AB INITIO CALCULATIONS OF VITAMIN B12 COMPOUNDS

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We would like to calculate properties of open-shell vitamin B12 (cobalamin) molecules and the intensities of the Raman spectroscopy for molecules on surfaces. In both cases a break down in the Born-Oppenheimer approximation can lead to vibronic interactions which distorts the molecular geometry ( PJT effect) for cobalamins or changes the relative Raman intensities for molecules on surfaces. We will calculate the properties of cobalamin molecules using the CASSCF method ; however , we will be investing other possible approaches using TD-DFT where the computation is more efficient. We will calculate Raman intensities by an ab initio method which should include both Franck-Condon and Herzberg-Teller type resonance Raman scattering. This approach will be used in order to obtain an understanding of the basic enhancement mechanisms of Surface Enhanced Raman Scattering (SERS). This calculation will be made for pyridine and 3-hydroxyflavone compounds on a Ag nanocluster. One possible calculation method involves using the Gaussian package of programs for solutions to the coupled perturbed Hartree-Fock equations ( CPHF) for multiconfigurational CASSCF wave functions in order to obtain the frequency-dependent polarizability components. The Raman scattering intensities are then found from the differential scattering cross section by evaluating numerical derivatives of the dynamic polarizability as a function of Cartesian displacements of all atoms with respect to the equilibrium geometry. Other approaches to this calculation will be investigated in the ADF or MOLPRO packages for quantum chemistry.