Vibrational solvatochromism and electrochromism of cyanide, thiocyanate, and azide anions in water

Abstract

Small IR probe molecules have been found to be useful to measure local electric fields in condensed phases and proteins and also to study nucleic acid and protein structure and dynamics by monitoring their vibrational couplings and frequency shifts. However, it is still difficult to accurately describe the vibrational solvatochromic frequency shifts of such IR probes, because the local electric fields produced by surrounding solvent molecules or by protein peptide and side groups are spatially non-uniform and highly inhomogeneous around a probe. We recently developed a distributed interaction site model to describe the vibrational solvatochromism and electrochromism of nitrile-, thiocyanato-, and azido-derivatized compounds and amino acids in solutions. Here, the nitrile or azido stretch is the maker mode. It was found that those interaction sites distributed over the IR probe molecule collectively act as an antenna sensing local electric field distributions around the IR probes. Once the vibrational solvatochromism of a given IR probe is understood, it becomes possible to quantitatively describe their vibrational Stark effects. Carrying out quantum chemistry calculations of cyanide, thiocyanate, and azide anions in water clusters, we extended the distributed site model for ionic IR probes and calculated the vibrational Stark tuning rates for direct comparisons with experimental results. It turns out that the charge transfers from an anionic solute to surrounding water molecules are significant, but their effects on vibrational solvatochromism and electrochromism of pseudohalide ionic IR probes are not. We anticipate that the present computational results will be of use to establish the relationship between vibrational frequency of an ionic IR probe and local electric field in condensed phases and protein matrices.