Molecular Size and Relaxation Mechanisms

The relation between molecular size, Tl, and the relaxation mechanisms is apparent from the homologous series of cycloalkanes. 7, and NOE data as well as the contributions of the DD and SR mechanisms accessible from these experimental values via eqs. (3.20) and (3.21) are listed in Table 3.15. The I3C nuclei of cyclopropane relax almost as fast by spin-rotation as by the DD mechanism (7, (SR) Ti (DD)). However, in cyclobutane the DD mechanism predominates (7, (DD) 7, (SR)), and the carbon atoms of cyclohexane and its higher homologs relax exclusively by the DD mechanism (71(DD 71(SR ) [160], 

The rl(DD) values themselves decrease fairly regularly with increasing ring size (Table 3.15). 

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