Exploring the non-covalent interactions behind the formation of amine-water complexes: the case of N-allylmethylamine monohydrate.

The conformational landscape of the monohydrated complex of N-allylmethylamine (AMA-w) was investigated for the first time using rotational spectroscopy from 8-20 GHz and quantum chemistry calculations. From a total of nine possible energy minima within 10 kJ mol, transitions for the two most stable conformers of AMA-w were detected, and assigned aided by DFT and ab initio MP2 predictions. The observed rotational transitions displayed characteristic hyperfine splittings due to the presence of the N quadrupolar nucleus. Quantum theory of atoms in molecules (QTAIM), non-covalent interaction (NCI) and natural bond orbital (NBO) analyses showed that the observed conformers of AMA-w are stabilized by two intermolecular interactions consisting of a dominant NH-O and a secondary C-HO hydrogen bond (HB) in which the water molecule acts simultaneously as a HB donor and acceptor. The HBs formed with water do not change the relative energy ordering of the most stable conformers of AMA but do affect the stability of higher energy conformations by disrupting the intramolecular forces responsible for their geometries. By comparing the intermolecular interaction energies with those of the monohydrates of the simplest primary (methylamine, MA), secondary (dimethylamine, DMA) and tertiary (trimethylamine, TMA) amines using symmetry-adapted perturbation theory (SAPT) calculations, we find that AMA forms the strongest bound complex with water. This is rationalized through the identification of subtle differences in stabilizing and destabilizing contributions across the amine-w series of complexes.