Theoretical calculation of isotope effects, kinetic energy release and effective temperatures for alkylamines

Abstract

The kinetic isotope effect (KIE) and kinetic energy release (KER) of protonated alkylamine dimers were studied by theoretical modeling. In the calculations on reaction kinetics one empirical parameter was used to describe the looseness of the transition state. Calculations are compared to experiments described by Norrman and McMahon [Int. J. Mass. Spetrom. 182/183 (1999) 381]. In the case of experiments using a high-pressure ion source the Rice–Ramsperger–Kassel–Marcus (RRKM) model, taking into account energy distributions and the time scale of metastable ion fragmentation, accurately describes the experimentally observed KIE’s of α-deuterated amines. The KER (available experimentally in one case only) is also correctly calculated, using no further parameters. In the case of low-pressure ion source, the internal energy distribution (IED) is not thermal, so it was empirically estimated based on the experimentally observed KIE. Using this estimate, it was possible to calculate the KER of α-deuterated amines accurately. Based on theoretical expectations it was found that the mean KER value is equal to 3/2 kT eff. This allows estimation of the KER in cases where it is determined by statistical factors. Energy distributions of various fragmenting ion populations are discussed in some detail. These may be helpful for a qualitative understanding of mass spectrometric processes and the theoretical basis of the kinetic method.