Consistent porphyrin force field. 1. Normal-mode analysis for nickel porphine and nickel tetraphenylporphine from resonance Raman and infrared spectra and isotope shifts

Resonance Raman spectra with variable-wavelength excitation are reported for NiII porphine (NiP) and for the pyrrole-d8, meso-d4, and (pyrrole + meso)-d12 isotopomers, as well as for NiII meso-tetraphenylporphine (NiTPP) and its pyrrole-15N4, pyrrole-d8, 13C4-meso, and phenyl-d20 isotopomers. All the Raman-active in-plane modes have been identified and are assigned to local coordinates which take into account the phasing of adjacent bond stretches within the pyrrole rings and at the methine bridges. The IR spectra of NiP and its isotopomers are also assigned. For most of the local coordinates good frequency agreement is seen for the different symmetry blocks, showing that longer range phasings have minor effects. These in-plane mode assignments are supported by normal-coordinate calculations with a physically reasonable valence force field, which is nearly the same for NiP and NiTPP. The principal force constants are in good accord with bond length relationships selected on the basis of scaled ab initio calculations. The phenyl substituents of NiTPP lower the frequencies of the asymmetric methine bridge stretching modes v10(B1g) and v19(A2g) by ∼60 cm-1; this shift is attributable partly to the loss of coupling with the CmH bending modes in NiP and partly to an electronic effect of the phenyl group. There are also near-resonant interactions in NiTPP between porphyrin and phenyl modes near 740 and 200 cm-1 resulting in strongly displaced modes. Otherwise the phenyl groups have little influence on the porphyrin skeletal mode frequencies. Several phenyl modes are subject to moderate RR enhancement, probably via intensity borrowing from nearby porphyrin modes. Large intensities associated with CH bending modes in NiP imply significant excited-state changes in the pyrrole CβH and methine CmH bond angles. RR spectra for NiII meso-tetramethylporphyrin (TMP) and its methyl-d12 isotopomer reveal methyl group modes and somewhat altered skeletal frequencies. Altered polarizations of v10 and v19 in NiTMP and NiTEP (meso-tetraethylporphine) reveal a lowering of the effective molecular symmetry, which is attributed to substituent orientation effects. All of these Ni porphyrins show strong overtone and combination band enhancement in resonance with the Q1 absorption band.