Density functional study of configurational, conformational, energetic, electronic and spectroscopic properties of fluorohydroxyformaldoxime and its dehydration products

Abstract

A detailed exploration of the conformational space of fluorohydroxyformaldoxime has been carried out with the aid of the B3LYP/6-31G* level of density functional theory. Among the eight possible isomers of fluorohydroxyformaldoxime the most stable structure corresponds to the Z-s-cis, s-trans isomer, while the highest energy Z-s-cis, s-cis conformer was found to be 15.3 kcal mol-1 higher in energy. Calculated structures, relative stability and bonding properties of the conformers are discussed with respect to computed spectroscopic properties, such as harmonic vibrational frequencies, UV-Vis electronic transitions and NMR chemical shifts. Upon dehydration fluorohydroxyformaldoxime could afford either fluoroformonitrile oxide or fluorocarbonylnitrene. Both dehydration processes are predicted to be endothermic, the heats of reaction being 54.1 and 52.2 kcal mol-1 respectively. The structures, energetic, electronic and spectroscopic properties (IR, UV-Vis and NMR spectra) of fluoroformonitrile oxide and fluorocarbonylnitrene have been investigated at various levels of abinitio molecular orbital and density functional theory. The fluoroformonitrile oxide was found to be 29.6, 6.4, 10.2 and 9.7 kcal mol-1 less stable than fluorocarbonylnitrene at HF/6-31G*, MP2/6-31G*, B3LYP/6-31G* and B3LYP/6-311++G** levels respectively. The fluoroformonitrile oxide at all levels of theory, except at the HF/6-31G* level, adopts the bent as the more stable geometry. However at the B3LYP/6-31G* level the linear geometry, corresponding to a saddle point with a doubly degenerate imaginary frequency, is only 4 kcal mol-1 higher in energy than the bent geometry. Finally, it was found that fluoroformonitrile oxide either in the bent or linear geometry and fluorocarbonylnitrene could be easily distinguished by 13C, 15N and 17O NMR spectroscopies.