Carbon-13 longitudinal relaxation time, observations

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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