Study of the Electronic Structure of Molecules. II. Wavefunctions for the NH3+HCl→NH4Cl Reaction

Abstract

Ab initio computations are presented for the reaction NH3+HCl→NH4Cl. The two reactants are studied at a large number of positions and for each point an SCF LCAO MO wavefunction and the corresponding total energy are obtained. These results are then collected in an energy surface diagram. All the electrons of the system are considered and a contracted Gaussian basis set has been used in this work. This study considers the two reactants as one system and both charge transfer as well as hydrogen-bonding characteristics are analyzed within the molecular-orbitals framework instead of the valence-bond approximation as customary in the past. The reaction has been analyzed for the case where the HCl approaches the NH3 molecule in a path normal to the plane of the three hydrogens of NH3; the H atom of HCl is between the N atom and the Cl atom. The first stage of the reaction (NH3 and HCl at large distances) indicates mutual polarization of the two molecules. In this region the SCF LCAO MO approximation is rather poor since it can not take into consideration dispersive forces. With HCl closer to the ammonia molecule, there is first hydrogen bonding formation and partial charge transfer. When NH4Cl is formed the molecule is stable with respect to the NH3 and HCl components by about 19 kcal/mole. No activation energy has been found in the reaction, contrary to previous assumptions. The computed geometry of the NH4Cl molecule at the equilibrium configuration is discussed in this work.