Ozone resonance

Scheme 2. Examples of resonance ozone, benzene and the allyl cation...

For singlet spin molecules at the equilibrium geometry, RHF and UHF wave functions are almost always identical. RHF wave functions are used for singlets because the calculation takes less CPU time. In a few rare cases, a singlet molecule has biradical resonance structures and UHF will give a better description of the molecule (i.e., ozone). 

To deal wifh circumslances such as fhe bonding m ozone fhe nolion of resonance befween Lewis sfrucfures was developed According fo fhe resonance concepf when more fhan one Lewis sfrucfure may be written for a molecule a single sfrucfure is msuf ficienl fo describe if Ralher fhe frue sfrucfure has an eleclron dislribulion fhaf is a hybrid of all fhe possible Lewis sfrucfures fhaf can be written for fhe molecule In fhe case of ozone fwo equivalenf Lewis sfrucfures may be wriffen We use a double headed arrow fo represenf resonance befween fhese fwo Lewis sfrucfures... 

If IS imporfanf fo remember fhaf fhe double headed resonance arrow does not mdi cate a process m which fhe fwo Lewis sfrucfures mferconvert Ozone for example has... 

There is evidence that dioxirane is an intermediate product in the low temperature ozonization of ethylene and is probably formed from the diradical resonance isomer of the 1,3-zwitterion (164). 

To deal with circumstances such as the bonding in ozone, the notion of resonance between Lewis structures was developed. According to the resonance concept, when more than one Lewis structure may be written for a molecule, a single structure is insufficient to describe it. Rather, the true structure has an electron distribution that is a hybrid of all the possible Lewis structures that can be written for the molecule. In the case of ozone, two equivalent Lewis structures may be written. We use a double-headed aiTow to represent resonance between these two Lewis structures. 

It is important to remember that the double-headed resonance anow does not indicate a process in which the two Lewis structures interconvert. Ozone, for exfflnple, has... 

By including electron correlation in the wave function the UHF method introduces more biradical character into the wave function than RHF. The spin contamination part is also purely biradical in nature, i.e. a UHF treatment in general will overestimate the biradical character. Most singlet states are well described by a closed-shell wave function near the equilibrium geometry, and in those cases it is not possible to generate a UHF solution which has a lower energy than the RHF. There are systems, however, for which this does not hold. An example is the ozone molecule, where two types of resonance structure can be drawn. Figure 4.8. 

The biradical resonance structure for ozone requires two singly occupied MOs, and it is clear that an RHF type wave function, which requires all orbitals to be doubly occupied, cannot describe this. A UHF type wave function, however, allows the a and /3 orbitals to be spatially different, and can to a certain extent incorporate both resonance structures. Systems with biradical character will often have a (singlet) UHF wave function different from an RHF. 

I Stratospheric ozone, O, protects life on Farth from harmful ultraviolet radiation from the Sun. Suggest two Lewis structures that contribute to the resonance structure for the 02 molecule. Experimental data show that the two bond lengths are the same. 

Seif-Test 2.1 IB Calculate the formal charges for the three oxygen atoms in one of the Lewis structures of the ozone resonance structure (Example 2.5). 

Ozone is a compound with the following resonance structures ... 

The structure of ozone, (a) The Lewis structure shows resonance that involves the bond (blue lines) and one lone pair (red dots), (b) The a bonding framework. 

A clue to the nature of the third itt MO can be found in the placement of electrons in the two resonance structures for ozone, which are shown with color highlights in Figure 10-36a. Notice that in one resonance structure, the left outer atom has three lone pairs and a single bond, while the right outer atom has two lone pairs and a double bond. In the other resonance structure, the third lone pair is on the right outer atom, with the double bond to the left outer atom. The double bond appears in different positions in the two stmctures, and one of the lone pairs also appears in different positions. These variations signal delocalized orbitals. 

A structure midway between the two resonance structures represents the ozone structure best. The bonds in this structure are stronger than a single bond but weaker than a double one. 

Fig. 13.1 Three important resonance structures for the ground state of the ozone molecule, Oj. The third structure represents a problematic diradical.

Ozone, O3, is an essential molecule in the ozone layer. It filters out some of the UV radiation that would otherwise destroy life on Earth. At ground level, ozone is a component in photochemical smog. Draw the Lewis structure for ozone, and indicate whether or not ozone is polar. Can ozone exist in resonance forms ... 

Advanced electro-optical methods (e.g., laser resonance absorption) capable of measuring average concentrations over long distances still requite extensive research and field testing to demonstrate their practical application to ozone monitoring. Because electro-optical methods have not yet been widely used, they are not discussed further here. 

The standard astm test method (D-1149-64) for rubber damage includes a test chamber (volume, 0.11-0.14 m ) through which ozonized air flows at a rate greater than 0.6 m/s. Because the residence time of the ozonized air in the test chamber is about 1 s, the ozone may be expected to reach the material in about 0.1 s. A somewhat similar test procedure (aatcc test method 109-1972 ansi L14, 174-1973) is used in testing colorfastness. The ozone generator is usually (but not necessarily) a mercury-vapor resonance lamp with emission lines at 184.9 and 253.7 nm. The 184.9-nm line is absorbed, and two ground-state oxygen atoms are produced ... 

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