Study of the Electronic Structure of Molecules. VI. Charge-Transfer Mechanism for the NH3+HCl⇆NH4Cl Reaction

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Abstract
In the NH3+HCl⇆NH4Cl reaction, analyzed via the population‐analysis method, we have determined that the nitrogen atom is capable of receiving pseudo‐π electrons from the hydrogens of the NH3 molecule and to transfer an equivalent amount of σ electrons to the chlorine atom. This π‐electron charge donated originally by the hydrogens, then accepted by nitrogen and finally donated by nitrogen as σ charge goes to the Cl atom. The hydrogen atom of HCl donates parts of its σ electronic charge to the Cl atom. Thus, the chlorine atom accepts σ electrons from the hydrogen of HCl and σ electrons from the nitrogen. Therefore, in addition to the familiar concepts of charge ``donor'' and ``acceptor'' we can make use of the concepts of charge ``transformer'' and charge ``dispatcher.'' The efficiency of the transformer is the ratio of the accepted charges of a given symmetry and the donated charges of a different symmetry. In the NH3+HCl reaction, the nitrogen atom is a π→σ transformer with nearly unit efficiency. The reaction NH3+HC⇆NH4Cl can be summarized as the process where the H3 group (of NH3) is π donor, the N atom is σ→π transformer (σ donor and π acceptor), the H (of HCl) is a charge dispatcher (transfer the σ charge of N to the Cl atom) and σ donor, and finally, the Cl atom is a σ acceptor. During the entire process the nitrogen atom has nearly constant charge of eight electrons.

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