College Park, Maryland      June 6 - 10 , 2004

TP16: Inelastic Neutron Scattering Spectra of Simple Porphyrins

N. Verdal, B. S. Hudson (Department of Chemistry, Syracuse University, Syracuse, NY), P. Kozlowski (Department of Chemistry, University of Louisville, Louisville, KY)

Porphine is the simplest of all porphyrins, a group of molecules pervasive in biological systems and thus in itself an interesting molecule. The biologically active porphyrin, typically an iron porphyrin, or heme group, is often involved in an oxidation/reduction reaction and thus small movements are important to biological function. In addition, it is used as a probe in resonance Raman studies in which the vibrational spectrum of the resonant group is used to probe changes in the protein environment associated with conformational changes. A more complete understanding of the porphine vibrations will place these probe applications on a more firm basis.

Inelastic neutron scattering (INS) spectra have been collected for both porphine and zinc porphine using the ISIS TOSCA spectrometer. These spectra have been compared with the results of both ab initio and the semi-empirical scaled quantum mechanical method calculations. The scaled quantum mechanical method uses a force field calculated by density functional theory using the B3LYP functional and the 6-31G* basis set. These calculations have been previously compared with IR and Raman spectra with good agreement [see J. Phys. Chem. 1996, 100, 7007-7013]. The advantage of INS is that it is not subject to optical selection rules and allows a more complete comparison with the frequency and intensity output from ab initio and semi-empirical calculations. Good agreement between the INS spectra and the scaled quantum mechanical results are observed.

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