Structure and spectroscopy of dihaloformaldoximes - He I photoelectron, photoionization mass spectroscopy, mid-IR, Raman and ab initio study

The electronic and geometric structures of dichloroformaldoxime, Cl2C=NOH, dibromoformaldoxime, Br2C=NOH and bromochloroformaldoxime, BrClC=NOH, are investigated for the first time in the gas phase by He I photoelectron, He I and H L(alpha,beta,gamma) photoionisation mass, and mid-IR spectroscopies, and in the solid state by Raman spectroscopy. Ab initio calculations at the MP2(fc)/6-31G** level provide an assessment of the geometric structures and, together with the IR and Raman data, permit an almost complete vibrational analysis for each molecule, which appear, in each case, to exist in one isomeric form. The calculations indicate that anti structures for the hydrogen atom position are the most stable in all cases. For BrClC=NOH, the calculations just prefer the Z-anti-structure as the thermodynamically more stable, whereas the vibrational spectroscopy, although not unambiguous, suggests the E-anti-isomer as the stereoselectively synthesised conformer. This conclusion is strengthened further by the observation that BrClC=NOH (with an exclusive E-structure), favours HCl elimination upon thermolysis, thus making it a good source of the transient BrCNO molecule. Photoelectron spectroscopy, supported by ab initio calculations and trends in the dihalo series, provides an analysis of the effect of halogen substitution on the orbitals of the parent oxime species, H2C=NOH.