Calculation of vibrational spectra of linear tetrapyrroles. 4. Methine bridge C-H out-of-plane modes

Quantum chemical force fields obtained by density functional theory (DFT) calculations systematically overestimate the frequencies of normal modes including ethylenic C-H out-of-plane (HOOP) coordinates. Compensation of this deviation requires a specific scaling factor for this type of coordinate that is distinctly lower than those applicable to out-of-plane coordinates in general. Such a specific scaling factor (0.900) has been optimized for the DFT(B3LYP) level of theory on the basis of vibrational analyses of training molecules including the HOOP coordinate. Thus, the root-mean-square deviation for the calculated frequencies of these modes is reduced from 16 to 8 cm(-1). Although Raman intensities are yet not reproduced in a satisfactory manner, implementation of the HOOP scaling factor into the set of global scaling factors determined previously (Magdo et al. J. Phys. Chem. A 1999, 103, 289-303) allows for a substantially improved reproduction of the experimental (resonance) Raman spectra of test molecules including linear methine-bridged tetrapyrroles. A very good agreement between calculated and experimental spectra is noted for the phycocyanobilin dimethylester dimer as well as for the protein-bound phycocyanobilin in the antenna pigment R-CPC. However, for the phycocyanobilin chromophore in the P-r state of the plant photoreceptor phytochrome phyA, considerable deviations remain in the spectral range between 800 and 500 cm(-1), which are attributed to the effect of specific protein-chromophore interactions. The influence of the protein environment is not considered in the present calculations that refer to the molecule in vacuo.