Molecular insight into the amine–water interaction: A combined vibrational, energetic and NBO/NEDA study

Hydrates of the neutral and protonated cyclohexylamine and of the cyclohexyldiamine isomers in the neutral, mono and diprotonated forms are studied theoretically at the B3LYP/aug-cc-pVDZ level of theory. Vibrational analysis of the optimized complexes shows variations in the frequency and intensity of the N–H and O–H stretching modes induced by charge variation and groups’ proximity. These results are quantified by calculating the energies of complex formation (ΔEcomplex). It is shown that the values obtained for this quantity result from the balance between water association (ΔEass) and amine–water interactions (ΔEint). Amino groups’ approximation leads to values of ΔEint different from those calculated by adding their individual contribution. This behavior is explained by the application of the Natural energy decomposition analysis (NEDA) and Natural Bond Orbitals (NBO) theory to the complexes and to the amine–water pairs. The Atoms in Molecules (AIM) theory is also used to interpret the hydration of the diprotonated diamines. Special attention is given to the characterization of the hydrogen bonds’ nature, as well as to the effect of the hydrogen bonding cooperativity and charges proximity on the amine–water interaction.