INVESTIGATION OF ASYMMETRIC PERTURBATIONS OF NICKEL(II) OCTAETHYLPORPHYRIN IN CH2CL2 BY RAMAN DISPERSION SPECTROSCOPY

We have measured the resonance Raman excitation profiles and the depolarization ratio dispersion of twelve nickel(II) octaethylporphyrin Raman lines of the A1 modes upsilon-3, upsilon-4, and upsilon-5, the B1 modes upsilon-10, upsilon-11, and upsilon-13, the B2 mode upsilon-29, the A2 modes upsilon-19, upsilon-20, upsilon-21, and the C1-C2 ethyl mode at 1021 cm-1. The data cover the resonance region of the Q and Q-upsilon bands and the preresonance region between the Q-upsilon and B bands. A self-consistent analysis of the depolarization dispersion was carried out by explicit consideration of multimode contributions to the polarizability tensor. We derive vibronic coupling parameters that suggest that the ruffled structure of nickel(II) octaethylporphyrin results from symmetry lowering A2 and B2 distortions with respect to D4 symmetry. We show by group theoretical reasoning that ruffling of the molecule causes antisymmetric A2 perturbations affecting the C(m) and N(p) atoms of the macrocycle. The B2 perturbations are caused by the interactions between the ethyl side chains and the C-beta atoms of the pyrroles. Our data clearly suggest that the symmetry of the NIOEP species predominant in CH2Cl2 is C2. It is therefore different from the corresponding molecular conformation in the tetragonal crystal structure.