Atom-Selective Vibrational Spectroscopy: Ferrocene Probed via the Iron Atom

Jayasooriya, Upali A.; Malone, Simon A.; Chumakov, A. I.; Rüffer, R.; Overweg, A.R.; Nicklin, Chris

doi:10.1002/1439-7641(20010316)2:3<177::aid-cphc177>3.3.co;2-2

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“…On the experimental side, nuclear resonance vibrational spectroscopy (NRVS), an emerging synchrotron-based technique, reveals the complete vibrational spectrum of a probe nucleus. − NRVS achieves the ultimate limit of selectivity for a single atom in a complex molecule, because only the vibrational dynamics of the probe nucleus contribute to the measured signal. 57 Fe NRVS is a particularly promising method to identify and characterize Fe−ligand modes at protein active sites. ,,,− For heme proteins, these include in-plane Fe vibrations, which have not been reported in resonance Raman investigations, and the Fe−imidazole stretch, which has not been identified in six-coordinate porphyrins.…”

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confidence: 99%

Quantitative Vibrational Dynamics of Iron in Nitrosyl Porphyrins

et al. 2004

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We use quantitative experimental and theoretical approaches to characterize the vibrational dynamics of the Fe atom in porphyrins designed to model heme protein active sites. Nuclear resonance vibrational spectroscopy (NRVS) yields frequencies, amplitudes, and directions for 57Fe vibrations in a series of ferrous nitrosyl porphyrins, which provide a benchmark for evaluation of quantum chemical vibrational calculations. Detailed normal mode predictions result from DFT calculations on ferrous nitrosyl tetraphenylporphyrin Fe(TPP)(NO), its cation [Fe(TPP)(NO)]+, and ferrous nitrosyl porphine Fe(P)(NO). Differing functionals lead to significant variability in the predicted Fe-NO bond length and frequency for Fe(TPP)(NO). Otherwise, quantitative comparison of calculated and measured Fe dynamics on an absolute scale reveals good overall agreement, suggesting that DFT calculations provide a reliable guide to the character of observed Fe vibrational modes. These include a series of modes involving Fe motion in the plane of the porphyrin, which are rarely identified using infrared and Raman spectroscopies. The NO binding geometry breaks the four-fold symmetry of the Fe environment, and the resulting frequency splittings of the in-plane modes predicted for Fe(TPP)(NO) agree with observations. In contrast to expectations of a simple three-body model, mode energy remains localized on the FeNO fragment for only two modes, an N-O stretch and a mode with mixed Fe-NO stretch and FeNO bend character. Bending of the FeNO unit also contributes to several of the in-plane modes, but no primary FeNO bending mode is identified for Fe(TPP)(NO). Vibrations associated with hindered rotation of the NO and heme doming are predicted at low frequencies, where Fe motion perpendicular to the heme is identified experimentally at 73 and 128 cm-1. Identification of the latter two modes is a crucial first step toward quantifying the reactive energetics of Fe porphyrins and heme proteins.

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