Molecular stereochemistry

Kimer JF, Dow W, Scheldt WR. 1976. Molecular stereochemistry of two intermediate-spin complexes. Iron(II) phthalocyanine and manganese(II) phthalocyanine. Inorg Chem 15 1685-1690. 

To date, only a few solution calculations for carbohydrates have been attempted (one such study of mannitol and sorbitol is described in the chapter by Grigera in this volume), but the results of these early studies bear out the expectation that solvation effects in carbohydrate systems can be both significant and difficult to predict. In the case of pyranoid rings, molecular solvation is further complicated by the close juxtaposition of these groups in essentially fixed relative orientations (assuming no conformational changes in the ring). Under such circumstances, molecular stereochemistry could play important physical roles, as is... 

Both VCD and ROA have been extensively developed along both experimental and theoretical lines (15). In the process, two sensitive new approaches to the direct investigation of molecular stereochemistry have become available. While other forms of vibrational optical activity (VOA) may yet be developed, the present two appear to be the most fundamental and the easiest to measure. In addition, they represent complementary rather than redundant approaches to the same stereochemical structural information. 

The comprehensive manner by which VOA intensities relate to the details of molecular stereochemistry can be appreciated by recognizing that the set of 3N-6 vibrational degrees of freedom is defined in the same space that specifies the parameters of molecular conformation. No other form of molecular spectroscopy is so closely related to molecular stereochemistry. It is literally trae that VOA spectra arise from stereospecific vibrational oscillations of a chiral molecule. A challenge facing VOA spectroscopy at the present is how to fully extract this stractural and conformational information from the spectra. 

In this section we describe some of the theoretical models that have been applied to the interpretation of VOA spectra, with an aim toward understanding the link between the spectral features and the molecular stereochemistry. The application of the models is included in the discussion of individual molecules in Sect. IV. 

It is clear from the preceding material that stereochemical applications of VCD are developing rapidly. This has been made possible by a growing body of VCD spectra from which the salient features relating to stereochemical details in molecules have been extracted. Progress in the area of ROA is somewhat more modest, in part due to the greater complexity of the relationship between the theoretical description of ROA intensities and molecular stereochemistry (14). 

The very useful lanthanide shift reagents, which facilitate analysis of molecular stereochemistry because of their line-broadening characteristics in NMR spectra, were studied when bound as a chelate complex to thietanes. X-Ray analysis of the adduct 3,3-dimethylthietane 1-oxide with tris(dipivalo-methanato)europium(III) [Eu(dpm)3] revealed the structure of a seven-coordinate complex (271). ... 

Prolonged standing in a THF solution results in its decomposition as outlined above however, by brief warming in THF followed by slow cooling, single crystals of the compound have been obtained. Its molecular stereochemistry is meridional in the solid state.9 Substitution reactions of the THF ligand by phosphine and halide ligands have been described.9,15... 

It is not within the scope of this book to describe the principles of crystal chemistry and molecular stereochemistry which have so far... 

Parker, D.H., Jalink, H. and Stolte, S. (1987). Dynamics of molecular stereochemistry via oriented molecular scattering, J. Phys. Chem., 91, 5427-5437. 

The fact that CPL and FDCD instruments are not commercially available, and, therefore must be constructed in individual research laboratories, has certainly affected the quantity of applications of these chiroptical techniques. Nevertheless, both have been shown to provide interesting, important, and often unique information concerning the structure and dynamics of optically active molecules. As needs develop for more selective and sensitive detection or imaging of chiral molecules or chiral molecular systems, the range of applications of CPL and FDCD will certainly increase, and these spectroscopic techniques should become even more important tools of molecular stereochemistry. 

An understanding of the spectroscopy of dissymetric molecules has been central to the science of inorganic and organic compounds. It is probably true that modern views of molecular stereochemistry have been determined through the systematic studies of chirality which have been conducted by numerous scientists. 

Even with these limitations, nuclear magnetic resonance has made significant contributions to four areas of the chemistry of the platinum group metals bonding problems, molecular stereochemistry, solvation and solvent effects, and dynamic systems—reaction rates. Selected examples in each of these areas are discussed in turn. Because of space limitations, this review is not meant to be comprehensive. 

Fig. S. The molecular stereochemistry of Cu4lr2[P(CeHs)3]a(C=CC6Hs)e. The phenyl group of the CeHsC=C group o-bonded to Ir(l) and 7r-bonded to Cu(2) has been omitted for the sake of clarity.

In the following sections we discuss in turn the experimental measurement of VCD spectra, the theoretical calculation of VCD spectra, and the application of VCD spectroscopy to the determination of molecular stereochemistry. 

We have described a methodology by means of which the IR and VCD spectra of a chiral molecule can be analyzed and molecular stereochemistry elucidated. Density... 

A description of the molecular stereochemistry is also included in both the structural formula and nomenclature. Each ring junction can exist in either a cis or trans conformation, yielding six centers of asymmetry or chirality (C-5, 8, 9, 10, 13, 14). Hence, 64 stereoisomers are possible for the ring system alone. With the C-17 side-chain forming a seventh site of asymmetry, the theoretical number of stereoisomers increases to 128. However, in practice, the isomeric possibilities are restricted by the overall conformational limitations of the ring system. 

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