Gas-Phase Reaction of Hydrated CO2•- Anion Radical with CH3I

Abstract

Hydrated CO 2 _ - anion radicals, CO 2 _ (H 2 O) N , are selectively prepared in an electron-impact free jet of CO 2 containing H 2 O. Mass spectrometric measurement reveals that CO 2 _ - (H 2 O) N reacts with CH 3 I to forman anion with [(CO 2 )(CH 3 I)] - stoichiometry. The product [(CO 2 )(CH 3 I)] - is further identified as the anion of acetyloxy iodide, CH 3 CO 2 I - , based on the observation that [(CO 2 )(CH 3 I)] - photodissociates at 532 nm into CH 3 CO 2 - + I or CH 3 CO 2 + I - channels. The CO 2 _ - (H 2 O) N + CH 3 I reaction thus presents a sharp contrast to the corresponding reaction in solutions: the gas-phase CO 2 _ - (H 2 O) N behaves as a carboxylation reagent for alkyl halides (RX), whereas in aqueous solutions the reaction proceeds as CO 2 _ - + RX → CO 2 + R + X - . Ab intio calculations suggest that CO 2 _ - (H 2 O) N can take on structures preferable for radical reactions: the hydration occurs on the O atoms of CO 2 _ - and the unpaired electron on the C atom remains uncovered with H 2 O solvents. The reaction mechanism of the CO 2 _ - (H 2 O) N + CH 3 I process is discussed in conjunction with previous results of (CO 2 ) N - + CH 3 I studies (J. Phys. Chem. 1997, A101, 5103).