Gas-phase SN2 reactions of chloride ion with alkyl bromides at atmospheric pressure. Temperature dependence of the rate constants and energies of the transition states

Rate constants for the gas-phase reactions Cl - +RBr→RCl+Br - , where R=Me, Et, and n-Bu, have been determined over the temperature range 35-150 o C by a kinetic ion mobility mass spectrometer (KIMMS) at 640 Torr of buffer gas pressure and by a pulsed e-beam high-pressure mass spectrometer (PHPMS) at 3 Torr of buffer gas pressure. For the case of R=Me, different rate constants and different temperature dependencies of the rate constants are observed at 640 versus 3 Torr of buffer gas pressure. This effect of pressure is thought to be due to increased collisional stabilization of the entrance-channel intermediate, (Cl - .MeBr), with increased pressure