75As-NMR

Preparation of the first-generation dendrimer appeared to yield chiral 75a,b without any problems. The CD spectra for both enantiomers showed the expected Cotton effects, but the mass spectra did not exhibit correct molecular peaks. As we found out by NMR spectoscropy, a mixture of stereochemically pure, once- and twice-substituted products was formed. The mass spectra showed peaks at half the mass of the molecular ion, but they do not arise from the tetranitriles 75a,b as predicted, but resulted from decomposition of the chiral amines 74a,b probably due to steric crowding. 

In contrast, introduction of a trigonal atom at the 9-position should decrease the flagpole-flagpole repulsion and thus increase the content of a CB conformer in the equilibrium. The study of the conformational equilibrium of bicyclo[3.3.1]nonan-9-one (75) at room temperature using shift reagents and H-NMR techniques made it possible to determine the content (22%) of conformations CB + BC (75a, 75c) (58). 

Blinc et al.10 have reviewed the present understanding of two recent advances in proton and deuteron glasses namely, (a) the determination of (Jea and W(p) in the "weak substitutional disorder" limit (J0>J) via 75As NQR and NMR and (b) the application of 2D "exchange" deuteron NMR to the study of deuteron inter- and intra-bond transfer rates in deuteron glasses on time scales 1-103 s. 

---