Infinite, undulating chains of intermolecularly hydrogen bonded (E,E)-2,2-dimethylcyclohexane-1,3-dione dioximes in the solid state. A single crystal X-ray, charge density distribution and spectroscopic study

Abstract

In the solid state (E,E)-2,2-dimethylcyclohexane-1,3-dione dioxime (1) and (E,E)-2,2,5,5-tetramethylcyclohexane-1,3-dione dioxime (2) give infinite, undulating polymer-like chains due to intermolecular dimeric oxime hydrogen bonding [R²₂(6) motif with C_i-symmetry; single crystal X-ray analyses]. Configurational stereoisomerism of the oxime groups is prevented by the two methyl groups at the 2-positions. Consequently, the oxime groups of both 1 and 2 are unequivocally defined and show no disorder. Whereas 1 has molecular C_S-symmetry, compound 2 lacks symmetry and two distinct intermolecular dimeric oxime hydrogen bonds are found.

In the case of 2, its charge density distribution was determined from high resolution X-ray data and subjected to a Bader type topological analysis giving for the first time insight into the chemical bonding of this dimeric intermolecular oxime hydrogen-bonding motif. The multipole populations and the properties of the (3, −1) bond critical points confirm the lack of symmetry for 2. All located (3, −1) bond critical points except those of the hydrogen bonds have negative values for the Laplacians ▽²ρ(r_p) in line with covalent bonding. Notwithstanding, the description of the two distinct O–N bonds of 2 is not fully adequate; to obtain negative Laplacian values at their bond critical points, hexadecapole parameters (l = 4) for C, N and O had to be used in the refinement. By comparison with B3LYP/6-311++G** results on acetone oxime it is shown that this anomaly can be attributed to deviations in the experimentally determined charge density distribution of the two distinct O–N bonds of 2. The positive Laplacians for the hydrogen bonds agree with closed shell interactions.

In addition, the spectroscopic properties of the intermolecular oxime hydrogen bonding R²₂(6) motifs of 1 and 2 were studied using ¹³C CP/MAS NMR and IR and Raman spectroscopy. ¹³C CP/MAS NMR showed that for 1 and 2 one and two distinct oxime hydrogen bonding motifs, respectively, are discernible. From their IR and Raman spectra unequivocal proof was obtained that the R²₂(6) motifs possess local C_i-symmetry.

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