Solid-state conformational heterogeneity of the 2,2,5,5-tetramethyl-3,4-hexandione monohydrazone: X-ray diffraction and MAS NMR experiments

The X-ray diffraction study of the 2,2,5,5-tetramethyl-3,4-hexandione mono- hydrazone 1 shows a solid solution of two screwed conformers in the crystal . In each of these conformers, the conjugated C=O and C=N double bonds have an approximately perpendicular orientation with Phi = 101.1 degrees (2) an d -93.4 degrees (2), respectively. AM1 theoretical calculations give the sa me result for the isolated molecule. The calculated rotational barrier arou nd the central single bond of the conjugated moiety is about 45.98 kJ mol(- 1) which is higher than the classical values observed for 1,3 conjugated sy stems (28.42 kJ mol(-1) in the 1,3-butadiene). Variable temperature C-13 CP MAS NMR experiments show hindered rotation around the COC(CH3)(3) tert-buty l group in the solid state. 1 crystallizes in the triclinic space group P-1 with a = 10.106(1)Angstrom, b = 11.698(1)Angstrom, c = 12313(1)Angstrom, a lpha = 62.108(1)degrees beta = 70.517(1)degrees gamma = 66.052(1), V = 1157 .0(3)Angstrom (3), D-calc = 1.06 with Z = 4.