Conformational analysis of N-isopropylbenzohydroxamic acids: crystal structure, DFT, and NMR studies |
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Authors: | Witold Przychodzeń Jaros?aw Chojnacki |
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Institution: | (1) Faculty of Chemistry, Gdańsk University of Technology, Narutowicza 11/12, 80-952 Gdansk, Poland |
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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.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. |
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Keywords: | Hydroxamic acids X-ray structure DFT calculations Substituent effects Conformations Absolute nuclear shieldings |
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