Isolating the spectral signature of H₃O⁺ in the smallest droplet of dissociated HCl acid

Abstract

The centrally important role of acids in aqueous chemistry has stimulated the search for the smallest droplet of hydrochloric acid. Based on several independent quantum calculations, this appears to be the HCl(H2O)4 cluster, which dissociates into the so-called solvent ion pair (SIP), H3O(+)(H2O)3Cl(-). Experimental verification of this prediction via infra-red spectroscopy is a major challenge and despite several recent reports of this SIP, there remains uncertainty about these observations. In this report, we present a calculation of the IR spectrum of the SIP in a fashion that isolates the contribution from the signature hydronium ion, H3O(+). The computed spectrum indicates that the vibrational states of H3O(+) are highly mixed, resulting in dispersed spectral features between 1300 and 3000 cm(-1), with the region between 2100 and 2900 cm(-1) being especially rich. These predictions point out the complexity of the SIP spectrum and offer guidelines for experiment. The energies of the HCl stretch fundamentals for three minima of the undissociated HCl(H2O)4 cluster are also reported.

Theoretical vibrational spectra of OH−(H2O)2: the effect of quantum distribution and vibrational coupling

“…In Table 4, we present the selected peak position and intensity calculated by the harmonic approximation, NM4D and NM6D models for conformer I that were used to make these figures. 27 Just like the present case, Mancini and Bowman mentioned that strong coupling of the stretching mode with other modes makes many combination bands and overtone bands in the vicinity of IHB OH stretching peak ''borrow'' intensity from the strong IHB OH stretching band. Furthermore, in the NM6D calculation, the combination bands at 2312 and 2331 cm À1 borrow intensity from the strong IHB stretching band, therefore the large intensity of the IHB stretching band is redistributed around a large frequency region contributing to the broadening of the spectra.…”

“…[25][26][27] Thereby in the present study we performed vibrational calculations including the anharmonic coupling of the IHB OH stretching modes to the IBH OH bending, and VDW stretching modes. However, recent studies on hydrated clusters have mentioned the importance of the coupling of the hydrogen bonded OH stretching mode to the low frequency intermolecular modes such as water wagging, water rotation, and van der Waals (VDW) water-water stretching modes.…”

Automatic determination of important mode–mode correlations in many-mode vibrational wave functions

“…Among these are internal coordinates, localized modes, 25,38-40 optimized coordinates, [41][42][43][44] and local modes 45-53 /local monomer modes. [54][55][56][57] All of the above choices have in common that they tend to be somewhat more local than normal modes. This locality might either be achieved by construction, 45-57 by optimization with respect to a localization criterion, 25,39 or by optimization of the VSCF energy in an anharmonic force field leading to a partial localization of the optimized coordinates.…”

Probing vibrational coupling via a grid‐based quantum approach—an efficient strategy for accurate calculations of localized normal modes in solid‐state systems

“…The concept to reduce the computational demand by focusing only on a reduced set of vibrational modes has also been employed in a number of investigations. Bowman and co‐workers proposed a local monomer model to investigate clusters of small molecules . Jacob, Reiher, and Neugebauer proposed an approach to calculate vibrational normal modes of quantum mechanical (QM)‐systems and combined quantum mechanica/molecular mechanical (QM/MM) simulations .…”