Effect of Substituents on Hydrogen Bond Strength in N-H•••O=C Hydrogen Bond Trimers

Theoretical calculations on a series of N-H center dot center dot center dot 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 center dot center dot center dot O distance and strengthen the NH center dot center dot center dot O=C hydrogen bond whereas electron withdrawing groups lengthen the H center dot center dot center dot O distance and weaken the N-H center dot center dot center dot O=C hydrogen bond. Natural bond orbital (NBO) analysis further indicates that a stronger hydrogen bond in NH center dot center dot center dot O=C results in a larger positive charge for the H atom and a larger negative charge for the O atom in the N-H center dot center dot center dot O=C bond. Natural bond orbital analysis also indicates that a stronger hydrogen bond in N-H center dot center dot center dot 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 center dot center dot center dot O=C molecules closer to the substituent will be affected more.