Probing the Role\nof Solvent Configurations and Local\nElectric Fields on HX (X = F, Cl, Br and I) Dissociation in Water\nClusters

The acidity of hydrohalic acids increases down the group,\nwith\nHF and HI being the weakest and strongest acids. Electronic structure\ncalculations suggest that the critical electric fields required for\nthe dissociation of HF, HCl, HBr, and HI are 347, 193, 163, and 153\nMV cm<sup>–1</sup>, respectively, which are proportional to\ntheir corresponding p<i>K</i><sub>a</sub> values and emphasize\nthat in these systems the bond dissociation energy determines the\np<i>K</i><sub>a</sub>. The solvent configuration plays a\nsignificant role in the acid dissociation process, which is illustrated\nby a particular configuration of three water molecules around HX and\nfavors dissociation of only HBr, even though the critical electric\nfield required for the dissociation of HI is lower than that of HBr,\nas depicted in the graphical abstract. Further, the Born–Oppenheimer\nmolecular dynamics (BOMD) simulations suggest that the spontaneity\nof HX dissociation depends on both the solvent configuration and thermal\nfluctuations, which hold higher significance in the case of weaker\nacids. In general, it was observed that the solvent electric field\nrequired for the acid dissociation process is marginally lower at\nhigher temperatures.