Coupled motion experimental results

An alternative to the traditional approach is to generate the electronic states as needed during the dynamics. This has been done for atomic collisions, where detailed calculations and comparisons with experimental results are possi-ble.(4-8) General treatments of the coupling of electronic and nuclear motions in molecular systems can be done in a variety of formulations. In particular, Ohrn, Deumens and collaborators have implemented a general variational treatment in... 

To recover the ideal case of Eq. (1.1) we would have to assume that (u ), vanishes. The analog simulation of Section III, however, will involve additive stochastic forces, which are an unavoidable characteristic of any electric circuit. It is therefore convenient to regard as a parameter the value of which will be determined so as to fit the experimental results. In the absence of the coupling with the variable Eq. (1.7) would describe the standard motion of a Brownian particle in an external potential field G(x). This potential is modulated by a fluctuating field The stochastic motion of in turn, is driven by the last equation of the set of Eq. (1.7), which is a standard Langevin equation with a white Gaussian noise defined by... 

Fig. 17. The ll DQ sideband patterns arising from DQ SQ correlation experiments on the OCH2 group of ethanol bound to amorphous silica surfaces.24 The sideband patterns are shown for two different DQ excitation times, as indicated. The solid lines are the experimental results the dotted lines are simulations assuming the motion indicated at the top of the figure and a distribution of effective dipolar coupling constants for the dipolar coupling within the OCH2 group.

Relaxation rates of nuclear spins can also be related to aspects of molecular structure and behaviour in favourable circumstances, in particular internal molecular motions. It is true to say, however, that the relationship between relaxation rates and structural features are not as well defined as those of the chemical shift and spin-spin coupling constants, and are not used on a routine basis. The problem of reliable interpretation of relaxation data arises largely from the numerous extraneous effects that influence experimental results, meaning empirical correlations for using such data are not generally available and this aspect of NMR will not be pursued further in this book. 

To date, the only experimental examples where a 2° Swain-Schaad relationship resulted in a breakdown of semidassical models and implicated tunneling and coupled motion were from studies of alcohol dehydrogenases (ADH). Furthermore, all these studies were conducted on the oxidation of the alternative substrate benzyl alcohol to aldehyde. The only attempt so far to conduct similar measurements used a very different system (DHFR). These experiments revealed no deviation from the semidassical EXP [45]. Until such experiments are extended to other systems or at least extended to the reduction of aldehyde to alcohol for the same system, the generalization of their interpretation should be taken with some discretion. These examples are discussed in great detail in Chapter 10, Section 10.5.1.1, and only a concise summary of two seminal examples is presented below. 

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