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Maas, Joop H. Van Der
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article
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
<p>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 [R<sub>2</sub><sup>2</sup>(6) motif with C<sub>i</sub>-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<sub>s</sub>-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 ▽<sup>2</sup>ρ(r<sub>p</sub>) 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 R<sub>2</sub><sup>2</sup>(6) motifs of 1 and 2 were studied using <sup>13</sup>C CP/MAS NMR and IR and Raman spectroscopy. <sup>13</sup>C 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 R<sub>2</sub><sup>2</sup>(6) motifs possess local C<sub>i</sub>-symmetry.</p>