Electron attachment to the fluoro-, bromo-, and iodomethanes studied by means of electron transmission spectroscopy and Xα calculations

Abstract

The electron transmission spectra of the fluoro~, bromo~, and iodomethanes (except CH 2 F 2 ) and of some monohaloalkanes are reported. The resonance energies for electron attachment to the halomethanes have also been evaluated by means of bound and continuum multiple scattering Xa calculations, which closely reproduce the low~energy experimental data. A large electron affinity increase is observed on going from the fluorine to the heavier halogen derivatives, while within each series of halides the electron affinity increases with increasing number of halogen atoms. The ground anion state is found to be stable in CHBr 3 , CBr 4 , and in all the iodomethanes except CH31. The fragment anion production, as a function of the electron impact energy, has been measured for some of the compounds under investigation, in order to obtain experimental information on anion states very close to zero energy or slightly stable, which are not accessible in electron transmission.

Electron cross section set for CHF3

“…Experimentally, resonances were observed by Modelli et al in electron transmission through CHF 3 at 6.3 and 9.3 eV. 23 Modelli et al assigned the 9.3 eV feature to overlapping A 1 and E resonances but were unable to assign the 6.3 eV feature. Symmetry decomposition of the SMC cross section shows that the double-hump structure is due to overlapping E and A 1 resonances centered, respectively, at about 11 and 13 eV.…”

Catalytic olefination. Estimation of the reactivity of polyhaloalkanes

“…Previously, qualitative correlations were revealed between the reactivity of polyhaloalkanes and such parameters as half-wave potential of electrochemical reduction, electron affinity, bond energy, and energy of the lowest unoccupied molecular orbital (LUMO) [13]. As a rule, electron affinity rises with increase in the number of halogen atoms (except for polyfluorinated compounds) [14]. In the recent time, global electrophilicity index ω is used to characterize the reactivity of chemical compounds.…”

Comparative studies of dissociative electron attachment to methyl halides

“…For the neutral curve, A and α were chosen to give the empirical values for the dissociation energy of CH 3 Br, 3.078 eV [9], as well as the vibrational (v = 0-1) spacing of 75.8 meV [32]. For the negative-ion curve, we have fi xed the vertical attachment energy to the electron transmission spectra estimate of [29] of 2.4 eV (B = 0.09754 au) and the asymptotic value using the electron affi nity of Br, 3.37 eV [3] (D =−0.00934 au). This leaves the parameter β undetermined and, as done previously for CH 3 I, it is used as a fi t parameter.…”

“…We note that we have not considered CH 3 F here. The vertical attachment energy for CH 3 F has been estimated from electron transmission spectroscopy (ETS) to be nearly 5 eV [29] and it might be assumed from this that the DEA cross section for CH 3 F is even smaller than that of CH 3 Cl. In section 4 we discuss the vibrational Feshbach resonances for all methyl halides.…”