MOLECULAR-STRUCTURE AND DYNAMICS OF C-1-ADAMANTYL SUBSTITUTED N-UNSUBSTITUTED PYRAZOLES STUDIED BY SOLID-STATE NMR-SPECTROSCOPY AND X-RAY CRYSTALLOGRAPHY

The influence of the 1-adamantyl group on the structure and the proton transfer dynamics of N-unsubstituted pyrazoles has been determined. F our compounds have been labelled with N-15 and studied by variable tem perature N-15 CP MAS NMR spectroscopy: 3(5)-(1-adamantyl)pyrazole 2, 4 -(1-adamantyl)pyrazole 3, 3,5-dimethyl-4-(1-adamantyl)pyrazole 4 and 3 ,5-di(1-adamantyl)pyrazole 5. Compound 2 (a 1:1 mixture of both tautom ers) is a long chain of hydrogen bonded molecules ('catemer') and as i n most catemers there is no proton transfer since it would imply an 'i nfinite' number of proton jumps. Compound 3, although also a 'catemer' , is possibly an exception to this rule, in that it seems to show prot on transfer. In the solid state, compounds 4 and 5 should be cyclic hy drogen-bonded structures, dimers or trimers, but the activation energi es for proton transfer, about 39 kJ mol(-1), are quite low compared wi th those of 3,5-dimethylpyrazole. It appears that the quasi-spherical shape of the 1-adamantyl substituent and its solid-state plasticity ma y play a role in lowering these barriers. The crystal structure of 2 h as been determined by X-ray analysis. Individual molecules of 2 form c hains through N-H ... N hydrogen bonds ('catemers') very similar to th ose already described for 4-(1-adamantyl)pyrazole and for pyrazole its elf; however, the packing of these catemers is different. Tautomers 2a and 2b are present in the crystal in a 1:1 ratio, forming alternating chains of hydrogen-bonded molecules (2a...2b...2a...2b...); the NH hy drogen atoms are linked to both nitrogen positions (N1 and N2) and sho w a 2:1 disorder.