Gas phase (CH 3) 3Si + affinities of alkylamines and proton affinities of trimethylsilyl alkyl amines

Abstract

The association reactions of (CH 3) 3Si + with primary, secondary and tertiary alkyl amines have been studied using a high pressure mass spectrometer with pulsed electron beam. Standard enthalpy and entropy changes have been measured for (CH 3) 3Si + transfer between amines and between some amines and a reference base, ethyl benzoate. The thermodynamic data for the association of (CH 3) 3Si + with ammonia were obtained using ethyl acetate as the reference base. CH 3Si + affinities increase in the order NH 3 < CH 3NH 2 < C 2H 5NH 2 < n-C 4H 9HN 2 and the absolute values increase linearly with the proton affinity of the amine. s- and t-Butyl amines have slightly lower (CH 3) 3Si + affinities than n-butyl amine which may be due to steric effects. Such effects appear to be responsible for the very low (CH 3) 3Si + transfer rate between trialkyl amines for which no thermodynamic data were obtained. It was ascertained, however, that (CH 3) 3Si + afinities increase in the order primary < secondary < tertiary amine. The proton affinities of trimethylsilyl amines derived from the (CH 3) 3Si + binding enthalpies are slightly lower (2–5 kcal mol −1) than those of their t-butyl analogues, showing, in agreement with theory, that an α-silicon is a little less effective at stabilizing positive charge on nitrogen than is an α-carbon.