Gauche y effects

While the comparison of the OMTS and the (CH2)12 spectra helped to learn something about the kind of information solid state chemical shifts can provide, we can obtain much more detailed data about the correlation of chemical shifts and the rotational isomeric states from the spectra of larger cycloalkanes. Usually conformational shift variations are discussed by (i) the so called y-gauche effect and (ii) the vicinal gauche effect, Vg 15) ... 

No Vg effect has to be considered for the CH3 carbon atoms. So it is reasonable to explain the splitting solely by the y-gauche effect. As can be seen from Fig. 14 the steric arrangement of the two CH3 carbon atoms within the segment differs by one y-gauche position of the carbons opposite of the CH—CH bond. With respect to the other carbons the steric situation is equivalent. Hence, the chemical shift... 

Conformational shift effects could be discussed in terms of discrete rotational isomeric states. Mainly two effects could be derived empirically to explain the shift differences due to conformational isomerism they-gauche and the Vg effect. However the spectra also indicate that the y-gauche effect is not a quantity with a universal numerical value. Furthermore the spectra of the cycloalkanes show that the conformational effects do not obey simple rules of additivity. With concern to our present knowledge great care has to be taken for the interpretation of NMR-spectra on the base of conformational shift increments which were not determined for the specific molecular structures. 

Other authors have successfully correlated changes in torsional angles with y-gauche effects (80,150). 

For more examples concerning various molecular systems in staggered conformations, the reader is referred to the compilations of diamagnetic y-gauche effects in refs. 7 and 9. The disappearance of a noticeable effect when the observed carbon atom is quaternary, that is, when no polarizable C-H bond is present, was also considered to support the above explanation (110,175). 

Up to this point our discussion of y-gauche effects has demonstrated that the transmission mechanisms are not yet well understood and still open to speculation. The original concept of a steric interaction is highly controversial, and as long as there is no convincing explanation, the use of the deep-rooted term steric compression shift must be discouraged. 

Finally, it should be noted that y-gauche effects are not confined to carbon atoms. Analogous shifts have been reported for l9F, 31P (191), and 15N nuclei (206). Distinct shieldings of I70 nuclei caused by y-gauchc-oriented methyl or methylene groups were found, too (207). 

Empirically, y-gauche effects are extremely useful in stereochemical analysis. For example, the cis- and trans-fusion of six-membered rings can be easily differentiated, since, in contrast to the trans-isomer of bicyclo[4.4.0]decane, there are gauche-butane fragments in the m-isomer (emphasized bonds) causing upheld signal shifts of the carbons involved (numbers refer to I3C chemical shifts in ppm)49-52. 

Similar to cyclobutanes, the chemical shifts of the ring carbons do not differ significantly in the diastereomeric methyl l-benzyl-4-methylazetidine-2-carboxylates475. The only clear difference is at the benzyl carbons (6 ppm) because of a y-gauche effect in the rran.s-isomer exerted by either... 

Medium-sized and large ring systems often show complicated conformational interconversions involving pseudorotations in one or even more conformational families. This makes stereochemical assignments in diastereomers rather difficult. Thus, very few systematic studies have been published. The situation is improved if such rings are embedded in polycyclic systems, or if they contain double bonds, which leads to restricted conformational mobility. An example is the differentiation of diastereomeric 2,3-dihydro-lf/-benzo[6]azepines 1 on the basis of y-gauche effects and on d(13C) and 3/H H values640. 

C NMR chemical shifts expected for the carbon atoms in propylene-vinyl chloride (P - VC) copolymers of low propyiene content are calculated as a function of copolymer stereosequence. Mark s conformational model of P - VC copolymers (C 004) Is coupled with the y gauche effect, which results in upfield chemical shifts for those carbon atoms in a gauche arrangement with carbon or chlorine substituents in the y position, to calculate the 13C NMR chemical shifts of the carbon atoms in the vicinity of a propyiene unit surrounded by vinyl chloride units. Agreement of the calculated chemical shifts and those which are observed is excellent. 

Typical y-gauche effects are observed in conformationally rigid systems such as methyl-cyclohexanes and methylnorbornanes [73, 87], e.g. ... 

For substituents three bonds away from the carbon of interest, a more complex pattern of behaviour appears. If the substituent is a carbon bearing hydrogen, and if a gauche orientation is possible, then the y-gauche effect is observed. This may be exemplified by comparing the C-19 chemical shifts of 5cr-cholestan-3/ -ol [12] and cholest-5-en-3/ -ol (cholesterol) [13]. In [ 13] there is one less y-gauche interaction owing to... 

Similar, shielding, y-gauche effects are evidenced at carbons 9 and 14 when a 7ct-hydroxy (compound 414) or a 7cr-hydroperoxy (compound 416) group is substituted into cholesterol (compound 211). The effects of bromine, hydroxyl, and hydroperoxy are —6 to —7 ppm for groups without hydrogen or with hydrogen and certainly of differing physical size. 

The y-gauche effect involving only C-H bonds has been recently investigated by means of a modified version of the INDO finite perturbation theory of 13C chemical shifts, using butane as a model. (41) While the expected geometrical dependence on the methyl chemical... 

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