Conformational analysis of fluoroacetoxime and of its O ‐methyl ether by 1 H, 13 C and 15 N NMR and theoretical calculations

The solvent dependence of the 1 H, 13 C and 15 N NMR spectra of ( E )‐fluoroacetoxime [( E )‐FAO] and of ( E )‐fluoroacetoxime O ‐methyl ether [( E )‐FAOME], was examined and the HF, CF and NF couplings are reported. Density functional theory (DFT) at the B3LYP/6–311++g(2df,2p) level with ZPE (zero point energy) correction was used to obtain the rotamer geometries. In both ( E )‐FAO and ( E )‐FAOME the DFT method gave two energy minima corresponding to the cis (F—C—CN, 0°) and gauche (F—C—CN, 124.1°) rotamers. In contrast, in ( Z )‐FAO the DFT method gave only one energy minimum corrsponding to the trans rotamer. The 4 J HF and 1 J CF couplings in ( E )‐FAO were analyzed by solvation theory assuming the cis and gauche forms to give $E_{cis}-E_{gauche} = 3.3 \, {\rm kcal\, mol}^{-1}$ in the vapor phase, decreasing to 1.54 kcal mol −1 in CCl 4 and −1.19 kcal mol −1 in DMSO (1 kcal=4.184 kJ. In ( E )‐FAOME the observed couplings, when analysed similarly by solvation theory, gave $E_{cis}-E_{gauche} = 2.2 \,{\rm kcal\, mol}^{-1}$ in the vapor phase, 0.91 kcal mol −1 in CCl 4 and −1.18 kcal mol −1 in DMSO. The 3 J NF coupling was independent of the molecular conformation, as it did not change with the solvent polarity. Copyright © 2003 John Wiley & Sons, Ltd.