Specific coupling between the 13-keto carbonyl and chlorin skeletal vibrational modes of synthetic 131-18O-(un)labelled metallochlorophyll derivatives

Metal complexes of methyl 131-18O-labelled pyropheophorbide-a1-M–18O (M = Zn, Cu and Ni) were prepared by metallation of the 18O-labelled free base (1-18O) and 18O-labelling of unlabelled nickel complex (1-Ni). The FT-IR spectra of 1-Zn and 1-Zn–18O in CH2Cl2 showed that the 13-keto carbonyl stretching vibration mode moved to about a 30-cm−1 lower wavenumber by 18O-labelling of the 131-oxo moiety. In 1-Cu–18O and 1-Ni–18O, the 13-C18O stretching modes were close to the highest-energy wavenumber mode of chlorin skeletal C–C/C–N vibrations at around 1650 cm−1 and they were coupled in CH2Cl2 to give two split IR bands (Fermi resonance). A similar coupling was observed in the resonance Raman scattering of 1-Ni–18O in the solid state. The hydrogen-bonded 13-C16O vibration mode of 1-Ni similarly coupled with the skeletal C–C/C–N mode in CCl4 containing 1% (v/v) 1,1,1,3,3,3-hexafluoro-2-propanol, while such a coupling was not observed in a neat CCl4 solution of 1-Ni possessing the 13-C16O free from any interaction. The skeletal C–C/C–N band selectively coupled with the 13-CO, not with the 3-CO, when the difference in their peak maxima was less than 20 cm−1.