Stationary points reactive systems

Noncovalent interactions between the two separate molecules define, in the gas phase analogue of this reactive system, the preferential channels of approach (in the simpler cases there is just one channel leading to the reaction) with shape and strength determined only by these interactions. As a general rule, these channels carry the reactants to a stationary point on the potential energy surface called the initial reaction complex. 

The case when the eigenvalues are equal to zero is special and must be treated separately. One is tempted to assume that such cases are rare. In fact these cases occur when the manifolds of stationary flow are not isolated points. The simplest of these cases give rise to parabolic resonances [77] however, they are beyond the scope of this review. These resonances have been observed in some of the simplest reactive systems [78]. 

In the phase-space treatment the situation is very similar. However, rather than study the morphology of the potential energy surface, we must focus on the total energy surface. The geometry of this surface, which is defined on phase space instead of coordinate space, can also be characterized by its stationary points and their stability. In this treatment, the rank-one saddles play a fundamental result. They are, in essence, the traffic barriers in phase space. For example, if two states approach such a point and one passes on one side and the other passes on the other side, then one will be reactive and the other nonreactive. Once the stationary points are identified, then the boundaries between the reactive and nonreactive states can be constructed and the dynamical structure of phase space has been determined. As in the case of potential energy surfaces, saddles with rank greater than one occur, especially in systems with high symmetry between outcomes, as in the dissociation of ozone. 

In this section we report our experimental findings relatively to three different reactions of CN radicals with simple alkynes, namely acetylene, methyl-acetylene and dimethyl-acetylene. We have selected these reactive systems for different reasons the reactions with C2H2 is the prototype for the class of reactions CN +- alkynes/polyynes, thus is expected to reveal key concepts for reactions with the higher members of the same series the reactions with methylacetylene and dimethylacetylene were selected to observe the effect of the H substitution with one or two alkyl groups. In all cases, the experimental results are discussed in the light of the ab initio electronic structure calculations for the stationary points of the relevant potential energy surfaces. 

We first expand the full 3N-dof potential energy surface about a chosen stationary point, that is, minimum, saddle, or higher rank saddle. By taking the zeroth-order Hamiltonian as a harmonic oscillator system, which might include some negatively curved modes, that is, reactive modes, we establish the higher order perturbation terms to consist of nonlinear couplings expressed in arbitrary combinations of coordinates. 

In order to draw a picture of the essential PES features, it is sufficient to know all stationary points in the regions of interest, as well as a certain number of route points connecting the stationary points (for instance along the descent paths from the saddles) Thus, for any aromatic compound including heterocycles, one can produce PES sections well-suited for textbook illustrations, and as a basis for a systematic comparison of the chemical and physical properties of the systems. Such figtures focus the striking features of the shape of the PES and do not aim at unimportant details. These representations should in particular stimulate chemist s intuition for the reactive processes in a given system. 

Sorption of Cu(tfac)2 on a column depends on the amount of the compound injected, the content of the liquid phase in the bed, the nature of the support and temperature. Substantial sorption of Cu(tfac)2 by glass tubing and glass-wool plugs was observed. It was also shown that sorption of the copper chelate by the bed is partialy reversible . The retention data for Cr(dik)3, Co(dik)3 and Al(dik)3 complexes were measured at various temperatures and various flow rates. The results enable one to select conditions for the GC separation of Cr, Al and Co S-diketonates. Retention of tfac and hfac of various metals on various supports were also studied and were widely used for the determination of the metals. Both adsorption and partition coefficients were found to be functions of the average thickness of the film of the stationary phase . Specific retention volumes, adsorption isotherms, molar heats and entropy of solution were determined from the GC data . The retention of metal chelates on various stationary phases is mainly due to adsorption at the gas-liquid interface. However, the classical equation which describes the retention when mixed mechanisms occur is inappropriate to represent the behavior of such systems. This failure occurs because both adsorption and partition coefficients are functions of the average thickness of the film of the stationary phase. It was pointed out that the main problem is lack of stability under GC conditions. Dissociation of the chelates results in a smaller peak and a build-up of reactive metal ions. An improvement of the method could be achieved by addition of tfaH to the carrier gas of the GC equipped with aTCD" orFID" . ... 

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