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We present the two‐photon absorption (2PA) spectrum of aqueous thiocyanate calculated using high‐level quantum‐chemistry methods. The 2PA spectrum is compared to the one‐photon absorption (1PA) spectrum computed using the same computational protocol. Although the two spectra probe the same set of electronic states, the intensity patterns are different, leading to an apparent red‐shift of the 2PA spectrum relative to the 1PA spectrum. The presented analysis explains the intensity patterns and attributes the differences between the 1PA and 2PA spectra to the native symmetry of isolated SCN, which influences the spectra in the low‐symmetry solvated environment. The native symmetry also manifests itself in variations of the polarization ratio (e.g., parallel vs. perpendicular cross sections) across the spectrum. The presented results highlight the potential of 2PA spectroscopy and high‐level quantum‐chemistry methods in studies of condensed‐phase phenomena.

 

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.