Collision complexes lifetime

When CET occurs between an atom and an excited polyatom the collision partners approach each other and at a certain center-of-mass, CM, or minimal distance, MD, from the closest atom of the polyatom, FOBS become operational. The colhding pan-forms or does not form a collision complex depending on how the latter is defined. At 300K the Ar-toluene collision complex lifetime is 0.68 ps while at 1500K it is 0.23 ps... 

Fig. 17. Association Rate constant Aj and collision complex lifetime Xj for the association of CH with NH3 as a function of KEcm./ represents stabilization efficiency in helium, which was found to be 0.6 (Saxer et al., 1987).

Femtosecond pump and probe experiments allow for real-time measurements of the collision complex lifetime, as we have seen earlier. Suppose the AB CD complex is formed in a supersonic expansion. If the AB molecule undergoes photofragmentation along a preferred direction on photolysis leading to hot A atoms, then not only can the ACD complex lifetime be measured, but also the time evolution of the AC + D products formed in the bimolecular reaction. 

The introductory remarks about unimolecular reactions apply equivalently to bunolecular reactions in condensed phase. An essential additional phenomenon is the effect the solvent has on the rate of approach of reactants and the lifetime of the collision complex. In a dense fluid the rate of approach evidently is detennined by the mutual difhision coefficient of reactants under the given physical conditions. Once reactants have met, they are temporarily trapped in a solvent cage until they either difhisively separate again or react. It is conmron to refer to the pair of reactants trapped in the solvent cage as an encounter complex. If the unimolecular reaction of this encounter complex is much faster than diffiisive separation i.e., if the effective reaction barrier is sufficiently small or negligible, tlie rate of the overall bimolecular reaction is difhision controlled. 

Since the forward peak is clearly from high J collisions, it is clearly produced via a rapidly rotating intermediate exhibiting an enhanced time delay. Further insight into the associated dynamics is provided by a classical trajectory simulation by Skodje. The forward peak results from the sideway collisions of the H atom on the HD-diatom (see Fig. 37). At the point where the transition state region is first reached, the collision complex is already oriented about 70° relative to the center-of-mass collision axis. The intermediate then rotates rapidly with an angular frequency of u> J/I, where / is the moment of inertia of the intermediate. If the intermediate with a time delay of the order of the lifetime r, the intermediate can rotate... 

Energy transfer occurs in a long-lived collision complex. An exited molecule is often very polarizable and may form a collision complex with the Q molecule in the ground state. The collision complex A Q has a longer lifetime than the corresponding AQ collision complex. The formation of an exciplex provides the energy transfer by a collision mechanism. 

Two pulses were used, the first to initiate the reaction and the second delayed to probe the OH product. The decay of [HOCO] was observed in the buildup of the OH final fragment in real time. The two reagents were synthesized in a van der Waals complex. The results established that the reaction involves a collision complex and that the lifetime of [HOCO] is relatively long, about a picosecond. 

Arnold et al.24 have calculated radiative lifetimes for the various collision complexes of singlet molecular oxygen on the basis of a collision time of 10"13 sec. The data for wavelengths and transition probabilities are presented in Table III. A recent paper25 describes the theory of double electronic transitions, and gives calculated oscillator strengths for the oxygen systems. 

Additional experimental investigations and theoretical treatments of collisional deactivation processes have recently been reported from several laboratories,250 253 Temperature effects on the lifetimes of intermediate adducts formed in the 0 -C02 interaction and in other relatively simple processes have been examined by Meisels and co-workers.252 254 Here the theoretical treatment involves application of a modified RRKM approach to the unimolecular dissociation of the adduct and/or of the termolecular collision complex consisting of the adduct plus the deactivating species M,. 

When the lifetime of the collision is of the order of several rotations a collision complex is formed, and the contour diagram can be approximately symmetric with respect to 90° (Figures 4.13 and 4.14). 

If the well is in the entrance valley the reaction unit can often need vibrational energy, but if it is in the exit valley translational energy is often more effective. Such features are indicative of a collision complex which lasts sufficiently long for many vibrations and some rotations to occur. The lifetime of a collision complex is long... 

Charge transfer as depicted by Mullilcen provides a single unifying basis for predicting arene reactivity based on the spectral, structural (both molecular and electronic), and thermodynamic properties of their intermolecular complexes, from stable organometallic derivatives to non-bonded collision complexes with very short lifetimes. 

Table 21 A comparison between free ethylene and ethylene in a sudden collision complex with a heavy atom (F, Cl and Br ) Transition moments (a.u.) and lifetimes (sec.) at different distances (A) between ethylene center of mass and heavy atom are displayed. From Ref. [150].

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