Imidazole, vibrational modes

The qualitative picture suggested by these calculations has recently been confirmed by new experiments on the UV photochemistry of phenol. Lee and collaborators have shown that loss of H atoms is the major process in phenol at 248 nm excitation [36], Ashfold and co-workers have applied high-resolution photofragment translational spectroscopy to obtain uniquely detailed insight into the photodissociation dynamics of phenol [37], The phenoxy radical is produced in a surprisingly limited subset of its available vibrational states [37], Such vibrational mode-specific dynamics is a signature of ultrafast radiationless decay through directly accessible conical intersections. Similar results have been obtained for pyrrole, imidazole and indole [38 10],... 

The reconstruction of the bandshape of the imidazole crystal was also performed using Car-Parrinello molecular dynamics (CPMD) simulation [73] of the unit cell of the crystal the results reproduce both the frequencies and intensities of the experimental IR spectrum of bands reasonably well, which we attribute to the application of dipole moment dynamics. The results are presented in Fig. 8 [70]. These and other recent CPMD calculations, on 2-hydroxy-5-nitrobenzamide crystal [71], oxalic acid dihydrate [72], and other systems [64-69], show that the CPMD method is adequate for spectroscopic investigations of complex systems with hydrogen bonds since it takes into account most of mechanisms determining the hydrogen bond dynamics (anharmonicity, couplings between vibrational modes, and intermolecular interactions in crystals). 

Heme complexes and heme proteins have also been the subject of NIS studies. Of specific interest have been three features the in-plane vibrations of iron, which have not been reported by Resonance Raman studies [108], the iron-imidazole stretch, which has not been identified in six-coordinated porphyrins before, and the heme-doming mode, which was assumed to be a soft mode. 

The peaks between 800 and 600 cm are caused by vibrations of the bases which are coupled to sugar vibrations and out-of-plane carbonyl bending vibrations. A mode attributed to a guanine imidazole ring vibration coupled via the Cl -N9 linkage to a sugar vibration in particular is extremely sensitive to the conformation. It therefore gives evidence of transitions between the different helical families of nucleic acids (Secs. 4.7.1.3.1, 4.7.1.3.2). 

N—H.. X system. (X = C1-, Br, 0, N). NH stretching and out-ofplane bending frequencies can also be simply related, as shown in Fig. 9 b. The points (Table 4) correspond to frequencies of imidazole and triazole molecules in different environments, i. e., physical state and in complexes with various metal salts. The range of the vNH (3550—2700 cm-1) and yNH(515—940 cm-1) frequencies is smaller than that of the O—H.. 0 system, the N—H.. X hydrogen bonds being weaker. The dyNH frequency shift is less than half that of the drNH shift and the dy/yo relative shift for imidazole crystal amounts to 83%. As shown by deuteration studies of these molecules (26,131) the NH out-of-plane mode of imidazole and triazole does not appear to be mixed with other out-of-plane vibrations. Yet the yNH(j 2i) vibration of pyrrole is strongly... 

Histidine vibrations The imidazole side chain of histidine often plays an important role in protein function. Unfortunately characteristic imidazole modes are rarely seen in protein Raman spectra. An extensive discussion of imidazole modes is given by Harada and Takeuchi [12]. 

The vNiN vibrations were assigned in 2,6-diacetylpyridine dioxime at 416, 341 and 276 cm and at 370 and 265 cm in hexaamine complexes, at 410 and 334 cm in triethylendiamine derivatives, around 240 cm" in pyridine derivatives and in imidazol complexes between 325 and 210cm"Gobemado-Mitre et al attributed the vCuN mode in copper complex of naphthalocya-nine to those bands observed at 341 and 221 cm" In Cu(II) tri-azamacrocycles it has been proposed the bands at 383 and 314 cm" as due to vCuN. This mode was observed in copper complexes of cyclam at 437 cm" and in Cu(II) hexaazacyclophane at 390 and 280 cm" In phenantroline Cu(II) complexes the vCuN vibration was identified with the bands at 300 and 430 cm" In the case of the Zn complexes, some tetraamine derivatives display the vZnN band at 432cm" and in triethylendiamine complexes were observed at 405 and 291 cm" in imidazole complexes the vZnN mode was assigned to the bands between 325 and 210 cm" The vZnN mode of bis(phenylhydrazine)-l, 10-phenantroline Zn(II) was attributed to the bands at 376 and 267cm" ... 

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