NMR parameters for conformational analysis

A comparison of the NMR parameters for (T>4-butadiene)M(CO)s (where M is Fe or Ru), carried out by von Philipsbom and co-workers (54, 55), provides an excellent example of the potential of a complete spectral analysis (Fig. 6). Values for the vicinal C—H and H—H couplings have been extracted and used to deduce the conformation of the complexed ligand. The authors conclude that the carbon skeleton is coplanar, that the... 

The conformational description suggested in this Viork is succinct and unequivocal. The use of the notation for analysis of NMR parameters for each bond (eg, J has been developed in a recent review... 

Application of NMR spectroscopy to heterocyclic chemistry has developed very rapidly during the past 15 years, and the technique is now used almost as routinely as H NMR spectroscopy. There are four main areas of application of interest to the heterocyclic chemist (i) elucidation of structure, where the method can be particularly valuable for complex natural products such as alkaloids and carbohydrate antibiotics (ii) stereochemical studies, especially conformational analysis of saturated heterocyclic systems (iii) the correlation of various theoretical aspects of structure and electronic distribution with chemical shifts, coupling constants and other NMR derived parameters and (iv) the unravelling of biosynthetic pathways to natural products, where, in contrast to related studies with " C-labelled precursors, stepwise degradation of the secondary metabolite is usually unnecessary. 

Fig. 1 Solid-state NMR structure analysis relies on the 19F-labelled peptides being uniformly embedded in a macroscopically oriented membrane sample, (a) The angle (0) of the 19F-labelled group (e.g. a CF3-moiety) on the peptide backbone (shown here as a cylinder) relative to the static magnetic field is directly reflected in the NMR parameter measured (e.g. DD, see Fig. 2c). (b) The value of the experimental NMR parameter varies along the peptide sequence with a periodicity that is characteristic for distinct peptide conformations, (c) From such wave plot the alignment of the peptide with respect to the lipid bilayer normal (n) can then be evaluated in terms of its tilt angle (x) and azimuthal rotation (p). Whole-body wobbling can be described by an order parameter, S rtlo. (d) The combined data from several individual 19F-labelled peptide analogues thus yields a 3D structural model of the peptide and how it is oriented in the lipid bilayer...

A conformational analysis of thiacycloalkanes including 2-thiacyclobu-tane, utilizing molecular mechanics for the calculation of the geometries and energies, was made by Allinger and Hickey. The calculated C—C, C—S, C—S—C, C—C—C, and C—C—S bond parameters for the thietane structure, in comparison to those that were derived from electron diffraction, NMR, and microwave measurement, are reported. The experimental and calculated heat of formation of the thiacyclobutane are 14.49 and 14.58 kcal/mol, respectively. 

The conformational analysis of oligothiophenes by use of a combined molecular dynamics (MD)/NMR spectroscopic protocol has been carried out. A series of MD simulations were performed for 2-(2-thienyl)-3-hexylthiophene 173, 2,5-bis(3 -hexyl-2 -thienyl)thiophene 176, and 2,5-bis(4 -hexyl-2 -thienyl)-thiophene 177, with a new MM2 torsional parameter set developed earlier for unsubstituted and methyl-substituted 2,2 -bithiophene. 

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