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

Abstract

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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Acknowledgments

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). https://doi.org/10.1007/s11224-008-9338-3

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Keywords

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