Circular dichroism spectrum of [Co(en)(3)](3+) in water: A discrete solvent reaction field study

In this work we investigate the circular dichroism (CD) spectrum of [Co(en)(3)](3+) in water, using the discrete solvent reaction field (DRF) model. The DRF model is a polarizable quantum mechanics/molecular mechanics (QM/MM) model. The implementation of the DRF model for CD spectra calculations based on time-dependent density functional theory (TDDFT) is presented. The combination of DRF with TDDFT allows for a computationally attractive solution for calculating chirooptical properties of molecules in solution when explicit solvent structures are of interest. Using a mixed coarse/fine-grained parallel computation, we show that average CD spectra from snapshots of the solvent structure can be obtained routinely. Classical polarizable molecular dynamics (MD) simulations have been used to obtain the solvent structure around the [Co(en)(3)](3+) (en = ethyldiamine) solute. We show that the final spectrum converges quickly with respect to the number of configurations. The DRF results were compared with results obtained from the much simpler conductor-like screening model (COSMO). Both models predicted similar blue shifts of the CD bands, but none of the models is in perfect agreement with the experiments. For instance, the calculated intensities are larger than what is found experimentally if reasonable empirical line width parameters are applied. From the DRF computations, we further show that almost all the solvent effects arise from ground-state solvation. Thus, ignoring the dynamic solvent response is a good approximation for a system like [Co(en)(3)](3+), where the solute is highly charged and the solvent is very polar. (C) 2006 Wiley Periodicals, Inc.