Vibrational assignment and definite harmonic force field for porphine .1. Scaled quantum mechanical results and comparison with empirical force field

The ground state geometry, force field, and vibrational intensity data of free-base porphine were determined by the scaled quantum mechanical (SQM) method, using a force field calculated by density functional theory. Calculations were carried out with the Becke-Lee-Yang-Paar composite exchange-correlation functional (B3-LYP) and with the 6-31G'' basis set. The resulting force field was transformed to nonredundant internal coordinates and scaled by a set of optimized scaling factors. There is very good agreement between the calculated and experimental frequencies and intensities and between experimental and simulated infrared (IR) spectra. The assignment of the harmonic frequencies is discussed and reassignment of some modes present in the IR spectra is suggested. Comparison between the empirical and scaled quantum mechanical force fields shows that the empirical field is qualitatively correct for the dominant terms but neglects many of the smaller but still significant force constants. The good reproduction of the b(3u) fundamentals, including the ''Kekule'' vibration is conclusive proof for the D-2h symmetry of the molecule.