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We describe several improvements to the reaction field model for the ab initio determination of solvation effects. First, the simple spherical cavity model is expanded to include higher-order electrostatic interactions. Second, two new and efficient implementations of the polarizable continuum model (PCM) are described, which allow a more realistic specification of the solute cavity as well as infinite-order electrostatics. Electron correlation effects are evaluated using the B3LYP density functional and Möller−Plesset perturbation theory to second order. An assessment of the importance of these various factors is made by comparing theoretical results to the experimentally known conformational equilibrium between syn and anti forms of furfuraldehyde and the C−C rotational barrier of (2-nitrovinyl)amine. Comparisons are also made with calculations that employ an ellipsoidal cavity with sixth-order electrostatics. Optimization using a simple Onsager model appears to be sufficient to evaluate the important geometry changes in solution. Energies obtained from the spherical and ellipsoidal cavity models often exhibit poor convergence in the truncated electrostatic series. Correlation to experiment is much improved when an infinite-order PCM method is used.

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Singlet and triplet vertical excitation energies of a series of acceptor parasubstituted N,N-dimethyl–anilines [NC–C6H4–N(CH3)2, NC–C6H4–NH2, OHC–C6H4–N(CH3)2, NC–C6H2(CH3)2–N(CH3)2, (H2N)OC–C6H4–N(CH3)2, (CH3)OC–C6H4–N(CH3)2, O2N–C6H4–N(CH3)2, named, respectively, 4DMAB–CN, 4AB–CN, 4DMAB–CHO, TMAB–CN, 4DMAB–CONH2, 4DMAB–COMe, and 4DMAB–NO2] have been calculated with TDDFT. Geometry optimization and excitation energy calculations have been performed, in most cases, with the B3LYP functional using a 6-31G(d) and a 6-311+G(2d,p) basis set (hereafter referred to as Sm and Bg, respectively). 4DMAB–CN and TMAB–CN have been investigated with particular care since gas-phase absorption spectra exist for those two molecules allowing thus a direct comparison with experimental results. The first and second singlet excited states of 4DMAB–CN, commonly named locally excited (LE) state and charge transfer (CT) state, are 0.1 and 0.04 eV higher than the experimental results at the B3LYP-Bg level, leading to a 0.06 eV underestimation of the gap between the two states. In the case of TMAB–CN, which is twisted in its ground state, B3LYP–(Sm/Bg) results show an error of 0.36 eV for the singlet CT state. Better agreement with experiment is obtained using the MPW1PW91 functional and Bg basis set with an underestimation of 0.17 eV for the singlet CT state and an overestimation of 0.16 eV for the second singlet state. Contrary to DFT/SCI results, the relative order and position of excitation energies of 4AB–CN and 4DMAB–CHO are well reproduced compared to solution spectra results. The singlet CT state using B3LYP and a Bg basis set is calculated 0.1 eV higher in energy than the experimental value obtained in isopentane for 4DMAB–CONH2, while the same excitation energy is predicted 0.08 and 0.28 eV too low compared to the gas-phase values for 4DMAB–COMe and 4DMAB–NO2, respectively. Finally, the CT excitation energy and its relative position to the LE state agrees with the acceptor strength concept.