Fig. 5-1 Two possible transient structures that may occur when the two halogenated ethanes CF3CHClF and CClF2CH3 undergo respective unimolecular decomposition leading to the production of HCl In the case of CF3CHClF, the H and Cl that are bonded to the same C-atom will approach to make a 3-centered structure, while in the case of CClF2CH3, elongation of C - Cl bond will be accompanied by a new bond formation between Cl and H, each belonging to an adjacent C-atom, to make a 4-centered structure.

Fig. 5-2 The change in the molecular structure and potential energy for the decomposition of CClF2CH3 producing HCl via a 4-centered structure

Fig. 5-3 Distribution of rotational and vibrational states of the HCl so produced, determined by laser spectroscopy In the HCl elimination from the 4-centered structure from CClF2CH3, highly vibrationally excited HCl is produced and its rotational states do not reflect the thermal equilibrium. In contrast, the 3-center elimination of HCl from CF3CHClF yields HCl that is populated in lower vibrational and rotational states in a Boltzmann distribution (solid curves).

While lasers are used to excite molecules selectively to a specific state, they can also be used to determine vibrational and rotational states, or translational states of a molecule. We have studied unimolecular decomposition of halogenated ethanes (CF3CHClF, CClF2CH3, etc) yielding HCl after infrared multiphoton excitation by using a method of crossed CO2-laser and supersonic molecular beams. On the way to decomposition the reacting molecule may take a transient structure, 3-centered or 4-centered, as shown in Fig. 5-1. In the case of a 4-centered structure, elongation of C - Cl bond is accompanied by Cl approaching the H originally bonded to the other C-atom (Fig. 5-2). Finally, the H-atom will rush to the Cl, resulting in a violent elimination of HCl from the molecule. Thus, the HCl produced is expected to be highly vibrationally excited. In the case of the elimination from the 3-centered structure, on the other hand, a new bond is formed between Cl and H, both of them being originally bonded to the same C, and a rather soft elimination of HCl results. Detailed calculation based on quantum mechanical (ab initio MO) method allowed the theoretical prediction of how the released potential energy at an exit barrier channel on the reaction coordinate will be distributed among possible energy states of the product, i.e. translational, rotational and vibrational states of HCl. We then attempted to determine experimentally the rotational and vibrational energy distributions of the HCl produced. The rotational and vibrational energy distribution of HCl produced from the 3- and 4-center elimination was measured using a 2 + 1 resonantly enhanced multiphoton ionization technique combined with time-of-flight mass spectrometry. The results (Fig. 5-3) indicate that the HCl from the 3-center elimination is rotationally and vibrationally cold (population: v = 0 > 1 > 2) and that the HCl from the 4-center elimination is in contrast hot (population: v = 2 > 1 > 0) and rotational distribution is far from the thermal equilibrium. The experimental data shown in Fig. 5-3 were obtained for the first time and are in good agreement with our theoretical prediction.

Reference A. Yokoyama et al., Rotational and Vibrational Energy Distributions of HCl Produced by Three- and Four-Center Elimination of HCl from Halogenated Ethanes, Chem. Phys. Lett., 307, 48 (1999).

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