Photofragment energy distributions and dissociation pathways in dimethyl sulfoxide

Abstract

Photolysis of dimethyl sulfoxide in a molecular beam with 210 and 222 nm photons reveals the decomposition mechanism and energy disposal in the products. Using vacuum ultraviolet light and a time-of-flight spectrometer, we identify CH 3 and CH 3 SO as primary fragments and CH 3 and SO as secondary fragments. From CH 3 quantum yield measurements, we find that secondary decomposition is minor for 222 nm photolysis, occurring in only about 10% of the fragments, but it increases to about 30% in the 210 nm photolysis. Laser-induced fluorescence measurements on the B 3 ⌺ Ϫ ←X 3 ⌺ Ϫ transition of SO in the 235 to 280 nm region determine the internal energy of that photoproduct. We compare our results to a simple statistical model that captures the essential features of the decomposition, predicting both the extent of secondary decomposition and the recoil energy of the primary and secondary methyl fragments.

Three-Body photodissociation of thionyl chloride

“…Since the SOCl + transients reflected the temporal evolution of the SOCl intermediate, the 8.7 ps component was consistent with the decay of hot SOCl produced in the stepwise pathway. This assignment was in agreement with the previous studies, which suggested that the photodissociation of SOCl 2 at 235 nm underwent a stepwise dissociation process and produced the fragement of Cl with quite high quantum yield [34,35].…”

Photoabsorption and photodissociation studies of dimethyl sulphoxide (DMSO) in the 35,000–80,000 cm−1 region using synchrotron radiation

“…It may be noted that appearance of spin forbidden transitions due to spinorbit interactions are not unusual in the spectra of [33]. It may be mentioned here that inverted vibrational population distributions for the photodissociated SO fragment have been reported earlier [2,26,27]. In particular for photoexcitation at 193, 210 and 220 nm, population has been found to be maximum in the v″ ¼2, 1 and 1 levels, respectively.…”

“…Table 6), which is well above the energy required to break the two C-S bonds in (CH 3 ) 2 SO, and results in the formation of the primary products CH 3 and SO [29]. Photodissociation via the electronic σ n 'n and π n 'n transitions at 210 and 222 nm (5.90 and 5.58 eV), respectively, have also been shown to release the primary products CH 3 and SO [2]. It has been established from these experiments [2,4,[26][27][28][29][30]] that most of the other possible fragments require extensive nuclear rearrangements that make their production unlikely.…”

“…Photodissociation via the electronic σ n 'n and π n 'n transitions at 210 and 222 nm (5.90 and 5.58 eV), respectively, have also been shown to release the primary products CH 3 and SO [2]. It has been established from these experiments [2,4,[26][27][28][29][30]] that most of the other possible fragments require extensive nuclear rearrangements that make their production unlikely. Most of these reports suggest that DMSO photolysis proceeds through initial loss of one methyl group at low energies followed by secondary decomposition of CH 3 SO to produce the second methyl group and SO fragments rather than a direct three-body fragmentation to release two methyl groups and SO as primary products.…”

“…Its ground state configuration is given by [core] (7a 0 ) 2 (8a 0 ) 2 (3a″) 2 (9a 0 ) 2 (10a 0 ) 2 (11a 0 ) 2 (4a″) 2 (5a″) 2 (12a 0 ) 2 (6a″) 2 (13a 0 ) 2 (7a″) 2 (14a 0 ) 2 : X 1 A 0 . DMSO is the sulphur analogue of the well-studied acetone molecule [2]. The simple molecular orbital (MO) picture in the acetone molecule where the highest occupied MO (HOMO) is a lone pair localized on the oxygen atom is disturbed in DMSO, as the sulphur d orbitals also contribute to the MOs and considerable delocalization is seen [4].…”