THE ROLE OF INITIAL CONDITIONS IN ELEMENTARY GAS-PHASE PROCESSES INVOLVING INTERMEDIATE COMPLEXES

When doing experiments with large numbers of gaseous molecules, it is difficult to distinguish the behavior characteristic of specific sets of molecules from the behavior of the entire ensemble. In this paper, several experiments involving unimolecular and bimolecular reactions are discussed which address this issue. In all cases, the initial conditions are well characterized, and reactions occur via intermediates for which the potential energy surface (PES) has pronounced local minima and no known exit channel barriers other than the reaction endoergicity. The role of such intermediate "complexes" can be to erase or lessen the system's memory of the initial conditions (i.e., conserving only energy and angular momentum), thus rendering the outcome "statistical". When lifetimes of the complex are long (e.g., 10-9 s), the experimental evidence suggests that systems behave statistically, and thus techniques such as IRMPD cannot be used to explore or exploit the nonstatistical regime. Exciting overtone transitions in carefully selected molecules having high-frequency vibrations and low D0's (e.g., H2O2) can lead to nonstatistical behavior, since reaction times can be short and local mode behavior pronounced. Electronic excitation followed by radiationless transitions allows very precise measurements to be made for a wide range of energies in excess of D0, E†, but in the most carefully studied systems to date (e.g., NCNO, E† < 5000 cm-1) there is no hint of nonstatistical behavior. With bimolecular reactions, we concentrate on the geometric orientations and alignments of reactants in the H + CO2 → OH + CO system, since E† can be made quite large, and HOCO is bound by only 107 kJ mol-1 relative to OH + CO. We discuss the use of a CO2HBr van der Waals precursor to orient and align the reactants relative to one another. In this case, photodissociation of the HBr constituent provides the H-atom reactant, and the ensuing reaction occurs with a very restricted set of angles and impact parameters relative to other methods. Nascent OH state distributions obtained with E† ∼ 150 kJ mol-1 (kuni ∼ 10-13 s) are different with and without orientation/alignment, and this difference may be due to the reactant initial conditions being carried over to the products. © 1986 American Chemical Society