Comparison of gas-phase and solution-phase reactions of dimethyl sulfide and 2-(methylthio)ethanol with hydroxyl radical

The reaction of the OH radical with dimethyl sulfide (DMS) and 2-(methylthio)ethanol (2-MTE) proceeds with the initial formation of a two-center-three-electron complex. In the gas phase the S-OH binding enthalpies (298 K) are 8.7 and 12.2 kcal/mol for DMS and 2-MTE, respectively. When entropy and aqueous solvation effects (via the CPCM method) are included, the free energies of association (298 K) of hydroxyl to DMS and 2-MTE become 3.0 and 3.2 kcal/mol, respectively. Calculations are based on DFT and/or MP2 optimizations and a G2-like method for evaluating energies. The most favorable (lowest free energy) conformation is often different between the gas phase and solution phase. Electron transfer from 2-MTE/2-MTE+ (1/1(+)) to OH/OH- has a positive free energy of 4.5 kcal/mol and is in competition with the acid-/base-catalyzed formation of CH3SCH2CH2O (2) plus water. The latter radical (2) undergoes intramolecular hydrogen transfer to form CH2SCH2CH2OH (3) or eliminates formaldehyde to form CH2SCH3+H2C=O, where the free energy barriers are 7.9 and 8.3 kcal/mol, respectively. The 2-MTE cation (11) can eliminate a C-H proton to form three different radicals that are within 2.0 kcal/mol of each other in free energy.