Abstract:

Theoretical calculations on a series of N—H…O=C hydrogen bond trimers were carried out using the MP2 method. We investigated the effect of substituents in hydrogen bond donor molecules on their hydrogen bond strength. The calculated 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 a stronger hydrogen bond in N—H…O=C results in a larger positive charge for the H atomand a larger negative charge for the O atomin the N—H…O=C bond. Natural bond orbital analysis also indicates that a stronger hydrogen bond in N—H…O=C results in a more charge transfer between the hydrogen bond donor and acceptor molecules, and results in a stronger second-order interaction energy between the oxygen lone pair and the N—H antibonding orbital. Hydrogen bonds in the N—H…O=C molecules closer to the substituent will be affected more.

Key words: Hydrogen bond trimer, Substituent, Hydrogen bond strength, Natural bond orbital analysis