Electric Field Effect on Condensed-Phase Molecular Systems. X. Interconversion Dynamics and Vibrational Stark Effect of Hydrogen Chloride Clusters in an Argon Matrix

In this study, the effect of a strong (≤4 × 10⁸ V·m^–1) dc electric field on hydrogen chloride (HCl) dimers and trimers isolated in a solid argon matrix has been investigated using the ice film nanocapacitor and reflection–absorption infrared spectroscopy methods. The H–Cl vibrational bands of the HCl dimers showed a linear Stark frequency shift and an increased intensity under the applied electric field, and these changes were reversible with the electric field strength. This behavior indicated that the dimers were reoriented by the applied electric field. The reorientation occurred via tunneling inversion of individual HCl subunits of the dimer, which interconverted the proton-accepting and -donating HCl subunits, as observed for the heterodimers HCl–DCl and DCl–HCl. The interconversion of dimers could occur even at low electric field strength (∼10⁷ V·m^–1) and was almost complete above the field strength of 1.0 × 10⁸ V·m^–1. In contrast, the asymmetric H–Cl stretching bands of the HCl trimers exhibited Stark broadening under the influence of the electric field without a shift in frequency or change in intensity. This behavior indicated that the cyclic structure of the HCl trimer was stable even when subjected to a strong electric field. The Stark sensitivity factor (Δμ) of H–Cl vibrations was deduced from the Stark effect analysis of the HCl dimer and trimer bands, which gave the following: Δμ_D1 = 2.3 ± 0.2 cm^–1/(10⁸ V·m^–1) for the proton-acceptor subunit of the dimer, Δμ_D2 = 5.1 ± 0.5 cm^–1/(10⁸ V·m^–1) for the proton-donor subunit of the dimer, and Δμ_T = 4.5 ± 0.5 cm^–1/(10⁸ V·m^–1) for the asymmetric stretching vibration of the cyclic trimer.