On the Relevance of Considering the Intermolecular Interactions on the Prediction of the Vibrational Spectra of Isopropylamine

Abstract

The effects of implicitly considering the effects of hydrogen bonding on the molecular properties, such as vibrational frequencies, were inferred on the basis of DFT calculations. Several clusters of isopropylamine were assembled and theoretically characterized. The results showed that maximum H-bond cooperativity is achieved when the amine group acts simultaneously as donor and acceptor. The effect of H-bond cooperativity manifests itself in the relative cluster stability and on the structural and vibrational frequency predictions. Referring to the vibrational frequencies it was found that the NH2 stretching and torsion vibrational modes are the most affected by the amine involvement in hydrogen bonding. Both stretching modes were found to be significantly redshifted relative to themonomer.TheNH2 torsionalmode, on the other hand, was found to be blueshifted up to 350 cm−1. Finally, the comparative study between the theory levels performed allows to conclude that the small 6-31G∗ basis set is able to stabilize weak C–H⋅ ⋅ ⋅N interactions as long as the new dispersion corrected DFTmethods are considered. The impairments observed with conventionalDFTmethods for describing weak interactionsmay be overcome with the improvement of basis set, but the associated increase of computational costs may turn the calculations unfeasible.