Matrix Infrared Spectra and Density Functional Calculations of CH₂Cl–Cl and CH₂Br–Br Produced by Laser-ablated Metal Plume Irradiation

CH₂Cl–Cl and CH₂Br–Br, photoisomers of CH₂Cl₂ and CH₂Br₂ , were identified in the matrix IR spectra from the precursors exposed to plume radiation of a laser-ablated transition metal. On the other hand, the corresponding photoisomers of CH₂F₂ and CH₂FCl (CH₂F–F, CH₂F–Cl, and CH₂Cl–F) were not observed because of their instabilities. The C[BOND]X bond of the product is unusually strong, leading to a high C[BOND]X stretching frequency. Natural bond orbital (NBO) analysis reveals that it is a true carbon–halogen double bond; in contrast, the X[BOND]X bond is largely ionic (H₂C[DOUBLE BOND]X^δ+ ∙∙∙X^δ− ), similar to the previously studied analogs from tri- and tetrahalomethanes. Intrinsic reaction coordinate (IRC) computation reproduces the smooth interconversion between the precursor and the energetically higher photoisomer, consistent with the disappearance of the product during visible photolysis