Activated molecular collision

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). 

At the other extreme is the associatively (a) activated associative (A) mechanism, in which the rate-determining step for substitution by 1/ proceeds through a reactive intermediate of increased coordination number, [M(H20) L](m x,+, which has normal vibrational modes and survives several molecular collisions before losing H20 to form [M(H20) 1L](m t,+, as shown in Eq. (8). Equation (9) indicates the linear variation with excess [I/-] anticipated for obs, which is similar in form to that of Eq. (5) when if0[I/ ] 1 and kohs + k. ... 

With all other factors held constant, decreasing the number of molecules decreases the chance of collision. Adding an accelerating catalyst has no effect on the rate of collisions. It lowers the activation energy, thereby increasing the chance for effective molecular collisions. Furthermore, it increases the rate of production. 

An active molecule or a complex of molecules is now generally regarded as one which has been raised either by the adsorption of radiation by an a particle, electron or molecular collision to a higher quantum state. 

Elementary reactions are initiated by molecular collisions in the gas phase. Many aspects of these collisions determine the magnitude of the rate constant, including the energy distributions of the collision partners, bond strengths, and internal barriers to reaction. Section 10.1 discusses the distribution of energies in collisions, and derives the molecular collision frequency. Both factors lead to a simple collision-theory expression for the reaction rate constant k, which is derived in Section 10.2. Transition-state theory is derived in Section 10.3. The Lindemann theory of the pressure-dependence observed in unimolecular reactions was introduced in Chapter 9. Section 10.4 extends the treatment of unimolecular reactions to more modem theories that accurately characterize their pressure and temperature dependencies. Analogous pressure effects are seen in a class of bimolecular reactions called chemical activation reactions, which are discussed in Section 10.5. 

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

Molecular collisions are a fundamental requirement for chemical reactions, and concentrations are important because they determine the frequency of these collisions. Nevertheless, most collisions do not result in a chemical reaction, because the energy of collision is insufficient to surmount the barrier to reaction. This barrier, the energy required to rearrange atoms in going from reactants to products, is very substantial for most reactions it is called the activation energy, or the enthalpy of activation (A//a). 

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 ... 

III. A greater temperature increases the chance that a molecular collision will overcome a reaction s activation energy. 

Equation (3.35) is a precursor to the Arrhenius equation relating the rate constant [k in Eq. (3.30)] to the probability of molecular collisions (to) and the activated energy (Ea) of a reaction. Polysaccharide viscous flow is characterized by a modified Arrhenius equation in which T j/i 0 replaces k ... 

The most natural source of energy for the activation of molecules is molecular collision, but since the rate is independent of the number of collisions it would appear at first sight that the simple collision mechanism can not be responsible for the activation. Three hypotheses have been proposed to account for the activation process the radiation hypothesis, the elaborated collision hypothesis, and the hypothesis of chain reactions. 

Thermodynamic Control at 40 °C At 40 °C, a significant fraction of molecular collisions have enough energy for reverse reactions to occur. Notice that the activation energy for the reverse of the 1,2-addition is less than that for the reverse of the 1,4-addition. Although the 1,2-product is still formed faster, it also reverts to the allylic cation faster than the 1,4-product does. At 40 °C, an equilibrium is set up, and the relative energy of each species determines its concentration. The 1,4-product is the most stable species, and it predominates. Since thermodynamics determine the results, this situation is called thermodynamic control (or equilibrium control) of the reaction. The 1,4-product, favored under these conditions, is called the thermodynamic product. 

Mass analysis determines the mass-to-charge ratio (tn/z) of ions derived from the analyte. For peptide ions, two characteristics can be obtained. The first characteristic is the molecular weight of the peptide, which can be calculated from the measured m/z of the source-generated intact peptide ion (the so-called molecular ion). The second characteristic is structural information that is obtained via an MS/MS analysis. An MS/MS experiment measures gas-phase dissociations of an activated molecular ion of the peptide to yield product-ion data that are diagnostic of the peptide sequence. The basic sequence of events in MS/MS includes 1. mass selection of the peptide ion of interest (that is population of ions of a single m/z) as a so-called precursor ion 2. activation of the precursor ion, most commonly through collisions with an inert gas, followed by dissociation of the activated precursor and formation of product ions 3. mass... 

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