The structure and some molecular properties of acetone-picrylhydrazone

Various physical measurements and quantum-mechanical computations to charac terize molecular 2-propanone(2,4,6-trinitrophenyl)hydrazone, alias acetone- picrylhydrazine (AH), are reported, including an X-ray diffraction structur al determination, an H-1 and C-13 NMR study of its internal hindered reorie ntation, and a theoretical (SCF-MO) interpretation of these observations. T he structure of AH was determined by X-ray crystallography. The space group is P (1) over bar, with a = 10.1768(9) Angstrom, b = 7.7968(18) Angstrom, c = 8.0018(5) Angstrom, alpha = 92.102(6)degrees, beta = 99.919(7)degrees, gamma = 105.926(6)degrees, Z = 2, wR2(F-2) = 0.1995 based on all 2748 uniqu e reflections. The (picryl) proton NMR thermal work yielded a Gibbs activat ion energy Delta G double dagger = 46.9 +/- 0.4 kJ mol(-1) in acetone-d(6) and 48.1 +/- 0.2 kJ mol(-1) in chloroform-d, whereas C-13 NMR (two pairs in the picryl ring) yielded 46.6 +/- 1.0 and 46.4 +/- 1.0 kJ mol(-1) in aceto ne-d(6). The SCF MO computations yielded a detailed model of the conformeri zation path. Various model conformations and tautomers of AH have been cons idered, as has removal of H+ or of H-0 from its hydrazinic linkage.