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The Journal of Physical Chemlstty, Vol. 83, No. 14, 1979
Powers et ai.
Infrared Spectra of Matrix-Isolated Sulfine D. E. Powers, C. A. Arrlnglon, W. C. Harrls, Department of Chemlstry, Furman University, Greenvilie, South Carolina 296 13
E. Block, Department of Chemlstry, University of Mlssourl- St. Louis, St. Louis, Missourl 63 12 1
and V. F. Kalaslnsky' Department of Chemistry, Mississippl State University, Mississippi State, Mlssissippi 39762 (Recelved January 30, 1979) Publication costs assisted by Mlsslssippl State University
The highly reactive sulfine molecule, H,CSO, has been trapped in an argon matrix at 18 K and studied in the infrared region from 4000 to 200 cm-l. Sulfine was generated by the vacuum pyrolysis of l,&dithietane 1-oxide or allyl methyl sulfoxide at 350 "C. The infrared absorptions due to sulfine have been identified by comparing the spectra obtained from the two pyrolysis reactions and by observing the temperature dependence of the spectral features. The data are consistent with previous studies of sulfine, and a complete vibrational assignment has been proposed.
Introduction While substituted sulfine derivatives, b C S 0 , have been known for some time,' the parent molecule has only recently been identified as a product in the pyrolysis reactions o€ dimethyl sulfoxide, 1,3-dithietane 1-oxide, methanesulfinyl chloride, allyl methyl sulfoxide, and other precursor^.^-^ Although sulfine has a very short lifetime, photoelectron,3 microwaveF4and limited infrared2 spectra have been reported. Microwave studies have resulted in a complete structure determination, and sulfine has been shown to be planar with a CSO angle of 114.7'. Additionally, relative intensity measurements have led to estimates of certain vibrational freq~encies.~Infrared spectra of the vapor phase have shown only the two strongest absorption bandsa2 Low temperature matrix-isolation techniques have been used to increase the effective lifetimes of highly reactive or unstable species which can be generated by photolysis or pyrolysis reactions.6 Consequently,we have undertaken a study of the infrared spectrum of sulfine trapped in an argon matrix at 18 K, and the results are reported herein.
the pyrolysis chamber, and deposition took place over periods ranging from 2-3 h. The most favorable results were obtained when pyrolysis was carried out at 350 'C. Spectra of pure argon were also recorded under similar conditions.
Experimental Section The samples of 1,3-dithietane 1-oxide and allyl methyl sulfoxide were prepared at the University of Missouri-St. Louis, according to previously published procedure^.^^^ They were stored at -20 "C in previously evacuated Pyrex tubes. The infrared spectra were recorded on a Perkin-Elmer 580 grating spectrophotometer which is interfaced to an Interdata 6/ 16 minicomputer, A signal averaging option is built into the software system, and the spectrum shown in Figure 1represents the average of ten scans accumulated over a 5-h period. The spectral resolution was better than 1 cm-l throughout, and the reported frequencies are expected to be accurate to within 2 cm-l. The argon matrices were supported on a CsI substrate maintained at 18 K by a Cryogenics Technology Inc. Model 21 closed-cycle helium cryostat fitted with CsI windows. The associated metal vacuum system was fitted with a segment of Pyrex tubing for the pyrolysis reactions. The Pyrex tube was wrapped with nichrome wire and asbestos, and the temperature was controlled with a Variac and monitored with a copper-constantan thermocouple. The argon and sulfine-precursor were mixed prior to entering
Results and Discussion The products of each pyrolysis reaction were trapped in an argon matrix, and the frequencies of the observed infrared bands are presented in Table I. A number of other species are formed during the reactions, and they can contribute significant interferences. Consequently, a number of experimentswere conducted in order to identify the sulfine absorptions. For example, the spectra of the starting materials were recorded, but no evidence of starting material was found after pyrolysis. After recording the spectrum at 18 K (see Figure 1)the matrix was allowed to warm to 31 K. At this temperature a number of bands were observed to grow in intensity while others became weaker. Since sulfine is very reactive we expect bands attributable to this molecule to disappear as migration within the matrix begins. Bands whose intensities increase significantly at the higher temperature are indicated in Table I. The bands at 1366 and 1153 cm-' are probably due to SO2,"and other bands correspond to f~rmaldehyde.~ Both of these are known to be decomposition products of sulfinea2 Additionally, there is evidence for thioformaldehyde,1° the other product expected in the pyrolysis of 1,3-dithietane 1-oxide.
0022-3654/79/2083-1890$01.OO/O
0 1979 American Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 14, 7979
Infrared Spectra of Matrix-Isolated HPCSO
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TABLE 111: Internal Force Constants for Sulfine force constant descriptn value,a mdyn1.A K, C-H stretch 4.86 t 0.04 KQ S= 0 stretch 11.2 i: 0.8 K, C=S stretch 8.2 t 0.7 Ha LCSObend 1 . 2 ?: 0.2 H6 LHCHbend 0.28 f 0.03 Ho LHCSbend 0.64 i 0.03 0.16 t 0.04 H, CH, wag H7. CH, twist 0.22 * 0.04 FR, C=S stretch/tHCH bend - 0.41 t 0.10 a
3200
2800 1500
1000
500
WAVENUMBER (crn-1)
Figure 1. Infrared spectrum of the products of the pyrolysis of 1,3dffhietane l-oxide trapped in argon at 18 K. Sulfine (H2CSO)bands are marked with 0 .
TABLE 11: Observed and Calculated Fundamental Vibrations (cm-l) for Sulfine species approx descriptn obsd calcda A‘
u1 u2 uj u4 us
u6 u7
A’
u8
ug
CH, antisymmetric stretch CH, symmetric stretch CH, scissors CSO antisymmetric stretch CSO symmetric stretch CH, rock (in-plane) CSO bend (in-plane) CH, twist (out-of-plane) CH, wag (out-of-plane)
3013 2960 1395 1357 1165 1055 394
3044 2926 1393 1359 1161 1055 394
972 767
972 767
Calculated with a modified valence force field and the force constants given in Table 111.
The pyrolysis of 1,3-dithietane 1-oxide at 350 “C provides a much “cleaner” spectrum of sulfine because of the simplicity of the accompanying products. However, the sulfine bands at 1395, 1357,1165,972,767, and 395 cm-l are clearly visible in the spectrum obtained after pyrolysis of allyl methyl sulfoxide. Other apparent coincidences listed in Table I exhibit anomalous relative intensities and are not assigned to sulfine. Of particular interest is the band of medium intensity at 987 cm-l. This feature does not appear in the spectra recorded for the pyrolysis products of allyl methyl sulfoxide and, hence, cannot be attributed to sulfine. A similar band has been associated with a byproduct of reactions designed to produce thioformaldehyde.1° Since thioformaldehyde is a major product of the pyrolysis of 1,3-dithietane 1-oxide but is not produced in the pyrolysis of allyl methyl sulfoxide, it is not surprising that the 987-cm-l band is only observed for the former reaction. The strongest sulfine bands, at 1165 and 767 cm-l in the matrix, have been previously reported for the gas phases2 A study of the relative intensities of excited state satellites in the microwave spectra indicated the existence of an additional vibrational level 369 f 20 cm-l above the ground state.4 Our data also confirm this observation, and the assignments of these and the other fundamentals are rather straightforward when sulfine is compared to specific model compounds. For a planar molecule like sulfine, which belongs to the C, point group, 7A’ and 2A” vibrations are expected. The assignments we propose for these nine fundamental modes are indicated in Tables I and 11. The CH2 vibrations appear to be “normal” for a vinylic moiety1’ rather than shifted as in the cases9JoJ2of H,C=O, H&=S, and H2C=C=0. The skeletal vibrations are similar in frequency to those for the isoelectronic sulfur dioxide molecule.s Accordingly we have designated v4 and
All bending coordinates have been weighed by 1 A .
as antisymmetric and symmetric skeletal stretches, respectively, rather than CS and SO stretches. Since a normal sample of sulfine consists of 4.2 % of the 34Sisotope, there exists the possibility that isotopically shifted bands for the skeletal vibrations will be observable in the spectra of sulfine. The inherent sharpness of the bands in the spectra of a matrix and the instrumental resolution are important in a search for the isotope bands. The only apparent manifestation of an isotope effect is the weak feature at 1158 cm-l which should correspond to the symmetric skeletal stretch for H2C34S0. The assignments in Table I1 readily lend themselves to a normal coordinate cal~ulation.’~The results of the calculations are only briefly indicated in Table 11. The frequency fit is very good, but the force constants in Table I11 were determined from only nine experimental frequencies. The force constants for the skeletal vibrations are very similar to those for sulfur d i o ~ i d e . ~Using J ~ the derived force constants, we predicted the vibrational spectra of H2C34S0and D2CS0. The only strong band which is expected to exhibit any significant shift for 34S substitution is the symmetric CSO stretch. The predicted shift is approximately 8 cm-l, while the observed shift is about 7 cm-l, as noted above. The predictions for D&SO will be tested in the near future as further experiments with sulfine proceed. vg
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society (E.B.),and to the Research Corporation (W.C.H.) for partial support of this research. W.C.H. also acknowledges the support of the Camille and Henry Dreyfus Foundation, provided by a TeacherScholar Grant. V.F.K. acknowledges the use of the facilities of the Thomas E. Tramel Computing Center. Miniprint Material Available: Full-sized photocopies of Table I (1page). Ordering information is available on any current masthead page. References and Notes (1) W. A. Sheppard and J. Diekmann, J . Am. Cbem. Soc., 88, 1891 (1964); W. M. Doane, 8. S. Shasha, C. R. Russell, and C. E. Rist, J. Org. Cbem.,30, 3071 (1965); C. N. Skold and R. H. Schlesslnger, Tetrahedron Lett., 791 (1970). (2) E. Block, R. E. Penn, R. J. Olsen, and P. F. Sherwin, J . Am. Cbem. Soc., 98, 1264 (1976). (3) E. Block, H. Bock, S. Mohmand, P. Rosmus, and B. Solouki, Angew. Cbem., Int. Ed. Engl., 15, 383 (1976). (4) R . E. Penn and R. J. Olsen, J . Mol. Spectrosc., 61, 21 (1976). (5) L. K. Revelle, Ph.D. Thesis, University of Missouri-St. Louis, 1979. (6) E. Whittle, D. A. Dows, and G. C. Plmentel, J. Chem. fbys., 22, 1942 (1954); R. R. Smardzewski and L. Andrews, ibid., 57, 1327 (1972). (7) E. Block, E. R. Corey, R. E. Penn, T. L. Renken, and P. F. Sherwin, J. Am. Cbem. Soc., 98, 5715 (1976). (8) R. N. Wiener and E. R. Nlxon, J . Cbem. Pbys., 25, 175 (1956); A. Anderson and M. C. W. Campbell, bid., 67, 4300 (1977). (9) G. Herzberg, “Molecular Spectra and Molecular Structure 11. Infrared and Raman Spectra of Polyatomic Molecules”, Van Nostrand, New York, 1945.
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The Journal of Pbysical Cbemistty, Vol. 83, No. 14, 1979
(10) M. E. Jacox and D. E. Mllligan, J . Mol. Spectrosc., 58, 145 (1975). (11) W. J. Potts and R. A. Nyquist, Spectrocbim Acta, 15, 679 (1959). (12) C. B. Moore and G. C. Pimentel, J . Cbem. fbys., 38, 2816 (1963); 40, 342 (1964); W. H. Fletcher and W. T. Thompson, J . Mol. Spectrosc., 25, 240 (1968); D. C. McKean and J. L. Duncan, Spectrocbim. Acta, 27, 1879 (1970); C. Pouchan, A. Dargelos, and
A. R. Watkins M. Chaillet, Spectrocbim. Acta, far? A , 33, 253 (1977). (13) J. H. Schachtschneider, "Vibrational Anatysls of Polyatomic Molecules, V and VI", Technical Report No. 231-64 and 57-65, Shell Development Company, Emeryville, Calif. (14) K. Ramaswamy and R. Srlnivasan, Acta fbys. folon., A51, 139 (1977), and references therein.
Electronic Processes of Exciplexes of Anthanthrene with Substituted Dimethylanilines A. R. Watkins" Max-flanck-Institut fur Biophysikalische Chemie, 3400 Gottingen-Nikolausberg,West Germany (Received February 14, 1979) fubllcatlon costs assisted by Max-flanck-Institut fur Biophysikallsche Chemie
Rate constants for fluorescenceemission, intersystem crossing,and radiationless deactivation have been measured for a series of exciplexes of anthanthrene in n-hexane solution as a function of exciplex energy. With decreasing exciplex energy fluorescence emission rapidly becomes inefficient; intersystem crossing within the exciplex shows little variation except when heavy atoms are present. No correlation could be found between exciplex energy and the rate of dissociation of the exciplex into the primarily excited molecule and the quencher.
Exciplexes, because of their importance as potential intermediates in photochemical and photobiological reactions and as possible chemical lasers, are becoming increasingly interesting as objects of study in their own right.' They are characterized by broad emission spectra, red-shifted with respect to the parent molecule, and generally possess highly polar structures. A good deal is already known about the thermodynamics and kinetics of their formation, and several theoretical studies have ap~ e a r e d . ~It B has, however, proved more difficult to obtain information about the exciplexes themselves; the broad spectra preclude high-resolution spectroscopic studies, and the sparse information that we posses about their structure comes mainly from laser studies of exciplex absorption spectra.' Even more sparse is information concerning the way in which exciplexes decay;4there have appeared very few studies having as their aim the measurement of all the rate processes leading to the decay of exciplex systems. This paper attempts to partially remedy this deficiency. The rate processes which lead to the disappearence of an exciplex can be regarded as being of four kinds: dissociation into the excited parent molecule and quencher molecule (the so-called "feedback" step1), emission of fluorescence, intersystem crossing (ultimately to the triplet state of the parent molecule or quencher, depending on which has the lower energy triplet state), and radiationless decay to the (repulsive) ground state of the exciplex. In polar solvents a fifth step (dissociation into radical ions of the component molecules) may occur; we are concerned here with nonpolar solvents and with exciplex systems which do not lead to any irreversible photochemical step. The investigations t o be described here deal with exciplexes of anthanthrene with a number of derivatives of dimethylaniline, the resulting similarity in the structure of the exciplexes providing the basis for comparing the properties of these exciplexes with one another. Experimental Section Anthanthrene (hereafter abbreviated to A) was purified by repeated recrystallization; 1,12-benzperylene (hereafter BP) was purified according to the method of Kajiwara et *Address correspondence to the author at CSIRO Division of Chemical Physics, P.O. Box 160, Clayton, Victoria 3168, Australia. 0022-3654/79/2083-1892$01 .OO/O
TABLE I: Oxidation Potentials, Exciplex Energies, and Quenching Kinetics for 1,12-Benzperylene and Anthanthrene Quenched by Substituted N,N-Dimethylanilinesa fluorescer
donor
1,12-benzperylene
p-Cl-DMA 3,5-DMDMA p-Me-DMA 2,4-DMDMA 3,4-DMDMA TMPD anthanthrene p-Cl-DMA 3,5-DMDMA p-Me-DMA 2,4-DMDMA 3,4-DMDMA TMPD
E ' , eV KL,M-' 2.96 2.88 2.77 2.65 2.58 2.28 2.66 2.58 2.47 2.35 2.28 1.98
328 504 1186 1840 2134 3000 127.5 152.0 183 187 217 213
k,, lo9 M-'s-' 1.73 2.65 6.24 9.68 11.23 15.8 17.2 20.5 24.7 25.3 29.3 28.8
a The anilines used, together with their abbreviations and oxidation potentials vs. SCE, are as follows: p-chloroN,N-dimethylaniline (p-C1-DMA, 0.84 V); 3,5-dimethoxyN,N-dimethylaniline (3,5-DMDMA, 0.76 V ) ; p-methylN,N-dimethylaniline (p-Me-DMA, 0.65 V ) ; 2,4-dimethoxyN,N-dimethylaniline (2,4-DMDMA, 0.53 V); 3,4dimethoxy-N,N-dimethylaniline(3,4-DMDMA, 0.46 V ) ; N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD, 0.16 V).
aL5 Purified samples of anthanthrene or 1,12-benzperylene
showed no indications of impurities in their fluorescence and flash photolysis spectra, and were used for all subsequent experiments. The amines used were all derivatives of N,N-dimethylaniline;they were prepared by methylation of the corresponding substituted amines followed by purification by vacuum distillation or by successive recrystallization. The abbreviations by which the amines are referred to in this paper as well as their oxidation potentials can be found in Table I. The solvent used in this work, n-hexane, was purified by distillation. All measurements were carried out at room temperature on samples which had been degassed by successive freezepump-thaw cycles. Fluorescence quenching measurements were carried out in a cuvet without stopcocks6with a Hitachi-Perkin-Elmer MPF2A spectrofluorimeter with digital readout; a typical fluorescence spectrum is shown in Figure 1. Corrected @ 1979 American Chemical Society
