Environmental Cherniatry
B. M. Fung Tufts University Medford, Massachusetts 02155
Molecular Orbitals and Air Pollution
In the teaching of undergraduate physical chemistry, the molecular orhital (R10) description of diatomic molecules constitutes an important part in the discussion of molecular structure and properties. For example, the paramagnetism of O2and the binding energies of N2, Nz+, Ox,and 0 2 + ( 1 ) can he very nicely explained by the MO theory. An equally important aspect of the 110 theory, namely the application of molecular orhital symmetry in explaining chemical reactions, has been well developed recently (2, 3). It can be adopted into the teaching of 1\10 theory to undergraduate students, and the time spent would he less than one lecture hour. The result is threefold: (1) it allows the students to further familiarize and understand the MO theory; (2) it introduces recent developments in chemistry into the undergraduate curriculum; and (3) it shows that principles of physical chemistry can really be applied in practical problems, as illustrated by the following example. Consider the reactions 2NO
-
N,
+ O1
AH" = -43 kcal/rnole
(1)
Both reactions are exothermic and thermodynamically allowed. Although reaction (1) is more favorable, i t is very slow. Had it been the predominant reaction, we would have much less smog and air pollution. On the other hand, reaction (2) is quite fast, especially in the presence of ultraviolet light and certain unsaturated hydrocarbons (4). Since nitric oxide (NO) is a major component of automobile exhaust, reaction (2) is largely responsible for the formation of smog in cities. The difference in the rates of these reactions can be qualitatively explained by molecular orbital symmetry. There are several symmetry rules for chemical reactions (2, 3). The most important one is the following. When two molecules approach each other in a reaction, electrons flow from the highest occupied molecular orbital (HOMO) of one to the lowest unoccupied molecular orbital (LUMO) of the other; electron movement cannot occur unless the HOMO and the LUMO have a net positive overlap, which means the signs of the overlapping MO loops should be the same. The energy level diagrams of Nz, NO, and O2 are shown in Figure 1. The relevant MO's of these molecules are either a or a* MO's, the shapes of which are depicted in most undergraduate physical chemistry textbooks. For the sake of clarity, let us consider the reverse reaction of eqn. (I), namely the reaction of Nz and 0 2 . From Figure 2A, it can be seen that electron flow from Nz to 0% is forbidden by symmetry. Figure 2B shows 26
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Journal of Chernicol Education
that electrons can transfer from O2 to Nz. However, since oxygen has a larger electronegativity than nitrogen, this is chemically unacceptable. Therefore the reaction between Nz and Oz would have a high energy harrier (f, 3). Because of microscopic reversihility of chemical reactions, the forward reaction would be very slow. It is believed that the first step in the oxidation of nitric oxide is the reaction between one molecule of NO and one molecule of O2(6,6) NO
+ 01-NOa
(3
Chemically, electron flow from NO to Oz is expected. Figure 2C shows that this is allowed by A40 symmetry. Consequently reaction (2) is .much faster than (11, although it is less preferred thermodynamically. The forward reaction of (I), namely the interaction between two A 1 0 molecules, has 110's like those in Figure 2C and is symmetry allowed. However, since the reverse reaction Nz OZ 2NO has a high barrier, reaction (1) is slow (2,s). Other examples, such as the reaction between Hz and Dz (S), and the reaction between HZand I2 (f), can also he easily understood by sophomore or junior students. The introduction of a t least one example of A 1 0 symmetry for chemical reactions would be a welcome addition to undergraduate physical chemistry.
+
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