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Conformational analysis of N-isopropylbenzohydroxamic acids: crystal structure, DFT, and NMR studies


X-ray crystal structure determinations together with density functional theory (DFT) calculations in vacuo and NMR studies in solution have been carried out for 4-MeOC6H4CONPriOH 2a and 3,5-(NO2)2C6H3CONPriOH 2b. The results were compared with that for the respective N-methyl benzohydroxamic acids. For crystal structures as well as for DFT-optimized geometries of 2 (both isomers) in vacuo, the effect of substituents in aromatic ring manifested by changing of charges is inconspicuous. Studies of potential energy surfaces showed that libration barrier around ω 1 = 0° is low enough to make electron conjugation feasible, and that for 2b rotation barrier around C(O)N bond is higher by 6 kcal/mol and additionally, that rotation around N–C bond is hindered. A careful analysis of low-temperature 1H NMR spectra confirmed the greater stability of Z-2a, the greater rigidity of E-2b and the influence of solvent on both isomers population. Despite solvent-dependent conformational alteration, both 2a and 2b crystallize exclusively as E isomers from ethyl acetate solution. Correlations of absolute 1H, 13C, and 15N shielding calculations with experimental data were also analyzed.

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We thank Dr. P. Sowiński for measurement of the NMR spectra. The studies were financially supported in part by the Polish Ministry of Science and Informatisation, grant No. 1T09A 07830.

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Correspondence to Witold Przychodzeń.

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Przychodzeń, W., Chojnacki, J. Conformational analysis of N-isopropylbenzohydroxamic acids: crystal structure, DFT, and NMR studies. Struct Chem 19, 637–644 (2008).

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  • Hydroxamic acids
  • X-ray structure
  • DFT calculations
  • Substituent effects
  • Conformations
  • Absolute nuclear shieldings