Charge-transfer-to-solvent states provide a sensitive spectroscopic probe of the local solvent structure around anions

This computational study characterises charge-transfer-to-solvent (CTTS) states of aqueous thiocyanate anion using equation-of-motion coupled-cluster methods combined with electrostatic embedding quantum mechanics/molecular mechanics (QM/MM) scheme. Equilibrium sampling was carried out using classical molecular dynamics (MD) with standard force-fields and QM/MM ab initio molecular dynamics (AIMD) using density functional theory. The two calculations yield significantly different local structure around solvated SCN - . Because of the diffuse character of CTTS states, they are very sensitive to the local structure of solvent around the solute and its dynamic fluctuations. Owing to this sensitivity, the spectra computed using MD and AIMD based snapshots differ considerably. This sensitivity suggests that the spectroscopy exploiting CTTS transitions can provide an experimental handle for assessing the quality of force-fields and density functionals. By combining CTTS-based spectroscopies with reliable theoretical modeling, detailed microscopic information of the solvent structure can be obtained. We present a robust computational protocol for modeling spectra of solvated anions and emphasise the use of an ab initio characterization of individual electronic transitions as CTTS or local excitations.

Automated summary

This computational study investigates the charge-transfer-to-solvent (CTTS) states of aqueous thiocyanate anion using equation-of-motion coupled-cluster methods and electrostatic embedding QM/MM scheme. The results from classical molecular dynamics (MD) and QM/MM ab initio molecular dynamics (AIMD) simulations show significant differences in the local structure around solvated SCN-. The sensitivity of the CTTS states to the solvent structure suggests that CTTS-based spectroscopy can be used to assess the quality of force-fields and density functionals. By combining CTTS-based spectroscopy with theoretical modeling, detailed microscopic information of the solvent structure can be obtained.