Beam studies of the energy dependence of the reactions of tritium atoms with n-hexane, cyclopentane, n-butane, and 1-chlorobutane

Abstract

The reactions of monoenergetic tritium atoms with simple alkanes and halocarbons have been studied in the 1–200 eV range by a molecular beam technique. Excitation functions σi (E) for the different reaction channels are derived from the product yields by a Kinetic Theory Analysis. It is found that: (1) T for H substitution cross sections (to form labeled parent alkanes) are similar for all alkanes studied (n-butane, n-hexane, cyclopentane, and cyclohexane): threshold at 1.5± 0.5 eV, a rapid rise to a maximum at 9–12 eV and 1–3 Å2 and a slow decline at higher energies. The asymptotic high energy parent yields correlate well with the degeneracy of the reaction path, a fact that further demonstrates the independence from structural details of T for H substitution for a single reaction path. (2) In the formation of degraded alkanes by C–C bond breakage, drastic structural dependence of the reaction cross section is found. (a) When the hydrogen atoms along a C–C bond are staggered, as in the thermodynamically stable conformation of most alkanes, the C–C bond is effectively shielded from direct attack by the hydrogen envelope, and the dominant mode of reaction consists of the sequence: H displacement followed by decomposition of energetic primary reaction products and stabilization of the fragment species. (b) When the hydrogens are eclipsed (as exemplified to a good approximation by cyclopentane in its thermodynamically stable half chair form), the C–C bond is exposed to direct attack by incoming T atoms, and direct alkyl displacement is facilitated. Direct alkyl displacement is a low energy process on our scale, with a threshold E0<1.2 eV, and with most of the reaction occurring below a few electron volts. At higher energies (E>4 eV), mechanism (a) takes over, since the products from (b) decompose. (3) T for Cl displacement in 1-chlorobutane occurs at higher average energies than H substitution. The cross section for the former process has an onset in the 1 eV region and goes through a maximum at ∼15 eV. The probability of displacing a Cl atom exceeds that for displacement of a given H atom by a factor of 2–3.