Modeling Hydrogen Transfer Reactions

Both quantum mechanical and force field-based methods have been employed in the attempt to model hydrogen transfer reactions. In free-radical chemistry, by far the most significant hydrogen transfer process involves the reaction between a carbon-centered radical and a chain-carrying reagent such as tributyltin hydride 

The synthetic utility of stannane-based reagents is discussed in other chapters of this compilation. While several workers have examined more general types of hydrogen transfer reactions by computational techniques, this chapter will focus on the modeling of hydrogen transfers of synthetic utility, in particular the ability of various modeling methods to accurately predict the stereochemical outcome of free-radical reductions. 

1 kJ mol ) [27]. It is interesting to note that Roberts had earlier reported an extended form of the Evans-Polanyi equation for predicting activation energies for a large cross-section of hydrogen transfer reactions to within +2.0 kJ mol with a correlation coefficient of 0.988 [29]. The role of polar effects operating in the tran- 

While the E method has distinct computational advantages over other molecular orbital methods, it does suffer from two main drawbacks. E is unable to provide geometries of reacting species and transition states involved in the reactions of interest, nor is it able to accommodate novel reagents and substrates because empirical bond strength data are required for the reacting species [28]. 

In order to address these issues, Beckwith and Zavitsas explored the use of the AMI semiempirical technique to model the reactivities and diastereoselectivities during hydrogen transfer from BujSnH to a series of radicals 66 derived from 

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The results in Table IV suggest that the condensation reactions cannot be described adequately by the ion-induced dipole model. In this regard the results agree with conventional studies which have frequently found a higher power inverse dependence of the cross-section on the field strength E for condensation reactions than for hydrogen transfer reactions. 

The deuterium KIE values are generally in the range expected for linear three-center hydrogen transfer reactions,44107 and they track nicely with the rate constants for the reactions with the faster, more exothermic reactions displaying smaller KIEs. The large KIE value for reaction of the benzyl radical is noteworthy in that it exceeds the theoretical maximum for the classical model in a manner apparently similar to that seen with tin hydride (see below). 

Hydrogen Transfer Reactions in Model Systems Representative of Recycle Solvents... 

To understand the fundamental photochemical processes in biologically relevant molecular systems, prototype molecules like phenol or indole - the chromophores of the amino acids tyrosine respective trypthophan - embedded in clusters of ammonia or water molecules are an important object of research. Numerous studies have been performed concerning the dynamics of photoinduced processes in phenol-ammonia or phenol-water clusters (see e. g. [1,2]). As a main result a hydrogen transfer reaction has been clearly indicated in phenol(NH3)n clusters [2], whereas for phenol(H20)n complexes no signature for such a reaction has been found. According to a general theoretical model [3] a similar behavior is expected for the indole molecule surrounded by ammonia or water clusters. As the primary step an internal conversion from the initially excited nn state to a dark 7ta state is predicted which may be followed by the H-transfer process on the 7ia potential energy surface. 

In this chapter Prof. Rice has advocated two techniques that should be useful for evaluations of optimal fields for laser control of chemical reactions (i) reduced space of eigenstates for representations of nuclear wavepackets and (ii) the use of effective reaction coordinates. Both techniques have already been used for efficient evaluations of reaction probabilities in model reactions. See, for example, Ref. 1 for the prediction of population inversion and Ref. 2 for the demonstration of rather strong deviations of chemical reactions from the reaction path, specifically in the case of hydrogen transfer reactions. 

Common Mechanistic Properties in the Enzymatic Catalysis for Enzyme-NAD+ Mediated Hydrogen Transfer Reactions—A Model System for the Study of the Evolutionary Process of Enzyme Catalysis ... 

Metal cluster complexes containing vinylidene ligands have been considered as models of species present when olefins or alkynes are chemisorbed on metal surfaces (114). Vinylidene has been detected in reactions of ethylene or acetylene with Fe(100), Ni( 111), and Pt(l 11) surfaces (115), and was shown to be an intermediate by theoretical studies on a manganese surface (116). The facile cleavage of C-H bonds which occurs in these systems, together with hydrogen addition or abstraction, also occurs on metal clusters. Typical of the reactions considered is the hydrogen transfer reaction... 

In this section, we discuss the photoinduced hydrogen transfer from phenol to water and ammonia in phenol-water and phenol-ammonia clusters, respectively, as a representative model of excited-state chromophore-to-solvent hydrogen transfer reactions. 

The Bell-Limbach model is not designed to give definite interpretations of Arrhenius curves of hydrogen transfer reactions which have to come from more sophisticated methods. However, it provides an opportunity to check whether the number of parameters describing a given set of Arrhenius curves matches or exceeds the number of parameters necessary to describe the same set in terms of sums of single Arrhenius exponentials. This check also tells whether it is useful... 

The results from the rudimentary model for isotope effects on a hydrogen transfer reaction shown in Fig. 11.3 are plotted in Arrhenius style as their logarithmic form versus inverse temperature. An analysis based on the temperature dependence of isotope effects is another standard method for detecting tunneling [25, 26], and again, non-tunneling models are helpful as a basis for comparison. For many ex-... 

Figure 13.1. Representation of the model for the hydrogen transfer reaction used to interpret the experimental data (see text, and Refs. [6, 84, 85], for more details) some of the parameters in Eqs. (13.1)-(13.3) are shown. The three axes are E, energy q, environmental coordinate (from which the transferred hydrogen atom is excluded) HC, hydrogen coordinate. The four vertical panels show the potential energy curve as a function of the hydrogen coordinate for three values of the environmental coordinate is for the...

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