Collision decomposition

Mies F H 1969 Resonant scattering theory of association reactions and unimolecular decomposition. Comparison of the collision theory and the absolute rate theory J. Cham. Phys. 51 798-807... 

Wlien the atom-atom or atom-molecule interaction is spherically symmetric in the chaimel vector R, i.e. V(r, R) = V(/-,R), then the orbital / and rotational j angular momenta are each conserved tln-oughout the collision so that an i-partial wave decomposition of the translational wavefiinctions for each value of j is possible. The translational wave is decomposed according to... 

A partial wave decomposition provides the frill close-coupling quantal method for treating A-B collisions, electron-atom, electron-ion or atom-molecule collisions. The method [15] is siumnarized here for the inelastic processes... 

Alternatively, ions of any one selected m/z value can be chosen by holding the magnetic field steady at the correct strength required to pass only the desired ions any other ions are lost to the walls of the instrument. The selected ions pass through the gas cell and are detected in the singlepoint ion collector. If there is a pressure of a neutral gas such as argon or helium in the gas cell, then ion-molecule collisions occur, with decomposition of some of the selected incident ions. This is the MS/MS mode. However, without the orthogonal TOF section, since there is no further separation by m/z value, the new ions produced in the gas cell would not be separated into individual m/z values before they reached the detector. Before the MS/MS mode can be used, the instrument must be operated in its hybrid state, as discussed below. 

By introducing a collision gas into Q2, collision-induced dissociation (CID) can be used to cause more ions to fragment (Figure 33.4). For example, with a pressure of argon in Q2, normal ions (mj ) collide with gas molecules and dissociate to give mj ions. CID increases the yield of fragments compared with natural formation of metastable ions without induced decomposition. 

Ions can be induced to fragment by increasing an electric potential known as a cone voltage, which speeds them. Accelerating the ions causes them to collide more energetically with neutral molecules, a process that causes them to fragment (collision-induced decomposition). 

The hybrid has other advantages of sensitivity, low signal-to-noise ratio, fast switching between MS and MS/MS modes, use with continuous or pulsed ion sources, and use with high- or low-energy collision-induced ion decomposition. 

Collision of normal ions from the first quadrupole with gas molecules in the second quadrupole increases fragmentation, a process known as either collisionally induced dissociation (CID) or collisionally activated decomposition (CAD). 

Collision-induced dissociation (or decomposition), abbreviated CID. An ion/neutral process wherein the (fast) projectile ion is dissociated as a result of interaction with a target neutral species. This is brought about by conversion during the collision of part of the translational energy of the ion to internal energy in the ion. The term collisional-activated dissociation (or decomposition), abbreviated CAD, is also used. 

Reactions involving collisions between two molecular species such as H2 and I2, or between two HI molecules are called bimolecular or second-order homogeneous reactions, because they involve the collision between two molecular species, and they are homogeneous since they occur in a single gas phase. The rates of these reactions are dependent on the product of the partial pressure of each reactant, as discussed above, and for the formation of HI, and the decomposition of HI,... 

The collision theory considers the rate to be governed by the number of energetic collisions between the reactants. The transition state theory considers the reaction rate to be governed by the rate of the decomposition of intermediate. Tlie formation rate of tlie intermediate is assumed to be rapid because it is present in equilibrium concentrations. 

Deployed airings after a collision are filled with nitrogan gas. The gas is a byproduct of the decomposition of sodium azide, which is triggered by the collision. 

Tandem quadrupole and magnetic-sector mass spectrometers as well as FT-ICR and ion trap instruments have been employed in MS/MS experiments involving precursor/product/neutral relationships. Fragmentation can be the result of a metastable decomposition or collision-induced dissociation (CID). The purpose of this type of instrumentation is to identify, qualitatively or quantitatively, specific compounds contained in complex mixtures. This method provides high sensitivity and high specificity. The instrumentation commonly applied in GC/MS is discussed under the MS/MS Instrumentation heading, which appears earlier in this chapter. 

Heath and Majer (H3) have recently used a mass spectrometer to study the decomposition of ammonium perchlorate. Decomposition was detected in the range from 110° to 120°C. At this temperature, there were ions in the mass spectrum caused by NH3, HC104, Cl2, HC1, nitrogen oxides, and 02. The appearance of the species NO, N02,02, and Cl2 in the decomposition products under very low pressure (i.e., in the absence of gas-phase molecular collisions) indicates that the principal decomposition reactions take place in the crystal and not in the gas phase. 

FIGURE 13.16 A representation of a proposed one-step mechanism for the decomposition of ozone in the atmosphere. This reaction takes place in a single bimolecular collision. 

The second alternative that can be considered is incomplete thermal-ization. Initial excess energy in the C2H4 + as well as excitation owing to energy released in the condensation reactions may not be completely removed between reactive encounters with C2H4. The accumulation of energy will cause increased decomposition. In 0.1-torr ethylene and 10-torr xenon 100 collisions with xenon will occur between a collision with ethylene. The above interpretation of the results suggests that 100 collisions are not sufficient for thermalization. 

More recently, certain MS-MS scans have been made available on the ion-trap instrument. This type of system differs from those described previously in that the MS-MS capability is associated only with the way in which the ion-trap is operated, i.e. it is software controlled, and does not require the addition of a collision cell and a further analyser. This is because ion selection, decomposition and the subsequent analysis of the product ions are all carried out in the same part of the instrument, with these processes being separated solely in time, rather than time and space as is the case for the instruments described previously. 

The only reactions that are strictly hrst order are radioactive decay reactions. Among chemical reactions, thermal decompositions may seem hrst order, but an external energy source is generally required to excite the reaction. As noted earlier, this energy is usually acquired by intermolecular collisions. Thus, the reaction rate could be written as... 

Intermediates are reactive chemical species that usually exist only briefly. They are consumed rapidly by bimolecular collisions with other chemical species or by unimolecular decomposition. The intermediate in Mechanism I is an oxygen atom that reacts rapidly with NO2 molecules. The intermediate in Mechanism II is an unstable NO3 molecule that rapidly decomposes. 

The rate-determining step of Mechanism II is a bimolecular collision between two identical molecules. A bimolecular reaction has a constant rate on a per collision basis. Thus, if the number of collisions between NO2 molecules increases, the rate of decomposition increases accordingly. Doubling the concentration of NO2 doubles the number of molecules present, and it also doubles the number of collisions for each molecule. Each of these factors doubles the rate of reaction, so doubling the concentration of NO2 increases the rate for this mechanism by a factor offour. Consequently, if NO2 decomposes by Mechanism II, the rate law will be Predicted rate (Mechanism n) = < [N02][N02] = J [N02] ... 

This mechanism has four steps. The first step is the unimolecular decomposition of Br2. The remaining steps all are simple bimolecular collisions. We can readily visualize each step as a reaction that the molecules can undergo, so the mechanism meets the first criterion. 

Mechanism II begins with fast reversible ozone decomposition followed by a rate-determining bimolecular collision of an oxygen atom with a molecule of NO. The rate of the slow step is as follows Rate = 2[N0][0 This rate expression contains the concentration of an intermediate, atomic oxygen. To convert the rate expression into a form that can be compared with the experimental rate law, assume that the rate of the first step is equal to the rate of its reverse process. Then solve the equality for the concentration of the intermediate ... 

Use reactant molecules to write appropriate elementary reactions that satisfy the following criteria (a) a unimolecular decomposition that generates I (b) a bimolecular collision that forms a square H2 I2 complex and (c) a bimolecular collision in which a hydrogen atom is transferred between... 

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