Calculation of Vibrational Spectra of Linear Tetrapyrroles. 1. Global Sets of Scaling Factors for Force Fields Derived by ab Initio and Density Functional Theory Methods

Abstract

An approach has been developed for calculating the vibrational spectra of linear methine-bridged tetrapyrroles constituting the chromophoric sites of various photoreceptor proteins. Using Pulay's scaling procedure (Pulay, P.; Fogarasi, G.; Pongor, G.; Boggs, J. E.; Vargha, A. J. Am. Chem. Soc. 1983, 105, 7037), scaling factors were determined for a set of 10 training molecules which mimic structural elements of the tetrapyrrole target molecules. Geometries and force fields were calculated at three theoretical levels, i.e., by the Hartree−Fock (HF), second-order Moller−Plesset perturbation (MP2), and B3LYP density functional theory methods using 6-31G* basis sets. A global optimization yielded sets of 14, 11, and 10 scaling factors for HF, MP2, and B3LYP, respectively. B3LYP provided the best results both with regard to the geometries and the vibrational frequencies. The root-mean-square deviation for the calculated frequencies was 11 cm-1 for B3LYP as compared to 13 and 17 cm-1 for HF and MP2, respectivel...