Thermal transitions between collisionally and tight-complex limited kinetics in the gas phase. Reactions involving carbonium ions

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Abstract
H− transfer reactions of C2H5+ and s‐C3H7+ with C4–C8 tertiary hydrocarbons were found to exhibit transition from slow kinetics at high temperatures to fast kinetics at low temperatures. Above the transition temperature, characteristic to each reaction, the reactions proceed with less than unit efficiency and the rate constants exhibit significant negative temperature dependencies with the functional form k=AT−1.5 to k=AT−2.5. Below its transition temperature, each reaction becomes a typical fast ion–molecule reaction. In these lower temperature regions the rates are comparable to the calculated ion–molecule collision rates and are independent of further lowering of the temperature. The transition between temperature dependent and temperature independent behavior is interpreted on the basis of a mechanism proceeding through two stages each of which may be rate limiting. The first step is an ion–molecule collision whose rate is independent of temperature. The second step involves the formation of a tight complex. On the basis of transition state theory considerations, the second process is expected to exhibit a negative temperature dependence. Effects of structure on reaction kinetics were investigated in H− transfer reactions from C4–C8 tertiary hydrocarbons to C2H5+, s‐C3H7+, t‐C4H9+ and t‐C5H11+. The reactions of t‐C4H9+ and t‐C5H11+ are slower by about two orders of magnitude than analogous reactions of s‐C3H7+ and show large negative temperature dependencies up to k=AT−5.5.