Unimolecular reactions of halogeno phenylarsenium ions: Kinetic energy release during the elimination of halogen hydride

The four dihalogeno phenylarsanes C6H5AsF2, 1, C6H5AsCl2, 2, C6H5AsBr2, 3, and C6H5AsI2, 4, produce in the 70 eV-EI mass spectra by loss of a halogen atom abundant halogeno phenylarseniun ions C6H5As+-X, 1a+-4a+. The further fragmentation reactions of ions 1a+-4a+ are elimination of a molecule halogen hydride HX and/or loss of a halogen atom X. The preferred route of fragmentation depends clearly on the strength of the As-X bond. The metastable fluoro ion 1a+ and chloro ion 2a+, respectively, fragment only by loss of HF and HCl, the metastable bromo derivative 3a+ exhibits losses of HBr and Br of about equal intensity, and the metastable iodo ion 4a+ fragments only by loss of an I atom. The loss of HX is associated with a large kinetic energy release (KER) which yield a dish-topped peak in the MIKE spectrum of 1a+ (〈T〉 = 845 meV) and 2a+ (〈T〉 = 550 meV) and a broad round-topped peak on the MIKE spectrum of 3a+ (〈T〉 = 369 meV). Theoretical calculations (UBHLYP/6-311 + G(2d,p)/-/UBHLYP/6-31 + G(d)) confirm that the elimination of HX requires an enthalpy of activation ΔH#, and that in the case of 3a+ this ΔH# and the reaction enthalpy for loss of Br are of similar size. The ΔH# of HX elimination is also responsible for the reverse enthalpy of activation ΔHrev# and the KER during this process. The observed 〈T〉 amounts to 78% (1a+), 55% (2a+), and 40% (3a+) of the calculated value of ΔHrev#. Thus, the KER as well as the distribution of the KER (KERD) and the competition between elimination of HX and loss of X are very specific for the halogen ligand at the As atom. Metastable (4-fluorophenyl) arsenium ions 1b+, F-C6H4As+-H, and (4-chlorophenyl) arsenium ions 2b+, Cl-C6H4As+-H+, eliminate HF or HCl with virtually identical KERD as metastable 1a+ or 2a+, C6H5As+-Cl, proving an identical transition state for both isomers. Accordingly, 1b+ and 2b+ rearrange to 1a+ and 2a+ prior to HCl elimination. Such a rearrangement by a reductive elimination/oxidative insertion of As+ into CH and CX bonds has been observed before and appears to be typical of arylarsane radical cation and arylarsenium cations.