Oximes in the isoxazolone, pyrazolone, and 1,2,3-triazolone series: experimental and computational investigation of energies and structures of e/z isomers of α-oxo-oximes in the gas phase and in solution

The structures of a series of heterocyclic alpha-oxo-oximes, viz. 4-oximinoisoxazolone-5(4H)-ones 1 and 2,4-oximino-5(4H)-pyrazolones 3-5, and 4-oximino-1-phenyl-1,2,3-triazol-5(4H)-one 6, were investigated experimentally and computationally. Whereas the intramolecularly H-bonded ZZ isomers of these oximes are usually the most stable in the gas phase, this preference is overcome by intermolecular H-bonding to a solvent or another molecule. For 1,3-dimethyl-4-oximino-5(4H)-pyrazolone 3b a turnaround is seen when going from the solid (predominantly Z isomer) to DMSO solution (predominantly E isomer), which can be ascribed to an intermolecular H-bond between the oxime OH function and a DMSO molecule. Such isomerization is not seen in CDCl3, where intermolecular H-bonding is unimportant. The Z/E-isomerization in DMSO solution is accelerated by photolysis. Calculations of the energies of different conformers, and of C-13 NMR data at the GIAO-omega b97xD/6-31G(d)//M06-2X/6-311G++(d,p) level permit a clear-cut correlation of conformer structures with observed C-13 NMR spectra