mode of N O , isolated in an argon matrix.18 Certainly the precedents set by pairs of triatomic molecules like OCS and C 0 2 and SzO and SOz19,20 argue strongly in favor of a decrease in the frequency of the deformation mode with the switch from O N 0 to SNO. Hence, although Tchir and Spratley ascribed absorptions between 777 and 790 cm-I to the deformation mode of SN0,8 we prefer to associate these features with the fundamental approximating to the N--C stretching vibration. Our conclusion receives support from the relative magnitudes of the 15N and I8O isotopic shifts exhibited by the absorption at 790.0 cm-’, namely 12.1 and 6.9 cm-’, respectively. N o definite assignment can be made to the deformation mode of the triatomic S N O molecule. Tchir and Spratley observed, but left unassigned, a very weak band at 385 cm-1;8this grew on irradiation of a matrix containing cis-”SO with the output from a mercury arc and exhibited a 15N isotopic shift of 5 cm-’. Our experiments too brought to light weak bands in the region 370-390 cm-l which grew on photolysis, but these are probably associated with S,O and N z 0 3formed in secondary reactions involving the ,.~~ oxygen atoms released by the photodissociation of C ~ S - N ~ OTo check the interpretation of the spectrum and to seek an estimate for the frequency of the third fundamental, we have carried out normal-coordinate analysis calculations for the three isotopomers SI4Nl6O,SI5Nl6O,and S 1 4 N 1 8 0 . Normal-Coordinate Analysis of SNO. For the purposes of the calculations, the following dimensions were adopted for the triatomic S N O molecule: rN-O = 1.20 A, rN-s = 1.60 A, and LSNO (a)= 139’. The N - 0 bond length was set close to that in NOz (1.195 A)” while the N-S bond length was estimated from the relationship rN-O(NO) rN-O(SNO) rN-S(Ns) rN-S(SNo)
-- rC-S(ocs)‘C-O(C0) rC-O(ocs) rC-S(cs)
(18) Tevault, D. E.; Andrews, L. Spectrochim. Acta, Part A 1974, 30A, 969. ( I 9) Nakamoto, K. “Infrared and Raman Spectra of Inorganic and Coordination Compounds”, 3rd ed.; Wiley-Interscience: New York, 1978. (20) Hopkins, A. G.; Daly, F. P.; Brown, C. W. J. Phys. Chem. 1975, 79, 1849. (21) See, for example: Hardwick, J. L.; Brand, J. C. D. Can. J . Phys. 1976, 54, 80.
TABLE I V General Valence Force Constants“for Nitrogen Oxide Sulfide soh 1 2 3
fN0
1058.7 1068.5 1075.7
fNS
479.5 499.4 517.5
fo
63.5
100.0 139.0
fNS,NO
fNS,o
fN0,a
105.0 129.3 152.2
34.0 64.3 93.2
0 0 0
Bond-stretching and stretch-stretch interaction constants measured in N m-I, stretch-bend interaction constants measured in N rad-’, and bending constant measured in N m rad-2.
and the known interatomic distances of NO?* NS,” OCS,Z3C0,22 and C S Z 2 A bond angle of 139’ was that which gave the best account of the measured vibrational frequencies (with due allowance for the effects of Fermi resonance) subject to the constraints of a simple quadratic force field. That nitrogen dioxide has a similar bond angle (133.8°)21gives us reason to believe that our estimate is not far wide of the mark. The calculated force constants appropriate to a general valence force field are presented in Table IV while Table V compares the calculated frequencies for the three isotopomers SI4Nl6O,SI5Nl6O, and SI4N1*Owith the observed values (corrected for the effects of Fermi resonance). Determination of a unique solution of the vibrational problem is denied for want of information about the deformation mode v3. Under the circumstances we present in Tables IV and V three solutions each reproducing the observed vibrational frequencies within experimental error. The major differences between the three solutions reside in the values of the interaction constants fNS,NO and fNs,aand the deformation frequency v3. These differences result from changes in the deformation force constant f a . The number of variables used to define = 0, a the force field was reduced to five by maintaining reasonable approximation since this is expected to be the least influential of the interaction constants. Several features of Tables IV and V merit discussion. Although differ by ca. the calculated and observed values of v l for S’4N160 2.0 cm-I, the agreement is well within experimental error. The correction of v 1 for Fermi resonance is least reliable for this particular isotopomer because the adventitious coincidence with absorptions associated with matrix-isolated water (present as a minor impurity) made it difficult to assess the intensity of the overtone band (2vJ at 1596.8 cm-’. An error in p of 11% would be sufficient to account for the difference between the resonance-corrected value for vI of 1554.9 cm-l given in Table I11 and the frequency of 1557 cm-I calculated by normal-coordinate analysis. Since the intensities of both components contributing to p carry some error, inaccuracies in the order of 6% in the estimated intensity of each absorption comprising the Fermi doublet would be sufficient to account for the discrepancy between the calculated and observed values of v I revealed for SI4Nl6Oin Table V. Without access to spectral subtraction and integrated absorbance routines, errors of this magnitude cannot be excluded. The wide range of values taken for the deformation force constant, f a , yields but a narrow range for both the stretching force constants, f N 0 and&,. The magnitudes of both f N 0 andfNS are entirely plausible for the triatomic molecule SNO. Thus, a value of 1068 f 9 N m-l for fNo is somewhat smaller than the corresponding parameter for NOz (1091 N m-1),24in keeping with the decrease in frequency of 60 cm-’ for v 1 of SNO relative to the antisymmetric N-0 stretching mode of NO,. It is also compatible with the relationship fNo = 3440/(rNO)5.97N m-I linking the NO stretching force constant and internuclear distance, rNOin angstroms, in a variety of oxo-nitrogen species.25 An NS stretching force constant in the order of 500 N m-l is perfectly reasonable in view of (i) the stretching frequency of 790.0 cm-’ for the isotopomer SI4Nl6O,(ii) the experience gained from other acyclic sulfur-nitrogen corn pound^,^^^*^ and (iii) the relationship fNs = (22) Huber, K. P.; Herzberg, G. “Molecular Spectra and Molecular Structure”; Van Nostrand Reinhold: New York, 1979; Vol. IV. (23) Maki, A. G.; Johnson, D. R. J . Mol. Spectrosc. 1973, 47, 226. (24) Laane, J.; Ohlsen, J. R. Prog. Inorg. Chem. 1980, 27, 465. (25) Ladd, J. A.; Orville-Thomas,W. J. Spectrochim. Acra 1966, 22, 919.
3046
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984
Hawkins and Downs
TABLE V Observed and Calculated Wavenumbers for the Vibrational Fundamentals of Three Isotopomers of Nitrogen Oxide Sulfide
calcd, cm-I soln 1
isotopomer
VI
v2
v3
VI
v2
3 3 14Ni80
1556.9 1526.6 1520.6
790.4 777.9 783.1
387.6 381.5 38 1 .O
1554.9 1526.7 1520.5
1556.9 1526.6 1520.5 1556.8 1526.6 1520.5
790.2 777.8 783.1 789.9 777.8 783.1
484.1 476.4 475.9 567.1 557.8 557.2
1554.9 1526.7 1520.5
790.0 771.9 783.1 790.0 777.9 783.1 790.0 777.9 783.1
32S14N160 32SlSN160
2
32Sl4N160 32SISN160 32Sl4N180
3
32Sl4Nl60 32SlSNl60 32S14NlBO
TABLE VI: Wavenumbers of the Deformation Mode of Vibration, va, in Selected Triatomic Molecules
molecule OCS
voi, cm-'
667a 520'
obsd, cm-I
ratio
av ratio
estd va(SNO), cm"
11.283
1554.9 1526.7 1520.5
irradiation (i) with the emission from the spectrophotometer source and (ii) with the filtered emission from an Hg arc (A > 375 nm), together with the lack of any other photoproduct arising from this treatment, argues strongly for a reversible photoinduced isomerization similar to that exhibited by N203in nitrogen matrices.12 The interconversion of the asymmetric (1) and symmetric (2) isomers of N2O3 proceeds according to eq b, and there is reason
567b OU
I
a Reference 19. Calculated by using v,(KO,)/v,(SNO) = 1.321 with va((NO,) = 749 cm-' (ref 19). 'Reference 20.
145/(rNS)'.ON m-l linking the NS stretching force constant and internuclear distance, rNs in angstroms, in a variety of sulfurnitrogen ~pecies.~' Thus, all three of the solutions listed in Table IV are acceptable, and the present data give no definitive basis for distinguishing between them. However, there are two independent relationships which suggest a preferred solution. Firstly, the frequencies associated with the deformation modes of the molecules CO,, OCS, SO2, and S20 are listed in Table VI. Allied to the frequency of the corresponding fundamental of NO2 (749 ~ m - ' ) , 2these ~ lead to an estimate of 567 cm-' for v3 of SNO. Secondly, GBzquez, Ray, and Parr have proposed an equation linking the bending force constant, f,, of a triatomic molecule to the two stretching force constants, f l and fi,2s viz.
f, = 11cfLf2)1/2- 0.55cf,
+ f i ) N m rad-2
Introduction of the values for fNo and fNS given in Table IV leads to estimates for f, of 134.2, 137.7, and 140.8 N m rad-2 for solutions 1, 2, and 3, respectively. Consequently, solution 3, in which v3 and f, are 567.1 cm-l and 139 N m rad-2, respectively, gives the closest approach to the vibrational properties expected for the SNO molecule. We think it likely therefore that the deformation mode of SNO occurs in the range 550-590 cm-'. Although several very weak absorptions were indeed observed hereabouts in the spectra of the photolyzed matrices, none appeared consistently with an intensity or response to 15N or I8Oisotopic enrichment allowing a confident assignment to the third fundamental of SNO. (ii) Species B and C. The absorptions ascribed to photoproducts B and C in Figure 3 and Table I1 were more pronounced after photolysis of carbonyl sulfide in argon matrices relatively rich in nitric oxide dimer, cis-N202. A high proportion of cis-N202was achieved by deposition of the matrix at 18-20 K rather than 13-14 K, and its phot~lysis'~ was prevented by irradiation of the matrix with the emission from a low-power Cd arc rather than the usual Hg arc. The dependence of the new bands on the cis-N202 concentration suggests that they originate in a species with the molecular formula SN202.Moreover, their behavior on successive (26) Banister, A. J.; Moore, L. F.; Padley, J. S. In "Inorganic Sulphur Chemistry"; Nickless, G., Ed.; Elsevier: Amsterdam, 1968; p 154. Peake, S. C.; Downs, A. J. J . Chem. Soc., Dalton Trans. 1974, 859. (27) Glemser, 0.; Muller, A.; Bohler, D.; Krebs, B. Z . Anorg. Allg. Chem. 1968, 357, 184. (28) GBzquez, J. L.; Ray, N. K.; Parr, R. G. Theor. Chim. Acta 1978,49, 1.
y3
1
2
to believe that the trans-trans isomer 2 is the most stable form of planar symmetric N2O3. By analogy with these observations, the species (B) responsible for the absorptions which decay under the action of the emission from the spectrophotometer source is presumed to be 3, the "asymmetric" isomer of S N 2 0 2 . Just as S
-+
spectrophotometer source
/N-N\,
A: 375-1200nm
0
*
3
5
4
the infrared spectrum of 1 in the region between 750 and 1900 cm-I is a superposition of those due to the molecules NO2 and N O only slightly perturbed by the interaction,12so the absorptions associated with B lie close to those due to the molecules SNO and NO (Table VII). A still closer correspondence is observed for the I5N isotopic shifts. On this basis, the bands at 1497.0 and 824.0 cm-' are assigned respectively to N-0 and N-S stretching fundamentals localized in the SNO fragment while the absorption at 1825.4 cm-I, close to the fundamental of the matrix-isolated N O molecule, is associated with a stretching mode localized in the NO fragment of 3. Since the vibrational spectrum of 3 is similar to that of SNO, it is unfortunate that there was no obvious candidate for the SNO deformation mode of 3 as this might have given a clue to the location of the third fundamental in SNO itself. There are two products, 4 and 5, which can result from the photoisomerization of 3, discounting the different geometric and conformational modifications of these molecules. Since the remaining absorptions associated with product C and located at 1669.2, 1567.3, and 322.6 cm-I decrease together on irradiation of the matrix with light having h > 375 nm, we have some confidence in assigning them to a single species. If 3 were to photoisomerize to a mixture of 4 and 5 , the three absorptions due to the product would not be expected to decrease with constant relative intensities on photoreversal since 4 and 5 are unlikely to have electronic transitions close enough in energy in the visible region to afford photoisomerization to 3 at exactly the same rates. We reject the possibility that the observed photochemistry is that of a reversible photofragmentation of 3 on the grounds either that the infrared spectra of possible fragments are w e l l - k n ~ w n ~ ~ ~ ~ ~ ~ ~ ~ ~
~~~
~~
~
(29) Teichman, R. A,, 111; Nixon, E. R. Inorg. Chem. 1976, IS, 1993.
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 3047
Photoproducts SNO and SN202
TABLE VII: Wavenumbers and I4N/l5N Isotopic Shifts Observed for the Molecules ON.NSO, ON.N02, ONSNO,and ONONO Compared with Those of the Molecules SNO, NOz, and NO 14N 14N I4N isotopomer, I5N isotopic isotopomer, I5N isotopic isotopomer, I5N isotopic cm-' shift, cm-' molecule cm-' shift, cm-l molecule cm-' shift, cm-I molecule 1839.7" 32.4 NO 1876.0b 33.1 ONmN02
ON-NSO ONSNO
1630.4' 1302.5" 775.7" 420.4'
35.8 15.7 10.8 10.9
1825.4' 1497.OC 824.OC
32.0 27.9 16.0
1669.2' 1567.3c
27.3 26.8
322.6'
3.6
'Nz matrix; ref 12.
NO2
1616.9b 1320.0b 749.76
34.8 14.0 9.7
SNO
1554.geld 79O.Oc
28.2d 12.1
ONONO
1689.7" 1661.O" 969.4' 877.0' 704.3' 387.4" 365.5'
28.1 23.6 14.8
NO
bGas phase; ref 24. 375 nm. These results confirm the earlier identification8 and extend the characterization of the triatomic molecule SNO and provide the first experimental evidence for a derivative of dinitrogen trioxide in which one of the oxygen atoms gives place to sulfur.
Acknowledgment. We thank SERC both for a grant to assist the purchase of equipment and for the award of a studentship (to M.H.). Registry No. 3,90171-80-5; 5, 90171-82-7; 0I5N-NSO, 90171-81-6;
015NS'5N0,90171-83-8; Ar, 7440-37-1; OCS, 463-58-1; NO, 1010243-9; "NO, 15917-77-8; N180, 15917-78-9; SNO, 56971-19-8; SI5NO, 90171-78-1; SNI80, 9017 1-79-2; N,O,, 16824-89-8.
