Metal-radical interactions frequencies

Vibrational frequencies measured in IR experiments can be used as a probe of the metal—ligand bond strength and hence for the variation of the electronic structure due to metal—radical interactions. Theoretical estimations of the frequencies are obtained from the molecular Hessian, which can be straightforwardly calculated after a successful geometry optimization. Pure density functionals usually give accurate vibrational frequencies due to an error cancellation resulting from the neglect of... 

Another type of DOUBLE ENDOR, called special TRIPLE , has been introduced by Dinse et al.90 to study proton hf interactions of free radicals in solution. In a special TRIPLE experiment two rf fields with frequencies vp + Av and vp — Av are swept simultaneously. For systems with Tln < T,i this leads to a considerable signal-to-noise improvement and to TRIPLE line intensities which are directly proportional to the number of nuclei with the same hf coupling constant. It should be remembered, however, that in transition metal complexes in the solid state the resonance frequencies are not, in general, symmetrically placed about the free proton frequency vp and that the condition Tln < Tj,i is not always fulfilled. 

Observed linewidths of NMR signals in paramagnetic systems vary enormously and the conditions that govern the observed widths are considerably more complex than in diamagnetic systems. Swift (30) reviewed the problem some years ago. Relaxation times of spin-j nuclei are governed by dipolar and hyperfine exchange (Fermi contact) relaxation processes. The dipolar interaction is normally dominant except in some delocalized systems in which considerable unpaired spin density exists on nuclei far removed from the metal ions (e.g. Ti-radicals). Distinction between the two processes can be made by consideration of the different mathematical expressions involved. For dipolar relaxation when o)fx 1 (t = rate constant for rotation of the species containing the coupled pair and to, = nuclear resonance frequency) ... 

The CC and CO vibrations are also sensitive to the molecular environment by virtue of electrostatic and hydrogen bonding interactions. The frequencies of phenoxyl and tyrosyl radicals complexed by macrocyclic hgands and generated in wVo were measured by resonance Raman and FTIR techniques. Thus a selective enhancement of the vibrational CC and CO stretch modes of the phenoxyl chromophores in metal-coordinated radical... 

Therefore, the absorption lines are separated by a/h in frequency. The precision of measurement of a is therefore determined by the width of each absorption line. In transition metal complexes line widths are almost always greater than 3 MHz while in favorable cases line widths a hundred-fold smaller can be observed in free radicals. Thus, in practice, hyperfine interactions can at best be determined to 0.1 MHz by ESR. 

The work by Fournel et al. [41] provides an instructional example of the use of multi-frequency ESR to obtain detailed data for a complex biochemical system (a bacterial enzyme), in particular the exchange interaction between a transition metal ion and a radical and the determination of the magnitude and sign of the zero-field coupling. Procedures are described for the less complex biradical systems in the following section. 

The interaction of nitric oxide (NO) with metal ions in zeolites has been one of the major subjects in catalysis and environmental science and the first topic was concerned with NO adsorbed on zeolites. NO is an odd-electron molecule with one unpaired electron and can be used here as a paramagnetic probe to characterize the catalytic activity. In the first topic focus was on a mono NO-Na" complex formed in a Na -LTA type zeolite. The experimental ESR spectrum was characterized by a large -tensor anisotropy. By means of multi-frequency ESR spectroscopies the g tensor components could be well resolved. The N and Na hyperfine tensor components were accurately evaluated by ENDOR spectroscopy. Based on these experimentally obtained ESR parameters the electronic and geometrical structures of the NO-Na complex were discussed. In addition to the mono NO-Na complex the triplet state (NO)2 bi-radical is formed in the zeolite and dominates the ESR spectrum at higher NO concentration. The structure of the bi-radicai was discussed based on the ESR parameters derived from the X- and Q-band spectra. Furthermore the dynamical ESR studies on nitrogen dioxides (NO2) on various zeolites were briefly presented. 

The ESR g-factor is also known as the Lande 7-factor or spectroscopic splitting factor and depends on the resonance condition for ESR (Eq. 3) and is independent of both applied field and frequency. The 7-factor of a free electron is 2.002322, while the 7-factors of organic free radicals, defect centers, transition metals, etc. depend on their electronic. structure. The 7-factors for free radicals are close to the free electron value but may vary from 0 to 9 for transition metal compounds. The most comprehensive compilations of 7-factors are those published in [75], [76]. The magnetic moments and hence 7-faclors of nuclei in crystalline and molecular environments are anisotropic, that is the 7-factor (and hyperfine interactions) depend on the orientation of the sample. In general, three principal 7-factors are encountered whose orientation dependence is given by ... 

The IR spectrum of C02 in a neon matrix, where the radical anion is free from interactions with metal cations (Sect. 1.4), exhibits marked differences with respect to that of the neutral parent molecule. Because of the bent geometry (C2v) of the anion, all the three normal vibration modes of C02 are IR-active. The changes of the stretching frequencies are pronounced. Their lower values reflect a reduced CO... 

The SERS of methyl viologen cations, and MV, and a wide variety of cationic metal complexes have also been observed (Table VIII). With the MV radical cation, both a SERS and a RR process can be observed from the surface Raman spectrum. " The MV and MV cations seem to interact with the Ag surface via adsorbed halides. On adding an electron, the SERS carbon-carbon stretching frequency shifts from 1292 cm for MV to 1352 cm" for MV for the carbons bridging the pyridine rings in the molecule. This upward frequency shift is indicative of increased electron density in the bridging carbon-carbon bond on electron transfer. 

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