Reaction mechanism collision complex

A significant recent experimental advance is the introduction of tandem mass spectrometers for studying ion-molecule reactions. Examining various isotope effects as a function of translational energy can provide detailed information about reaction mechanisms. Tandem experiments can also observe many of the possible reaction channels for a given collision complex. Such information provides valuable clues to the chemical and physical nature of the intermediates in ion-neutral interactions. 

The mechanism of this reaction describes what happens at the molecular level. Two NO2 molecules collide, forming a collision complex. In this collision complex, a bond may form between the two nitrogen atoms, producing an N2 O4 molecule. 

The above nonstatistical view of reaction (2) has been reinforced by recent experiments made under LP conditions and presents a great challenge to the GPIC community. It now appears that the LP rate constants obtained for any nonstatistical reaction of this type will be extremely difficult to interpret in terms of candidate mechanisms and potential energy surfaces envisioned for that reaction. An accurate prediction of for such reactions would have to include a set of very complex factors, some of which are not presently well understood. These factors would include the initial distributions of energy within the set of collision complexes, X, formed under all possible collision impact conditions the rates of energy transfer between all vibrational modes within the species, X and Y and the mode-dependent rate constants for the motion of individual species within the sets of ion complexes, X and Y, in both directions on the reaction coordinate. 

The first mechanism implies k19 = k5AK11A 11A the second leads to ki9 = k2iKlm- . Independent evidence suggests the existence of both intermediate species in the nitric oxide-oxygen system, and both mechanisms involve entirely reasonable collision complexes. In both, the equilibrium step is rapid, and the overall kinetics are third order. Theoretical calculations based on the activated complex theory were made by assuming a true termolecular reaction the predicted rates agree well with experiment.161 The experimental rate constants are summarized in Tables 4-3 and 4-4. 

W.B. Miller, S.A. Safron, D.R. Herschbach, Exchange reactions of alkali atoms with alkali halides—A collision complex mechanism, Discuss. Faraday Soc. 44 (1967) 108. 

Chemical reactions occurring because of a single kinetic act, i.e., because of a single collision between two molecules, are defined as elementary reactions. More complex laws of dependence on concentrations can be explained by complex reaction mechanisms, i.e., by the idea that most reactions occur as a sequence of many elementary reactions, linked in series or in parallel. As an example, the following... 

Subtle mechanistic distinctions have been achieved in some gas-phase ion-molecule reactions. One interesting example concerns the reaction of equation 17 which can, in principle, occur via three distinct mechanisms H+ transfer from the ion to the neutral, H transfer from the neutral to the ion, and an intermediate collision complex mechanism. 

These results, obtained on a FT-ICR mass spectrometer, led to the proposal that these reactions proceed through long-lived ion-molecule collision complexes which can undergo secondary reactions within the complex. The mechanism, sketched in Scheme 41, predicts the formation of products originating from attack of F on the neutral by proton transfer, SN2 or elimination reactions. 

---