Matrix isolation FTIR study of hydrogen-bonded complexes of methanol with heterocyclic organic compounds

Abstract

Acting as hydrogen bond acceptors, heterocyclic compounds could interact with a hydrogen bond donor, where either the heteroatom or the π system is the bonding site. The hydrogen bonded complexes of heterocyclic compounds with methanol (MeOH) were studied using matrix isolation FTIR spectroscopy and theoretical calculations based on density functional theory. Four heterocyclic compounds, furan (Fu), 2,5-dihydrofuran (DHF), pyrrole (Py) and thiophene (Th) were selected as representative examples for acceptors. For each of the MeOH-Fu, MeOH-DHF and MeOH-Th complexes, the O-H⋯π and O-H⋯Y (Y=O, S) hydrogen bonded structures were obtained, while an N-H⋯O instead of O-H⋯N hydrogen bonded conformer was found in the MeOH-Py complex. The measured OH-stretching transitions of the complexes in the IR spectra were assigned to the O-H⋯O bonded MeOH-Fu (b) and MeOH-DHF (b) conformers and the N-H⋯O bonded Py-MeOH (b) conformer, respectively. However, it was hard to assign the spectra to the exact MeOH-Th conformer, because all the hydrogen bond characteristic features obtained for different MeOH-Th conformers are too close. DHF forms a stronger O-H⋯O hydrogen bond than furan, and the O-H⋯O hydrogen bonded MeOH-Fu complex is more stable than the O-H⋯S bonded MeOH-Th complex. Atoms in molecules analysis was also performed to understand the nature of interaction in the MeOH complexes. This analysis allowed us to characterize the new bond critical point generated in the complexes. The present results help to evaluate the atmospheric behavior of some heterocyclic compounds, and indicate their importance in the pre-nucleation mechanism at the molecular level.