Temperature-dependent studies

Fig. 14. Comparison of the VTMCD spectra ( 1.6 K and 6 T) of the valence-delocalized S = 9/2 [Fe2S2] cluster in the aUtEdine form of the C60S mutemt of C. pasteurianum 2Fe Fd with those of the [ZnFesSJ Emd [Fe3S4] clusters in P. furiosus Fd. Magnetization and temperature dependence studies show that the bands marked with an asterisk in the valence-delocEdized S = 9/2 [Fe2S2] spectra result from a valence-localized S = 1/2 component.

Experimentally, one can only measure a total cr. However, the two contributions can be separated by performing a temperature dependence study of a or by having an a priori knowledge of o-vib from vibrational spectroscopy. 

A significant technical development is the pulsed-accelerated-flow (PAF) method, which is similar to the stopped-flow method but allows much more rapid reactions to be observed (1). Margerum s group has been the principal exponent of the method, and they have recently refined the technique to enable temperature-dependent studies. They have reported on the use of the method to obtain activation parameters for the outer-sphere electron transfer reaction between [Ti Clf ] and [W(CN)8]4. This reaction has a rate constant of 1x108M 1s 1 at 25°C, which is too fast for conventional stopped-flow methods. Since the reaction has a large driving force it is also unsuitable for observation by rapid relaxation methods. 

Another example is the temperature-dependent study of the 31P-NMR spectra of the hydrogenation catalyst shown in Figure 11.11 [28]. At 240 K the 31P-NMR signals are equivalent, whereas at 123 K two sets of signals are observed which are attributed to a seven-membered ring in a twist-chair and a distorted boat conformation. 

In the case of the pseudo-me/o-isomer (m-[38]), signals due to a quintet state were not observed. Instead, a set of signals due to the thermally populated triplet Tj appeared at 25 K. The temperature dependence data were analysed in terms of an equation similar to (13) [3 exp —AEjRT) in place of 5 exp (-iAEIRT) in the numerator] to give AE j = 98cm" ( = 280 cal mol" ). The behaviour of p-[38] was more like o-[38], although the former was more reactive and did not allow a satisfactory temperature-dependence study. 

Removal of calcium from HRP C has a significant effect not only on enzyme activity and thermal stability, but also on the environment of the heme group. The calcium-depleted enzyme has optical, EPR, and H NMR spectra that are different from those of the native enzyme (211). Temperature dependence studies indicate that the heme iron exists as a thermal admixture of high- and low-spin states. Kinetic measurements at pH 7 show that ki, the rate constant for compound I formation, is only reduced marginally from 1.6 0.1 x 10 to 1.4 x lO M s , whereas k, the rate constant for compound II reduction, is reduced from 8.1 1.6 x 10 to 3.6 x lO M s (reducing substrate p-aminobenzoic acid), 44% of its initial value (211). There can be little doubt that this is the main reason for the loss of enzyme activity on calcium removal. 

The quasi-ideality of the (1 x l)Co/Cu(lll) and (1 x l)Co/Cu(110) monolayer interfaces allows a temperature dependent study of the polarisation dependent Debye Waller damping of the EXAFS oscillations i.e the analysis of the amplitude of the mean square relative displacements of the Co atoms parallel to the adsorbate layer, or perpendicular to it. The results are based on the analysis of data collected with the sample temperature T = 77 K and T = 300 K. The S—S and S—B (see above)... 

Inukai et al. [512] have used STM to study Hg UPD on Au(lll) in sulfuric and perchloric acid solutions. For sulfuric acid, the influence of adsorption of bisulfate was indicated. It has been found that after the formation of the first UPD adlayer, two different structures were simultaneously formed on the same terrace. For perchloric acid, only a single structure was found. These results reflected a significant influence of the supporting electrolyte anions on the UPD structure. Recently, Abaci et al. [513] have presented the temperature-dependent studies on the influence of counteranions on Hg UPD on Au(lll). 

Martinez, E B. Cabanas, A. Aranda, and P. Martin, Kinetics of the Reactions of NOj Radical with Selected Monoterpenes A Temperature Dependence Study, Environ. Sci. Technol., 32, 3730-3734 (1998). 

Evidence for photoassociation in the triplet manifold is at present inconclusive. Although Hoytink et al.20 have reported excimer phosphorescence from cooled ethanolic solutions of phenanthrene and naphthalene, concentration and temperature-dependent studies of the emission characteristics must be extended in order to distinguish photoassociation of the triplet state from intersystem crossing of the singlet excimer and possible triple-triplet annihilation. Certainly the decay constant of the molecular triplet state in fluid media is relatively insensitive to solute concentration21 although this... 

It is, in fact, true that the temperature dependence studies provide the only substantial evidence suggestive of an additional mode of interaction between the alkyl halide and the complex. The magnitudes of the observed activation energies are much smaller than those usually associated with nucleophilic substitution at the saturated carbon atom (15 to 30 kcal. per mole) (13, 22). This might arise from some sort of pre-equilibrium. 

In Table 1 are summarized representative examples of self-exchange rate constant data for a variety of different types of redox couples based on metal complexes, organometallic compounds, organics and clusters. Where available the results of temperature dependence studies are also cited. For convenience, data obtained from temperature dependence studies are presented as enthalpies and entropies of activation as calculated from the reaction rate theory expression in equation (14). 

Temperature dependence studies of irradiated UP showed that B14 and B23 interconvert reversibly above about 30 K, with B14 dominating at all temperatures at which Species B is stable. In Figure 14, a plot of InK versus 1/T gives a straight line for spectra taken above 30 K, but more or less random values for spectra taken below that temperature. The abrupt change near 30 K shows that below that temperature, interconversion is slow, and nonequilibrium populations are obtained. 

When the viscosity of the solution increases by using ethyleneglycol or glycerol water mixtures as solvent, the rotational correlation time increases. This determines (1) higher relaxivity values at low frequencies (2) a shift toward lower frequencies of the a>s dispersion (3) the appearance of a second dispersion (ascribed to the a>i dispersion) at high fields. Temperature dependence studies show that the observed rates are not controlled by exchange, but arise from variation of the rotational correlation time. 

In view of the ambiguity involved in the detailed model calculations and lack of data concerning temperature dependences of the 14C KIEs presented in this part of the chapter, the numerical values of 14C KIEs collected in Table 2 and 3 should be given also the alternative simple interpretation which could be verified by determination of the 14C KIE for 14C-labelled, ylides and by 14C KIE temperature-dependence study. 

Temperature-dependence studies of the NMR spectra of protonated alcohols... 

Recently, Shin and co-workers907 have studied the cyclization of the trienol 279 and isolated two compounds (280 and 281) (Scheme 5.94). Temperature-dependence studies and the transformation of related compounds resulted in the conclusion that both a concerted process (route a) and a stepwise mechanism are operative (route b). 

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