Quantum chemical study of the trimethylamine—hydrogen chloride complex

Abstract

The results of an ab initio quantum chemical study, carried out largely at the MP2 level of theory, are reported for the trimethylamine—hydrogen chloride complex. The resulting geometry, harmonic frequencies and properties such as dipole moment and electric field gradients at the Cl and N nuclei are compared with those of the parent monomers as well as those of the trimethyl-ammonium ion. The computed N…Cl distance, intermolecular stretching frequency and field gradients agree with experiment. The asymmetric NHCl stretch is found to be quite anharmonic and consequently vibrational averaging with respect to that mode has a significant effect on the dipole moment and electric field gradients. The calculated structure and frequencies point to a large degree of proton transfer and hence ionic character. This is confirmed by an analysis of the SCF binding energy by the constrained spatial orbital variation method as well as the results of population analyses that also suggest significant covalency.