Calculation of Chlorin IR Spectrum by Density Functional Theory

The structure, frequencies of normal vibrations, and absolute intensities of bands in the IR spectra of chlorin and four of its symmetric isotopomers were calculated using DFT/B3LYP with the 6-31G(d) basis set. The force field was scaled by the Pulay method in independent and dependent natural coordinates. A method for obtaining effective force fields without using experimental data for the frequencies of fundamental vibrations was proposed. It has been demonstrated that most vibrations of the porphyrin macrocycle have characteristic frequencies upon hydrogenation of the pyrrolenine ring and only 12 vibrations differed significantly. The IR spectra of chlorin and its isotopomers were modelled. Frequencies were assigned and normal vibrations were interpreted for the examined molecules.