The Determination of the Transition State Structure from the J Dependence of the Dissociation Energy E₀ (J): The Methane and Ethane Ion Dissociation

Abstract

A threshold photoelectron photoion coincidence experiment has been setup which allows to measure breakdown curves of energy selected ions at different sample temperatures. These studies reveal the rotational energy of the transition state in unimolecular reactions, which in turn is used as a probe for the transition state structure. According to a model recently suggested this should allow the distinction of tight and loose transition states. The breakdown curves of methane and ethane ions in the region of the lowest reaction channel have been measured at 300 K and close to 0 K. In the case of the H loss from methane ions the shift between these two breakdown curves reveals that the dissociation energy Eo varies significantly with the rotational temperature as expected for a simple bond fission (loose transition state). In the case of H2 loss from ethane ions the shift between the breakdown curves at 300 and close to 0 K shows that E0 (J) does not vary significantly with the rotational temperature. This is to be expected for a concerted elimination reaction (tight transition state).

Translational energy release: Experiment and theory. H2 elimination reactions of small gas phase ions, and correspondence to HH bond activation

“…Both distributions tail off at T 30 kJ mol À1 . The most precise measurement of the critical energy is E 0 55.2 kJ mol À1 from a threshold photoelectron photoion coincidence study (Weitzel, Mahnert, & Baumgartel, 1993). This value is in good agreement with earlier photoionization and PEPICO values (Chupka & Berkowitz, 1967;Suzuki & Maeda, 1977;Bombach, Dannacher, & Stadelmann, 1984), and gives E 0,r 24 kJ mol À1 .…”

Statistical and non-statistical reactions in energy selected fluoromethane ions

“…that which divides the area under the thermal probability distribution into two identical parts. For molecules of this size [20] this median internal energy and thus the expected shift of the cross-over point in converting from 300 to 0 K is roughly 3/4 of the corresponding average rovibrational energy. Since the average rovibrational energy as calculated from the frequencies of the neutral molecule [23] is 41 meV at room temperature the cross-over energy for 0 K would be expected at 13.37 ± 0.02 eV.…”

“…Details of the experimental setup have been described previously [20]. Only a brief description will be given here.…”

High-resolution energy-selected study of the reaction NH3+→NH2++H: Accurate thermochemistry for the NH2/NH2+ and NH3/NH3+ systems

“…A more reliable method for AE determinations is the conventional vacuum ultraviolet (VUV) threshold photoelectron (TPE)-photoion coincidence (TPEPICO) technique, 4,5 which involves the detection of correlated TPE-photoion pairs. The TPEPICO scheme has been widely used for the study of state-or energy-selected ion dissociation dynamics.…”

“…4,5 The analysis of the breakdown curves for parent and fragment ions based on TPEPICO time-of-flight (TOF) data is made to recover information concerning the AE for the fragment ion. 5 The simulation of the breakdown curves applies statistical theories and takes into account the internal energy population of the parent ion. However, the precision of AE values derived from this method has been limited by the relatively low TPE resolution.…”