Discrimination between O-H…N and O-H…O=C Complexes of 3-Methyl-4-pyrimidone and Methanol. A Matrix-isolation FT-IR and Theoretical DFT/B3LYP Investigation

Abstract

FT-IR matrix-isolated spectra for 3-methyl-4-pyrimidone and its H-bonded complexes with methanol in Ar were studied with the aim of discriminating between O-H…N and O-H…O=C complexes. Theoretical calculations were carried out using the DFT/B3LYP/6-31+G(d) methodology in an attempt to predict the preferred interaction site of the 3-methyl-4-pyrimidone molecule with proton donors. The observed frequency decrease of the ν(C=O) mode of 3-methyl-4-pyrimidone and the appearance of a broad ν(OH…O) band in the spectrum of the complex with methanol suggest that H-bonding with methanol occurs at the carbonyl group. Computed binding energies of the hydrogen-bonded complexes (ΔE_c) and computed intermolecular distances (r(O…H)) confirm that the O-H…O=C complex is preferred with methanol. However, for H-bonding with stronger acids such as HCl, the computational data suggest that the H-bonding occurs at the N₁ ring atom of 3-methyl-4-pyrimidone.

Keywords: Matrix-isolation, 3-methyl-4-pyrimidone, methanol, FT-IR spectroscopy, DFT/B3LYP calculations