Pyrazole hydrogen-bonded trimer

The geometries of hydrogen bonded trimers and tetramers in solid 3,5-sub-stituted pyrazoles (Fig. 6) have been studied from consideration of both 15N chemical shift tensors and dipolar interactions involving 15N [122]. 

Pyrazoles with free NH groups form hydrogen-bonded cyclic dimers (195) and trimers (196) as well as linear polymers, depending on the substituents at positions 3 and 5. For R = H, Me or Et, the oligomers are preferred, but for R = Ph, the cyclic dimer and the linear polymers exist. The cyclic trimer (196 R = Ph) is) is not formed because of steric hindrance (B-76MI40402). 

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