One- and Two-Dimensional Translational Energy Distributions in the Iodine-Loss Dissociation of 1,2-C2H4I2+ and 1,3-C3H6I2+: What Does This Mean?

Threshold photoelectron photoion coincidence (TPEPICO) has been used to study the sequential photo-dissociation reaction of internal energy selected 1,2-diiodoethane cations: C2H4I2+ -> C2H4I+ + I -> C2H3+ + I + HI. In the first I-loss reaction, the excess energy is partitioned between the internal energy of the fragment ion C2H4I+ and the translational energy. The breakdown diagram of C2H4I+ to C2H3+, i.e., the fractional ion abundances below and above the second dissociation barrier as a function of the photon energy, yields the internal energy distribution of the first daughter, whereas the time-of-flight peak widths yield the released translational energy in the laboratory frame directly. Both methods indicate that the kinetic energy release in the I-loss step is inconsistent with the phase space theory (PST) predicted two translational degrees of freedom, but is well-described assuming only one translational degree of freedom. Reaction path calculations partly confirm this and show that the reaction coordinate changes character in the dissociation, and it is, thus, highly anisotropic. For comparison, data for the dissociative photoionization of 1,3-diiodopropane are also presented and discussed. Here, the reaction coordinate is expected to be more isotropic, and indeed the two degrees of freedom assumption holds. Characterizing kinetic energy release distributions beyond PST is crucial in deriving accurate dissociative photoionization onset energies in sequential reactions. On the basis of both experimental and theoretical grounds, we also suggest a significant revision of the 298 K heat of formation of 1,2-C2H4I2(g) to 64.5 +/- 2.5 kJ mol(-1) and that of CH2I2(g) to 113.5 +/- 2 kJ mol(-1) at 298 K.