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Calculation of Vibrational Spectra of Linear Tetrapyrroles. 4. Methine Bridge C−H Out-of-Plane Modes
Citation key Mroginski2006
Author Mroginski, Maria Andrea and Murgida, Daniel Horacio and Hildebrandt, Peter
Pages 10564-10574
Year 2006
ISSN 1089-5639
Journal J. Phys. Chem. A
Volume 110
Number 36
Month sep
Publisher American Chemical Society
Abstract Quantum chemical force fields obtained by density functional theory (DFT) calculations systematically overestimate the frequencies of normal modes including ethylenic CH 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, 289303) 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 alpha-CPC. However, for the phycocyanobilin chromophore in the Pr 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 proteinchromophore interactions. The influence of the protein environment is not considered in the present calculations that refer to the molecule in vacuo.
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