ABSTRACT
A new computational protocol relying on the use of electrostatic embedding, derived from QM/QM' ONIOM calculations, to simulate the effect of the crystalline environment on the emission spectra of molecular crystals is here applied to the ß-form of salicylidene aniline (SA). The first singlet excited states (S1 ) of the SA cis-keto and trans-keto conformers, surrounded by a cluster of other molecules representing the crystalline structure, were optimized by using a QM/QM' ONIOM approach with and without electronic embedding. The model system consisting of the central salicylidene aniline molecule was treated at the DFT level by using either the B3LYP, PBE0, or the CAM-B3LYP functional, whereas the real system was treated at the HF level. The CAM-B3LYP/HF level of theory provides emission energies in good agreement with experiment with differences of -20/-32 nm (cis-keto form) and -8/-14 nm (trans-keto form), respectively, whereas notably larger differences are obtained using global hybrids. Though such differences on the optical properties arise from the density functional choice, the contribution of the electronic embedding is rather independent of the functional used. This plays in favor of a more general applicability of the present protocol to other crystalline molecular systems.
ABSTRACT
The popular AMBER force-field has been extended to provide an accurate description of large and flexible nitroxide free-radicals in condensed phases. New atom types have been included, and relevant parameters have been fitted based on geometries, vibrational frequencies and potential energy surfaces computed at the DFT level for several different classes of nitroxides, both in vacuo and in different solvents. The resulting computational tool is capable of providing reliable structures, vibrational frequencies, relative energies and spectroscopic observables for large and flexible nitroxide systems, including those typically used as spin labels. The modified force field has been employed in the context of an integrated approach, based on classical molecular dynamics and discrete-continuum solvent models, for the investigation of environmental and short-time dynamic effects on the hyperfine and gyromagnetic tensors of PROXYL, TEMPO and INDCO spin probes. The computed magnetic parameters are in very good agreement with the available experimental values, and the procedure allows for an unbiased evaluation of the role of different effects in tuning the overall EPR observables.
