Mode isotope correlations

The most direct information on the state of cobalt has come from Mossbauer spectroscopy, applied in the emission mode. As explained in Chapter 5, such experiments are done with catalysts that contain the radioactive isotope 57Co as the source and a moving single-line absorber. Great advantages of this method are that the Co-Mo catalyst can be investigated under in situ conditions and the spectrum of cobalt can be correlated to the activity of the catalyst. One needs to be careful, however, because the Mossbauer spectrum one obtains is strictly speaking not that of cobalt, but that of its decay product, iron. The safest way to go is therefore to compare the spectra of the Co-Mo catalysts with those of model compounds for which the state of cobalt is known. This was the approach taken... 

Until now, the isotopic effect was discnssed only in relation to the reactants. In electron-transfer reactions, the solvent plays an eqnally important role. As mentioned, different solvate forms are possible for reactants, transition states, and products. Therefore, it seems important to find a reaction where the kinetic effect resulting from the introduction of an isotope would be present for solvents, but absent for reactants. For a published work concerning this problem, refer Yusupov and Hairutdinov (1987). In this work, the authors studied photoinduced electron transfer from magnesium ethioporphyrin to chloroform followed by a dark recombination of ion-radicals in frozen alcohol solutions. It was determined that the deuteration of chloroform does not affect the rate of transfer, whereas deuteration of the solvent reduces it. The authors correlate these results with the participation of solvent vibrational modes in the manner of energy diffraction during electron transfer. 

Normal vibration calculations, if based on a correct structure and correct potential field, would supposedly permit a unique correlation to be made between predicted and observed absorption bands. In most cases this ideal situation is far from being achieved in the study of high polymer spectra. More usually the structure and force field are to some extent unknown, or normal mode calculations are not available, so that other methods must be used in order to establish the origin of bands in the spectrum. Even if complete calculations were available it would be desirable to check their predictions by means other than a comparison of observed and predicted frequency values. One method of doing so is by studying isotopically substituted molecules, and the most useful case is that in which deuterium is substituted for hydrogen. 

In recent years, these donor-acceptor correlations to the reaction mode [49], substituent effects to stabilize meta intermediates [50-52], exo/endo selectivity [53-55], kinetics by means of fluorescence study [56] and deuterium isotope effect [57,58], formation and deactivation studies of exciplex [59,60], influence of pressure [61], and theoretical calculations [62-64] have been extensively studied. 

The ground-state vibrational normal modes of uracil have also been extensively studied, both experimentally and computationally. The IR and Raman spectra in Ar matrix have been measured for the 5-d, 6-d, 5,6-d2, 1,3-d2, l,3,5-<73, 1,3,6-d3, and d4 isotopomers [120-122], Vibrational spectra in the crystalline phase have been reported for the 5,6-d2, 1,3-d2, and d4 isotopomers of uracil [123] and of the 2-1S0, 4-lsO, 3-d, 5-d, 6-d, 5,6-d2 and l-methyl-<73 isotopomers of 1-methyluracil [124], UV Resonance Raman spectra have been reported for natural abundance, 2-lsO, 4-lsO, and 2,4-1802 uracil in neutral aqueous solution [125]. These data have been modeled successfully by both ab initio [94, 117, 126-132] and semi-empirical [133, 134] calculations. However, most of these caculations ignore electron correlation effects on the vibrational properties of uracil, particularly the Raman and resonance Raman spectra. However, the most robust reconciliation of experiment and computation is a recent attempt to computationally reproduce the experimentally observed isotopic shifts in 4 different uracil isotopomers [116], The success of that attempt is an indication of the reliability of the resulting force field and normal modes for uracil. The resonance Raman vibrations of uracil, and their vibrational assignments, are given in Table 9-2. 

The monosubstituted species M( C" 0)5( C" 0) occnrs at natural abundance to the extent of nearly 7%. The species belongs to the point group C4 and the b2 and e modes correlate with the eg and modes, respectively, of M( C 0)6. The remaining modes are of symmetry a. Thus, the monosubstituted species should, in principle, give rise to three isotope bands. In the IR spectrum, one of these will be a lower frequency satellite of the strong mode, while the other two will be at a higher energy and much weaker. 

Experimentalists often rely on motional models, based on hydrodynamics, in order to interpret their liquid state spectra. MD simulations, can be considered as model-free in the sense that they do not assume the molecular motion to be in any specific regime. MD can be used to evaluate the motional models and even replace them. MD simulations can be used to calculate both the correlation times and the whole correlation functions. This is useful in those cases when correlation times cannot be deduced from measurements of other isotopes in the same molecule or when there is no method available at all. Correlation functions give information about intermolecular interactions and reveal cases when several motional modes are contributing to relaxation mechanisms at slightly different time scales. This can be observed as multiple decay rates. Time correlation functions from MD simulations can be Fourier transformed to power spectra if needed to provide line shapes and frequencies. 

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