Example 2 Quantitative Vibrational Dynamics of Iron Ferrous Nitrosyl Tetraphenylporphyrin

3 Example 2 Quantitative Vibrational Dynamics of Iron Ferrous Nitrosyl Tetraphenylporphyrin 

The current example illustrates PVDOS formulation as an effective basis for comparison of experimental and theoretical NIS data for ferrous nitrosyl tetraphe-nylporph3Tin Fe(TPP)(NO), which was done [101] along with other ferrous nitrosyl porphyrins. Such compounds are designed to model heme protein active sites. In particular, the elucidation of the vibrational dynamics of the Fe atom provides a unique opportunity to specifically probe the contribution of Fe to the reaction dynamics. The geometrical structure of Fe(TPP)(NO) is shown in Fig. 5.16. 

Experimental and theoretical PVDOS for Fe(TPP)(NO) single-crystal and powder samples are shown in Fig. 5.17. The DFT calculations were performed, using the B3LYP and BP86 functionals. 

The upper panel in Fig. 5.17 compares the experimentally determined PVDOS for the oriented array of Fe(TPP)(NO) crystals with that for Fe(TPP)(NO) powder. The crystal data is recorded with the incident X-ray beam 6° above the mean porphyrin plane. Since the NIS measurement is only sensitive to Fe motion along a probe direction, the spectral contribution of modes with predominantly in-plane Fe motion are enhanced, and the contribution of out-of-plane modes is suppressed in the oriented crystal data. 

The comparison of crystal and powder results in Fig. 5.17 identifies three dominant modes at 74, 128, and 540 cm with Fe motion perpendicular to the porphyrin plane. In contrast, the Fe motion parallel to the porphyrin plane makes the dominant contribution to modes in the 200-500 cm range. These observations are consistent with the assignment of the 540 cm mode to the out-of-plane Fe-NO stretch (because its relative amplitude is reduced in the in-plane crystal spectrum). The corresponding experimental value = 0.30 was based on fitting a 

---