Electronic Absorption Spectra and Solvatochromic Shifts by the Vertical Excitation Model: Solvated Clusters and Molecular Dynamics Sampling

Marenich, A. V.; Cramer, C. J.; Truhlar, D. G.
J. Phys. Chem. B 2015, 119, 958 (doi:10.1021/jp506293w).

A physically realistic treatment of solvatochromic shifts in liquid-phase electronic absorption spectra requires a proper account for various short- and long-range equilibrium and nonequilibrium solute-solvent interactions. The present article demonstrates that such a treatment can be accomplished using a mixed discrete-continuum approach based on the two-time-scale self-consistent state-specific vertical excitation model (called VEM) for electronic excitation in solution. We apply this mixed approach in combination with time-dependent density functional theory to compute UV/Vis absorption spectra in solution for the n → π* (¹A₂) transition for acetone in methanol and in water, the π → π* (¹A₁) transition for para-nitroaniline (PNA) in methanol and in water, the n → π* (¹B₁) transition for pyridine in water, and the n → π* (¹B₁) transition for pyrimidine in water. Hydrogen bonding and first-solvation-shell specific complexation are included by means of the explicit solvent molecules, and solute-solvent dispersion is included by using the solvation model with state-specific polarizability (SMSSP). Geometries of microsolvated clusters were treated in two different ways: (i) using single liquid-phase-global-minimum solute-solvent clusters containing up to two explicit solvent molecules and (ii) using solute-solvent cluster snapshots derived from molecular dynamics (MD) trajectories. The calculations in water involve using VEM/TDDFT excitation energies and oscillator strengths computed over 200 MD-derived solute-solvent clusters and convoluted with Gaussian functions. We also calculate ground and excited state dipole moments for interpretation. We find that inclusion of explicit solvent molecules generally improves the agreement with experiment and can be recommended as a way to include the effect of hydrogen bonding in solvatochromic shifts.