Structure, energetics, and electronic coupling in the (TCNE2)-* encounter complex in solution: a polarizable continuum study

For the prototypical dyad (TCNE2)-*, previous in vacuo calculations indicate that sizable distortion of the equilibrium gas-phase structure may be required to reduce the donor/acceptor electronic coupling element (HDA) to the solution-phase experimental estimates. We employ the polarizable continuum model (PCM) to simulate the solvation environment for several polar solvents, finding noticeable structure change associated with the promotion of charge localization due to solvation. We have extended the counterpoise (CP) correction procedure so as to include fragment relaxation energies within the PCM model, and it would be of interest to incorporate this approach into schemes for optimizing coordinates on CP-corrected energy surfaces. The calculations include face-to-face encounter geometries as well as several lateral and twist distortions of the face-to-face structures. In proceeding from vacuum to solution, the calculated stabilization energy is reduced from -18 to -3 kcal/mol, and the calculated energy surface becomes flatter, with a somewhat larger minimum-energy separation of the monomer units (rDA). The corresponding minimum-energy structures are, respectively, delocalized and charge-localized. Using TD-DFT, spin-projected MP2 (PUMP2), and state-averaged two-configuration SCF (SA-TCSCF) calculations to evaluate HDA for symmetric encounter complex geometries (models for transition-state structures) indicates that HDA has comparable magnitude in the gas phase and in solution for a given dimer structure. SA-TCSCF calculations comparing HDA based on symmetric charge-delocalized structures and their asymmetric (minimum-energy) charge-localized counterparts (at a given rDA) yield very similar values. Even with account taken of the energetically accessible configurations probed by the PCM calculations, the HDA values remain significantly higher than the experimental estimates inferred from solution spectra and assumption of rDA based on crystal data. Clearly, additional calculations based on molecular-level solvent models would be of value in helping to characterize the intermolecular structures accessible to the encounter complex in polar solution.