Dissociation chemistry of hydrogen halides in water

Abstract

To understand the mechanism of aqueous acid dissociation, which plays a fundamental role in aqueous chemistry, the ionic dissociation of HX acids (X=F, Cl, Br, and I) in water clusters up to hexamer is examined using density functional theory and Møller-Plesset second-order perturbation methods (MP2). Further accurate analysis based on the coupled clusters theory with singles and doubles excitations agrees with the MP2 results. The equilibrium structures, binding energies, electronic properties, stretching frequencies, and rotational constants of HX(H(2)O)(n) and X(-)(H(3)O)(+)(H(2)O)(n-1) are calculated. The dissociated structures of HF and HCl can be formed for n>/=4, while those of HBr and HI can be formed for n>/=3. Among these, the dissociated structures of HX (X=Cl, Br, and I) are more stable than the undissociated ones for n>/=4, while such cases for HF would require much more than six water molecules, in agreement with previous reports. The IR spectra of stable clusters including anharmonic frequencies are predicted to facilitate IR experimental studies. Undissociated systems have X-H stretching modes which are highly redshifted by hydration. Dissociated hydrogen halides show three characteristic OH stretching modes of hydronium moiety, which are redshifted from the OH stretching modes of water molecules.

Communication: A new ab initio potential energy surface for HCl–H2O, diffusion Monte Carlo calculations of D and a delocalized zero-point wavefunction

“…In those studies, the fragments' translational energy distributions were measured, allowing for the determination of the dimer's dissociation energy (D 0 ), which was reported to be 1334 ± 10 cm −1 . 1 Previous theoretical studies 19,21,36,37 have reported values of D 0 ; at the highest level of theory, CCSD/aug-cc-pVDZ+, 19 with an approximate anharmonic zero-point energy (ZPE) calculated from MP2/aug-cc-pVDZ+, a value of 1189 cm −1 was reported. This is not in good agreement with experiment.…”

“…[19][20][21][22][36][37][38] The global minimum with labeled coordinates is shown in Figure 1. As seen, this has a non-planar structure and, as can be surmised, there is an equivalent nonplanar minimum with a planar C 2v saddle-point connecting them through a bend of the θ 2 angle and a slight bend in the θ 3 angle.…”

Proton transfer and autoionization in HNO3·HCl·(H2O)n particles

“…In the first place, the obvious factor is the number of water molecules in the cluster (hereafter hydration number). It is known that increasing hydration number enhances proton transfer in anionic systems, 36,37 as well as in neutral aggregates of water and one acid (HCl, HBr or HNO 3 ) [38][39][40] where at least four water molecules seemed to be required to stabilize the ionic pair formed. We find here a similar effect, albeit with a smaller hydration number, for HCl.…”

Electric Dipole Moments of Nanosolvated Acid Molecules in Water Clusters

“…One inquiry that persistently threads its way through this subject is: what is the minimum quantity of water molecules, (H 2 O) n , required to dissociate the acid [18]? According to calculations, the acidically dissociated structure is supported starting from n=4; most of the theoretical approaches agree that at this size the dissociated form represents the global minimum on the potential energy surface [18][19][20][21][22][23][24]. Experimentally, for neutral clusters the question has been probed by laser spectroscopy (see, e.g., [25][26][27][28]) but finding a conclusive spectroscopic signature of the onset of dissociation in a nanocluster is not straightforward.…”