DFT-SQM force field for nickel porphine: Intrinsic ruffling

Nonlocal density functional theory [DFT] has been used to compute vibrational frequencies and intensities of nickel porphine and of several isotopomers via a scaled quantum mechanical [SQM] force field [FF]. The results support and extend those obtained from a revised empirical FF. The force constants are similar for the two FF's, although the SQM FF has a complete set of off-diagonal elements. The SQM FF produces somewhat more accurate frequencies and isotope shifts than the empirical FF for the in-plane NIP modes. In addition, the SQM FF calculates out-of-plane modes that are in good agreement with available infrared [IR] data. Also, the SQM FF satisfactorily reproduces the relative intensities of both IR and [off-resonance] Raman bands. A striking result is the calculation of large Raman intensities for nontotally symmetric B-1g modes, in conformity with experimental FT-Raman spectra. This effect is attributed to the phasing of local polarizability components of the pyrrole rings and methine bonds. The DFT-computed bond distances and angles are in good agreement with crystallographically determined values. The lowest energy structure is a true minimum with D-2d symmetry. It is slightly distorted from the planar geometry along the ruffling coordinate. Constraining it to be planar [D-4h] raises the energy slightly [similar to 0.1 kcal/mol] and leads to an imaginary frequency for the ruffling mode. This finding provides theoretical confirmation of Hoard's empirical observation that metal ions with M-N[pyrrole] bonds significantly shorter than 2.00 Angstrom produce an out-of-plane distortion of the macrocycle. The computed degree of ruffling is small, as are the calculated shifts in vibrational frequencies [<6 cm(-1)]. Although the symmetry lowering relaxes selection rules, the induced intensity in IR- or Raman-forbidden modes is calculated to be negligible, except for a single IR band associated with an out-of-plane mode [E-g, 420 cm(-1)], which is indeed observed experimentally. The agreement of both frequencies and intensities with experiment provides further validation of the accuracy of the DFT, even for molecules as complex as metalloporphyrins.