Theoretical Study On The Effects Of Substituents On The Hydrogen Bonding Strength In N-H···O=C Hydrogen Bond Trimers

Hydrogen Bonds plays an important role in chemistry, physical and life science area, so the research for its essence, structure and energy is definitely crucial.In series of every kind of hydrogen bond, the N-H···O=C hydrogen bond is of great significance on protein, nucleic acid dns any other biology molecule system. The research that substituents, hydrogen cooperative effect and any others influence on the hydrogen bonding strength of N-H···O=C hydrogen bond is far-reaching for the hydrogen bond interactions on life systems.Theoretical calculations on a series of the N-H···O=C hydrogen bond trimers were carried out using the MP2 method and the effects of substituents in the hydrogen bond donor molecule on the hydrogen bond strength were therefore explored. The calculated results show that the electron donating groups shorten the H···O distance and strengthen the N-H···O=C hydrogen bond whereas the electron withdrawing groups lengthen the H···O distance and weaken the N-H···O=C hydrogen bond. The natural bond orbital analysis further indicate that the stronger the N-H···O=C hydrogen bond is, the more positive charge the H atom possesses and the more negative charge the O atom possesses in the N-H···O=C hydrogen bond, the more charge transfer take places between the hydrogen bond donor and acceptor molecules, and the stronger the second-order interaction energy becomes between the oxygen lone pair and the N-H antibonding orbital. The N-H···O=C hydrogen bond, closer to the substituent will be affected more greatly.