Complexes long-lived intermediate

The anomalous plasma decay suggests that the deionization coefficient is larger at higher electron densities and higher H2 densities. The second interpretation proposes that electrons and H2 molecules can act as stabilizing third bodies and that this process involves the same long-lived intermediate complexes that are observed in merged beam experiments (see Section IV.B). 

The combination of quantum mechanical and classical modes is particularly important for larger systems which prohibit a complete quantal treatment. It is most suitable for direct processes with short interaction times and it is less applicable for long-lived intermediate complexes. The ultimate step in the hierarchy of time-dependent approximations is a complete classical treatment of all degrees of freedom. This is the topic of the next chapter. 

The main purpose of this chapter is to emphasize the intimate relation between the topology of the dissociative PES and the vibrational excitation of the fragment molecule. In Sections 9.1 and 9.2 we consider exclusively direct processes. The photodissociation of symmetric molecules with two equivalent product channels is the topic of Section 9.3. Finally, vibrational state distributions following the decay of a long-lived intermediate complex will be discussed in Section 9.4. The theory... 

The linear Boltzmann plot obtained in the photodissociation of H20 in the first continuum merely reflects the rotational FC distribution for the zero-point bending motion in the electronic ground state. It has nothing to do with energy randomization in a long-lived intermediate complex ... 

For example, for a bimolecular nucleophilic substitution (Sn2) reaction like Cl- + CHoBr — CICHo-l-Br, the potential energy has a double-well shape, i.e., two minima separated by a central barrier. The minima for this reaction reflects the stability (an effect that is also well known within classical electrostatics) of the ion-dipole complexes Cl- CHoBr and CICH3 Br . For other indirect (or complex mode) reactions one finds two saddle points separated by a well on the path from reactants to products. The existence of a well along the reaction path implies that the collision may be sticky , and a long-lived intermediate complex can be formed before the products show up. Examples of complex mode reactions are H + O2 — OH + O (with the intermediate H02), H+ + D2 and KC1 + NaBr. 

Unimolecular reactions induced by electromagnetic radiation are often further divided into direct and indirect reactions. In a direct reaction, the unimolecular reaction is over within the order of a vibrational period after the initial excitation. In an indirect reaction, the unimolecular reaction starts long after the initial excitation, since a long-lived intermediate complex is formed. Evidently, the cyclo-butene reaction is not... 

These reactions usually proceed by incorporation of Si into the product ions by way of long-lived intermediate complexes which allow a rearrangement of the existing bonds and the formation of new bonds. The reactions of ground state Si+ (2P) with HC1, H2O, H2S and NH3 have been studied as a function of the ion-neutral center of mass kinetic... 

Some atomic recombinations catalized by a rare gas atom ° and some reactions involving a long-lived intermediate complex 206-208) studied... 

Clerc and Barat " flash-photolyzed CO2 in the vacuum ultraviolet and watched CO formation by kinetic spectroscopy. They found [CO] to increase rapidly to a maximum (at about 25 i sec when [CO2] = 3 torr, [Ar] = 300 torr) and then remain constant. They attributed the observation to the reaction of excess 0( /)) with the CO produced. They calculated a rate coefficient for 0( Z)) + CO recombination of 10 l. mole . sec or 2x 10 l.mole sec depending whether or not the reaction required a third body. They noted that the third-order rate coefficient was too large for a normal three-body reaction, and suggested a long-lived intermediate complex. In two later papers " ° ° they reported the value of the second-order rate coefficient to be first 6x 10 and then 1.2 x 10 ° l.mole" sec As mentioned above, Clerc and Reiffsteck " recently reassessed the relative rates of addition of 0( J9) to CO2 and CO, and found the latter reaction to be approximately 55 times faster than the former. 

The (a) (b) < 1 ratio could be explained, as was done in [22], on the basis of a statistic partitioning, based on the assumption that a long-lived intermediate complex is formed. However, product angular distribution measurements showed that this is not the case even at very low energies [25], so that an alternative explanation had to be sought. 

Reactions occurring via the formation of a long-lived intermediate complex. The long-lived intermediate complex usually corresponds to the strongly vibrationally excited molecule AB (e). This complex does not decompose within the time interval, which exceeds at least several rotation periods. Many vibrations occur in the complex during this time interval, so that the validity of the statistical description of the vibrational energy redistribution can naturally be assumed. 

A more detailed analysis of the kinetics of the reaction proceeding through several long-lived intermediate complexes can be exemplified by the reaction of -NH2 with NO. Interest in this reaction is caused by the fact that in it the chemically active nitrogen atom is transformed into the inert N2 molecule. This determined the significance of this reaction for the atmospheric chemistry and purification of industrial gases fiom nitrogen oxides. N2, H2O, and -OH were observed in the reaction products. The presence of these products assumes two reaction chaimels... 

The negative temperature dependence of the overall rate constant immediately indicates that reaction 13 proceeds through a long-lived intermediate complex. To explain the available experimental material on this reaction, theoretical calculations... 

---