POLARIZED INFRARED-SPECTRA OF PHOTOORIENTED MATRIX-ISOLATED FREE-BASEPORPHYRIN ISOTOPOMERS

Polarized single-site IR spectra of free-base porphyrin-d(4), -d(8), - d(12), and -N-15(4), isolated in xenon matrix and aligned by action of linearly polarized visible light, have been measured and compared wit h the previously obtained spectra of unlabeled porphyrin and porphyrin -d(2). There is much agreement but also a fair amount of disagreement with the symmetry assignments previously proposed in the literature. T he results are compared with those of the empirical force fields propo sed in the literature and with ab initio calculations at the 3-21G lev el of Hartree-Fock theory. It is concluded that experimental informati on on additional isotopomers and particularly, on gerade symmetry vibr ations, will be needed before a reliable harmonic force field can be d eveloped. The calculations require the introduction of electron correl ation before the equilibrium ground state geometry and force constants can be calculated reliably enough to be useful.