Methylsulfonyl and Methoxysulfinyl Radicals and Cations in the Gas Phase. A Variable-Time and Photoexcitation Neutralization−Reionization Mass Spectrometric and ab Initio/RRKM Study

Abstract

The title radicals were produced by femtosecond collisional electron transfer in the gas phase and studied by the methods of variable-time neutralization−reionization mass spectrometry combined with fast-beam laser photoexcitation and G2(MP2) ab initio/RRKM calculations. The methylsulfonyl radical (CH3SO2 •, 1) was calculated to be bound by 59 kJ mol-1 against the lowest-energy dissociation to CH3 • and SO2 at 0 K and to have a heat of formation Δ H f,298(1) = −211 ± 4 kJ mol-1. When formed by vertical electron transfer, radical 1 dissociated rapidly due to a large Franck−Condon energy, E FC = 141 kJ mol-1. The reverse addition of CH3 • to the sulfur atom in SO2 had a potential energy barrier of 1.3 kJ mol-1 and Arrhenius parameters, log A = 12.19 and E a = 5.4 kJ mol-1. The calculated addition rate constant, k 295 = 1.7 × 1011 cm3 mol-1 s-1, was in excellent agreement with the previous measurement of Simons et al. The methoxysulfinyl radical (CH3OSO•, 2) was calculated to exist as an equilibrium mixture of syn (2s) and anti (2a) conformers. The Boltzmann-averaged heat of formation of 2 was calculated as Δ H f,298(2) = −230 ± 4 kJ mol-1. Vertical neutralization of ions 2s + and 2a + produced substantial fractions of stable 2s,a. Dissociating 2s,a formed CH3 • and SO2 through unimolecular isomerization to 1. Direct dissociation of the C−O bond in 2s,a to form CH3 • and SO2 was calculated to have a large activation barrier (152 kJ mol-1 from 2a) and did not compete with the isomerization to 1, which required 111 kJ mol-1 from 2a. Photoexcitation of 2s,a resulted in a slightly increased formation of 2s,a +. This was interpreted with the help of CIS/6-311+G(3df,2p) calculations as being due to the formation of a bound excited B state of 2s upon electron transfer. The B state was photoexcited at 488 and 514.5 nm to high Rydberg states which were predicted to have large cross sections for collisional ionization. The A state of 2s was calculated to be bound but photoinactive. The C through E states of 2s were unbound and predicted to dissociate exothermically to CH3OS and (3P)O.

Infrared absorption of CH3SO2 observed upon irradiation of a p-H2 matrix containing CH3I and SO2

“…11 Several theoretical computations predict the energy, geometry, and vibrational wavenumbers of CH 3 SO 2 and methoxysulfinyl radical (CH 3 OSO). [12][13][14][15][16][17][18] According to the most extensive computations, reactions of CH 3 with SO 2 might proceed via two paths: a barrierless channel to produce CH 3 SO 2 and a channel with a barrier of 49-58 kJ mol −1 to produce two conformers of CH 3 OSO, syn-CH 3 OSO, and anti-CH 3 OSO; anti-CH 3 OSO might transform via a barrier of ∼1 kJ mol −1 to syn-CH 3 OSO that is more stable than anti-CH 3 OSO by ∼8 kJ mol −1 , and more stable than CH 3 SO 2 by 21−37 kJ mol −1 . [16][17][18] Isomerization from CH 3 SO 2 to syn-CH 3 OSO is unlikely because of a large barrier of ∼200 kJ mol −1 .…”

Dissociation dynamics of the methylsulfonyl radical and its photolytic precursor CH3SO2Cl

“…The formation of SO 2 and its subsequent conversion into the sulfate anion ͑SO 4 2− ͒ is of significant environmental concern due to the latter's role in acid rain and its effect on global climate. While the major anthropogenic source of SO 2 is the combustion of sulfur-containing fossil fuels, the major natural sources of SO 2 in the troposphere are from volcanic injection and the atmospheric oxidation of both DMS and carbon disulfide ͑CS 2 ͒.…”

“…Previously published theoretical work pertaining to the CH 3 SO 2 system highlights the need for concrete experimental data regarding the energetics along the dominant reaction channels. [4][5][6] Table I gives the barrier heights for the dissociation of CH 3 SO 2 to CH 3 +SO 2 , as calculated at various levels of theory. Zhu and Bozzelli 5 compared their computational results and concluded that, while G3 methods gave good energetics, the CCSD͑T͒ method was unreliable.…”

Isomerizations of CH3SO2: Quantum Chemistry and Topological Study

“…[3][4][5] CH 3 SO 2 might be produced either from reactions of CH 3 SO with NO 2 and O 3 , 4,5 or from reactions of OH or O atom with dimethyl sulfoxide. 6,7 The neutralization-reionization mass spectrometry, infrared absorption fourier-transfrom and ESR spectroscopy of CH 3 SO 2 and CH 3 OSO have been detected by Frank, 8 Lee 9 and Walton et. al.…”

“…The decompositions of CH 3 SO 2 and CH 3 OSO to CH 3 and SO 2 have also been studied by theoretical method. 8,9,12,13 However, there has been no theoretical or experimental study on the stability and isomerizations of other isomers. In order to gain some insight into the character of CH 3 SO 2 isomers, in this paper, the stability and isomerization were investi-gated by theoretical method.…”