Unimolecular and collision-induced processes of ethyne and ethyne- d1 dications and charge separation of dications derived from butadiene

Abstract

Unimolecular charge separation reactions for C 2H 2+ 2, C 2HD 2+, C 4H 2+ 2, and C 4H 2+ 3 are reported where the dications are formed in the source of a mass spectrometer. Charge separation reactions of ethyne and ethyne- d 1 dications have been observed as deprotonation only and the measured releases of ion kinetic energy, T, interpreted in terms of intercharge distances in the transition state. Measured values of T, which are identical within the limits of experimental error and yield an average value, 3.44±0.05 eV, corresponding to an average intercharge separation in the transition state of 4.19±0.06 Å, are in good agreement with theoretical calculations of the energy barrier to deprotonation plus the exothermicity of deprotonation. The energy barrier to dedeuteronation of C 2HD 2+ has been estimated to exceed that for deprotonation by some 1.5 eV (i.e. ∼ 50% of the barrier to deprotonation) on the basis of the ion kinetic energy release measurements appropriately corrected for collision-induced charge separation. The re-ionization of fast C 2HD molecules to the ethyne- d 1 dication, C 2HD 2+, in two consecutive single charge transfer processes with oxygen as target gas has been observed and thus confirms an earlier proposal of the similar re-ionization of fast C 2H 2 to the ethyne dication, C 2H 2+ 2. The energetics of the charge stripping process for C 2H + 2, C 2HD + , C 2H +, and C 2D + have been determined from the translational energy losses accompanying the removal of the second electron from each cation. Deconvolution of two overlapping charge separation peaks of C 4H 2+ 2 dications derived from butadiene has been effected by means of a mass analysis stage in tandem with a mass spectrometer of reversed geometry. C 4H 2+ 2 undergoes charge separation to yield C 3H + +CH + and C 3H + 2 +C + with markedly different releases of ion kinetic energies: the products separate with releases of 3.11±0.03 eV (corresponding to an intercharge separation of 4.63±0.04 Å) and 3.77±0.03 eV (3.92±0.03 Å), respectively. The intercharge separations indicate that C 4H 2+ 2 may exist in two forms with a C 3 cyclic structure. Charge separation of C 4H 2+ 3 leads to the formation of but a single set of products, C 3H + +CH + 2, and yields a kinetic energy release of 3.23±0.03 eV; the corresponding intercharge distance (4.46±0.04 Å) suggests a C 3 cyclic structure for C 4H 2+ 2. Deprotonation of neither C 4H 2+ 2 nor C 4H 2+ 3 was observed.