Theoretical study of formation of ion pairs in (NH3•HCl)(H2O)6 and (NH3•HF)(H2O)6

We have performed theoretical studies on sixteen molecular cubes for both (NH3 center dot HCl)(H2O)(6) and (NH3 center dot HF)(H2O)(6). We use an empirical gauge, based upon the N-H and H-X bond lengths, to categorize the degree to which the cubes are neutral adduct or ion pair in character. On this basis, we describe all sixteen cubes of the former as highly ionized, but only five of the latter as greater than 85% ionic in character. Addition of one or two bridging water molecules to form (NH3 center dot HF)(H2O)(7) or (NH3 center dot HF)(H2O)(8) raises the percent ionic character to greater than 85% for these systems. The relative energy of the cubes can be categorized based on simple chemical principles. The computed vibrational frequency corresponding to the proton stretch in the N-H-F framework shows the highest degree of redshifting for systems near 50% ion-pair character. Molecular cubes close to neutral adduct or to ion-pair character show less redshifting of this vibrational motion.