An accurate in-plane force field for porphine. A scaled quantum mechanical study

The ground state geometry, force field, and vibrational intensity data of free-base porphine were determined by density functional theory. Calculations were carried out with the Becke-Lee-Yang-Parr composite exchange-correlation functional and 6-31G∗ basis set. The calculations predict a stable structure of D2h symmetry. The resulting force field was transformed to internal coordinates and scaled by transferable scale factors from our previous work. There is very good agreement between the calculated and experimental frequencies and intensities, and between experimental and simulated spectra. The assignment of the in-plane modes is discussed and some reassignments are proposed. Finite field perturbation theory was used to obtain the Raman intensities.