Theoretical Study On The Hydrogen Bond Strength In Complexes Containing Base And N-methylacetamide

Theoretical calculations on a series of hydrogen-bonded complexes containing base andN-methylacetamide were carried out using B3LYP and MP2methods. The effects ofsubstituents on the hydrogen bond strength were explored. This paper contains two parts asfollowing:1.Using the N－H…O＝C hydrogen-bonded complexes containing1-methyluracil andN-methylacetamide as models, the effects of substituents on the hydrogen bond strength wereexplored. The calculation results show that the electron donating groups shorten the H…Odistance and strengthen the hydrogen bond whereas the electron withdrawing groups lengthenthe H…O distance and weaken the hydrogen bond when the substituents are in the hydrogenbond acceptor molecule. The effects of substituents in the hydrogen bond donor molecule areopposite in results.Natural bond orbital (NBO) analysis indicates that there exists a linear relationshipbetween the hydrogen bond strength and the charge transfer between the proton donor andacceptor molecules in the hydrogen bond. There also exists a linear relationship between thehydrogen bond strength and the second-order interaction energy between the oxygen lone pairand the N－H antibonding orbital in the N－H…O＝C bond. Atoms in molecules (AIM)theory analysis indicates that there exists a linear relationship between the hydrogen bondstrength and the electron densities at hydrogen bond critical points.2. Using the hydrogen-bonded complexes containing base and N-methylacetamide asmodels, the hydrogen bond strength were calculated. The calculation results show that baseand N-methylacetamide are more apt to form hydrogen-bonded complexes thanN-methylacetamide and N-methylacetamide.