Evidence from dynamic studies

Direct or kinematic evidence for the formation of an intermediate complex was first obtained by Henglein and Muccini [85—87] in the same simple experiments as those which gave the evidence for stripping. From what was described in Section 4.1.1, it is evident that the non-occurrence 

More immediate kinematic evidence for the formation of a complex which shows an isotropic decay in the centre-of-mass coordinate system was obtained by Gislason et al. [138] in the endothermic reaction 

The contour map of the intensity of the DO product showed, shortly above the threshold, a very nearly isotropic distribution and the greatest intensity occurred at the centre-of-mass velocity. This is in agreement with the data of Ding and Henglein [139] who obtained the same result by measuring the distribution of translational energy in the forward direction. 

The D2O intermediate represents a potential well 2.6 eV deep with respect to reactants, excluding activation barriers. Since, in addition, the reaction is endothermic by 1.9 eV, the threshold for dissociation of the complex to products is correspondingly high. Moreover, for an endothermic reaction to proceed via stripping, the impact energy relative to one atom must be greater than the endothermicity. This condition is met 

Herman et al. [140] then obtained decisive evidence for complex formation in their crossed beam experiments, described above, in which both velocity and angular spectra were measured. In the reaction 

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Anomeric effects are evident from dynamic NMR studies on at least one substrate, N-benzyloxy-Af-chlorobenzamide (2c) ". In acetone-de the benzyl aromatic signal (S 7.85) de-coalesced into two signals (ratio 2 1) close to 200 K, corresponding to a free energy barrier of ca 10-11 kcalmoH Amide isomerization appeared to be faster than N—0 rotation since benzoyl resonances were largely unaffected. 

It is evident from these studies that dielectric relaxation spectroscopy provides a direct and unambiguous method for studying the molecular dynamics and alignnient properties of liquid crystalline side chain polymers. It has the distinct advantage over the NMR, ESR and DSC methods that a wide frequency range (10 to 10 Hz) can be covered at each temperature in the liquid crystal, biphasic and isotropic ranges of these anisotropic materials. 

From a study of overall rate constant k(T) for a reaction in the bulk and its dependence on concentrations of reactants, catalyst/inhibitor, temperature etc., the kinetics come up with a mechanism by putting together a lot of direct and indirect evidences. The determination of the overall rate constant k(T) using transition state theory was a more sophisticated approach. But the macroscopic theories such as transition state theory in different versions are split to some extent in some cases, e.g. for very fast reactions. The experimental and theoretical studies in reaction dynamics have given the indications under which it becomes less satisfactory and further work in this direction may contribute much more to solve this problem. 

As a final comment there is the question whether the cation observed with LFP is the same as the one produced in a ground-state reaction. With nanosecond LFP, the time interval from excitation to observation is likely sufficient to ensure that this is the case. Two pieces of evidence can be cited. (1) There is a good correspondence of ultraviolet-visible (UV-vis) spectra for the transient cations with ones obtained for solutions of ground-state cations under strongly acidic solutions. (2) Ratios of rate constant obtained directly by LFP agree with selectivities measured for the ground-state reactions. Diffusional separation of ion pairs is complete within 1-10 ns, so that a transient cation observed with nanosecond LFP is a free ion. At shorter times, that is, in picosecond LFP, ion pairs can be observed and their dynamics studied. ... 

In recent molecular dynamics studies attempts were made to reproduce the shapes of the intercollisional dip from reliable pair dipole models and pair potentials [301], The shape and relative amplitude of the intercollisional dip are known to depend sensitively on the details of the intermolecular interactions, and especially on the dipole function. For a number of very dense ( 1000 amagat) rare gas mixtures spectral profiles were obtained by molecular dynamics simulation that differed significantly from the observed dips. In particular, the computed amplitudes were never of sufficient magnitude. This fact is considered compelling evidence for the presence of irreducible many-body effects, presumably mainly of the induced dipole function. 

For decabenzylferrocene (Fig. 13) it was reasoned, based on qualitative model studies, that a 180° rotation about the (benzyl)CH2—C5 bond should not be possible (Section IV,B,6) (98). No stronger evidence from molecular mechanics calculations or dynamic NMR studies is, as yet, available. Ambient temperature NMR measurements of other bent-metallocene or half-sandwich pentabenzyl-Cp complexes (cf. Figs. 2, 10, and 11)... 

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