Effect of Substituents on Hydrogen Bond Strength in Hydrogen-Bonded <em>N</em>-methylacetamide and Uracil Complexes

Abstract

Theoretical calculations on a series of N-H…O=C hydrogen-bonded complexes containing 1-methyluracil and N-methylacetamide were carried out using B3LYP and MP2 methods. Substituent effects in the hydrogen bond acceptor molecule (1-methyluracil) on the hydrogen bond strength and hydrogen bond cooperativity were explored. The calculation results show that electron donating groups shorten the H…O distance and strengthen the N-H…O=C hydrogen bond, whereas electron withdrawing groups lengthen the H…O distance and weaken the N-H…O=C hydrogen bond. Natural bond orbital (NBO) analysis further indicates that electron donating groups result in a larger positive charge on the H atom and a larger negative charge on the O atom in the N-H…O=C bond, and result in increased charge transfer between the proton donor and acceptor molecules. Electron withdrawing groups show the opposite results. NBO analysis also indicates that electron donating groups result in larger second-order interaction energies between the oxygen lone pair and the N - H antibonding orbital when compared to the 1-methyluracil-containing complex (R=H), while electron withdrawing groups result in smaller second-order interaction energies.