Methylsulfonyl and methoxysulfinyl radicals and cations in the gas phase. A variable-time and photoexcitation neutralization-reionization mass spectrometric and ab initio RRKM study

The title radicals were produced by femtosecond collisional electron transf er in the gas phase and studied by the methods of variable-time neutralizat ion-reionization mass spectrometry combined with fast-beam laser photoexcit ation 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 Delta H-f,H-298(1) = -211 +/- 4 kJ mol(-1). When formed by vertical electro n transfer, radical 1 dissociated rapidly due to a large Franck-Condon ener gy, 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 param eters, log A = 12.19 and E-a = 5.4 kJ mol(-1). The calculated addition rate constant, k(295) = 1.7 x 10(11) cm(3) mol(-1) s(-1), was in excellent agre ement with the previous measurement of Simons et al. The methoxysulfinyl ra dical (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 Delta H-f,H-298(2) = -230 +/- 4 kJ mol(-1). Vertical n eutralization of ions 2s(+) and 2a(+) produced substantial fractions of sta ble 2s,a. Dissociating 2s,a formed CH3. and SO2 through unimolecular isomer ization 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 f ormation 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 o f 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 b ut photoinactive. The C through E states of 2s were unbound and predicted t o dissociate exothermically to CH3OS and (P-3)O.