Cycloalkanes shifts 115

Table 2.12. chemical shifts (relative configurations of cycloalkanes, pyranoses and alkenes (application of y-effects) The shifts which are printed in boldface reflect y-effects on C atoms in the corresponding... 

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) ... 

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. 

Table 4 13C NMR chemical shifts of ring carbon atoms of some spiro(cycloalkane-1,2 -[1,2,4]triazolo[1,5-c]pyrimidine carbonitriles measured in DMSO-d6 solutions...

It can be seen that the HOMO energy of cyclopropane is higher than that of cyclobutane or cyclohexane, and that the much more reactive bicyclo[1.1.0]butane has a much higher HOMO energy, which is close to that of propene. Another important factor is the polarizability, which reflects how easily the electron density may be shifted in the presence of an electric field (such as that developed by a proton). Here again, cyclopropanes have significantly higher polarizability than other cycloalkanes.52... 

The acid-catalyzed isomerization of cycloalkenes usually involves skeletal rearrangement if strong acids are used. The conditions and the catalysts are very similar to those for the isomerization of acyclic alkenes. Many alkylcyclohexenes undergo reversible isomerization to alkylcyclopentenes. In some cases the isomerization consists of shift of the double bond without ring contraction. Side reactions, in this case, involve hydrogen transfer (disproportionation) to yield cycloalkanes and aromatics. In the presence of activated alumina cyclohexene is converted to a mixture of 1-methyl- and 3-methyl-1-cyclopentene 103... 

Empirical additive substituent increments obtained by analysis of substituted alkanes, alkenes, cycloalkanes, aromatic and heteroaromatic compounds have proved to be useful for the prediction of 13C chemical shifts. These substituent increments will be tabulated for the various classes of organic compounds in Section 4.13. 

Table 4.4. 13C Chemical Shifts and One-Bond Coupling Constants JC H of Cycloalkanes (c5c in ppm) [206].

Fig. 4.1. Plot of UC chemical shifts of cycloalkanes versus ring size [206].

Table 4.10. 13C Chemical Shifts of Alkenes [90, 232-235], Dienes [233] and Methylene-cycloalkanes [236] (Sc in ppm).

The shifts of the protons of alkanes and cycloalkanes fall in the range 0.9-1.5 ppm with C—H protons coming at the low-field end of this range and —CH3 protons coming at the high-field end (see Table 9-4). 

Although proton spectra are not very useful for identification purposes, 13C nmr spectra are very useful. Chain-branching and ring-substitution normally cause quite large chemical-shift changes, and it is not uncommon to observe 13C shifts in cycloalkanes spanning 35 ppm. Some special features of application of 13C nmr spectra to conformational analysis of cycloalkanes are described in Section 12-3D. 

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