Resonance Raman, Infrared, and Normal Coordinate Analysis of Free-Base Tetraphenylbacteriochlorin: A Model for Bacteriopheophytins

Resonance Raman (RR) and FT-IR spectra are reported for free-base tetraphenylbacteriochlorin and its pyrrole- 15 N 4, β-D 8, meso - 13 C 4, and phenyl-D 20 isotopomers. A normal mode analysis is carried out in order to assign observed RR and IR modes, using a NiTPP-based force field, with bond-distance scaling of force constants and the introduction of long-range interaction constants. The modes correlate well with those of NiTPP, once due allowance is made for direct effects of pyrrole ring reduction: inequivalent C β −C β and C α −C β bonds and extra C β −H bending modes. Although the C α −C m and C α −N bonds of the 16-membered inner ring are not strictly equivalent, there is little tendency for mode localization with respect to these bonds. The absence of a central metal lowers the frequencies of C α −C m stretching and pyrrole translational modes. The RR enhancement pattern is quite different than for porphyrins. Totally symmetric modes dominate spectra in resonance with the Q x band, due to a larger transition moment than in porphyrins, while non-totally symmetric modes are strongly enhanced in the region of the close-lying B x and B y transitions, reflecting vibronic coupling between them.