Exploring the potential energy surface associated with the HBr loss from 2-bromobutane radical cations

Abstract : The loss of HBr from 2-bromobutane radical cation, 1, was investigated by means of mass spectrometry techniques. In the gas phase, the spontaneous unimolecular reaction of metastable ion 1 yields only a C4H8*+ (m/z 56) fragment and neutral HBr. In all the deuterated ions the loss of HBr is always strongly dominant, the loss of DBr remaining under 1%. Ion-molecule reactions indicate that the C4H8*+ ions correspond to a mixture in which ionized 2-butene and methylcyclopropane were detected. These results are discussed in the light of calculations carried out by using high-level density functional theory techniques. For this purpose we have located the relevant stationary points of the [H9, C4, Br]*+ potential energy surface (PES). The geometries and the vibrational frequencies were obtained at the B3LYP/6-31G* level, and for the final energies a B3LYP/6-311+G(3df,2p) approach was used. From these data, it is proposed (i) that the loss of HBr involves 1,3- and 1,4-elimination reactions and several elimination pathways involving tunneling mechanisms and (ii) that the dissociation is preceded by an extensive H-exchange whose mechanism involves the intermediacy of ionized methylcyclopropane weakly bonded to a HBr molecule.
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Submitted on : Wednesday, December 25, 2013 - 3:02:01 PM
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M.C. Oliveira, M.A. Almoster Ferreira, O. Mó, M. Yáñez, Henri Audier. Exploring the potential energy surface associated with the HBr loss from 2-bromobutane radical cations. Journal of Physical Chemistry A, American Chemical Society, 2000, 104 (41), pp.9287-9294. ⟨10.1021/jp001402w⟩. ⟨hal-00922245⟩

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