Unraveling the dissociation of dimethyl sulfoxide following absorption at 193 nm

Abstract

We have studied the photodissociation of dimethyl sulfoxide, DMSO-h 6 and DMSO-d 6 , at 193 nm using the technique of photofragment translational spectroscopy with a tunable vacuum ultraviolet product probe provided by undulator radiation on the Chemical Dynamics Beamline at the Advanced Light Source. In contrast to previous investigations we have found the dissociation to proceed via a stepwise mechanism involving multiple reaction channels. The primary dissociation, S-C bond cleavage to eliminate a methyl radical, was found to have two competing channels with distinct translational energy distributions. The translational energy distribution for the major primary dissociation channel suggests that it proceeds in a statistical manner on the ground electronic surface following internal conversion. In competition with this channel is a primary dissociation that exhibits a translational energy distribution suggestive of dissociation on an excited electronic surface with most of the available energy partitioned into translational and electronic degrees of freedom. Secondary decomposition of the CD 3 SO intermediate was found to proceed exclusively via C-S bond cleavage, CD 3 SO→CD 3 ϩSO. However, secondary decomposition of the CH 3 SO intermediate was found to exhibit competition between CH 3 SO→CH 3 ϩSO and CH 3 SO→CH 2 SOϩH. The dissociation to CH 3 and SO was the major secondary decomposition channel with the translational energy distribution indicating a barrier to recombination of Ͼ8 kcal/mol. While a minor hydrogen atom elimination channel was found to play a role in secondary decomposition of CH 3 SO intermediates, no analogous secondary C-D bond cleavage was detected from the CD 3 SO intermediates indicating the importance of tunneling in the secondary decomposition of CH 3 SO.

Photofragment energy distributions and dissociation pathways in dimethyl sulfoxide

“…The first studies of the 193 nm photolysis using laser-induced fluorescence ͑LIF͒ detection of SO and resonance-enhanced multiphoton ionization ͑REMPI͒ of CH 3 suggest a concerted decomposition, 7 but a subsequent molecular beam photofragmentation experiment using photoionization mass spectrometric detection observes the CH 3 SO fragment and points to a stepwise process in which the CH 3 SO fragment dissociates if it contains sufficient energy. 8 Another photofragment measurement 9 finds that about 53% of the CH 3 SO fragments dissociate in the 193 nm photolysis, in agreement with the methyl yield of 1.4 Ϯ0.2 obtained using a diode laser to probe the methyl products of the 193 nm photolysis. 10 Our results complement the data from the short wavelength photolysis and allow us to explore the role of the initial electronic state by exciting either the *←n(d←n) transition at 210 nm or the *←n transition at 222 nm.…”

High resolution VUV photoabsorption cross section of dimethyl sulphoxide (CH3)2SO

“…Previous studies [8][9][10] have used laser photofragmentation at 193 nm (6.425 eV) to study the dissociation of DMSO, and have shown that DMSO undergoes a stepwise dissociation with the main dissociation channel (CH 3 ) 2 SO + hv ! CH 3 SO + CH 3 then CH 3 SO !…”

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

“…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.…”

“…2). Since it is very unlikely that the parent molecule produces SO 2 on photodissociation [4,[26][27][28][29][30], these are attributed to residual SO 2 , which is used for wavelength calibration. To the best of our knowledge, this is the first report of an extended vibronic band system associated with the e 1 Π-X 3 Σ À transition.…”

“…These details are discussed in what follows. The photodissociation dynamics of DMSO have been studied by several workers using 193 nm (6.42 eV) laser excitation [4,[26][27][28][29][30]. The electronic excitation at 6.42 eV corresponds to the π n 'π so transition (cf.…”