Conformational effects, chemical

Hassel shared the 1969 Nobel Prize in chemistry with Sir Derek Barton of Imperial College (London) Barton demonstrated how Hassel s structural results could be extended to an analysis of conformational effects on chemical reactivity... 

In the main, the physical and chemical properties of saturated and partially unsaturated alicyclic compounds closely resemble those of the analogous acyclic compounds formally derived by cleavage of the carbon ring at a point remote from any functionality. Relatively small, but often significant, differences in properties arise from conformational effects, and from strain effects in small rings, and these differences can be striking in properties which are particularly sensitive to molecular shape. 

The 1H NMR spectra of the epimeric cyclohexanols in DMSO reveal that the hydroxyl proton in the axial alcohol shows a resonance absorption at a higher field than in the equatorial one, indicating that the conformational effect of the hydrogen bond influences the XH NMR chemical shifts128. 

C. J. Carman I guess it could be, but again I m not certain because there will be a distribution of chemical shifts which perhaps contributes to the line widths as well. In addition to the latter, other factors can contribute to line broadening. As was indicated by other speakers before me, conformational effects do play a role in polymer spectra, I m convinced of it. I m not certain of how to extract it but I m convinced it is there. In the center of the EPDM spectrum is the bulk of the structure which arises from the long runs of methylene carbons. There are several peaks which are very temperature dependent and very solution dependent. I am not sure how much of the line width is really due to molecular motion as opposed to distribution of chemical shifts. The shifts may differ from those found for the polymer when it is in solution. This is why I am a little hesitant to assign the broadening to crystallinity effects. 

The intimate relationship between NMR parameters such as chemical shifts and spin-spin coupling constants and molecular geometry is particularly evident for derivatives with rigid frameworks. Therefore, structural and conformational effects are treated first as a separate topic and then in conjunction with specific compounds. As data on hydroxylamines, oximes and hydroxamic acids are not as extensive as those for other types of systems containing nitrogen or oxygen, comparisons with their respective parameters or effects have also been included wherever they are considered relevant. 

Discussion of NMR data have been organized according to the nucleus and type of parameter. They are used to exemplify general aspects of NMR data and are followed by more specific examples. This material is arranged by parameter (chemical shifts, coupling constants) and then by nucleus, in their order in the periodic table. Wherever general aspects of discussions of substituent or conformational effects are at hand, they are treated... 

Carbon chemical shifts were employed in studies of conformational effects on aldoximes and ketoximes. They appear in the region 145 to 163 ppm and several values are illustrated in Table 2. Such resonances are some 50 ppm to higher field relative to the corresponding carbonyl resonance. Where substimtion is asymmetrical, differences in oxime carbon chemical shifts are observed, depending upon the conformation of the oxime N—OH. Such conformational isomerism also has a profound effect upon the chemical shift of the a-carbon. 

In the absence of secondary effects as part of the RPC distribution process(es), i.e., when no temperature- or ligand-mediated conformational effects, slow chemical equilibrium, or pH-dependent ionization events occur, then the resolution, Rsi, between two adjacent peptides separated under equilibrium or near-equilibrium conditions can be expressed as... 

In the application of Anet s equations (see Section II,B,5 below) to the estimation of AG and AG° by low temperature 13C-NMR spectroscopy, the magnitude of Av (the chemical shift difference between the exchanging sites) is required. Because this cannot always be observed, resort has to be made to some indirect method of estimation of Av. This has been done, for example, in the case of the l,2,4-trimethylhexahydro-l,2,4-triazine equilibrium (Section III,F,3) by estimating the chemical shifts in the various conformers from chemical shift effects based on model systems (Table VIII). Utilization of... 

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