J. Phys. Chem. 1992, 96, 6579-6585
6579
Vibronic Absorption Spectra of S3 and S, in Solid Argon Paniz Hassanzadeb and Lester Andrews* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 (Received: April 20, 1992)
Molecular S4 species show two distinct electronic absorptions, a broad green band centered at 518 nm and a structured red band between 560 and 660 nm in solid argon. The red-absorbingspecies is converted to the green-absorbing species on irradiation with red light while the green-absorbingspecies is converted to the red-absorbing species on green light photolysis. These bands are assigned to two different structural isomers of S4 which undergo photoisomerism. The red band shows two vibronic progressions with vibrational intervals of 320 and 590 cm-'. The electronic spectra are correlated with infrared spectra and ab initio calculations for different structural isomers of S4. Molecular S3shows relatively sharper bands between 350 and 440 nm,which are assigned to three argon matrix trapping sites based on annealing, vibrational spacings, and isotopic shifts. The origin of the most stable site is only 250 cm-' above the gas phase band origin. Vibronic analysis provides Y,' and vi values of approximately 450 and 340 cm-I, respectively, for S3.
Introduction
TABLE I: Major Absorption Peaks (nm)Observed for S, Species in
Recent matrix infrared studies of sulfur vapor from a superheater or microwave discharge provided evidence for two different structural isomers of S4.* Elegant ab initio calculations have determined that a singlet (IAJ cis planar structure is the global minimum for S4with singlet trans planar, puckered ring, branched ring, and rectangular ring structures up to about 12 kcal/mol higher.2 More recent ab initio calculations with CI are also in accord with this finding but point out that the rectangular isomer is very close in energy to the cis ~ t r u c t u r e . ~ ? ~ An electronic absorption band at 530 nm was first assigned by Meyer et alSsto S4, and recently a second absorption band at 620 nm has also been assigned to Se6 Meyer et al. also produced a structured red absorption on annealing krypton matrix samples but failed to identify the carrier of the red band ~ y s t e m .Recent ~ laser Raman studies of superheated sulfur vapor have reported and considered several signals for S4 species.' A very recent low-resolution gas phase absorption investigation of sulfur vapor has idcnMied the " I absorption of S3and the 530-nm S4band but the 600-nm region was not scanned? The S4molecule is of interest, in part, because there is no known oxygen analog. The ground state of thiozone, S3,is a bent molecule like 0 ~ 0 n e ~which * ~ Jexhibits ~ a structured absorption between 350 and 440 nm., However, due to hot bands the gas-phase spectrum is difficult to interpret, and previous matrix spectra have given a series of broad bands for S3.5The present matrix absorption study shows that there are three matrix trapping sites for S3in solid argon just as found in the infrared spectrum.'
Solid Arson'
RWult.9
ExperimentPl Section The vacuum system and chamber for matrix-isolation studies have been described previously.'J1 A closed-cycle refrigerator (CTI-Cryogenics, Model 22) and an indicator/controller were used to cool and monitor the temperature of the 12 & 2 K sapphire window. Absorption spectra in the 800-200-nm region were recorded on a Cary 2415 UV-vis absorption spectrometer which was interfaced to a personal computer for data collection and processing. A slit width of 0.1 nm, scan speeds of 0.05 and 0.1 nm/s, and mechanical resolutions of 0.02 and 0.05 were used for high resolution and fast scans, respectively. Natural sulfur (Electronic Space Products, Inc., recrystallized) and enriched sulfur-34 material with 98% 34S(EGBrG Mound Applied Technologies) were used as received. The quartz tubes for producing sulfur species in an argon stream by microwave discharge or superheating sulfur vapor have been described previ~usly.~ The gas streams were condensed at 12 f 2 K,and spectra were recorded before and after annealing and photolysis with a 175-Wmedium pressure (Philip)or a 1OOO-W high-pressure (T. J. Sales) mercury arc using water, band pass, and long-wavelength pass filters for 30-min periods.
Sulfur clusters were produced by microwave discharge and superheater sources and spectra were recorded before and after photolysis and annealing. Microwave Discbarge. A series of experiments was carried out with the sulfur reservoir temperature at 25, 50,70, and 100 OC and spectra were collected from 800 to 200 nm. With the sulfur reservoir temperature at 25 OC (intense pink discharge), only very intense absorptions below 250 nm were observed. At 50 OC (pink discharge), a series of peaks between 320 and 220 nm which belong to S2were also observed.'* At 70 OC (purple discharge) another series of absorptions between 430 and 350 nm, which belong to S,,were also produced. Two additional absorptions, a broad band centered at 518 nm and a series of structured absorption bands between 560 and 660 nm, were produced together as the sulfur reservoir temperature was increased to 100 OC (blue discharge). No other new band was observed with the sulfur reservoir temperature above 100 OC (intense blue discharge). The photochemistry and annealing behavior of the bands between 350 and 660 nm were fully studied as follows. The S3bands did not show any change upon photolysis with 590-, 510-, 490-, 290-, and 254-nm light. However, annealing at 25 f 2 K caused some of the absorption bands labeled A to sharpen and increase
0022-3654/92/2096-6579$03.00/0
3 2 s
A A A
A A
B B A A B, C A B A A A C
43 1.70 430.77 425.23 424.50 423.33 418.73 414.39 413.61 410.11 409.30 408.41 407.38 404.50 403.61 402.83 400.77 399.05 398.1 1 397.89 397.23 396.50 395.39 394.45
34s
430.61 425.23 424.56 419.04 414.92 414.06 410.48 409.96 408.98 408.1 1 405.05 404.53 403.67 401.10 400.1 1 398.86 398.42 398.17 397.49 396.42 395.30
B A
C B A C
B C
B C B C
B C
3 2 s
34s
391.83 389.51 388.17 387.61 384.85 383.05 382.06 381.33 318.61 378.11 316.14 315.95 312.55 372.23 369.95 369.17 366.64 363.83 361.39 360.89 358.30 356.05
392.61 390.73 389.17 388.86 386.04 383.23 382.80 319.92 379.58 377.42 377.16 314.05 313.55 371.36 311.11 368.11 364.42 363.23 359.92 357.80
'Accuracy is *0.05 nm ( k 3 cm-I). A, B, and C are band peaks.
0 1992 American Chemical Society
6580 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992
Hassanzadeh and Andrews
~~
I
340
I
I
360
_
I
I
I
380
I
400
460
I
420
440
Wavelength (nm) Figure 1. Absorption spectra in the 340-440-nm region recorded after deposition of discharged sulfur vapor/argon stream on a 12 f 2 K window: (a) spectrum after 3 h of deposition; (b) spectrum after 25 2 K annealing; (c) spectrum after 30 2 K annealing.
*
*
slightly while the other two series of bands labeled B and C decreased. On annealing at 30 f 2 K, the latter series of peaks decreased faster, although to different extent, than the former. The spectra before and after annealing at 25 f 2 and 30 f 2 K are shown in Figure 1 and the absorption bands are collected in Table I. The absorption bands between 460 and 660 nm were photolyzed with the medium-pressure mercury arc lamp using long wavelength pass filters. On irradiation with 590 nm filtered light the bands between 560 and 660 nm (red bands) decreased slightly while the broad 518-nm band (green band) increased slightly. With 490 nm filtered light the opposite trend was observed, the green band decreased and the red band increased; further 590-nm photolysis reversed this change but to a smaller extent. Further photolysis with 290 nm filtered radiation and the full arc (254 nm) lamp caused the green band to decrease and the red band to increase very slightly. Finally annealing to 30 f 2 K decreased the green band markedly while the red band increased. In another experiment the matrix was photolyzed with 520 nm filtered light; the result was similar to the 490 nm filtered light photolysis, that is, the green band decreased and the red band increased. The spectra before and after photolysis with 590 and 490 nm filtered light and annealing at 30 f 2 K are shown in Figure 2. Similar experiments were carried out with the 98% enriched sulfur-34 isotopic material. The spectra for natural and heavy isotopic sulfur in the 340-440 nm and 570-650 nm regions are contrasted in Figures 3 and 4, respectively. Superheater. Several experiments were carried out with a double oven superheater' where the temperatures of the sulfur reservoir and the superheater were varied to obtain optimized conditions for producing sulfur clusters. The reservoir temperature was set to 100 O C (6-9 mTorr vapor pressure)13and the superheater region was heated to 700 O C . The spectrum showed the S2bands, the broad S4band centered at 530 nm, very weak bands
500
540
580
620
660
Wavelength (nm) Figure 2. Absorption spectra in the 460-660-nm region recorded after deposition of discharged sulfur vapor/argon stream on 12 & 2 K window: (a) spectrum after 3 h of deposition; (b) spectrum after 30 min photolysis with h > 590 nm; (c) spectrum after 30 min photolysis with h > 490 nm; (d) spectrum after 30 2 K annealing. The medium pressure mercury arc was used in this experiment.
due to S3,and a weak, broad band centered at 630 nm. All the bands gradually decreased on annealing at 25 and 30 K, and no new band appeared. As the temperature of the superheater region was increased, the relative intensity of the S3bands and the red band (630 nm) to the green band (530 nm) increased; a transient faint blue emission was also noticed on annealing in this case. With the superheater temperature around 800 OC,all bands were observed with relatively high and comparable intensities and the red band showed structure. However, the relative intensities of the S3bands and the red bands to the intensity of the green band were still lower than those observed in the microwave discharge experiments. The red and green bands were consecutively photolyzed with the high-pressure mercury arc lamp using band pass filters A (combination of Pyrex, red glass (590 nm long wavelength pass), and water filter) to irradiate only the red band and B (combination of Pyrex, yellow glass (470 nm long wavelength pass), and saturated NS04 solution, 4 0 % transmission at 620 nm) to irradiate only the green band. The red band was converted to the green band upon photolysis with filter A; the green band was converted to the red band upon irradiation with filter B. The interconversion of the red and green bands upon selective photolysis with band-pass filtered light is shown in Figure 5. Upon annealing at 40 f 2 K, the S3bands d d while both the red and the green bands increased, and the red band lost vibronic structure. Upon further annealing at 50 f 2 K the intensities of all the bands decreased (Figure 5). Finally, the spectrum in the blue visible region is contrasted (a) for microwave discharge and (b) for superheater sources in Figure 6. Infwd In another experiment the infrared spectrum of the sulfur clusters S3(680.0,676.2, and 674.5 cm-I) and S4 (661.6 and 642.4 cm-l) produced from the sulfur-doped argon discharge
Vibronic Absorption Spectra of
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6581
S3and S4 in Solid Argon
S67cm'
I
,
571 cm-'
-311 cm-'
312 cm-'
A
E
e s!
9
I
I
340
I
I
I
380
360
I
I
I
400
I
420
440
Wavelength (nm) Figure 3. Absorption spectra in the 340-440 nm region for sulfur clusters produced in microwave discharge experiments: (a) spectrum of natural isotopic sulfur; (b) spectrum of 98% enriched isotopic sulfur-34. TABLE II: Observed Absorption (am) for the Structured Red Absorption B.nd0 3 2 s
34s
32s
34s
642.5 629.6 619.0 607.0
642.0 629.4 619.3 607.6
597.3 587.1 575.5
598.3 587.4 577.6
'Accuracy +0.1 nm (A2 cm-I),
were studied. The matrix was photolyzed by the high-pressure mercury arc lamp using filters A and B consecutively. The 6 4 2 . h - ' band decreased upon irradiation with the filter A while the 661.6-cm-I band increased; opposite behavior was observed when the matrix was irradiated with filter B. The S3bands did not show any detectable change in these photopmcews. Infrared spectra showing the red-green photochemical rearrangement are illustrated in Figure 7. Note that the photochemical rearrangement can be reversed, traces a-e, without loss of intensity. Also note that annealing to 24 f 2 K caused a slight increase of both 66 1.6- and 6 4 2 . h - I infrared bands with a slight decrease in the 680.0- and 676.2-cm-' S3bands. More annealing to 38 f 2 K further increased the 661.6- and 642.4-cm-I bands with the latter increasing most, decreased the 680.0- and 676.2-cm-' S3 bands, increased the 674.5-cm-' S3site absorption, and produced the new 683.2-cm-I band, as discussed earlier.' Cdculntions. Very important structural information can be obtained from the mixed 32.3'S isotopic 4 multiplet in the infrared spectrum of the two S, species.' The 661.6-cm-' band exhibited a mixed isotopic sextet, and the 642.4-cm-' absorption gave rise to a mixed isotopic triplet.' Calculations of all mixed sulfur isotopic frequencies for the strongest fundamental were done using the HONDO 7.0 progn~n'~9'~ and the DZP basis set for the cis,trans, rectangular, and branched ring structures fixed at the CISD (DZP)geometries calculated by Quelch et al., Although the
, 560
I
I
I
580
I
ho
1
I
620
I
I
640
I
I
660
Wavelength (nm) Figure 4. Expanded scale absorption spectra in the 560-660-nm region for sulfur clusters produced in microwave discharge experiments: (a) spectrum of natural isotopic sulfur; (b) spectrum of 98%enriched isotopic sulfur.
absolute values of these frequencies are not as reliable as those produced by higher level calculations, the isotopic pattern and splittings are expected to be reliable. Calculations for the cis, trans and rectangular structures all predicted isotopic multiplets (Table 111) which would give a resolved sextet for the experimental bandwidth of 1.0 cm-I. The isotopic sextet for the observed 661.6-cm-' band is compatable with cis, trans or rectangular structures. Although one calculated rectangular (us) fundamental (782 cm-')2 is much too high to match the 661.6-cm-' observed band, another calculated value (694 ~ m - is9 within ~ an acceptable range. Both cis and trans calculated values (693 and 68 1 cm-I, respectively)2 are comparable with the observed band. In the mixed experiment, the 642.4-cm-l fundamental revealed a slightly broadened triplet structure with a 642.4-cm-' band, a strong central feature at 633.0 cm-' with full-width at half-maximum (fwhm) of 1.3 cm-I, and a counterpart at 623.4 cm-' with fwhm of 1.0 cm-'. The mixed isotopic multiplet calculated for the branched ring exhibits a central band split by 0.17 cm-l, which would not have been resolved in the infrared experiment with 0.5 cm-' resolution and a bandwidth of 1.0 cm-'. The branched ring structure is compatable with the observed broadened mixed isotopic triplet. F d y several attempts to distort tetrahedral S4 along a C, axis to give rigorously equivalent S2 submolecules with equivalent S atoms resulted in the puckered ring structure. The strongest calculated fundamentals for the puckered ring are below 500 cm-I, as expected for sulfur rings, and clearly not compatable with either of the observed S4 species. The 16 calculated mixed isotopic exo stretching fundamentals for the branched ring S4 isomer are listed in Table 111. Within the experimental bandwidth of 1.O cm-l, the calculated fundamentals form a 1:21 triplet, in agreement with the observed 32734S
6582 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992
Hassanzadeh and Andrews
\
\
8
e P
9
400
500
600
700
Wavelength (nm) Figure 5. Absorption spectra in the 350-700-nm region recorded after codeposition of superheated sulfur vapor (800 "C)with excess argon on a 12 f 2 K window: (a) spcctrum after 2 h deposition; (b-f) spectra after consecutive photolyses of the red and green bands selectively with the filtered high-pressure mercury arc; (g, h) spectra after annealing at 40 2 K and 50 2 K, respectively.
*
spectrum. In other words, coupling of the exo S2 subunit with the basal S2 subunit is very small. The 34-34 exo isotopic fundamentals with four basal isotopes range from 638.57 to 638.67 cm-'; the 32-34 and 34-32 ex0 isotopic fundamentals with four basal isotopes range slightly more from 648.39 to 648.56 cm-I. This accounts for the 1.3-cm-' line width of the central (633 an-') component of the experimental triplet and the 1.O-cm-' width of the low-frequency (623-cm-') component.
Discussion Vibronic bands for S3and two S4 isomers in solid argon will be identified through two methods of sulfur cluster production, photochemistry of trapped species, and annealing to allow diffusion and reaction of trapped atoms. Earlier studies in this laboratory have shown that by controlling the temperature of the sulfur reservoir input to the microwave discharge, it is possible to produce atomic sulfur at 25 OC,S and S2 at 50 OC,S,S2,and S3at 70 OC,and S,S2,S3,and Sa at 100 OC as was evidenced from SO, S20, S3,and S4 vibrational absorptions.16 The present UV-vis absorptions are in accord with the previous work, and thus, the absorption bands below 250 nm, observed with the sulfur reservoir temperature at 25 OC,belong to SO or SO2,I7the bands between 220 and 320 nm belong to S2I2 without any contribution from S3,the bands between 350 and 440 nm belong to S3without any contribution from S4, and the bands between 460 and 660 nm belong to S, species. No other new band was observed above 660 nm even with higher sulfur reservoir temperatures indicating that higher sulfur species lose the conjugated structure and absorb below 250 nm. S, The fine structures of the S3spectrum were reproducible in all of the experiments, which suggests that all of the peaks
I , , , , I , , , , I 350
400
450
Wavelength (nm) F i 6. Absorption spectra in the 350-450-nm region for sulfur clusters prepared by microwave discharge (a) and by superheater (b).
belong to S3species. Based on the changes in intensities for these bands on annealing at 20 f 2 and 30 f 2 K, these bands are separated into three groups labeled A, B, and C (Table I). These groups of bands were also observed with different relative intensities in the superheater experiments where the group A bands were in much lower intensity than the group B and C bands. Spectra for the S3species produced by the microwave discharge and the superheater sources are contrasted in the Figure 6; the spectrum obtained from the superheater is also shifted slightly to the red. This classification is also supported by the fact that each group shows slightly different spacings (Figure 3). The origin for the series A at 430.77 nm is just below the gas phase origin predicted at 426.17 nm. The sharp bands, which increased on annealing (series A) formed a clear 343,325,295, and 265 cm-I progression which is probably in the bending mode (vi) of the upper S3electronic state; the large decrease in separation in the progression indicates substantial anharmonicity in this upper state bending mode. The above series gives way to a series with 441-, 448-, 448-, and 437-cm-' spacings, which is appropriate for the totally symmetric stretching mode (v,') in the upper state. There appears to be a change-over in vibronic intensity from v2) to vl' when 2 ~ 2= ) vl'. This is borne out in the other two sites (series B and C) which decreased on annealing (Figure 3) and were dominated by vibrational intervals ranging from 460 to 400 cm-I. Two bending mode intervals were found at 240 and 230 cm-'in the middle of the strongest B progression; presumably as the origin is approached series B and C bands are dominated by series A. The matrix absorption spectrum indicates that v2) = 340 cm-I, which is larger than v; (256 cm-I), and that vI' = 450 cm-l, which is smaller than Y," (575 cm-'),'* but larger than the 420 cm-I value predicted from the low-resolution gasphase The origins of series B and C are not clear, and if the observed vibrational frequencies are extrapolated, the vibrational energies for each site are expected to be within 10-20
Vibronic Absorption Spectra of S3and S4 in Solid Argon
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6583 the peaks, which is dependent on the methods of the production, indicate that these three series of absorption bands belong to three different packing sites of S3in the argon matrix. Thiozone also showed three matrix sites at 674.5, 676.2, and 680.0 cm-' for v3 in the infrared absorption study.' The 674.5-cm-' band increased while the 676.2- and 680.0-cm-' bands decreased on annealing.' In the present infrared investigation, the 676.2-cm-' band decreased more than the 680.0-cm-' band on annealing. Accordingly we correlate series A to the 674.5-cm-' infrared band, series B to the 676.2-cm-I infrared band, and series C to the 680.0-cm-' infrared band. Finally, it is presumed that the 350-440 nm transitions for S3 is analogous to the 260-340 nm A(IB1) X('A,) transition for SOz." In the case of S3,the increase in v i implies an increase in charge density on the terminal atoms which sharpens the bending potential function and increases the upper state bending frequency. S,. The present matrix isolation studies show that the structured red absorption and the broad green absorption are due to structural isomers of the S4 cluster species. The structured red band system is converted into the unstructured green band when the former is irradiated by 590-1000-nm filtered light; however, when both are excited with 490-1000-nm radiation, the green band is converted into the red band system (Figure 2). Although both S4 species grow on annealing in most experiments, the red-absorbing species grows relatively more than the green-absorbing species (Figures 2 and 5 ) . Similar behavior was found in the infrared spectra; both 661.6- and 642.4-cm-' bands grew on annealing but the latter band grew relatively more (Figure 7). As shown in Figure 5 the red- and green-absorbing species produced by the superheater were interconverted on consecutive photolyses with the selectiveband pass filters A and B, respectively. It is important to notice that Sz and S3bands do not show any detectable change during these photolysis processes. Photochemical interconuersion of the red and the green bands clearly demonstrates that these bands belong to structural isomeric species. Since the green band is well-known to belong to the S4 species, consequently the red absorbing band also belongs to another structural isomeric form of the S4 species. As has been shown for isomers for octatetraene, decapentaene, and substituted styrene cations in solid argon, the matrix environment is a good medium for photochemical i s ~ m e r i s m . ' ~ - ~ ~ Selective excitation of one isomer initiates a dynamic equilibrium between isomers; however, the isomer not absorbing light is relaxed and preserved by the matrix. The same phenomenon is documented here for structural isomeric S4 species. The earlier infrared absorption studies assigned two bands at 661.6 cm-l and 642.4 cm-' to S4 species where the former band decreased and the latter band increased on annealing. These two bands showed a higher dependence on sulfur concentration than S3absorptions.' The present band-pass photolysis studies showed that the 661.6-cm-' infrared band increased and the 642.4-cm-' infrared band decreased upon irradiation with the 590-1000 nm filtered light; an opposite trend was observed when the matrix was irradiated with the green light. This behavior is similar to that observed for the green- and red-absorbing species upon photolysis with the same filtered light. Interconversion of the 661.6- and 642.4-cm-' infrared bands upon photolysis with the same spectral light that photoconverts the green and red absorption bands clearly shows that the green-absorbing band and the 661.6-cm-I infrared band belong to the same S4 isomer, and the red-absorbing band and the 642.4-cm-' infrared band belong to another S4 isomer. Ab initio electronic structure, energy, and frequency calculations have been done for 11 possible S4 structures at different levels of theory." The matrix values for the strongest infrared fundamental and the mixed32v34S4 isotopic multiplets can be used to relate calculation and experiment and to further characterize the observed isomers of S4. Which level of theory can best predict the observed 661.6- and 642.4-cm-' infrared bands? We seek calculated values in the 675-695-cm-' range, which after scaling, will approximate the above infrared bands. The CISD (DZP) level of theory provides fundamentals slightly above the observed
-
, 720
'
760
'
.
680
. 660
.
610
.
620
Wavenumber Figure 7. Infrared spectra in the 720470-cm-' region for sulfur cluster prepared by microwave discharge of SBvapor in argon stream: (a) 3 h deposition at 12 & 1 K; (b) 590-1000-nm photolysis for 30 min; (c) 470420-nm photolysis for 30 min; (d) 590-1000-nm photolysis for 30 min; (e) 470-620-nm photolysis for 30 min; (0 after annealing to 24 f 2 K for 2 min; ( 8 ) after annealing to 38 f 2 K for 2 min. TABLE Iu: Strongest Infrared Absorption (em-') Calculated with DZP B M Set ~ for Mixed **%I isotope branched ring 32-3 2-3 2-32 32-32-32-34 32-32-34-32 32-32-34-34 32-34-32-32 32-34-32-34 32-34-34-32 32-34-34-34 34-32-32-32 34-32-32-34 34-32-34-32 34-32-34-34 34-34-3 2-3 2 34-34-3 2-34 34-34-34-32 34-34-34-34
isotope 32-32-32-32 34-32-32-32 32-34-32-32 32-32-34-32 32-32-32-34 34-34-32-32 32-34-34-32 32-32-34-34 34-32-32-34 34-32-34-32 32-34-32-34 34-34-34-32 32-34-34-34 34-32-3634 34-34-32-34 34-34-34-34
658.20 658.16 658.16 658.13 648.56 648.53 648.53 648.49 648.48 648.43 648.43 648.39 638.67 638.62 638.62 638.57
rectangu 1ar
cis
644.78 1 639.82 639.82 639.821 639.82 635.29
636.71 631.35 631.35 631.92 63 1.92 626.80 627.30 627.88 627.30 625.68 625.68 621.87 622.37 622.37 621.87 617.73
635.24
634'36 634.36
6:;
630.20 630.20 625.55
1
1 1
1 1
1
I 1
trans 600.02 595.30 595.30 595.37 595.37 591.14
1
1
:;::::1
1
1 1
591.21 590.01 590.01 586.41 586.34 586.34 586.41 582.12
1
1
1
Isotopic molecules in branched ring, rectangular, cis, and trans planar structures. Geometries from CISD (DZP)calculations of ref 2 were used. cm-' of each other. This fact plus the different stability of these peaks on annealing as well as different intensity distribution of a
6584 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992
bands.2 Three structures, the cis, trans, and branched ring singlets, have calculated strongest bands in this range? The cis, trans, and helical triplet structures all have energies 7-9 kcal/mol higher than the cis singlet, but of more importance here, the calculated strongest fundamentalsare 596, 595, and 587 cm-I, respectively. These calculated fundamentals are too low to match the observed values, and hence, triplet structures can be ruled out. Another competitive structure, the puckered ring, has calculated fundamentals below 500 cm-l, and this structure can also be ruled out. Although the best theoretical calculations favor the cis-planar structure over the rectangular structure for the global minimum of S4, the energy difference is too small to be clear cut, and this preference is not certain. We will refer to the lower energy structure, which is probably the cis isomer, as the green absorbing species, which we believe is definitive. Accordingly, the 661.6-cm-' S4 absorption is assigned to the cis planar structure on the basis of CISD (DZP) calculated fundamentalsZand the mixed isotopic sextet observed for this band.' Although the appearance of a triplet mixed isotopic spectrum is usually offered as evidence for an S2species, we must reiterate the marked sulfur concentration dependence for both 661-6- and 642.4-cm-' bands,' which show that both of these bands are due to sulfur clusters with more than three sulfur aroms. The present DZP isotopic multiplet calculations (Table 111) show that only the branched ring structure can account for the experimental "triplet" spectrum. The strong ex0 fundamental for the branched-ring isomer is predicted to be 13 cm-' below the cis planar us mode. Therefore, we assign the 642.4-cm-I band to the branched ring isomer of S4. Although, calculations have characterized more than five relatively low energy S4 structural isomers, this laboratory can document the existence of only two structural isomers of S.,. The 683-cm-I infrared band observed in the earlier study' shows a still higher order sulfur concentration dependence and is believed to be due to a larger exo cyclic ring S=S, ( x 1 4). The expanded scale spectra between 570 and 650 nm for 3zS4 and yS4 are shown in Figure 4. Spacings between the band heads suggest two progressions with vibrational frequencies of 320 and 590 cm-' for the excited state of branched-ring Sg.The sulfur-34 band origin is blue shifted by 12 cm-' and shows vibrational frequencies of 310 and 570 cm-I which are reduced 10 and 20 cm-I from the sulfur-32 bands. These isotopic shifts are in good agreement with those expected for sulfursulfur vibrational modes. The vibronic intervals of 320 and 590 cm-' must be compared with 400 and 680 cm-I values calculated for the symmetric and exo S-S bond stretching modes for the ground state S4 isomera2 Since the calculations predict the exo stretching mode about 40 cm-' too high, the 680 cm-' calculated value must be reduced to the 642 cm-' observed value. This ground-state value is slightly higher than the 590-cm-' matrix value for the excited electronic state, which is in excellent agreement between theory and experiment. The 400-cm-' value for the ground state symmetric bond stretching mode is slightly above the 320-cm-' matrix value for the excited state. This comparison suggests that the ring S-S bonds are weakened in the excited state and helps rationalize the ready photochemical rearrangement of the branched-ring isomer in the relaxation process. Although a number of Raman lines have been assigned to sulfur clusters including S4,no assignments have been made to branched-ring S4.' However, 635 and 400 cm-' Raman signals were resonance enhanced by 647-nm excitation, and these bands could be due to ul" and up of branched-ring S4. If this possible assignment is correct, straightforward correlation with the 590- and 320-cm-I vibronic intervals for the u,' and Y; modes follows. The energy levels of the cis and branched ring S4 isomers are summarized in Figure 8. On the basis of the ab initio calculations of Quelch, Schaefer, and Marsden, the cis isomer is the global minimum and the branched ring isomer is about 8 kcal/.nd higher in energy.z The strong infrared fundamental is shown for each ground-state structure. The excited electronic states, the broad green and structured red bands, are placed according to the present matrix absorption spectra. Notice the considerable energy overlap
Hassanzadeh and Andrews
S,SKS.S
s I 0
J-
S
.--------___________
Figure 8. Schematic energy level diagram for two S, isomers. On the basis of the calculations of ref 2, cis-S4 is the global minimum and the branched-ring isomer is about 8 kcal/mol higher in energy. The obscrved infrared fundamental is shown for each isomer. The excited states are placed according to their matrix absorption spectra. Note the energy overlap for the two excited states.
of the two excited states. Even if the branched ring isomer is higher by as little as 5 or as much as 13 kcal/mol, there is still energy overlap between the isomer excited states. Since the two excited states are isoenergetic, dynamic rearrangement between them is readily accomplished. It follows that there is little barrier to the rearrangement between S4 excited electronic states. Hence, the excitation of one isomer gives an equilibrium population of the other excited isomer, which, in the absence of exciting radiation for the other isomer, can be relaxed and preserved by the matrix, as discussed The broad green band implies a dissociative upper state whereas the structured red band indicates a bound upper state for these two structural isomers. The orbital interaction diagram for the cis isomer shows that the two central sulfur atoms are not bound in the upper cis's4 state.z It is suggested that the green (cis) upper state is unbound with respect to the two excited singlet Szsubunits. Relaxation of this unbound cis upper state [(S2)(S2)]* can easily form the branched-ring S4 isomer by a simple 90° rotation and translation of one S2 submolecule, as is observed here. In fact less spatial rearrangement of terminal sulfur is required for the cis 2 branched-ring rearrangement than for the conventional cis a trans isomerization. Owing to the diffuse nature of sulfur orbitals, the branched-ring isomer is competitive in energy, whereas most compounds with cis and trans isomers cannot as easily form the branched-ring "intermediate" structure. There is no matrix spectroscopic evidence for the trans form of S4,which of course, requires a 180° rotation about the central S-S bond. Finally, the observed growth of the higher energy branched-ring isomer on annealing (probably from the S + S3reaction) indicates that the barrier to the branched-ring cis-planar rearrangement is greater than the Sz-S2 binding energy, but less than the 642-nm photon energy that initiates the photochemical rearrangement. This 44 kcal/mol photochemical upper limit for the barrier is more
J. Phys. Chem. 1992,96,6585-6592 in accord with the barrier for FSSF (24 kcal/m01)~~ than the very small value (1.5 kcal/mol)2 calculated for the &-trans isomerism of S4. The observation of both isomers growing on matrix annealing requires a barrier separating the two structures that ex4 s the S2-S2binding energy. Reaction Mecbinisnrs. In the superheater experiments at 700 "C the spcctrum showed mainly S2and a weak S4 band centered at 530 nm. Decrease of the S4 band on annealing indicates that the reaction S2 S2 S4 involving ground state S2molecules does not occur in solid argon. The red S4 band and the S3band intensities also increased together as the superheater temperature was increased. On the other hand, the red S4 band was observed with relatively higher intensity in the discharge experiments, and it increased on annealing; discharge experiments are known to produce sulfur atoms.' These observations suggest that the superheater also produces S atoms and that S3is formed from the combination reaction S2 S S3and the structured red band belongs to the species formed from the reaction S + S3 S4 in solid argon. The earlier formation of the structured red S4band on annealing solid krypton samples3containing sulfur is proposed to arise from the S S3 reaction as well. This suggests that addition of S to S3in the out-of-plane branched-ring orientation is less inhibited by steric effects than for the in-plane cis orientation. Although S3and both S4 isomers clearly exist in superheated gaseous sulfur,6* a considerable fraction of the S3and S4 observed in the present matrix studies must be due to matrix combination reactions involving sulfur atoms.
+
-
+
-
-
+
Conclusions Distinctly different electronic absorptions have been observed for S2, S3,and two different S4 clusters as a function of sulfur concentration in matrix discharge and superheater experiments. The S4 isomers give rise to broad 5 18-nm and structured 560660-nm absorptions, which exhibit reversible photochemical isomerism on selective irradiations. Infrared bands at 661.6 and 642.4 cm-I exhibit identical photochemical isomerism with the same photolysis treatment. Correlation of ab initio electronic energy, structure, and vibrational spectra calculations2with the observed visible and infrared absorption spectra with mixed sulfur isotopic multiplets' characterizes cis-planar and branched-ring nonplanar S4 structural isomers. This study provides strong evidence for a cis-planar s branched-ring photoisomerism for S4,
6585
which requires less atomic motion than the conventional cis s trans isomerization, and suggests that the branched-ring isomer may be involved in other photochemical rearrangements. Similar electronic spectra and photochemical rearrangements have been observed for Se4 and Te4, which will be reported in later papers. Acknowledgment. We gratefully acknowledge financial support from NSF Grant CHE 88-20764 and helpful conversations with H. F. Schaefer, K. Raghavachari, and C. Trindle.
References and Notes (1) Brabson, G. D.; Mielke, Z.; Andrews, L. J . Phys. Chem. 1991,95,79. (2) Quelch, G. E.; Schaefer, H. F., 111; Marsden, C. J. J. Am. Chem. Soc. 1990, 112, 8719. (3) Raghavachari, K.; Rohlfing, C. M.; Binkley, J. S.J. Chem. Phys. 1990, 93,5862. Raghavachari, K., personal communication of more recent calcu-
lations which find the cis isomer slightly lower in energy than the rectangular isomer, 1992. (4) Von Niessen, W. J . Chem. Phys. 1991, 95, 8301. (5) Meyer, B.; Oommen, T. V.; Jensen. D. J . Phys. Chem. 1971, 75,912. Meyer, B.; Stroyer-Hansen,T.; Oommen, T. V. J . Mol. Spectrosc. 1972,42,
335. (6) Steudel, R.; Jensen, D.; Godel, P. Eer. Bunsen-Ges. Phys. Chem. 1988, 92, 118. (7) Lenaine, P.; Piquenard, E.; Corset, J.; Jensen, D.; Steudel, R. Eer. Bunsen-Ges. Phys. Chem. 1988,92,859. ( 8 ) Billmers, R. I.; Smith, A. L. J . Phys. Chem. 1991, 95, 4242. (9) Rice, J. E.; Amos, R. D.; Handy, N. C.; Lee, T. J.; Schaefer, H. F. J . Chem. Phys. 1986,85,963. (IO) Andrews, L.; Spiker, R. C., Jr. J. Phys. Chem. 1972, 76, 3208. (11) Andrews, L.; Mielke, Z. J . Phys. Chem. 1990, 94, 2348. (12) Brewer, L.; Brabson, G. D.; Meyer, B. J. Chem. Phys. 1965,42,1385. (1 3) Mills, K. C. Thermodynamic Data /or Inorganic Sulfides, Selenides and Tellurides; Butterworth: London, 1974. (14) Dupuis, M.; Rhys, J.; King, H. F. J . Chem. Phys. 1976, 65, 1 1 . (15) Dupuis, M.; Watts, J. D.; Villar, H. 0.;Hurst, G. J. B. Computer Phys. Commun. 1989,52, 415. (16) Hassanzadeh, P.; Andrews, L. J . Am. Chem. Soc. 1992, 114, 83. (17) Greenough, K. F.; Duncan, A. B. F. J. Am. Chem. Soc. 1%1,83,555.
(18) Lenain, P.; Piquenard, E.; Lesne, J. L.; Corset, J. J . Mol. Stmcr. 1986, 142, 355. (19) Kelsall, B. J.; Andrews, L.; Schwarz, H. J . Phys. Chem. 1983, 87, 1413. (20) Dunkin, I. R.; Andrews, L.; Lurito, J. T.; Kelsall, B. J. J . Phys. Chem. 1985, 89, 2528. (21) Andrews, L.; Dunkin, I. R.; Kelsall, B. J.; Lurito, J. T. J . Phys. Chem. 1985. 89. 821. (22) Andrews, L.; Lurito, J. T. Tetrahedron 1986, 42, 6343. (23) Marsden, C. J.; Oberhammer, H.; Losking, 0.;Willner, H. J . Mol. Srruct. 1989, 193, 233.
A Surface-Enhanced Raman Spectroelectrochemical Study of a Number of p-Amino-Substituted Tetraphenylporphyrins in Aprotic Media Charles M. Rosten, Ronald L. Birke,* and John R. Lombardi Department of Chemistry, The City College, City University of New York, New York, New York 10031 (Received: February 14, 1992)
The surface-enhanced Raman scattering (SERS) spectra of five paminesubstituted tetraphenylporphyrins were investigated at a silver electrode in the aprotic solvent acetonitrile. Good quality spectra were obtained for both the neutral porphyrin and its first reduction product, the radical anion. Fluorescence interference from the porphyrin was completely quenched by the surface. SERS spectra allowed for the identification of two geometries of the adsorbate. Cyclic voltammetry and UV-vis thin-layer spectroeiectrochemistry were used to substantiate information obtained by SERS. The utility of SERS as a technique for identifying electrochemical products in a nonaqueous environment, where elucidation of the electrode mechanism is simpler and radical anions are more stable, is demonstrated.
