SIROHEME MODEL COMPOUNDS - VIBRATIONAL-SPECTRA AND NORMAL-MODES OF COPPER(II) OCTAETHYLISOBACTERIOCHLORIN

Resonance Raman (RR) and infrared spectra are reported for the Cu(II) complexes of octaethylisobacteriochlorin (OEiBC) and the meso-deuteriated compounds OEiBC-gamma-d1, OEiBC-beta,gamma,delta-d3, and OEiBC-alpha,beta,gamma,delta-d4. These compounds represent a series in which each of the three symmetry inequivalent methine bridges of the macrocycle is systematically perturbed. The RR spectra of the complexes are obtained at a variety of excitation wavelengths which span the B(x), B(y), Q(x), and Q(y) absorption bands of the complexes. The observed RR intensity enhancement patterns provide insight into the scattering mechanisms that are active for the tetrahydroporphyrin macrocycle. All of the vibrational data are used in conjunction with semempirical normal coordinate calculations to obtain a set of assignments for the observed high-frequency (above 1000 cm-1), in-plane skeletal modes of CuOEiBC. The normal coordinate calculations and, more importantly, the isotope-shift patterns observed for the complexes deuteriated at specific methine bridges indicate that the forms of the vibrational eigenvectors of many of the modes of the OEiBC macrocycle differ substantially from those of porphyrins, chlorins, or bacteriochlorins. The differences in the general appearance of the vibrational eigenvectors of these various classes of macrocycles can be rationalized in terms of structural changes that occur upon reduction of the pyrrole rings.