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

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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 Φ = 101.1°(2) and −93.4°(2), respectively. AM1 theoretical calculations give the same result for the isolated molecule. The calculated rotational barrier around 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 systems (28.42 kJ mol−1 in the 1,3-butadiene). Variable temperature 13C CPMAS NMR experiments show hindered rotation around the COC(CH3)3 tert-butyl group in the solid state. 1 crystallizes in the triclinic space group P1 with a = 10.106(1)Å, b = 11.698(1)Å, c = 12.313(1)Å, α = 62.108(1)° β = 70.517(1)° γ = 66.052(1), V = 1157.0(3)Å3, Dcalc = 1.06 with Z = 4.

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