Vinyl influences on protoheme resonance Raman spectra: nickel(II) protoporphyrin IX with deuterated vinyl groups

Infrared absorption and variable excitation (4067, 4131, 5145, and 5682 Å, near the B, Q, and Q0 absorption maxima) resonance Raman spectra are reported for NiPP (PP = protoporphyrin IX dimethyl ester) and for NiPP-dα (vinyl α-carbons deuterated) and NiPP-d2β (vinyl β-carbons dideuterated). The deuteration shifts and comparison with NiOEP (OEP = octaethylporphyrin) permit identification of most of the vinyl modes and correlation of the protoporphyrin skeletal modes with those of OEP. Strong interactions with skeletal modes are evident in splittings of some of the vinyl modes and are probably responsible for the vinyl mode Raman enhancements. They also shift some of the skeletal mode frequencies, relative to NiOEP, but several skeletal modes are unshifted, including some previously identified as heme structure markers. The vinyl groups induce Raman activity into several infrared modes, reflecting the asymmetric conjugation in PP. No IR activity is seen for Raman modes, however, suggesting that vinyl conjugation occurs primarily in the excited state.