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

Autor: Hans-Friedrich Grützmacher, Dirk Kirchhoff, Hansjörg Grützmacher
Rok vydání: 2006
Předmět:
Zdroj: International Journal of Mass Spectrometry. :130-137
ISSN: 1387-3806
Popis: The four dihalogeno phenylarsanes C6H5AsF2, 1, C6H5AsCl2, 2, C6H5AsBr2, 3, and C6H5AsI2, 4, produce in the 70eV-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 degrees. 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 degrees 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 Delta H-#, and that in the case of 3a(+) this Delta H-# and the reaction enthalpy for loss of Br degrees are of similar size. The Delta H-# of HX elimination is also responsible for the reverse enthalpy of activation Delta H-rev(#) and the KER during this process. The observed < T > amounts to 78% (1a(+)), 55% (2a(+)), and 40% (3a(+)) of the calculated value of Delta H-rev(#). Thus, the KER as well as the distribution of the KER (KERD) and the competition between elimination of HX and loss of X degrees are very specific for the halogen ligand at the As atom. Metastable (4-fluorophenyl) arsenium ions W, 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 C-H and C-X bonds has been observed before and appears to be typical of arylarsane radical cation and arylarsenium cations. (c) 2006 Elsevier B.V. All rights reserved.
Databáze: OpenAIRE
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