DFT-SQM force field for nickel porphine: Intrinsic ruffling

by Kozlowski, Pawel M.; Rush, III, Thomas S.; Jarz?cki, Andrzej Arkadiusz; Zgierski, Marek Z.; Chase, Bruce; Piffat, Christine; Ye, Bao-Hui; Li, Xiao-Yuan; Pulay, Peter; Spiro, Thomas G.

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.

Journal of Physical Chemistry A
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1520-5215; 1089-5639