Scaled quantum mechanical and experimental vibrational spectra of magnesium and zinc porphyrins

The ground state geometries, scaled quantum mechanical (SQM) quadratic force fields, and infrared and Raman intensities of magnesium and zinc porphyrins (MgP and ZnP), and of (H2O)(2)MgP have been determined from density functional theory, using the Becke-Lee-Yang-Parr composite exchange-correlation functional (B3-LYP) and the 6-31G* basis set. The geometry of (H2O)MgP has also been determined. The calculated force fields have been transformed to non-redundant natural internal coordinates and scaled by six scaling factors optimized earlier for free base porphyrin. Infrared and normal Raman spectra (the latter excited with 1064 nm infrared radiation) of the title compounds have been recorded. The calculated spectra compare excellently with the experimental ones, showing the accuracy of the SQM force field, and enabling a complete assignment of the fundamentals. According to the calculations, both MgP and ZnP are planar. The Mg ion in (H2O)MgP is about 0.28 Angstrom out of the N-4 plane. The calculations give reliable estimates for the force constants associated with the inactive vibrations. Some of these, e.g. the ruffling of the porphyrin ring, are important and are difficult to determine experimentally. (C) 1997 Elsevier Science B.V.