NO2 molecule

In the event that the CO and NO2 molecules do not have sufficient energy to attain the summit, they reach a point only part way up the left side of the barrier. Then, repelling one another, they separate again, going downhill to the left. 

The NO2 molecule is nonlinear. It has nine degrees of freedom three translational, three rotational, and 3rj - 6 = 3 vibrational. The complex vibrational motion of this molecule can be resolved into three fundamental... 

Hydroxyl, OH, acts as a catalyst for the oxidation of NO to NO2. NO2 molecules can react with the OH radical to produce HNO3, which may be removed in precipitation. This is how most tropospheric NO, eventually gets removed, either in wet or dry deposition. 

N02(g) N2 04(g) Imagine this reaction foiiowing the two-step pathway that appears in Figure 6-19. In the first step, 2 mol of NO2 molecules decompose into nitrogen molecules and oxygen molecules ... 

The second part of Figure 14-1 shows a molecular view of what happens in the two bulbs. Recall from Chapter 5 that the molecules of a gas are in continual motion. The NO2 molecules in the filled bulb are always moving, undergoing countless collisions with one another and with the walls of their container. When the valve between the two bulbs is opened, some molecules move into the empty bulb, and eventually the concentration of molecules in each bulb is the same. At this point, the gas molecules are in a state of dynamic equilibrium. Molecules still move back and forth between the two bulbs, but the concentration of molecules in each bulb remains the same. 

Nitrogen dioxide, a red-brown gas, is stable at room temperature. However, Figure 15-1 shows that reducing the temperature causes a sample of NO2 to become colorless. The color change that happens at low temperature occurs because two NO2 molecules combine to form one molecule of a colorless gas, N2 O4. ... 

The mechanism of this reaction describes what happens at the molecular level. Two NO2 molecules collide, forming a collision complex. In this collision complex, a bond may form between the two nitrogen atoms, producing an N2 O4 molecule. 

The formation of N2 O4 requires more than a simple collision between two NO2 molecules. The product contains a bond between the nitrogen atoms, so the collision must form a collision complex that brings the two... 

The most common type of elementary process is a bimolecular reaction that results from the collision of two molecules, atoms, or ions. The collision of two NO2 molecules to give N2 O4 is a bimolecular reaction. Here is another example ... 

Notice that the characteristic feature of a bimolecular elementary reaction is a collision between two species, giving a coiiision compiex that resuits in a rearrangement of chemical bonds. The two reaction partners stick together by forming a new bond (N2 O4 forming from two NO2 molecules), or they form two new species by transferring one or more atoms or ions from one partner to another (2 H2 O forming from OH and H3 ). 

A second possible mechanism for NO2 decomposition starts with a bimolecular reaction. When two fast-moving NO2 molecules collide, an oxygen atom may be transferred between them to form molecules of NO3 and NO. Molecules of NO3 are unstable and readily break apart into NO and O2. This reaction sequence can be summarized as Mechanism It for NO2 decomposition ... 

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. 

NO2 NO -I- O. Oxygen atoms are known to be highly reactive, so it is reasonable to predict that this intermediate reacts rapidly with NO2 molecules. Compared with the fast second step of this mechanism, the step that forms the oxygen atoms is e.xpected to be slow and rate-determining. Similarly, the first step of Mechanism It, NO2 NO3 + NO, produces NO3. This species is known to be unstable, so it will decompose in the second step of Mechanism II almost as soon as it forms. Again, the second step of the mechanism is expected to be fast, so the step that forms the reactive intermediate is slow and rate-determining. Later in this chapter, we discuss experiments that make it possible to distinguish between Mechanisms I and II. 

The first step in Mechanism I is the unimolecular decomposition of NO2. Our molecular analysis shows that the rate of a unimolecular reaction is constant on a per molecule basis. Thus, if the concentration of NO2 is doubled, twice as many molecules decompose in any given time. In quantitative terms, if NO2 decomposes by Mechanism I, the rate law will be Predicted rate (Mechanism I) = [N02 ] Once an NO2 molecule decomposes, the O atom that results from decomposition very quickly reacts with another NO2 molecule. 

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] ... 

In another possible mechanism, two NO2 molecules collide in the rate-determining step to form NO and NO3. In a second and faster step, the highly reactive NO3 intermediate transfers an oxygen atom to CO in a bimolecular collision ... 

Example shows that no energy barrier exists for the combination of two NO2 molecules to form N2 O4. The activation energy for this reaction is zero because NO2 is an odd-electron molecule with a lone electron readily... 

In a vessel that contains only NO2 molecules, the production of N2 O4 is the only reaction that takes place. However, once N2 O4 molecules are present, the reverse reaction also can occur. An N2 O4 molecule can fragment after collisions give It sufficient energy to break the N—N bond. These fragmentations regenerate NO2 N2 O4 2 NO2... 

Collisions between NO2 molecules produce N2 O4 and consume NO2. At the same time, fragmentation of N2 O4 produces NO2 and consumes N2 O4. When the concentration of N2 O4 is veiy low, the first reaction occurs more often than the second. As the N2 O4 concentration increases, however, the rate of fragmentation increases. Eventually, the rate of N2 O4 production equals the rate of its decomposition. Even though individual molecules continue to combine and decompose, the rate of one reaction exactly balances the rate of the other. This is a dynamic equilibrium. At dynamic equilibrium, the rates of the forward and reverse reactions are equal. The system is dynamic because individual molecules react continuously. It is at equilibrium because there is no net change in the system. 

In the NO2 /N2 O4 system, molecules of NO2 combine to give N2 O4 molecules, and N2 O4 molecules decompose to give NO2 molecules. This Is an example of the reversibilityRoasting of molecular reactions. 

Look again at Figure 16-1 If two NO2 molecules can form a bond when they collide, then that bond also can break apart when an N2 O4 molecule distorts. The concept of reversibility is a general principle that applies to all molecular processes. Every elementary reaction that goes in the forward direction can also go In the reverse direction. As a consequence of reversibility, we can write each step in a chemical mechanism using a double arrow to describe what happens at chemical equilibrium. 

The NO2 molecule offers an example which illustrates this point. The spectrum of N02 molecules rigidly held on MgO at —196° is characterized by gxx = 2.005, gyv = 1.991, and gzz = 2.002 (29). If this molecule were rapidly tumbling, one would expect a value of Qa.v — 1 999. The spectrum of NO2 absorbed in a 13X molecular sieve indicates an isotropic gzv = 2.003 (.80), which is within experimental error of the predicted value for NO2 on MgO. The hyperfine constants confirm that NO2 is rapidly tumbling or undergoing a significant libration about some equilibrium position in the molecular sieve (81). 

A fairly detailed treatment of the theory for hyperfine interactions has been given in Appendix D, and it is our intention to show how the results of this development can be used to determine molecular structure. Perhaps the most straightforward way to introduce the subject is to examine the experimental results for the NO2 molecule adsorbed on MgO (29). This molecule has been extensively studied in the gas, liquid, and solid phase, so that there is ample data for comparison purposes. 

This rather lengthy example, using the hyperfine tensor for the adsorbed NO2 molecule, has illustrated the type of information that one can obtain... 

Nitroarenes are recognized from their characteristic neutral losses due to the NO2 substituent. Normally, all theoretically possible fragment ions, the plausible [M-N02] and [M-O] ions as well as the unexpected [M-NO] ion, are observed. It is worth noting that molecular ions are 1,2-distonic by definition, because nitroarene molecules are best represented as zwitterion (Chap. 6.3). The molecular ion may either dissociate directly by loss of an oxygen atom or a NO2 molecule or it may rearrange prior to loss of NO. For the latter process, two reaction pathways have been uncovered, one of them involving intermediate formation of a nitrite, and the other proceeding via a three-membered cyclic intermediate. [208]... 

Ellis and coworkers studied the effect of lead oxide on the thermal decomposition of ethyl nitrate vapor.P l They proposed that the surface provided by the presence of a small amount of PbO particles could retard the burning rate due to the quenching of radicals. However, the presence of a copper surface accelerates the thermal decomposition of ethyl nitrate, and the rate of the decomposition process is controlled by a reaction step involving the NO2 molecule. Hoare and coworkers studied the inhibitory effect of lead oxide on hydrocarbon oxidation in a vessel coated with a thin fQm of PbO.P l They suggested that the process of aldehyde oxidation by the PbO played an important role. A similar result was found in that lead oxide acts as a powerful inhibitor in suppressing cool flames and low-temperature ignitions.P l... 

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