The role of the adiabatic principle in ion chemistry: a personal history

Prior to their measurement in the laboratory, several critical ionospheric ion–molecule reactions were excluded from models on the basis of the adiabatic principle of Ehrenfest. Following the development of versatile laboratory measurement techniques, a number of these reactions were found to be fast, implying that the adiabatic principle was generally inapplicable to thermal energy ion–molecule reactions. Examples of rare exceptions are shown. More recently, there have been successful applications of this theory as evidenced in Landau–Teller plots of vibrational quenching and excitation of various ions.