A ab initio moleular orbital study of molecular interactions between formic acid and ammonia.

A molecular orbital study of molecular interactions between formic acid and ammonia was performed using the ab initio LCAO-SCF-MO method. As primitive functions, the STO-3G basis set, in which valence properties are comparable to those of the STO set, was used. The interaction energies between NH4+ and HCOO-, between NH4+ and HCOOH, between NH3 and HCOO-, and between NH3 and HCOOH were calculated. For the NH4+-HCOO- complex, the structure containing two hydrogen bonds and in which the C2v axis of NH4+ coincides with that of HCOO- was the most stable. For the complex NH4+-HCOOH, the structure in which the NH bond of NH4+ forms a hydrogen bond with the carbonyl oxygen was more stable. For the complex of NH3 and HCOO-, O of HCOO- and the NH bond of NH3 form a linear hydrogen bond as the most stable structure. For the NH3-HCOOH complex, the structure in which the nitrogen lone pair of NH3 forms a linear hydrogen bond with the OH bond of HCOOH was the most stable. Those four structures are discussed in connection with the interaction between lysine and glutamate (or aspartate), and the hydrolysis of esters, taking account of the interaction energies between HCOOH and H2O and between HCOO- and H2O, and the stabilities in vacuum and in nonpolar solvents.