Effects Of Substituents On The Hydrogen Bonding Energies In Hydrogen-bonded Dimers

Theoretical calculations on a series of the hydrogen-bonded dimers are carried out usingthe MP2 and B3LYP method. The geometry optimizations, Single point energies and Basis set superposition error (BSSE) are calculated. and the effects of substituents on the hydrogen bonding energies are therefore explored. The work of this paper can divide into the two parts as following:1. Using the formamide dimer which has one hydrogen bond as model, the effects of substituents on the hydrogen bonding energies are explored. Calculated results indicate that both electron donating groups and electron withdrawing groups can regulate the strength of the N-H…O=C hydrogen bond. We found that the electron donating groups, such as -Et, have little effect on the N-H…O=C hydrogen bond whereas the strong electron withdrawing groups, such as -NO₂ on the proton acceptor molecules can weaken the hydrogen bond as much as 2.6 kcal/mol, -CN on the proton donor molecules can strength the hydrogen bond as much as 4.6 kcal/mol.2. Using the benzoic acid-pyridine dimer as model, the effects of substituents on the hydrogen bonding energies are explored. Calculated results indicate that the -NH₂,-OCH3,-OH groups in acceptor molecule strengthen the O-H…N hydrogen bond but weaken the O-H…N hydrogen bond in donor molecule. The -COCH₃,-CHO,-NO₂ groups in acceptor molecule weaken the O-H…N hydrogen bond but strengthen the O-H…N hydrogen bond in donor molecule.