Quantum Induced Bond Centering in Microsolvated HCl: Solvent Separated versus Contact Ion Pairs

Nuclear quantum effects on the structure of dissociated HCl(H2O)4 clusters are studied using ab initio path integral simulations. This cluster supports two distinct zwitterionic species serving as minimal microsolvation models for solvent-shared and contact ion pairs (SIP and CIP, respectively) involving the dissociated acid, i.e., Cl– and H3O+. The SIP structure is not much affected by quantum effects apart from expected broadening of distance distribution functions. In stark contrast, the CIP structure is qualitatively changed: the hydrogen bond that directly connects the ion pair, i.e., the Cl– and H3O+ species, becomes both centered and fluxional as a result of zero point motion. Thus, the detached proton is “pushed back” to Cl– and thereby stabilizes a Zundel-like structure motif, Cl···H···OH2, in a low–barrier hydrogen bonding scenario, which is relevant to acid dissociation in the bulk at ultrahigh concentrations.

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