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

Abstract

AbstractThe solvent dependence of the 1H, 13C and 15N 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—CN, 0°) and gauche (F—C—CN, 124.1°) rotamers. In contrast, in (Z)‐FAO the DFT method gave only one energy minimum corrsponding to the trans rotamer. The 4JHF and 1JCF 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 CCl4 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 CCl4 and −1.18 kcal mol−1 in DMSO. The 3JNF coupling was independent of the molecular conformation, as it did not change with the solvent polarity. Copyright © 2003 John Wiley & Sons, Ltd.