Carbon-13 longitudinal relaxation time measurement

In 2-substituted adamantanes 25 both types of 8-positioned carbon atoms (8syn and 8 ) exist within one molecule (Scheme 37). Early measurements with limited spectral resolution (176) did not differentiate between their signals. Later (124,244), differences of up to 0.7 ppm were detected, and application of various independent methods, including addition of lanthanide shift reagents (245), determination of longitudinal relaxation times T, (246), and evaluation of deuterium... 

On these systems the C longitudinal relaxation times T of the C5Me5 moieties have been measured by means of the Torchia s pulse sequence [48]. The Ti( C) values obtained for ring carbons have been rationalized on the basis of two relaxation interactions (dipole-dipole and chemical shift anisotropy) modulated by the motions involving the permethylated cyclopentadienyl rings. Interestingly, a qualitative comparison between solution and solid state C relaxation data shows that the same relaxation mechanisms are operative in both physical states. 

Under conditions of proton decoupling there are two relaxation parameters of general interest the longitudinal relaxation time, and NOE, the nuclear Over-hauser enhancement. The third relaxation parameter, T2 or spin-spin relaxation time is not readily measurable for in decoupled systems. Both and NOE are directly related to the transition probabilities associated with nuclei changing energy levels. However, their functional dependence on transition probabilities differs and they can therefore be used in combination to resolve ambiguities. NOE has been found to be a very sensitive measure of mobility of carbon atoms of proteins and to provide reliable qualitative measures of the state of carbon atoms [3]. 

Further interesting is azacalix[4]arene 13b with a 1,3-alternate conformation, which has been demonstrated to be inflexible in solution [33]. Conformational behavior of 13b in solution was examined by means of relaxation time measurements (Fig. 10). A much smaller longitudinal relaxation time of 1.03 s was observed for the aromatic protons of 13b, as compared with 2.51 s reported for conformationally flexible p-fert-butylthiacalix[4]arene [34], demonstrating that the 1,3-conformation of 13b was inflexible in solution. This result was further supported by two additional experimental facts. First, NMR spectra of 13b were temperature independent [22, 33]. Second, the observed nuclear Overhauser effects were properly explained by considering a sole contribution of an inflexible 1,3-conformation of 13b [22]. X-ray crystallographic analysis revealed that a small annulus of 13b was responsible for the conformational immobilization by the small, but yet sufficiently bulky 0-methyl groups [33], which were too small for carbon-bridged calix[4]arenes to keep their conformations in solution [1,3,35,36]. 

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