Activation by collisions

Typical MS/MS configuration. Ions produced from a source (e.g., dynamic FAB) are analyzed by MS(1). Molecular ions (M or [M + H]+ or [M - H]", etc.) are selected in MS(1) and passed through a collision cell (CC), where they are activated by collision with a neutral gas. The activation causes some of the molecular ions to break up, and the resulting fragment ions provide evidence of the original molecular structure. The spectrum of fragment ions is mass analyzed in the second mass spectrometer, MS(2). 

Lindemann Mechanism of unimolecular reactions — activation by collisions... 

At low cM, the rate-determining step is the second-order rate of activation by collision, since there is sufficient time between collisions that virtually every activated molecule reacts only the rate constant K appears in the rate law (equation 6.4-22). At high cM, the rate-determining step is the first-order disruption of A molecules, since both activation and deactivation are relatively rapid and at virtual equilibrium. Hence, we have the additional concept of a rapidly established equilibrium in which an elementary process and its reverse are assumed to be at equilibrium, enabling the introduction of an equilibrium constant to replace the ratio of two rate constants. 

According to Lindemann s hypothesis, the activation by collision can still give rise to first order kinetics if the activated molecules decompose only slowly compared to the rate at which they are deactivated. There is a time-log between the moment of activation and the moment of decomposition and in such a case, a stationary concentration of the activated molecules gets built up. Since the activated molecules will be in equilibrium with the normal molecules, their concentration will be proportional to that of normal molecules. The activated molecules disappear through two parallel processes, i.e. through deactivation and decomposition, represented as follows ... 

In a purely photochemical reaction the absorption of radiant energy is plainly responsible for the activation. This suggested the possibility that thermal reactions are also due to activation by the thermal radiation which is present at every temperature. The argument was very forcibly presented by Perrin who showed that if the specific rate of a imimolecular gas reaction remains constant, with indefinite diminution in pressure, activation must be by radiation since the number of opportunities for activation by collision also diminishes without limit. In fact, the decomposition of nitrogen pentoxide, the first gas reaction shown to be unquestionably unimolecular, was found to have a specific reaction rate constant over a wide range of pressure, and apparently increasing at very low pressures. ... 

Molecular Statistics of the Bimolecular Hydrogen Iodide Decomposition. The theory of activation by collision. 

Another mechanism has been suggested by Christiansen and Kramers in which activation is by collision and yet there is an apparently unimolecular reaction. It depends upon the possibility that the products of reaction, possessing the energy corresponding to the chemical heat of reaction as well as the original heat of activation, are able immediately to activate fresh molecules of reactant. In this way reaction-chains are set up. The assumption is made that every molecule of product can at once activate by collision a fresh molecule of the reactant. In this way each activated molecule removed from the system by chemical transformation is replaced by a new activated molecule. 

The first problem is to decide between three possibilities (1) activation by radiation, (2) activation by collision, (3) activation by both agencies simultaneously, always bearing in mind, however, that the activation process is not necessarily the same for all unimolecular reactions. 

If activation by collision is assumed two questions arise (i) how the velocity constant of the unimolecular reaction remains independent of pressure, and (ii) whether the number of collisions taking place in the gas is great enough to activate molecules sufficiently fast to account for the observed rate of chemical change even at the lowest pressures. 

The only example of all the unimolecular reactions known where such a difficulty has actually arisen in an acute form is the decomposition of nitrogen pentoxide. It appears that at low pressures nitrogen pentoxide reacts at a rate which is considerably greater than the maximum possible rate of activation by collision, however great a value of n be assumed. There is a limit to the maximum rate theoretically possible, since, when n is increased beyond a certain point, the increase in the term E — EArrhenius + n- )RT produces a decrease in the calculated rate which more than compensates for the increase due to the term (E/RT)1l2n 1 multiplying the exponential term. 

The most satisfactory explanation is that the rate is increased beyond the maximum rate of activation by collision through the operation of a chain mechanism. The observations of Sprenger on the peculiar behaviour of nitrogen pentoxide at low pressures suggest strongly that chains are propagated. Moreover, if the rate of the azoisopropane reaction at the lowest pressures should prove to be greater than can be accounted for on the basis of the simple collision mechanism, a chain mechanism can be assumed without difficulty since the reaction is quite markedly exothermic. 

It is the existence of this time lag between activation by collision and reaction which is basic and crucial to the theory of unimolecular reactions, and this assumption leads inevitably to first order kinetics at high pressures, and second order kinetics at low pressures. 

Other elementary reactions can be handled in the same fundamental way molecules can become activated by collision and then last long enough for there to be the same two fates open to them. The only difference lies in the molecularity of the actual reaction step ... 

We have seen that molecules must be activated before they can react and that in thermal reactions they are activated by collisions with high-velocity molecules, while in photochemical reactions they are activated through displacement of the outer electrons produced by the absorption of light. Molecules in the gas phase may be activated also by electronic displacement caused by collisions with electrons or ions. This activation may be brought about directly by cathode rays, by electrical discharges of various types, or indirectly by alpha rays and X-rays, which seem to be effective chiefly through the secondary electrons and ions which they produce. 

Lennard-Jones and Goodwin2 have, however, shown that the adsorbed atoms can be activated by collisions with the free electrons in the underlying metal there is no reason to expect a simple connexion between activation and the thermionic work function. 

Although the Lindemann theory is often satisfactory, it is incomplete since it does not fully recognise the relation between translational and internal energies. In many reactions the rate of activation by collision is not itself explicable unless it is assumed that activation can also occur by the transfer of vibrational energy from one molecule to another. This possibility was recognised by Hinshelwood and by Lewis and may be equivalent, in effect, to multiplying the frequency factor by 10" or more. 

A represents a molecule activated by collision. The rate of decomposition of A is given by... 

The remaining postulate, activation by collision, has been the subject of considerable speculation.4 Simple activation by collision has been... 

Summary.—The mechanism of the activation process in gaseous systems has been investigated from the point of view of (1) activation by radiation (2) activation by collision. An increase in the radiation density of possible activating frequencies has resulted in no increased reaction velocity. The study of the bimolecular decomposition of nitrous oxide at low pressures has led to the conclusion that the reaction is entirely heterogeneous at these pressures. A study of the unimolecular decomposition of nitrogen pentoxide between pressures of 7io mm. Hg and 2 X 10 3 mm. Hg shows no alteration in the rate of reaction such as was found by Hirst and Rideal but follows exactly the rate determined by Daniels and Johnson at high pressures. No diminution of the reaction velocity as might be ex-expected from Lindemann s theory was observed. 

The activated-complex theory provides a plausible explanation of the first-order rate of unimolecular gaseous reactions. In such a reaction the reacting molecules gain the energy of activation by colhsion with other molecules. This might be thought of as a second-order process, since the number of collisions is proportional to the square of the concentration. However, Lindemann showed in 1922 that activation by collision could result in first-order rates. If A is an activated molecule of reactant, the equilibrium between A and A and reaction to products B can be represented as... 

This prediction is also borne out experimentally (cf. Table 4.1) since the experimental rate equation is of exactly the same form in other words the partial order of reaction with respect to each reactant is 1 and overall the reaction is second-order. In the case of a unimolecular reaction, the situation is more complicated since a single reactant particle has to become energized or activated by collisions, either with other reactant particles or other bodies that are present, in order for reaction to occur. However, although we shall not go into detail, a theoretical treatment shows that under most circumstances the rate of reaction will be directly proportional to the concentration of the single reactant species so that the theoretical rate equation can be written... 

Modem mass spectrometric methods greatly help to speed up degradation studies by identifying the molecular mass of peptide fragments. Various desorption techniques, which produce protonated protein ions in the gas phase, are applied. Prominent examples are fast-atom bombardment (FAB) and matrix- assisted laser desorption/ionization (MALDI). Most commonly these techniques provide only the mass of the molecular ion, but they can be coupled with a collision-induced dissociation (CID) procedure. The ions are activated by collisions with neutral target gases, and the peptides dissociate in the gas phase. The resulting mass spectra... 

In 1922, Lindemann proposed a mechanism by which the molecules could be activated by collision and yet the reaction could, nonetheless, be first order. The activation of the molecule is by collision... 

The apparent first-order rate constant decreases at low pressures. Physically the decrease in value of the rate constant at lower pressures is a result of the decrease in number of activating collisions. If the pressure is increased by addition of an inert gas, the rate constant increases again in value, showing that the molecules can be activated by collision with a molecule of an inert gas as well as by collision with one of their own kind. Several first-order reactions have been investigated over a sufficiently wide range of pressure to confirm the general form of Eq. (32.61). The Lindemann mechanism is accepted as the mechanism of activation of the molecule. 

The results obtained by considering activation by a third body must now be compared to those described earlier for activation by collision of reactant molecules. 

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