Structure correlation method

Concerted bond-forming/bond-breaking processes at tetrahedral carbon (the familiar SN2 reaction) are not easily studied by the crystal structure correlation method. The preferred approach of a nucleophile is sterically more encumbered than the approach to a singly or doubly bonded centre, and the transition states involved are generally of high energy. Intramolecular displacements, such as those described on pages 117-118, are a possible way round this problem, but no systematic study is available. 

The changes in the geometric parameters of the CISiC/jO coordination centre are typical of a bimolecular nucleophilic substitution reaction at a tetrahedral atom and consistent with the results obtained using the Burgi-Dunitz structure correlation method with the experimental results of Macharashvili and coworkers26. 

Gilli, R, Bertolasi, V., Ferretti, V., and Gilli, G., Covalent nature of the strong homonuclear hydrogen bond. Study of the 0—H"0 system by crystal structure correlation methods, J. Am. Chem. Soc. 116, 909-915 (1994). 

The above summary shows that mechanistic parlance implies energetic and structural aspects. As discussed in Chapter 5, structure correlation does not allow detailed energetic deductions to be made and can usually not distinguish one step from multistep mechanisms, or early from late transition states. Other aspects of mechanism, however, can be addressed by structure correlation methods, e.g. the detailed structural changes necessary to deform the ground state of the reactants into a transition state, and the structural characteristics of the latter such as looseness or tightness. 

The examples presented in this chapter also illustrate a development of the structure correlation method itself. Initially, it was applied to whatever representatives of a specific fragment happened to be available (Cd). Later, directed searches in the CSD [6] led to sometimes surprising new types of correlations (Sn). More recently, compounds have been synthesized in a planned way and their structures determined, so that a specific structural feature or reactivity problem could be studied (Si, B). We have also seen that the methods and interpretations of structure correlation can be applied to results of quantum-chemical calculations. Combinations of kinetic, mechanistic, and computational studies together with structure correlations, are just beginning to illuminate as yet poorly understood problems of chemical reactivity and selectivity, e.g. the factors differentiating between substitutions proceeding with retention or inversion at Si. 

The structure correlation method has been applied to derive information about a variety of other reaction paths, for example, for weakening and ultimate fission of one bond of a tetrahedral MX4 molecule to give a planar MX3 species (5n1-reaction type) [48] and nucleophilic addition at carbonyl C-atom [49]. From analysis of the conformations of Ph3P=0 fragments [50], the stereoisomerization path could be mapped and identified as corresponding to a two-ring flip mechanism [51]. 

Spectral analysis based on group frequencies and the spectra-structure correlation method is well established in IR and Raman spectroscopy. These methods are also becoming popular in NIR spectroscopy, and recently Weyer and Lo (30) wrote a valuable review article on the relationship between NIR bands and chemical structure. They compared NIR spectra of selected aliphatic hydrocarbons, those of selected aromatics, those of selected alcohols, and so on and discussed detailed spectra-structure correlations in the NIR region. Tables for group frequencies in the NIR region are also available in some NIR books (3,4). 

This picture closely agrees with predictions of the structural correlation method (Sect. 1.3.5.3, Fig. 1.18) and conforms well to the predictions founded on a study of the regions of charge concentration and charge depletion, as defined by the Laplacian of electron density, to identify the sites of nucleophilic attack of formaldehyde and acetaldehyde [112]. It may easily be interpreted by considering the interactions between the frontier orbitals of reactants. The main... 

Burgi, H.-B. Dunitz, J. D. (Eds), Structure Correlation, 1994 VCH, Weinheim Ferretti, V. Gilli, R Bertolasi, V. Gilli, G. Structure correlation methods in chemical crystallography, Crystallography Reviews 1996, 5, 3-104. 

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