Gas phase relative proton affinities from selected‐ion flow tube (SIFT) rate constant measurements

A new method of measuring equilibrium constants of proton transfer gas phase reactions is proposed. The method is based on measuring only the endothermic direction. The expression developed is: Kk_exp(BR)k_c/{(k_ck_exp)k′_c}, where k_exp is the measured rate constant, BR is the branching ratio of the proton transfer channel, k_c and k′_c are the forward and reverse collision rates. Two reaction systems were measured: the equilibrium between methanol and chloroacetonitrile and the equilibrium between ethanol and acetaldehyde. Both equilibrium constants were calculated by measuring forward end reverse rate constants and by the new method. There is good agreement between the two methods and with previously published values.