79
J . Phys. Chem. 1991, 95, 79-86 tetrahedral bridged P4S species. The 473.9-cm-' band occurs only in P4 Ss discharge experiments. The 470.5-cm-' band in the 50% 34Sexperiment is twice as intense as the 473.9-cm-I band, suggesting a triplet absorption with the pure % component under P4. Since P4S3absorbs strongly in the region, the 473.9-cm-' band is tentatively assigned to P4S2 with two bridged sulfur atoms. Semiempirical SCF-MO calculations suggest the structure with adjacent P-S-P edges is more stable than that with opposite in such a structure the sulfur motions couple and reveal a mixed isotopic component. Unfortunately, a weak band was also observed at 473.7 cm-' in sulfur discharge experiments,I5and although the present 473.9-cm-I band is much stronger, a sulfur bearing impurity cannot be ruled out. Mechanisms. The simple phosphorus sulfides identified here, namely, PS, P2S, and PS2, were produced in the microwave discharge of P4 and S8 in argon and trapped in a solid argon matrix. Photolysis increased the yields of several products. Photoexcitation of P2 adjacent to sulfur atoms should initiate reaction to form more P2S. Visible photolysis of discharged P4 samples increased P3 owing to diffusion and reaction of P atoms,21 and in the present experiments the P S2 reaction is likely to occur with P atoms dissociated from larger phosphorus clusters. On the other hand, the S3 and S4 speciesI5 can be expected to photodissociate to give S atoms and S2 molecules for further reaction with P atoms in the matrix produced in the discharge of P4, In addition, photodetachment of PS2-contributes to the photochemical yield of PS,.
+
+
(29) McCluskey, M.; Andrews, L. J . Phys. Chem., in press. (30) Lohr, L. L. J . Phys. Chem. 1990, 94, 4832. (31) Glidewell, C. fnorg. Chim. Acta 1984, 81, 187.
Secondary reactions during matrix condensation led to the formation of other products: PS2 PSZ SPSPSZ
+
P2S + P2
-
cyclic PIS
Also notable is the reaction of P2 from the discharge with S atoms produced by photodissociation of OCS on the matrix surface P, + s P2S
-
Annealing the samples to 32 f 2 K allowed for the diffusion and further reaction of trapped sulfur atoms as attested by the observed blue chemiluminescence. Other S atom reactions on annealing markedly reduced PS, PS,, and P2S absorptions and increased SPSPS2.
ConcIusi ons Microwave discharge of P, and S8 mixtures in excess argon produced three simple primary reaction products (PS, P2S, and PS,) that were isolated in solid argon for infrared spectroscopic study. These species were also prepared from other phosphorus and sulfur compounds. The use of enriched S-34 provided isotopic shifts to characterize the normal mode and multiplets to define the sulfur stoichiometry. Photolysis and annealing behavior helped identify the primary and several secondary reaction products including PS,, P4S,and SPSPS2. Sulfur atoms are clearly less reactive than oxygen atoms, but many of the same phosphorus species are produced. Acknowledgment. We gratefully acknowledge financial support from NSF Grant C H E 88-20764 and ab initio calculations performed by M. McCluskey.
Infrared Spectra and Structures of Isotopically Enriched S3 and S4 in Solid Argon G. Dana Brabson, Zofia Mielke,+ and Lester Andrews* Chemistry Department, University of Virginia, Charlottesville, Virginia 22901 (Received: June 12, 1990)
Sulfur vapor was seeded into a microwave-poweredargon discharge and condensed at 12 K. Infrared spectra revealed sharp strong bands at 680.0, 676.2, 661.6, and 642.4 cm-I, which were also observed by thermal dissociation of Ss flowing in a quartz tube heated to 400-900 O C . Increasing the sulfur concentration in the discharge favored the latter two relative to the former two absorptions. Enriched 34Ssamples gave isotopic shifts for pure sulfur fundamentals and isotopic multiplets that identify these species. Sample annealing produced a strong S2chemiluminescence and a sharp new 674.5-cm-l band. This feature revealed a sharp 1 / 2 / l / l / 2 / l sextet with 50/50 32S/34S, which confirms the observation of C, thiozone; similar multiplets were found for the 680.0- and 676.2-cm-' bands. Calculations from four pairs of symmetrical isotopic u j values gave I I6 f 2' for the valence angle. The 66 1.6- and 642.4-cm-I absorptions behaved differently in the experiments and revealed isotopic multiplets appropriate for S;, these absorptions are assigned to two different open-chain S4 isomers. Weaker bands at 720.5 and 710.1 cm-I, which appeared on annealing, are assigned to ( S J 2 dimer species.
Introduction Sulfur chemistry is important because of its relevance to acid rain and pollution problems and the increasing use of sulfur in organic and inorganic synthesis. For three decades, the photolysis of small molecules such as hydrogen sulfide, H2S, and carbonyl sulfide, OCS, has served as a convenient technique for the generation of sulfur atoms, which then participate in reactions with a variety of organic and inorganic substrates.'g2 The stable and transient products of these reactions have been trapped and studied in frozen inert gas Because of their inherent limitations, electric discharges have been used much less commonly as a source of sulfur atoms.9 Nevertheless, diffusion flames employing discharged sulfur as one reagent have been employed in a series of phosphorus-sulfur University of Wroclaw, Wroclaw, Poland.
0022-3654/9 1/2095-0079$02.50/0
experiments in order to isolate reaction products in solid argon for study by FTIR spectroscopy.I0 Since pure sulfur molecular species are formed in the discharge and/or by reactions in the matrix, IR spectra of the products of discharged sulfur were explored in some detail so that bands due to pure sulfur species ( 1 ) Meyer, B. Chem. Rev. 1976, 76, 361. (2) Gunning, H. E. In Elemental Sulfur; Meyer, B., Ed.; Interscience: New York, 1965, p 265. (3) Long, S . R.; Pimentel, G. C. J . Chem. Phys. 1977, 66, 2219. (4) Smardzewski, R. R. J . Chem. Phys. 1978, 68, 2878. (5) Wight, C. A,; Andrews, L. J . Mol. Spectrosc. 1978, 72, 342. (6) Lee, Y.-P.; Pimentel, G . C. J . Chem. Phys. 1979, 70, 692. (7) Hawkins, M.; Downs, A. J. J . Phys. Chem. 1984, 88, 3042. (8) Hawkins, M.: Downs. A. J. J . Phys. Chem. 1985, 89, 3326. (9) Meyer, B. J . Chem. Phys. 1962, 37, 1577. (10) Mielke, Z . ; Brabson, G.D.; Andrews, L. J . Phys. Chem., preceding paper in this issue.
0 199 1 American Chemical Society
80
The Journal of Physical Chemistry, Vol. 95, No. I , 1991
could be separated from those of other discharge products. The composition of saturated vapor in equilibrium with elemental sulfur (solid or liquid) has been investigated by several workers' and modeled by Rau et aI.l1 More recently, Lenain et al. have explored the composition of saturated and superheated sulfur vapors from 300 to 900 O C using Raman spectroscopy.12 The vapor contains all species of S, with 2 5 n I8, plus small amounts of molecules with n > 8. While the vapor undoubtedly consists of both ring and open-chain molecules, it is generally assumed that the species with n 2 5 are rings. Near the melting point (1 12.8 "C), sulfur vapor is dominated by Sa. At about 1000 K, S2becomes a major species, and as the temperature is increased above this point, the importance of the large sulfur species ( n = 6-8) decreases while the importance of S3and S4 increases. It has been suggested that the latter species contribute to the color of molten s ~ I f u r . l ~ * ' ~ The vibrational spectrum of S4 has been studied both in the gas phase and in a variety of m a t r i c e ~ . ~ J ~ The * ~ ~last - ' ~work of the Meyer group reported bands between 636 and 688 cm-I, and based largely on comparison of the behavior of the infrared bands and the 530-nm S4 absorption, the infrared bands were assigned to S4.I7 The products of a discharged SO2 stream gave Raman bands at 688, 601, and 440 cm-' which were assigned to S4.'* More recently, the Raman spectrum of ultramarine red revealed S4 bands in this region.19 The vibrational spectroscopy of S3is better understood. Meyer et al. observed fine structure in the gas-phase absorption spectrum and assigned a value of 590 cm-I to the symmetric stretching frequency.13 Raman bands have been reported at 651 and 585 cm-' for S3in a frozen SO2 matrixla or at 662 and 583 cm-I in solid argon.20 A more recent gas-phase Raman study observed the three fundamentals at 656, 575, and 256 cm-1.21 Although the latter study employed relatively pure sulfur-34, none of the previous S3investigations have identified thiozone by mixed isotopic statistics. Although S3 is thought to be a bent molecule with C2, symmetry,21s22the geometry of S4 is not known. A number of electronic structure calculations have been reported for S4. The most systematic study employed the STO-3G basis set and concluded that a gauch structure was more stable than planar forms and that triplet were lower than singlet states.23 The most recent calculation suggests that S4 consists of two weakly bound Sz moieties in a rectangular ring, but an open-chain configuration was calculated to lie only slightly higher in energy.24 The above evidence suggests that two, or possibly more, different S4isomers might be observable. It should be noted that the terminal S-S bonds in an open-chain isomer have higher vibrational frequencies than an internal S-S bond.'2,25 In this context, sulfur rings have fundamental frequencies below about 500 cm-I, while the stretching frequencies for a terminal S-S subgroup should be above 600 cm-]. ( I I ) Rau, H.; Kutty, T. R.; Guedes de Carvalho, J. R. F. J . Chem. Thermodyn. 1973. 5, 833. ( 1 2) Lenain, P.; Picquenard, E.; Corset, J.; Jensen, Bunsen-Ges. Phys. Chem. 1988, 92, 859.
D.:Steudel, R. Ber.
( 1 3 ) Meyer, B.; Stroyer-Hansen, T.; Oommen, T. V . J Mol. Spectrosc.
-. .
1972. 42. -. - -135 --
(14) Meyer, B.; Stroyer-Hansen, T.; Jensen, D.; Oommen, T. V . J . Am. Chem. SOC.1971, 93, 1034. (15) Meyer, B.; Schumacher, E. Helu. Chim. Acta 1960,43, 1333; Nature 1960, 186, 801. (16) Brewer, L.; Brabson, G.D.; Meyer, B. J . Chem. Phys. 1965,42,1385. ( I 7) Meyer. B.; Stroyer-Hansen, T. J . Phys. Chem. 1972, 76, 3968. (18) Hopkins, A . G.; Tang, S.-Y.;Brown, C. W. J . Am. Chem. SOC.1973, 95, 3486. (19) Clark, R. J. H.; Cobbold, D. G. fnorg. Chem. 1978, 17, 3169. (20) Tang, S.-Y.;Brown, C. W. fnorg. Chem. 1975, 14, 2856. (21) Lenain, P.; Picquenard. E.; Lesne, J. L.; Corset, J. J . Mol. Struct. 1986, 142, 355. (22) Rice, J. E.; Amos, R. D.; Handy, N . C.; Lee, T. J.; Schaefer, H. F. J. Chem. Phys. 1986. 85, 963. (23) Kao, J. Inorg. Chem. 1977, 16. 2085. (24) Hohl, D.; Jones, R. 0.; Car, R.; Parrinello, M. J . Chem. Phys. 1988, 89, 6823. (25) Schmidt, M. In ElementalSulfur; Meyer, B., Ed.; Interscience: New York, 1965; p 301.
Brabson et al.
n I
........ . r e
%,I.....< I
'
A
I "
1
RESERVOIR HEATER
-.
SUPERHEATER
. . .. .
\\
TARGET
RADIATION SHIELD
c_.
Figure 1. Apparatus used for dissociation of sulfur vapor in matrix isolation experiments: (a) coaxial quartz tube with finger for evaporating SBinto microwave-powered argon discharge; (b) quartz double heater for evaporation and thermal dissociation of S8.
Here follows a matrix infrared investigation of S3and S;, mixed isotopic sulfur spectra provide identification and structural information on the S3and S4 molecular species. Experimental Section Two different sets of experiments were performed; in the first set, sulfur atoms and small molecules were generated by a microwave discharge in argon seeded with sulfur vapor, while in the second set, small sulfur molecules were generated by thermal dissociation of sulfur vapor in a double oven. The vacuum cryogenic and gas supply Systems were typical of those described earlier .26,27 Microwave Discharge. The coaxial quartz discharge tube illustrated by Figure l a is similar to the tube used in phosphorus experiments.10s28 Natural isotopic sulfur (Electronic Space Products, Inc., recrystallized) and enriched sulfur (85% S-34, Oak Ridge National Laboratory) were used as received; a 50/50 mixture of the two samples was also studied. The vapor pressure of sulfur located in the side arm was controlled by the resistively heated windings around the side arm and measured by a thermocouple in mechanical contact with the side arm; a secondary winding prevented condensation of the sulfur in the zone not filled by the discharge. The microwave discharge was sustained in the argon-sulfur mixture by a Burdick MW200 diathermy (operated at 50-80% of the maximum power level) with an Evenson-Broida cavity and extended from a region about 5 cm downstream of the sulfur reservoir to the end of the discharge tube. The argon flow rate was between 2 and 3 mmol/h, and the estimated molar Ar/S8 ratio was from 200 to 2000, depending on the experiment. The presence of significant quantities of S2 in the discharge was indicated by the sky-blue in experiments with the highest concentration of sulfur, the vapor pressure of sulfur (as indicated by the temperature of the side arm) was 1 order of magnitude higher than that just necessary to give a bluish coloration to the (26) Withnall, R.; Andrews, L. J . Phys. Chem. 1988, 92,4610. (27) Andrews, L.; Withnall, R. J . Am. Chem. SOC.1988, 110, 5605. (28) (a) Mielke, Z.; McCluskey, M.; Andrews, L. Chem. Phys. Lett 1990, 165, 146. (b) Andrews, L.; Mielke, Z. J . Phys. Chem. 1990, 94, 2348.
The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 81
Isotopically Enriched S3 and S4 in Solid Ar TABLE I: Infrared Absorptions (in cm-') Produced by Condensing a Microwave-Discharged Argon/Sulfur Vapor Stream at 12 K
S-32 725.7 720.5
s-34 704. I 699.2
718.8 710.1 708.4 683.2 680.0 676.2
699.5 689.3 687.6 663.1
674.5
654.6
661.6
641.9 623.4
659.9 656.1
642.4 624.6
594.2 584.7 58 I .5 473.7
annealing/420/220
ident
+/Of0
+IO/-IO/-101+IO/-/-/+I-/+/ lo/-/-IO/-IO/+/-/-
Results Infrared spectra of small sulfur molecular species were recorded using discharge and thermal dissociation of sulfur vapor. Discharge. Solid sulfur in the finger near the entrance of the discharge tube (Figure l a ) was heated to 82 f 2 O C to entrain sulfur vapor into the argon discharge stream. This gas mixture was condensed at 12 f I K, and weak SOz, SiS, and S 2 0 bands at 1355, 1351, 739, and 672.8 cm-' and the stronger S 2 0 band at 1 157 cm-I were produced by reaction between sulfur and the quartz tube.1°*20*29*M A weak blue S2 luminescence was observed in the d i s ~ h a r g e . ~Figure . ~ 2a shows the 750-570-cm-' region of the infrared spectrum; sulfur species absorptions were observed at 683.2, 680.0, 676.2, 674.5 (shoulder), 661.6, 642.4, and 594.2 cm-I in the deposited sample. Annealing the sample to 33 f 2 K increased absorbances of the sharp 674.5-cm-' shoulder, the sharp 642.4-cm-l band, and weak bands at 725.5, 720.5, 683.0, 610.1, and 624.6 cm-l. In contrast, the 680.0- and 676.2-cm-' band absorbances were halved, the 661.6-cm-l band absorbance decreased 20%, and the weak 594.2-cm-' band was essentially unchanged, as shown in Figure 2a'. A weak new band also appeared at 473.7 cm-l. The new bands are listed in Table I. ~~
( 2 9 ) Atkins, R. M.; Timms,
m l
+/o/o +/o/o +/o/o +IOIO
normally pink argon discharge. The products of the discharge were collected on a CsI target at 12 f 1 K, and the spectra were recorded with a Nicolet 7199 FTIR from 4000 to 425 cm-' at a resolution of 0.5 cm-l and a wavenumber accuracy of f O . l cm-I. Spectra were recorded after each deposition phase, after photolysis with a high-pressure mercury arc (filtered by water and glass filters), and after annealing sequences. Thermal Dissociation. The 6-mm-diameter quartz double oven used in thermal dissociation experiments is illustrated by Figure 1 b. This effusion cell was located entirely within the vacuum chamber and was shielded from the target by two stainless steel radiation shields. The resistively heated windings at the rear of the cell permitted control of the sulfur vapor pressure, while the front windings enabled superheating of the vapor and thermal fragmentation of the SBmolecules. The temperature of the sulfur reservoir was measured by a thermocouple in mechanical contact with the bottom of the sulfur reservoir, and the temperature of the superheated zone was estimated from Leeds and Northrup optical pyrometer measurements of the temperature of the heating wires near the 1-mm-diameter orifice of the cell. A separate calibration was done for lower temperatures with a thermocouple. Argon was introduced through a separate spray-on nozzle at a flow rate of about 4 mmol/h. Thermolysis products were collected on a Csl target at 12 f 1 K, and the spectra were recorded with a Perkin-Elmer 983 spectrophotometer from 750 to 180 cm-' at a resolution of about 1.5 cm-' and a wavenumber accuracy of f0.3 cm-I. Spectra were recorded after each deposition period and after annealing sequences.
~
I
P. L. Spectrochim. Acta 1977, 33A, 853. (30) Schnockel, H.; Koppe, R. J . Am. Chem. Soc. 1989, I l l , 4583.
SO
$20
660 630 WFiVENUVBERS
690
600
g70
Figure 2. Infrared spectra in the 750-570-cm-' region for sulfur species produced by evaporating S8into argon discharge stream and condensing the gas mixture at 12 f 1 K: (a) sulfur t = 82 f 2 O C , (a') spectrum after annealing to 33 f 2 K over IO min, (b) sulfur I = 95 f 2 "C, and (c) sulfur t = 107 f 2 OC.
During the annealing operation, a very bright blue chemiluminescence was observed from the sample, similar to the discharge emission. Sharp weak new absorptions at 1599.8 and 3723.4 cm-' in the water region increased by 50% on annealing as did the sharp 725.7-cm-l absorption. Similar experiments were done with 95 f 2 and 107 f 2 OC sulfur finger temperatures, which represent factors of 3 increase in vapor pressure," and the spectra are contrasted in Figure 2b,c. With higher input sulfur vapor pressure, the blue emission was more intense. Increasing sulfur concentration markedly increased the intensities of the 661.6- and 642.4-cm-' bands relative to intensities of the 680.0- and 676.2-cm-I bands; in Figure 2c the latter are stronger than the former, and the sharp 642.4-cm-I band exhibitied a sharp 633.5-cm-l mixed isotopic counterpart. The 683.0-cm-I band intensity showed a marked sulfur concentration dependence, and weak 594.2-, 584.7-, 5 8 1 5 , and 473.7-cm-' bands were also observed. In these experiments, annealing had the same effect as described above, and a very strong blue chemiluminescence was observed from the sample. Photolysis with 420-nm radiation decreased the 66 1.6- and 642.4-cm-I band intensities slightly with the latter being more sensitive, decreased the 594.2-cm-l band by 30%, destroyed the 473.7-cm-' band, and had no effect on the 680.0- and 676.2-cm-' and 584.7- and 581 S-cm-I bands. Full arc (220 nm) photolysis decreased the intensities of all bands, but the 642.4- and 594.2-cm-l bands decreased more (50%). Two experiments were done with high sulfur concentration using a discharge tube constricted to a 1-mm orifice, which markedly reduced irradiation of the condensing sample. The product distribution was similar to that shown in Figure 2b, except that the 594-cm-l band was reduced 5-fold in intensity. Annealing produced more dramatic changes than shown in Figure 2a. An experiment was performed with 85% S-34-enriched sulfur in the finger at 90-95 O C , and the spectral region of interest is shown in Figure 3a. The Si3% and Si34Sbands at 739.1 and 728.9 cm-' confirm the isotopic enrichment of the sulfur sample. The strong absorptions shifted approximately 20 cm-l to 659.9,656.1, 641.9, and 623.4 cm-I, as listed in Table I. Annealing to 34 f 2 K markedly decreased the strongest 659.9- and 656.1-cm-' bands and significantly increased intensities of a sharp 654.6-cm-I band
Brabson et al.
The Journal of Physical Chemistry, Vol. 95, No. 1, 1991
82 N 3.
l1
O
1
1
Ii
a. C
r 3
.7:@
735
7+0
7b5
640 6?5 WAVENUMBER
660
645
633
6i5
Figure 3. lnfrared spectra in the 750-61 5-cm-' region for isotopic sulfur species produced by argon discharge dissociation: (a) sulfur t = 105 f 5 OC.85% 34S-enrichedsulfur, and (b) after annealing sample to 35 2 K over IO min. Bars connect mixed isotopic absorptions of same species.
*
TABLE II: Mixed Sulfur Isotopic Multiplets Observed in Discharge Experiments"
strone
strone
680.0 676.2 672.3 668.2 663.4 659.9
676.2 672.3 668.2 664.3 660.5 656.1
674.5 670.9 666.5 662.9 569.1 654.6
sham
683.2 673.2 663.1
661.6 655.5 651.3 649.9 645.5 641.9
642.4b 633.0 (633.5) 623.4 (624.1)
720.5 710.3 699.2
710.1 704.6 698.3 697.0 694.1 689.4
~
725.4 7 14.9 704.1
*
'All values are in cm-I. bBand overlap in mixed isotopic experiment.
and isotopic counterparts marked overhead, the sharp 641.9-cm-l band with isotopic satellite at 645.5 cm-I, and the sharp 623.4-cm-I band and component at 633.0 cm-I. Sample warming also revealed a 663.1-cm-' band and produced weak new bands in the 720690-cm-' region, as shown in Figure 3b. Again the blue luminescence was pronounced. Mixed isotopic species, also observed in the 720-690-cm-' region, are listed in Table 11. A similar investigation with P4added to the discharge gave the same isotopic absorptions for the major sulfur species.I0 One experiment was performed with a 50% S-34, 50% S-32 sample, and the important spectra are shown in Figure 4. Overlapping of four major bands with extensive isotopic dilution makes correlation of the bands in Figure 4a difficult. However, annealing to 34 f 2 K increased the sharp band at 674.5 cm-l and its isotopic counterparts and decreased the 680.0- and 676.2-cm-' bands and their isotopic counterparts; this made possible the clear recognition of a sharp 1/2/ 1/ 1/2/ 1 isotopic sextet that is marked in Figure 4b and listed in Table 11. Following this example, similar isotopic sextets can be located for the strong bands in Figure 4a in spite of band overlaps; the 3.8-4.8-cm-l isotopic interval for each component in the sextet aids in peak location. A strong isotopic 1/2/1 triplet is also recognized at 642.4, 633.0, and 624. I cm-I. The strong 661.6-cm-' absorption also increased on annealing, which facilitated recognition of a different isotopic sextet pattern whose components are marked in Figure
E '
,640
680
6jO
660
660
640
630
650
WAVENUMBER
Figure 4. Infrared spectra in the 69C-620-cm-l region for isotopic sulfur species produced by argon discharge dissociation: (a) sulfur t = 105 f 5 OC,50% 34S-enrichedsulfur, and (b) after annealing sample to 35 f 2 K for 10 min. Bars connect mixed isotopic absorptions of same species.
4b. The 730-690-cm-' region also revealed a large number of weak bands on annealing; these are grouped in multiplets and listed in Table 11. An Ar/H2S = 50/ 1 sample was passed through the center tube under similar discharge conditions and the effluent trapped at 12 K. The infrared spectrum was similar to that in Figure 2b in terms of band yield and relative intensities except that the weak 725.7-cm-l band was not observed. In several experiments the argon/sulfur discharge stream was codeposited with Ar/PF3 mixtures to serve as a trap for sulfur atoms and to test for S atom yield. Strong SPF, absorptions observed in this sample were comparable to those produced by photolysis of Ar/PF3/OCS mixtures.8 Finally, several S8 discharge ex eriments were done for visible-ultraviolet spectroscopic study?8 A strong vibronic absorption with structure from 250 to 300 nm, a weak absorption centered at 400 nm with partially resolved structure, and a weak, broad, 530-nm absorption were observed. Similar absorptions have been assigned by earlier workers to S2, S3, and S4, r e ~ p e c t i v e l y . ' ~ ~ ' ~ Thermal. Sulfur was evaporated from the quartz double oven (Figure Ib) in several experiments to determine the optimum reservoir temperature for evaporation of Sx and superheater temperature for dissociation of Sxin order to complement the discharge observations. With the reservoir temperture at 105 f 5 "C and the superheater at 120 f 5 "C, Sxwas condensed on the 12 f 1 K CsI window along with an argon stream. The spectrum shown in Figure 5a revealed only sharp bands at 475.8 ( A = absorbance = 0.08), 243.3 ( A = 0.12), and 193.5 cm-l ( A = 0.21) which are in good agreement with gas-phase and solution data for S8,l2and the Ar/Sx ratio was on the order of 200/1. The reservoir temperature was maintained at 105 f 5 "C, the superheater temperature was increased through a range of temperatures, and S8 evaporated from the reservoir was passed through the superheater for I-h periods on the way to the cold matrix. With the superheater tube at approximately 400 f 50 "C, some decomposition was observed as evidenced by sharp weak bands at 680.0, 676.2, and 661.6 cm-' and the continued growth of S8 (Figure 5b). Increasing the superheater wire to 900 f 50 "C markedly increased the growth rate of the 680.0-, 676.2-, and 661.6-cm-' bands and produced a strong 642.4-cm-' band, a 683-cm-' shoulder, and a weak 725-cm-' band (Figure 5c) while S8 absorptions did not increase. This trend continued for superheater wire temperatures up to 1000 f 50 "C. In another experiment using a 100 +z 5 "C reservoir temperature, the S8 sample was passed through the superheater at intervals over the 500-900 f 50 "C range. The 680- and 662-cm-' bands appeared first, as in Figure 5b, and with increasing superheater temperature,
t
Isotopically Enriched S3 and S4 in Solid Ar
'
2 . 0 . t '
1
1
'
'
'
The Journal of Physical Chemistry, Vol. 95, No. I . 1991 83 '
'
'
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WAVENUMBERS Figure 5. Infrared spectra in the 750-180-cm-' region for sulfur species produced by thermal dissociation of SBvapor codeposited with argon at 12 K: (a) experiment with reservoir at 105 5 OC and superheater tube at 120 f 5 O C for 2-h deposition, (b) superheater tube at 400 f 50 O C for 2.5 h more deposition, (c) superheater tube wire at 900 f 50 OC for 1.5 h more deposition, and (d) different experiment with reservoir at 100 f 5 OC and superhcnter wire increased through 500-950 f 50 OC range over 5-h period.
these bands increased, a weak 642-cm-' band appeared, and the growth rate of the 642-cm-l band increased until the 680-, 662-, and 642-cm-' band intensities were comparable. The spectrum in Figure 5d followed 5 h of thermal dissociation and collection in solid argon; the 683- and 725-cm-' bands appeared only with higher superheater temperatures. These samples were annealed to 25, 30, 35, and 40 f 2 K; the product bands were broadened and decreased slightly in absorbance. In all cases, no chemiluminescence appeared from these samples.
Discussion The new observations will be considered with regard to the identification and structures of S3and S4 and reactions of S atoms and S2 in the matrix. Identification. The strong infrared bands observed here in solid argon at 680.0,661.6, and 642.4 cm-I using discharge and thermal dissociation of S8have been observed previously in a variety of matrices and tentatively assigned to the S4 molecule.17 These infrared absorptions were produced by condensing sulfur vapor superheated to 1000 K where it was assumed that S2was the only species present in the vapor. However, ultraviolet-visible spectra show that S, and S4 are present in addition to S2in the thermal experiments. The work with PF3 shows that S atoms are also trapped in the matrix, and the S2 chemiluminescence observed on annealing demonstrates that S atom reactions take place during sample warming in the samples produced by discharge. Clearly, equilibrium dissociation of s8 was not attained in the earlier thermal experiments, since we will now show that the strong 6 8 0 . 0 - ~ m -band ~ and its 676.2-cm-l satellite are due to S , . Two important pieces of new evidence shed light on this matter. First, increasing sulfur concentration in the discharge over an order of magnitude range (Figure 2) demonstrates that the molecules responsible for the 661.6- and 642.4-cm-I absorptions contain more sulfur than the 680.0- and 676.2-cm-' absorbers. If the former are due to S4, then the latter must be due to S3,since the S2
fundamental is at 716 cm-l in solid argon." Second, and most important, are the mixed sulfur isotopic data (Figure 4). The growth of a sharp, strong sextet on annealing with 1/2/1/1/2/1 relative intensites, identical with that for mixed isotopic ozone, confirms that the new 674.5-cm-l band is due to a species that contains two equivalent sulfur atoms and a third inequivalent sulfur atom. This molecule is thiozone, isostructural with ozone. The same sextet pattern can also be recognized for the strong nearby 680.0- and 676.2-cm-l bands, although band overlap complicates the issue; these bands are therefore due to S3molecules in different matrix trapping sites. The S4 molecule can have planar structures of C,, or C,, symmetry and a nonplanar gauch structure of C2symmetry. The 66 1.6- and 642.4-cm-' bands behave differently on annealing and photolysis and exhibit different growth rates on thermolysis of s 8 as a function of temperature; they are assigned here to two different open-chain structural isomers of S4. The isotopic sextet observed for the 661.6-cm-l band is similar to that reported for 04-, which contains two equivalent O2subunits weakly bound t ~ g e t h e r . ~ ~Although ,~, the sextet in Figure 4b is not completely free of overlapping bands, this sextet contains three intermediate bands stronger than the 661.6-cm-I band, which is free of overlap with other species. This multiplet is sufficient to characterize the 661.6-cm-I absorption as due to a chain S4 species of cis or trans planar structure. On the other hand, the sharp isotopic triplet for the 642.4-cm-l band indicates a vibration of two equivalent sulfur atoms, but the central component for the 32S-34Svibration depends on whether most of the remaining sulfur is 32S(633.5 cm-l) or 34S(633.0 cm-I). This indicates a small coupling to another sulfur unit, which is most likely S2. (31) Barletta, R. E.; Claassen, H. H.; McBeth, R. L. J. Chem. Phys. 1971, 55, 5409. ( 3 2 ) Andrews, L. J . Chem. Phys. 1971, 54, 4935. (33) Smardzewski, R. R.; Andrews, L. J . Phys. Chem. 1973, 77, 801.
04 The Journal of Physical Chemistry, Vol, 95, No. 1 , 1991
Brabson et al.
100, 7841.
of FTIR allowed the weak 584.7-cm-] band ( A = 0.005) to be observed over an A = 0.001 noise level. The u3 fundamental 680.0-cm-' band ( A = 0.3) is substantially stronger than u I ; the Unforsame intensity relationship has been observed for 03.40 tunately, the u2 fundamental is too weak to be observed on the blue tail of the 243-cm-' S, band. The isotopic multiplet for S3 provides a basis for calculation of the thiozone valence angle from pairs of isotopic v3 fundamentals The unique assignment as done previously for SO, and 03.40q41 to mixed isotopic S3species is made possible by comparing peak intensities in Figure 4b where the symmetrical mixed isotopic absorptions at 666.5 and 662.9 cm-I are assigned to 34-32-34 and 32-34-32, respectively, on the basis of isotopic abundance which slightly favors the S-32 isotope. The isotopic pairs 32-32-32132-34-32 and 34-32-34134-34-34 involving central isotopic substitution predict lower limits of 112.5' and 115.2' for the S3valence angle whereas the pairs 32-32-32134-32-34 and 32-34-32134-34-34 involving terminal substitution predict upper limits of 119.5' and 116.6' for the valence angle. As discussed previously for SO, isotopic calculations, the upper limit-lower limit average represents an excellent determination of valence angle for isotopes with very little anharmonic difference.,' For SO2 isotopic calculations, the upper limit is 122.0', the lower limit is 116.3', the average is 119.2', and the best experimental value is 119.3O. The average value (1 15.9') for S3determined here is in excellent agreement with several ab initio structure calculations A similar vibrational isotopic calculation for S3(1 16', 1 17°).22324338 for O3has predicted the valence angle within 1 ' of the experimental value.40 Hence, the matrix isotopic spectra determine the S3valence angle as 116 f 2'. It is interesting to compare bond force constants for 02, 0, and S,, S3. Vibrational analysis of S, was performed using the FG matrix method. On going from 0, to 03, the 0-0stretching force constant decreases from 11.35 to 5.88 mdyn/A,q whereas a much smaller decrease is found for the S-S force constant from S2to S3,namely, 4.83-to 3.85 mdyn/A. The implication here is that x bonding is sufficiently weak in S2that the reduction in K bond order for S3makes much less of a difference to the overall bonding scheme. S4. In a Raman spectroscopic study of heated sulfur vapor, Lenain et al. report a 635-cm-l band with 6471-A excitation, which was assigned to sulfur chains, and 605- and 680-cm-I signals using 5145-A excitation, which were considered for S4species.I2 Raman spectra of films formed by condensing a pure SO2 gas stream subjected to discharge revealed bands at 688 and 601 cm-I that were assigned to S4 on the basis of their decrease with growth of s8 on sample warming.18 Hence, the present 661.6- and 642.4-cm-' absorptions originally assigned to S4 by Meyer fall in the region expected for S, molecules. Ab initio STO-3G calculations for S4isomers predict that the triplet gauche diradical is slightly more stable than the trans and cis d i r a d i ~ a l s .Although ~~ molecular dynamics calculations predict a rectangular species to be lower in energy than the trans form, the above fundamentals are too high for a ring species2, There is clearly a need for a systematic study of S4 structures with high-quality ab initio calculations. The isotopic sextet for the 661.6-cm-l band indicates strong coupling among four sulfur atoms in this species. The observed sextet contains three intermediate components that are stronger than the 661.6-cm-] band. In the 85% S-34 experiment, the 645.5-cm-' band is broader than the 641.9-cm-l band and half the intensity expected for the (32-34) - (34-34) isotope assuming equivalent positions in each subunit. However, for an S4 chain the ends of each S2 subunit are not equivalent and two bands separated by 0.5-0.9 cm-' could account for the observed band with unresolved components for the 32-34-34-34 and 34-32-34-34 isotopic species. As described above, the cis or trans planar S4 species could give rise to such a spectrum. With increased coupling between terminal Szsubuniits through the in-
(36) Laidlaw, W. G.; Trsic, M . Chem. Phys. 1979, 36, 323. (37) Feng, W. L.; Novaro. 0.;Garcia-Prieto, J . Chem. Phys. Lett. 1984. 1 1 1 , 297. (38) Morin, M.; Foti. A. E.: Salahub, D. R. Can. J . Chem. 1985,63, 1982. (39) Laidlaw, W. G.; Trisic, M . Can. J . Chem. 1985, 63, 2044.
(40) Andrews, L.; Spiker, Jr., R. C. J . Phys. Chem. 1972, 76, 3208. (41) Allavena, M.; Rysnik, R.; White, D.; Calder, V.; Mann, D. E. J . Chem. Phys. 1969, 50, 3399.
S 3 . Thiozone has been studied in the gas phase by visible absorption and resonance Raman s p e c t r o s ~ o p i e s . ' ~The * ~ ~resonance Raman spectrum was interpreted in terms of strong combinations of the symmetric fundamentals uI (575 cm-I) and v 2 (256 cm-') and weak bands involving the antisymmetric mode u3 (656 cm-I). Reactions of S 2 0 gave S3 for Raman observation of a strong 583-cm-' band for the u I mode in solid argon; a very weak 662-cm-I band was tentatively assigned to Y , . ~ O The present assignments of strong 680.0-, 676.2-, and 6 7 4 . 5 - ~ m -bands ~ to v3 of S3show that the weak 662-cm-l matrix Raman band is due to some other product. In view of the 8-cm-l blue matrix shift for v , of S3,a blue shift for u3 is predicted as well for this large molecule. Although a 20-cm-' blue shift is larger than expected, such a shift is not out of the question. The u3 Raman band is weak, however, and this assignment could be in error. The 656-cm-' band from the gas-phase Raman spectrum appeared at 639 cm-I with S-34, which is not in good agreement with the calculated 636.4-cm-I position. On the other hand, the present sharp 674.5-cm-I argon matrix band is predicted at 654.4 cm-I by using the harmonic approximation, as compared to the observed 654.6-cm-' absorption. The sharp sulfur isotopic sextet reported here confirms the present assignment of u3 for open S3 in solid argon. A large number of electronic structure calculations have been done for thiozone concerned largely with the ground-state structure since ozone has a low-lying cyclic isomer and S3 is expected to have open (C,) and closed (D3J forms.22.24*3"39Although many calculations have suggested that first one and then the other form is lower i n energy, the most recent work favors the open form.22 A high-quality MC SCF-CI calculation with large extended basis set predicts the open structure to lie 8.2 kcal/mol below the ring structure. Frequencies calculated for the C2, structure are 769 cm-' (b]), 668 cm-I (al), and 287 cm-' (al), and for the D3,, structure are 667 cm-I (a,') and 508 cm-I (e'). The ab initio frequency calculations support this assignment to open S,. If one uses the 583-cm-I Raman observation of v , for open S3in solid argon as a standard, the scale factor 0.87 must be multiplied by the calculated frequencies to obtain experimental values. This predicts the strongest absorption for open S, at 671 cm-' in solid argon, within 3-9 cm-I of the observed sharp matrix bands, and for closed S3at 442 cm-I, in a region of the spectrum free of absorption in both thermal and discharge experiments. We conclude from the matrix observation of open S3and the failure to detect closed S3that the open form is more stable. These matrix observations confirm the theoretical predictions that the C, open structure is the lowest energy form of S3. The S3observed here was prepared in two ways. The 680.0and 676.2-cm-' absorptions result when S, from thermal or discharge dissociation of SBis trapped in solid argon. The sharp 674.5-cm-I band is due to S3formed in the matrix reaction of S atoms and S2molecules in discharge experiments, which clearly produce sulfur atoms, based on chemical evidence and observation of the strong blue S2 chemiluminescence from the matrix on annealing. Absence of the 674.5-cm-' band in thermal decomposition experiments is noteworthy as is the lack of S2chemiluminescence arising from S atom combination. Clearly, open S3 is the more stable structure in the gas phase and the only structure formed from the matrix reaction of S and S2 during 32 f 2 K annealing; therefore, the C,, open structure is more stable for S,. The weak bands at 584.7 and 581.5 cm-I (arrows in Figure 2c) track with the strong 680.0- and 676.2-cm-' bands on photolysis and annealing. General agreement with the 583-cm-' Raman band supports assignment of the former weak bands to u I of the same two matrix sites of S, as the latter strong bands. The sensitivity (34) Carlsen. N. R.; Schaefer, H. F. Chem. Phys. Lett. 1972, 76, 3208. (35) Salahub, D. R.; Foti. A . E.; Smith, V. E. J . Am. Chem. SOC.1978,
Isotopically Enriched S3 and S4 in Solid Ar
The Journal of Physical Chemistry, Vol. 95, No. I , 1991 85
ternal S-S bond, the antisymmetric terminal stretching mode (661.6 cm-I) should be separated from the symmetric terminal stretching mode. Hence, the 680-cm-' gas-phase Raman band is compatible with this S4 structure and assignment. Observation of the Raman band with green excitation also associates this band with the 530-nm absorption assigned to S4. The isotopic multiplet for the 642.4-cm-' absorption shows weak coupling between the terminal S2 subunits. The basic triplet absorption exhibits slight shifts for changes in isotopic composition. For example, the central 32S-34Scomponent at 633.5 cm-I with natural sulfur (4.2% 34S)shifts to 633.0 cm-I with 85% 34Swhich is a consequence of different coupling between 32S34Sand 32S2 or 34S2.Likewise, the pure 34Scomponent at 623.4 cm-l shifts to 624.1 cm-I with 50/50 32S/34S.Hence, the basic triplet indicates a vibration of an S2subgroup, and the small differences in isotopic fundamentals denote weak coupling to another sulfur unit, which is most probably another S, molecule. Accordingly the matrix infrared band at 642 cm-' due to out-of-phase stretching of the terminal S2subgroups characterizes a different structure than the 661.6-cm-' band. Ab initio calculations in progress in this lab using the DZP basis suggest that the singlet gauch structure is a saddle point between cis and trans and predict that the triplet gauch structure antisymmetric fundamental is too low and too weak to be compatible with the 642.4-cm-' band. Hence gauch structures are ruled out, and we are left with the one of the planar forms not required for the 661.6-cm-I absorption as the best structure for the 642.4-cm-' absorbing isomer. Although the 635-cm-' Raman band was assigned to chains with length greater than four units,', it is not clear why S4 was excluded. The 635-cm-I Raman band could be due to the same S4 isomer as the 642-cm-l infrared band. If so, this isomer is associated with the red electronic absorption. It appears that both S4 species are produced in the gas phase by discharge and thermal dissociation of SBvapor and then are trapped in the argon matrix. The growth of the 642.4-cm-I band on annealing suggests that this species can also be made by addition of S atoms to S3in the discharge experiments. The S atom reaction with S3can also produce S2dimer as will be discussed below. Of the two S4 structural isomers, the 642.4-cm-l absorber is much more sensitive to annealing and photolysis than the 66 1.6-cm-l absorber. The minimal vibrational coupling between S2 subunits in the former may suggest more tenuous bonding between S, subunits. On photolysis it is likely that the former isomer dissociates to (S,), although this could not be confirmed. The greater stability of the 6 6 1 . 6 - ~ m -band ~ on annealing and photolysis and its appearance at lower superheater temperatures suggests that this absorption is due to the more stable S4 isomer. The two S4 isomers characterized here probably have cis and trans planar structures, but the present data cannot determine which absorption arises from which structure. Cyclic Species. The 683.7-cm-' infrared absorption exhibits a higher order sulfur dependence from its increase in relative yield at higher sulfur concentration and growth on annealing. This band increases an order of magnitude while S4 bands increase a factor of 4-5 and S3 bands are approximately constant (Figure 2b,c). The 683.2-cm-' band decreased substantially on full arc photolysis and was almost completely regenerated by further sample warming. The j4S counterpart at 663.1 cm-I denotes a sulfur fundamental, and one stronger mixed isotopic counterpart was identified at 673.2 cm-l owing to growth on annealing in this congested region, which suggests a vibration of two sulfur atoms not coupled to other sulfur. The most likely molecule with a unique S2subgroup expected to absorb in this region is a branched sulfur ring containing the terminal S=S subgroup, as has been discussed previously.I2 Such a molecule could photodissociate and re-form in the matrix cage on annealing. Accordingly, the above evidence suggests assignment of the 683.2-cm-' band to a branched ring, probably Ss=S, formed by association of S4 and S, in the gas phase and on sample warming. Ring stretching counterparts are expected to be much weaker and are not observed here. The weak 473.7-cm-l band in discharge experiments grows on annealing and is destroyed by photolysis. Since it comes just below
the 475.8-cm-l s8 band, a cyclic sulfur species is indicated. The 473.7-cm-I band could be due to s8 formed by dimerization of S4 or the cyclic S6species formed by association of S4 and S2,but there is insufficient evidence for a definite identification. S2 Clusters. No growth of absorptions was found on annealing the thermally prepared samples, but new bands appeared at 725.7, 720.5, and 710.1 cm-' in the S2 fundamental region (720 cm-I gas, 716 cm-I solid a r g ~ n )on~ annealing ~ . ~ ~ the discharged samples. The 725.7- and 720.5-~m-~ bands exhibit 1/2/1 triplets with mixed sulfur isotopes, but the 710.1-cm-' band became a higher multiplet (Table 11). These bands showed harmonic diatomic S-34 shifts in keeping with S2 fundamental vibrations. The 720.5- and 7 IO.1-cm-' bands are assigned to (S,), dimers of different structures made by S2association and/or S atom reaction with S3 on annealing. It is tempting to suggest structures analogous to the S4 molecules responsible for the 642.4- and 661.6-cm-l bands, since similar mixed sulfur isotopic multiplets were observed. The 720.5-cm-' band revealed only a triplet; hence, the (S,) subunits are loosely interacting, and this interaction results in a small blue shift from the 716-cm-' diatomic fundamental in solid argon3' The 710.1-~m-~ band exhibiited a mixed isotopic sextet similar to the 661.6-cm-' band; this red shift results from a slightly stronger interaction and is consistent with a C,, or C , structure for this (S2)2species. These van der Waals dimers of S2have an analogue in O,, in contrast to S4, which has no oxygen analogue. Since the 725.7-cm-' band was present in thermal experiments at high temperature and increased on annealing in discharge experiments, it may be due to an S2complex. Its appearance above the S2 fundamental suggests a possible hydrogen-bonding interaction since blue shifts in N2 and 0, fundamentals have been ~ , ~725.7-cm-' band observed for HF and H 2 0 c o m p l e ~ e s . ~The is tentatively assigned to the S2--H20complex. Bands in the water regions at 1599.8 and 3723.7 cm-I increased on annealing with the 725.7-cm-' band. The blue shift on the water bending mode, and red shift on the water stretching mode, are indicative of a hydrogen-bonding interaction. Since larger water displacements were found for the S2complex, the S2-- H 2 0 interaction is stronger than the N2--H,O interaction, an expected conclusion. S3-. The weak band at 594.2 cm-' in discharge experiments survived annealing, but it was reduced by 30% with 420-nm photolysis and more by 220-nm radiation. It is important to note that this band was not detected in thermal dissociation experiments, which of course can only produce neutral fragments, and that the relative yield of this band was markedly reduced when the discharge tube orifice was constricted to minimize sample irradiation. The proximity to v3 of S3- at 580-569 cm-' in solut i o n ~and ' ~ the photochemical behavior suggest tentative assignment of the 594.2-cm-' band to s,- isolated in solid argon. Here photoionization of sulfur species by vacuum-ultraviolet radiation from the argon discharge produced photoelectrons for capture by molecules present in the matrix. Both PO< and 03-have been formed and trapped in similar experiments with discharged P4 and 03.28 Finally, observation of the major product bands with thermal dissociation ensures that they are due to neutral species.
Conclusions Sulfur vapor was decomposed by microwave discharge and thermal methods and condensed with excess argon at 12 K. Strong infrared product bands at 680.0,676.2,661.6, and 642.4 cm-I have been assigned to S3 and S4 on the basis of isotopic shifts and isotopic multiplets. Sample annealing produced a strong S, chemiluminescence and a sharp new 674.5-cm-' absorption from S atom reactions in the matrix. Of most importance, the 674.5-cm-' feature revealed a sharp 1/2/1/1/2/ 1 sextet with 50/50 32S/34S, which characterizes a species with two equivalent and one inequivalent sulfur atoms and confirms identification of the open (C2&form of thiozone. Similar multiplets were found (42) Rosen, B., Ed. Spectroscopic Data Relative to Diatomic Molecules; Pergamon Press: New York, 1970. (43) Davis, S. R.; Andrews, L. J. Chem. Phys. 1985,83, 4983. (44) Hunt,R. D.;Andrews, L . J. Chem. Phys. 1987,86, 3781.
86
J. Phys. Chem. 1991, 95, 86-90
for the 680.0- and 676.2-cm-I absorptions, which arise from S3 prepared by gas-phase discharge and thermal dissociation of ss. The strong 661.6- and 642.4-cm-I absorptions behaved differently on annealing and photolysis, showed different growth rates on thermal dissociation of sg, and revealed isotopic multiplets appropriate for two different open-chain structural isomers of S4. The 661.6-cm-’ band exhibited an isotopic sextet in accord with a cis or trans planar S4 molecule with strong coupling among the sulfur atoms while the 642.4-cm-‘ band yielded a basic triplet for an S2vibration with small shifts due to weak coupling to the other
S2 submolecule with a different overall structure. The former species is probably more stable than the latter species, which required a higher temperature for production from the dissociation of Ss. These stable S4 species are distinctly different from (s2)* dimers, which absorb at 720.5 and 710.1 cm-I, close to the isolated S2 fundamental at 716 cm-I in the matrix Raman s p e c t r ~ m . ~ ’ Acknowledgment. We gratefully acknowledge financial support from NSF Grant CHE-88-20764 and the assistance of R. B. Bohn with a discharge experiment.
Infrared Studies of Matrix-Isolated and Neat Solid Nitrosyl Chloride L. H. Jones and B. I. Swanson* Los Alamos National Laboratory, Isotope and Nuclear Chemistry Group, INC-4, Los Alamos, New Mexico 87545 (Received: April 10, 1990; In Final Form: June 18. 1990)
The infrared spectra of nitrosyl chloride (ONCI) isolated in rare gas solids have been studied in an attempt to verify the existence of a dimer. The results indicate that with increased concentration and with high-temperature annealing one or more types of dimers or possibly trimers form in argon and krypton matrices. In a xenon matrix we see the presence of dimers at concentrations of 0.2%,and eventually multimers and aggregates form with high-temperature annealing. In liquid solutions of ONCl in krypton, xenon, and carbon disulfide no evidence of dimer formation is seen. In the solid state at 150-160 K we see absorption peaks at 1925 and 520 cm-l, which are attributable to the molecular solid showing Raman peaks at 1900 and 475 cm-I. For the ionic phase we observe an infrared NO+ stretching frequency of about 2090 cm-l, close to that observed in Raman spectra. The frequency of the NO stretch for the molecular solid is strongly influenced in an upward direction by the presence of intimately dispersed ionic solid, leading us to believe that the molecular solid consists of long chains with strong intermolecular coupling rather than discrete dimers.
Introduction Raman studies of liquid and solid nitrosyl chloride (ONCI) by Killough, Swanson, and Agnew’ show great changes in the vibrational spectra from that of the vapor phase. Their conclusion is that well-bonded molecular dimers make up the neat solid above 85 K but that the stable phase below 85 K is ionic, consisting of NO+ and CI-. The internal mode Raman frequencies at 74 K are 1909,458, and 233 cm-’ for the molecular solid and 2086 cm-I for the NO+ stretch of the ionic solid. Since it appears that the attractive forces between individual ONCl molecules are quite strong, it seemed that an infrared study of ONCl in rare gas matrices, before and after annealing, might yield significant information on the structure of dimers formed in the neat solid above 85 K. Before discussing the studies it seems appropriate to consider the structure of and bonding in the monomeric ONCl molecule. Bonding in Monomeric Nitrosyl Chloride The structure of the ONCl molecule is rather remarkable and still not adequately explained. It was determined by microwave spectra of the gas phase to be2 R(N-0) = 1 .I405 A; R(N-CI) = 1.9734 A, and LONCI = 1 13.24’, leading to R(O-CI) = 2.64 A. The N-CI bond length is extremely long compared to the expected single-bond length of about 1.7 A. The reason for this is not clear; however, we note that the 0-CI distance of 2.64 A is much less than the van der Waals distance of 3.2 A, suggesting significant interaction between CI and 0, probably leading to the long N-CI distance. The general quadratic valence force field has been determined3 for ONCl and essentially verified more r e ~ e n t l y . However, ~ as pointed out in the past,sv6 force constants
TABLE I: Interaction Coordinates for ONCl
(NO)Nci = -0.1 (NO), = 0.003 A/rad (NO)oci = 0.03 (NCI),, = 1.2 (NCI), = -0.1 A/rad (NCI),, = 0.7
((Y)NO = 0.034 rad/A ((Y)NcI = 0.09 rad/A (a)ocI= 0.5 rad/A
(OCI),o = -0.4 (OCI)NCI= 0.8 (OCI), = 0.7 A/rad
are not unique as they depend strongly on which internal coordinates are considered important, which is generally an arbitrary decision. On the other hand, compliance constants were shownsp6 to be unique and lead directly to interaction coordinated which are useful in describing the bonding and correlation with molecular orbital theory. In Table I we give the interaction coordinates2 for ONCI in the gas phase. As shown earlier: there are redundancies among the coordinates so that in this case only six of the compliance constants are independent; however, they are all unique and are well-defined. The interaction coordinates are calculated form the relation6 ( j ) k = Cjk/Ckkand are also well-defined. The meaning of (NO)N,-l = -0.1, for example, is that a small unit stretch” of the N-CI bond leads to a contraction of the N - 0 bond by 0.1 unit for minimum energy: the OCI distance and the angle a also change by +0.8A/A and -0.09 rad/& respectively. This is expected as it should increase the triple-bond character of the NO group, approaching NO’. Stretching N-CI also leads to a significant increase in the 0-CI distance, presumably because any attraction between the 0 and C1 atoms is not strong. The resulting decrease in the ONCl angle, a, suggests some attraction of 0 for C1 or significant bond directional forces coming into play. A recent
( I ) Killough, P. M.; Swanson, B. 1.; Agnew, S . F. J . Phys. Chem. 1989, 93, 7953.
( 2 ) Cazzoli, G.:Degli Esposti. C.; Palmieri, P.; Simeone, S. J . Mol. Spectrosc. 1983, 97, 165. ( 3 ) Jones, L. H.; Ryan, R. R.; Asprey, L. B. J . Chem. Phys. 1968,49, 581.
0022-3654/91/2095-0086$02.50/0
(4) McDonald, J. K.; Merrit, J. A.; Kalasinsky, V. F.; Heusel, H. F.; Durig, J. R. J . Mol. Spectrosc. 1986, 117, 69. (5) Decius, J. C. J . Chem. Phys. 1963, 38, 241. (6) Jones, L. H.; Ryan, R. R. J . Chem. Phys. 1970, 52, 2003.
0 1991 American Chemical Society
