J. Phys. Chem. 1981, 85,3603-3607
3603
transport between vesicles. For pyrenenonanoic acid the relaxation time for the transfer between dimyristoylphosphatidylcholine vesicles at 28 "C was reported as 0.185 s at pH 7.4 and 12.5 s at pH 2.8. The flip-flop rates in these cases are closer to the transfer rates of very small molecules like ~ a t e r and ~ ~ acetic J ~ acid.13 Systematic studies of the transfer rates of small molecules through a lipid bilayer remains to be done. Fluorescence stoppedflow methods based on excimer formation, energy transfer, or fluorescence quenching would then be very powerful t00ls.l~ Summary and Conclusions Fluorescence quenching techniques may be used to estimate the inside/outside distribution ratio of a vesicle-
bound fluorescent molecule. Protection against quenching does not necessarily mean, however, that the fluorescent molecule and the quencher are on opposite sides of the vesicle membrane. In the case of Ru(bpy)gP+in dicetyl phosphate containing vesicles, time-resolved measurement and spectral changes in the presence of quenchers show the presence of at least two different solubilization sites. Rapid transfer of the quencher through the vesicle bilayer makes the estimation of distribution ratios by static methods impossible. As shown in the case of cetylpyridinium quenching of pyrene in positively charged vesicles, the distribution ratio may be obtained from fluorescence stopped-flow measurements which also are powerful means for studying the dynamics of the transmembrane transport process.
(12)Lawaczeck, R. Ber. Bunsenges. Phys. Chem. 1978,82, 930. (13)Alger, J. R.;Prestegard, J. H. Biophys. J. 1979,28, 1-14. (14)Verkman, A. S.Biochim. Biophys. Acta 1980,599, 370-9.
Acknowledgment. This project has been supported by the Swedish Natural Science Research Council.
The 147-nm Photolyses of Tetramethylene Sulfide, Tetramethylene Sulfoxlde, and Tetramethyiene Sulfone A. A. Scala," Ismael Colon, and W. Rourke Department of Chemistty, Worcester Polytechnic Institute, Worcester, Massachusetts 0 1609 (Received: June 30, 198 1; In Flnal Form: August I 1, 198 I )
Tetramethylene sulfide, sulfoxide, and sulfone have been photolyzed with 147-nm radiation in the gas and condensed phases. Mechanisms which assume diradical intermediates are presented for the photodecomposition of each compound. The major reaction in all three cases is the elimination of ethylene by a 0-cleavage of the initially formed diradical: .XCH2CH2CH2CH2- .XCH2CH2.+ CzH4. The competing 0-cleavage produces X=CH2 and C3He This second 6-cleavage is less important in the sulfoxide and the sulfone than it is in the sulfide. The direct elimination of the sulfur atom, and any attached oxygen atoms, is of no importance in the sulfide and of increasing importance as the oxidation state of the sulfur atom increases. The similarity between the mass spectral fragmentation and the vacuum ultraviolet photochemistry of these compounds is noted and the possible involvement of triplet states is mentioned.
-
Introduction Although there is a wealth of information on the photochemistry of heterocyclic organic compounds in the near-ultraviolet, there are few studies of simple heterocycles which absorb only in the vacuum ultraviolet. There are even fewer studies which examine the photochemistry of heterocycles as a function of the heteroatom. Sulfur heterocycles are of particular interest because of their stability in a variety of oxidation states. While the photochemistry of alicyclic sulfides has been examined to some extent,'+ the photochemistry of alicyclic sulfoxides10and sulfonesll has received little attention. This paper presents the results of an investigation of the vacuum ultraviolet photolyses of tetrahydrothiophene (THT), tetramethylene sulfoxide (TMSO), and tetra(1)W.E.Haines, G. L. Cook, and J. S. Ball, J. Am. Chem. Soc.. 78, 6213 (1956). (2)K.S. Sidhu, Can. J. Chem., 46,3305 (1968). (3)H. A. Wiebe and J. Heicklen, J.Am. Chem. Soc., 92, 7031 (1970). (4)S.Braslavskv and J. Heicklen. Can. J. Chem.. 49. 1316 (1971). (5) D. R. Dice &d R. P. Steer, J . khys. Chem., 77; 434 (1973). (6)D. R. Dice and R. R. Steer, J. Am. Chem. Soc., 96, 7361 (1974). (7)D. R. Dice and R. P. Steer, Can. J. Chem., 52, 3518 (1974). (8) D. R. Dice and R. P. Steer, Can. J. Chem., 53, 1744 (1975). (9)D. R. Dice and R. P. Steer, Can. J. Chem., 56, 114 (1978). (10)F. H.Dorer and K. E. Salomon, J. Phys. Chem., 84,1302 (1980). (11)A. A. Scala and I. Colon, J. Phys. Chem., 83,2025 (1979).
0022-3654/81 /2085-3603$01.25/0
1 THT
TMSO
0
\ /p
TMSO,
methylene sulfone (TMSOJ at 147 nm. The purpose of this study is not only to present the various reaction channels for each of these compounds but also to elucidate how changing the oxidation state of the sulfur atom affects the modes of fragmentation. Since the mass spectral behavior of heterocycles is often similar to their photochemistry in the vacuum ultraviolet, some discussion of the mass spectra of these compounds is of value. The mass spectrum of THT12indicates that loss of a two-carbon fragment from the molecular ion is the predominant reaction channel, followed in importance by the loss of a three-carbon fragment. The only other significant processes involve the loss of HS and H2S. The mass spectrum of TMSO is drastically altered from that of the simple sulfide.13 The major fragmentation involves loss of HSO, which suggests that rearrangement (hydrogen (12)A. M.Duffield, H. Budzikiewicz, and C. Djerassi, J. Am. Chem. Soc., 87, 2920 (1965).
(13)R. Smakman and T. J. DeBoer, Org. Mass Spectrum., 3, 1561 (1970).
0 1981 American Chemical Society
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Scala et ai.
The Journal of Physical Chemistry, Vol. 85, No. 24, 1981
TABLE I: Photolyses of Tetrahydrothiophene at 147 nm press., torr 1 5 10 10 10 additive 10% 10% product 0 2 ea NO CH4 CZHZ 'ZH4 'ZH6 C3H6
C3H8 C3H, c-C3H6 1,3-C,H6
2.9 3.9 80.1 0.72 4.9 0.45 1.2 1.3 4.6
1.8 4.8 78.4 0.7 5.3 0.6 1 .o 1.6 5.8
1.4 3 .I 79.6 0.5 5.5 0.7 0.9 1.7 6.1
2.5 19.7
90.1
7.5
6.4
0.13 2.6 7.6
2.2 1.3
0.033 0.044 1.06 0.010 0.064 0.0006 0.016 0.019 0.073
TABLE 111: Photolyses of Tetramethylene Sulfoxide at 147 nm product
gasa
liquidb
solidC
CH4 CZHZ CP4 C2H6
ND 7.5 61.3 5.3 10.4
4.3 0.5 72.6 1.3 10.5 1.4 5.1 3.9
ND 1.1 60.0 1.7 12.0 1.4 20.4 3.4
C3H6
c-C3H6 1-C4H, c-C4H,
1 .o
6.6 1.9
a Equilibrium vapor pressure of TMSO at 25 "C was -0.01 torr. 298 K. 77 K.
OSlb
Quantum yields measured at 2 torr of THT. tion efficiency (q). a
Ioniza-
TABLE 11: Isotopic Distribution of Products from THT-or-d, Photolysesa Me thane Ethylene CHD, 21% CZHZDZ 88% 27% CZH3D 7% CHZD, 10% 'ZH4 5% CH,D CH4 42% Propylene and Cyclopropane 1,3-Butadiene C3H4DZ 92%C4H; D4 82% C3H5D 8% C,H3D3 18% a Data have been corrected for actual THT isotopic composition which was 89%a-d4 and 11%a-d3.
migration) takes place. Unlike the sulfide, in the sulfoxide, loss of three-carbon fragments is more important than the loss of two-carbon fragments. The mass spectrum of sulfolane (TMS02) is dominated by hydrocarbon fragments and indeed resembles the mass spectrum of 1butene.13 Loss of SOzis the major fragmentationpathway. The large peak at mle 28, which cannot be ascribed to decomposition of a C4 fragment, results from either the elimination of an ethylene ion from the parent ion or the cyclic decompositionsuggested by Weinberg et all4 The loss of HSOz from the molecular ion suggests some rearrangement with hydrogen migration.
Results The product analyses of the 147-nm photolyses of THT at various pressures and in the presence of NO are presented in Table I. As the mass spectrometry would lead one to expect, ethylene accounts for 80% of the hydrocarbon products. The only other products of any importance are propylene, cyclopropane, and 1,3-butadiene. The relative amounts of these latter products increase slightly with pressure, with the effect being a little more pronounced for 1,3-butadiene than for C3H6. The effect of NO is to decrease the ethane, propane, and allene yields, while the yields of propylene, cyclopropane, and 1,3-butadiene appear to increase slightly. Table I1 presents the isotopic composition of the methane, ethylene, C3H6,and 1,3-butadiene formed in the 147-nm photolysis of 5 torr of THT-a-d4. Since tetrahydrothiophene ionizes at 147 nm with an efficiency (7)of 0.11,15quantum yields could be determined by use of ion-current measurements. These are also displayed in Table I. The total quantum yield for the hydrocarbon products is 1.32 even though 11%of the radiation absorbed goes into the production of ions. Since 147 (14) D. s. Weinberg, C. Stafford, and M.w. Scoggins, Tetrahedron, 24. 5409 (1968). '(15) A: A. Scala and D. Salomon, J. Chern. Phys., 62, 1469 (1974).
nm is at the onset of ionization, the ions which are produced have no excess energy and do not undergo decomposition. Photoionization mass spectrometry16indicates that the only ion-molecule reaction which takes place at this wavelength is dimerization. The fate of these dimers is uncertain. They probably undergo further solvation by THT and although the ultimate neutralization of these cluster ions may contribute to product formation, this is unlikely. The results of photolysis of tetramethylene sulfoxide at 147-nm are presented in Table 111. The low vapor pressure of TMSO prevented the undertaking of pressure studies, but liquid-phase as well as solid-state photolyses (77 K) were conducted to evaluate collisional quenching effects. It is evident from the ratios of propylene to cyclopropane and 1-butene to cyclobutane that singlet trimethylene and tetramethylene diradicals are not important intermediates in the photolyses. Even at 77 K in the solid, the olefins predominate over the cycloalkanes. It should be noted that in comparison to THT, the yields of C3 and C4 hydrocarbons are higher in the photolyses of TMSO. The results of photolyses of TMSO2 at 147 nm are presented in Table IV. As in the case of TMSO the very low vapor pressure of this compound prevented the determination of quantum yields and the usual pressue studies; however, the pressure effects were determined by photolyses in the presence of SF6as well as in the solid phase at 77 K. Gas-phase photolyses were conducted at 25 and 80 "C. It is evident from the acetylene yield that secondary decomposition is less significant at 80 "C than at 25 "C. Aside from pressure-quenching effects,secondary decomposition might be expected to increase at lower pressure (25 "C) due to the fact that the products begin to absorb a significant proportion of the incident radiation. These complications at 25 "C are manifest in the lack of a clear trend in the pressure dependence of the l-butene/cyclobutane ratio a t 25 "C. A t 80 "C there is a consistent trend in this ratio, and it appears that all of the 1-butene which is quenchable to cyclobutane is quenched at a pressure of 75 torr of SF6. This is evident from the values of the 1-butene to cyclobutane ratio at 75 torr of SF6 and in the solid phase at 77 K (infinite pressure) which are 0.39 and 0.38, respectively. The isotopic composition of the ethylene produced from the 147-nm photolysis of 5 torr of TMS02-cu-d4in the presence of 10% oxygen was 8% CzD4,2% CzHD3,81% C2H2D2,1% CZH~D, and 8% C2H4 The most surprising result of the photolyses of TMSOZ is the selective production of methylcyclopropane. This is a very intriguing product which has not been observed in the photolysis of any other unsubstituted five-membered ring heterocycle. This implies that a specific process pe(16) J. D'Angona, M.S. Thesis, Worcester Polytechnic Institute, Worcester, MA, 1975.
The Journal of Physical Chemistry, Vol. 85, No. 24, 198 1 3605
Photochemistry of Alicycles Sulfoxides and Sulfones
TABLE IV: Photolyses of Tetramethylene Sulfone at 147 nm additive
press., torr
none none
SF, + 10760, a
25 80 25 25 25 80 80 80 -196 25
5 10 20 50 75 120 solid 5
SF, SF, SF, SF, SF, SF,
temp, "C
C,H, 9.86 0.68 2.09 3.26 4.42
2.33 3.53
C,H,
82.2 83.2 84.7 82.3 79.6 79.1 79.5 76.8 65.2 56.0
l-C,Hb
l-C,H,
MCPa
c-C,H,
1.50 1.50 1.52 1.06 0.96 2.40 2.19 1.64 0.94 0.32
0.94 6.44 1.97 2.30 2.03 4.56 3.89 4.74 5.33 1.10
1.21 1.91 4.01 3.98 5.41 4.35 4.54 4.80 12.3 15.8
4.31 3.47 5.72 7.11 7.59 9.57 9.87 12.1 14.0 23.2
Methylcyclopropane.
TABLE V : Mass Spectrum of l-Oxa-2-tnietane-2-oxide
/p
CHz-S
I
CH2-O
mle 92 91 90 64 62
intensity 79 19 16 29 100
I mle 48 46 30 28
intensity 44 36 7.8 19
culiar to TMSOz is involved in its genesis. The results of photolyses in the solid phase at 77 K are especially interesting, since there is a dramatic increase in the amount of methylcyclopropane produced. When oxygen is present the relative yields of both methylcyclopropane and cyclobutane increase dramatically. Although sulfur dioxide cannot be detected by using a flame-ionization detector, the relative amount of SO2 produced was determined by mass spectrometry. While mass spectral analyses of the gas-phase photolyses mixtures revealed that SOzproduction was in accord with the amounts of hydrocarbon produced, analysis of the solidphase photolyses product mixtures indicated that the amount of SOz produced was significantly less than one would have expected from the hydrocarbon yield. In addition photolyses in condensed media produced a compound of molecular weight 92 which was not apparent in the gas-phase photolyses. From the mass spectrum of this compound (Table V) the structure l-oxa-2-thietane-2-oxide shown in Table V was assigned. This assignment is consistent with the propensity of this type of compound to undergo four-center cleavage to yield the sulfine cation radical (mle 62).17 When a-d4sulfolane is photolyzed mle 92 shifts to m l e 94 and mle 62 shifts entirely to mle 64. In addition to the results reported, the following observations should be noted. Hydrogen which was a product in all experiments was not routinely measured. When it was determined its yield was usually about 15% of the ethylene yield.
Discussion Photolysis of THT at 147 nm in the gas phase leads to ionization and the formation of neutral electronically excited molecules, reactions 1 and 2. The ions produced in
is close to the collision rate. The fate of these dimer ions is uncertain; however, they probably undergo further solvation and upon ultimate neutralization of the cluster ions do not contribute to the formation of products.16 In dealing with heterocyclic molecules, fragments of molecular formula C,Hb are usually major products which are insensitive to the addition of trapping molecules. When this is the case and in the absence of other data one can ascribe these products to either molecular reactions or short-lived biradical intermediates. In the absence of definitive proof of one or the other of these mechanisms the distinction between them is largely semantic. Because in certain cases evidence for a biradical is available, and because the existence of a biradical intermediate provides a unifying characteristic to the mechanisms for the photolyses of a large number of heterocyclic molecules, this mechanism is to be preferred and will be used in this paper. The simplest primary act in the photolysis of THT which is consistent with the facts, and promotes understanding of the system, is the initial breaking of the weakest bond in the molecule, the carbon sulfur bond (reaction 3). Essentially all of the observed products can (3)
I be understood in terms of competing reaction channels 8f I. The major reaction channel of I is a @-cleavageto form ethylene and CzH4S(reaction 4). Since the quantum yield I
A
C2H4
+
CH2CHzS
(4)
of the ethylene is 1.06, it is evident that a fraction of the CzH4Sproduced in reaction 4 undergoes secondary decomposition. If reaction 4 has a maximum quantum yield of ca. 0.7 (1.0 - 4(C4)- 4(C3)- q ) then at 2 torr about half of the C2H4Sproduced in reaction 4 must decompose to ethylene. The C3H6products, propylene and cyclopropane, are formed by an alternative @-cleavageof I (reaction 5) I
A
[CHz=SI
-k C 3 H 6
(5)
which competes with reaction 4. The factor which determines the competition between reactions 4 and 5 may be the strength of the a bond which is formed. Since the carbon-sulfur a bond is quite weak reaction 5 does not compete very effectively with reaction 4. On this basis it would be expected that in tetrahydrofuran photolyses the process corresponding to reaction 5 would be more competitive and it is.lgZo The only other significant reaction in the 147-nm photolyses of THT is the loss of H2S and the formation of 1,3-butadiene. This reaction also is easily understood in terms of the biradical I undergoing a @-
reaction 1,$ = 0.11, undergo dimerization at a rate which (17) E. Block, R. E. Penn, R. J. Olsen, and P. F. Sherwin, J. Am. Chem. SOC., 98, 1264 (1976).
(18) Z.Diaz and R. D. Doepker, J. Phys. Chem., 82,10 (1978). (19) N. Kizilkilic, H.-P. Schuchmann, and C. V. Sonntag, Can. J. Chem., 58, 2819 (1980). (20) A. A. Scala and W. Rourke, unpublished results.
3000
Scala et al.
The Journal of Physical Chemistty, Vol. 85, No. 24, 1981
hydrogen atom transfer to form the unsaturated thiol which loses H2Sand forms 1,3-butadiene (reaction 6). All
H2S
+
CHz=CH-CH=CHz
(6)
of the initially formed products may contain excess vibrational energy and undergo further secondary fragmentation to yield the other minor products observed. The excellent correlation of the 147-nm photolysis with the mass spectrometry of THT should be noted. In both instances loss of ethylene is the major reaction pathway, with loss of C3 fragments and H2S being the only other major competing channels. It is significant that the only C4 hydrocarbon is 1,3-butadieneand that no 1-butene or cyclobutane is formed. Although the two weakest bonds in THT are the carbon-sulfur bonds, direct elimination of sulfur does not occur. Photolysis of TMSO at 147 nm produces only neutral electroncally excited molecules. There are four major channels by which the excited molecules fragment. The predominant path involves the formation of ethylene and C2H4S0by a @-cleavageof the initially produced biradical, I1 (reaction 7). Since no C2H4S0could be detected, it is 0 *
II
’S,
C ~ Z ,CHz CHz-CHZ
C2H4
+
C2H4SO
As in the case of THT, the ionic reactions occurring in the mass spectrometer provide considerable insight into the reaction of highly excited neutral TMSO. Photolyses of TMSOz at 147 nm produces only electronically excited molecules (reaction 10). As in the 0
\\
4 0 ,S HzC. CH2 HIC-CCk2
+
SO2
*CH2CH2CH2CH2*
l
1-
(10)
1 C4Hs
CH -C H ~ *
I
2C2H4
I
CH2-CH2
photolyses of trimethylene sulfone elimination of SOz is a major reaction.ll Similar to the photolysis of cyclopentanone, some of the ethylene, cyclobutane, and 1butene are coupled through the intermediacyof the singlet tetramethylene diradicalazZ If a singlet tetramethylene diradical was an intermediate it would be possible to quench the “hot” cyclobutane in the solid phase at 77 K and obtain cyclobutane as a major product. This trend is clearly seen in Table IV. The fact that oxygen is able to quench a significant fraction of the 1-butene requires that 1-butene is also produced by some other reaction. The formation of ethylene is also quenched by the presence of oxygen. Some of the ethylene is no doubt formed in a nonconcerted process such as reaction 11. A
(7 1
I1
likely that substantially all of the C2H4S0decomposes to ethylene and SO (reaction 8). Direct loss of SO does I1
-
SO
+
C4H8
(8)
appear to be a significant process. The C4H8species produced is a significant source of ethylene since there is a substantial change in the ethylene to C4H8ratio when the gas- and solid-phase photolyses are compared. Alt&ough the formation of cyclobutane certainly implicates a singlet tetramethylene diradical, most of the 1-butene is formed from some other C4H8species, possibly a triplet tetramethylene diradical, since there is little difference in the cyclobutane yield in the gas and solid phases. The last reaction of any consequence in TMSO photolyses involves the formation of C3H6(reaction 9). This reaction is best 11
A
CH2=S=O
-k C3H6
(9 1
concerted process such as reaction 12 is rendered unlikely
9\/p H2Ccs‘,cH2 HzC-CH2
so2
+
2C2H4
(12)
by the results in the solid phase, where the yield of SOz is much less than expected from reaction 12 and material balance considerations. In addition substantial amounts of S02C2H4have been detected. The available data suggest that most of the ethylene is produced in stepwise fashion beginning with reaction 11. The loss of a molecule of ethylene from the electronically excited sulfolane produces a diradical intermediate which possesses excess vibrational energy and can undergo further decomposition to yield another molecule of ethylene and SO2,reaction 13. In the *S02CH2CHy SO2 + C2H4 (13) condensed phase this radical is stabilized and undergoes intramolecular cyclization, reaction 14, to yield the cyclic sulfinate ester whose mass spectrum is shown in Table V. +
understood in terms of a competing @-cleavae of the diradical I1 to form the highly unstable sulfinefl and C3H6. The overwhelming predominance of propylene over cyclopropane in the gas, liquid, and solid phases rules out the intermediacy of a singlet trimethylene diradical as a major source of the C3H6.As in the case of the C4H8 products, the triplet diradical is a possible intermediate. In their investigation of the near-ultraviolet photolyses of TMSO Dorer and SalomonlOfound that the only products of triplet-sensitized decomposition were ethylene and propylene. They proposed that the triplet trimethylene diradical was generated. Previous work from this laboratory proposed that the triplet trimethylene is responsible for the formation of propylene but not cyclopropane in the vacuum ultraviolet photolyses of trimethylene sulfone and cyclobutanone.ll It appears that the triplet trimethylene diradical is produced in the 147-nmphotolyses of TMSO. (21) W. A. Sheppard and J. Diekmann, J. Am. Chem. SOC.,86,1891 (1964).
The genesis of the oxygen-quenchable 1-butene and ethylene is of considerable interest. If we assume that the quantum yields of methylcyclopropane and cyclobutane are unaffected by the presence of oxygen, then the difference in the results at 5 torr and 25 “C in the presence and absence of oxygen requires that a substantial fraction of the ethylene and 1-butene is quenchable by oxygen. It is tempting to implicate a triplet tetramethylene diradical as an intermediate because this radical would be incapable of cyclizing and could produce only 1-butene and ethylene, (22) A. A. Scala and D. G. Ballan, Can. J. Chem., 50, 3938 (1972).
The Journal of Physical Chemistiy, Vol. 85, No. 24, 198 1 3607
Photochemistry of Alicycles Sulfoxides and Sulfones
Scheme I 3*CHzCHzCHzCH2.
c - C ~ H ~
-
CEHL
+
31-C4H~
3CzH4
-
-
excited states produced in vacuum ultraviolet photolyses by investigating the behavior of ions in the mass spectrometer, in an empirical area such as vacuum ultraviolet photochemistry presently is, any insight is of value.
CZH~
l-C4H8
Scheme I. Either the triplet tetramethylene diradical or some other species, i.e., triplet 1-butene or triplet ethylene, would be expected to react with oxygen. A similar scheme proposing a triplet trimethylene diradical has been proposed to explain the results from cyclobutanone and trimethylene sulfone photolyses.'l The similarities in these two homologous studies are striking. The existence of triplet states of complex organic molecules at these energies is unknown and surprising. This point awaits further clarification. The mechanism for the formation of methylcyclopropane is completely unknown. Collin has shown that methylcyclopropane is not formed in the 147-nm photolysis of l - b ~ t e n e .The ~ ~ facts are that it is a primary photolysis product which is not susceptible to quenching by oxygen and whose yield increases substantially in the solid phase a t 77 K. It is of interest that, in the photolyses of the sulfoxide, TMSO, methylcyclopropane is not a significant product. In summary changing the oxidation state of the sulfur atom has a profound effect on the photolyses of fivemembered ring sulfur heterocycles at 147 nm. Although in each case the major reaction is reaction 15 the relative
('
=
"
-
CzH4
+
XCHzCHz
hydrogen rearrangement X
+
C4H8
(17)
(18)
importance of the other reactions is highly dependent upon the sulfur oxidation state. Reaction 16 is relatively unimportant for the sulfide, TMSO, and becomes very unimportant for the sulfone, TMS02. Reaction 17 is of relatively minor importance for all three compounds and in fact produces different products in each case. THT yields H2Sand butadiene, TMSO yields SO and 1-butene, and TMS02yields SO2and methylcyclopropane. Process 18 does not occur at all for THT and is a minor reaction for TMSO. For TMS02 things are substantially different and elimination of SO2 is the second most important reaction. Photolysis of TMS02 is different in another respect in as much as a triplet state may be implicated. The singlet excited state appears to decay primarily by loss of ethylene and SO2. The loss of SO2 produces the tetramethylene diradical which forms cyclobutane and the nonquenchable ethylene and 1-butene. The triplet forms an oxygenquenchable diradical which can lose either ethylene or sulfur dioxide. As a final point the utility of mass spectrometry in elucidating gas-phase vacuum ultraviolet photolysis mechanisms should be emphasized. While the excellent correlation between the 147-nm photolysis of THT and its fragmentation in the mass spectrometer may be fortuitous, it is significant that the qualitative changes observed in the mass spectrometer on changing the oxidation state of the sulfur atom are all observed in the 147-nm photolyses of these compounds. Although one cannot expect to completely unravel the mechanisms of reaction of neutral (23) G. J. Collin, Can. J. Chem., 51, 2853, (1973).
Experimental Section Materials. THT, TMSO, and TMS02 were purchased from Aldrich and were purified by distillation on a 30 theoretical plate spinning-band column (TMSO and TMS02 were distilled under reduced pressure). Nitric oxide (Matheson) was purified prior to use by low-temperature distillation on a vacuum line. Oxygen (Airco) and sulfur hexafluoride (Matheson) were used without further purification. Sulfolane-cu-d4was prepared by dissolving sulfolane in D20and adding a small piece of sodium metal. The mixture was heated to 40 "C and allowed to exchange for 48 h. After neutralization the solution was extracted with ethyl acetate. The ethyl acetate was removed under reduced pressure, and the sulfolane-a-d4was purified by distillation under reduced pressure. Irradiation and Analysis. The design and preparation of the microwave-powered discharge lamps used in this study have been described before.24 The photolyses of T H T were conducted in a 1-L vessel to which the lamp was attached through a ground glass joint. The vessel was connected to a vacuum line, evacuated, and then filled with the specified pressure of THT. The pressure was measured by a Wallace & Tiernan FA-160 pressure gauge. The lamp and bulb assembly was then placed near the antenna of a microwave generator (2450-MHz Raytheon PGM 101, 125 W). The lamp discharge was initiated by sparking with a tesla coil and was maintained by the microwave generator. Quantum yields were determined by using saturation current measurements with a 500-cm3cell containing two parallel circular stainless steel electrodes. The potential across the plates was applied by using a John Fluke Model 415B regulated high-voltage power supply and the current was measured on a Victoreen Model 1001 pic~ammeter.'~ The photolyses of TMSO and TMS02in the gas phase were conducted in the 1-L vessel mentioned above, while liquid- and solid-phase photolyses were conducted in a 50-cm3cylindrical cell. Due to the extremely low vapor pressure torr) of these materials at room temperature, quantum yield measurements were not attempted. After photolysis the gaseous products were analyzed by injecting a 25-cm3aliquot of the photolysis mixture into an F&M 810 gas chromatograph equipped with a flameionization detector and a 30 ft, 30% squalane on Chromosorb P, column. Because cis-2-butene and methylcyclopropane have the same retention time on the 30-ft squalane column, methylcyclopropane was identified by using a 6-ft Alumina column.26 Mass spectral analysis of the products of photolysis was accomplished by introducing the mixture through the gas inlet system of the mass spectrometer (DuPont Instruments 21-491) for SOz analysis, or by trapping the sample from the gas chromatograph and then introducing the sample into the mass spectrometer. Acknowledgment. We thank the US.Department of Energy, Contract No. DE-AC02-76ER03569, for support of this work. (24) P. Ausloos and S. G. lias, Radiat. Res. Rev., 1, 78 (1968); R. Gordon, Fr., R. E. Rebbert, and P. Aueloos, Natl. Bur. Stand. Tech. Note,
No. 494, (1969).
(25)We are indebted to Professor C. von Sonntag and the staff of the Institut fur Strahlenchemie, Max-Planck-Institut ftir Kohlenforschung, Mulheim a.d. Ruhr, West Germany for their help in identifying the methylcyclopropane.
