Ortho and para states

Equilibrium distribution of ortho and para states. Defined value. 

Stock, M. and Weber, H.G. (1974). An optical pumping-experiment of ortho- and para-states of Ne2-molecules, Phys. Lett. A, 50, 343-350. 

For H2/ at temperatures below 1000 K, the rotational constant B = 59.4 cm 1 is so large that integration is invalid, and one must sum directly the leading terms of the two sums. The same holds for D2, where B = 29.9 cm-1. For the heavier homonuclear diatomics, like 02 (B = 1.437 cm ) or N2, B is so small that the high-temperature integration of Problem 5.3.11 works well, and the difference between ortho and para states becomes experimentally indistinguishable. 

Theory shows that the vapour pressures of ortho- and para- states of diatomic molecules ( 23.IV) should be diflferent, and this has been confirmed experimentally for hydrogen.3 According to Herzfeld and Teller, the heavier isotope may have a lower or higher vapour pressure than the lighter one. For neon isotopes, the vapour pressure is a linear function of the atomic weight. ... 

Because the transition rate between the ortho- and para- states is low except in the presence of magnetic catalysts, we can equilibrate the gas at a given temperature in the presence of a magnetic catalyst. For low temperatures, in the presence of a catalyst, the system will become virtually entirely para while for high temperatures it will tend to 25% para and 75% ortho. We can therefore do neutron scattering experiments with different predetermined ratios of the two forms so that both cross-sections can be extracted. 

Topic 10 of volume 5 of the International Encyclopedia of Physical Chemistry and Chemical Physics gives a clear account of the perfect gas. Following an introductory chapter on thermodynamics, there is a chapter on the measurement of heat capacities which surveys the principles and limitations of various experimental methods. A large part of the work deals with the calculation of thermodynamic properties, including consideration of the ortho- and para-states, residual entropy, hindered internal... 

For those who are familiar with the statistical mechanical interpretation of entropy, which asserts that at 0 K substances are nonnally restricted to a single quantum state, and hence have zero entropy, it should be pointed out that the conventional thennodynamic zero of entropy is not quite that, since most elements and compounds are mixtures of isotopic species that in principle should separate at 0 K, but of course do not. The thennodynamic entropies reported in tables ignore the entropy of isotopic mixing, and m some cases ignore other complications as well, e.g. ortho- and para-hydrogen. 

Canada has no known basic producers of chlorobenzene. There is one company that isolates small quantities of ortho and para from purchased mixed dichlorobenzenes. Some of the isolated product is exported. The primary portion of Canada s chlorobenzenes comes from the United States. 

Monochlorobenzene. The largest use of monochlorobenzene in the United States is in the production of nitrochlorobenzenes, both ortho and para, which are separated and used as intermediates for mbber chemicals, antioxidants (qv), dye and pigment intermediates, agriculture products, and pharmaceuticals (Table 5). Since the mid-1980s, there have been substantial exports of both o-nitrochlorobenzene, estimated at 7.7 million kg to Europe and -nitrochlorobenzene, estimated at 9.5 million kg to the Far East. Solvent use of monochlorobenzene accounted for about 28% of the U.S. consumption. This appHcation involves solvents for herbicide production and the solvent for diphenylmethane diisocyanate manufacture and other chemical intermediates. 

In the case of nitrobenzene, the electron-withdrawing nitro group is not able to stabilize the positive charge in the complex intermediate. In fact, it strongly destabilizes die intermediate. This destabilization is greatest in the o- and />-intermediates, which place positive charge on the nitrosubstituted caibon. The meta transition state is also destabilized relative to that for benzene, but not as much as the ortho and para transition states. As a result, nitrobenzene is less reactive than benzene, and the product is mainly the meta isomer. 

If the transition state resembles the intermediate n-complex, the structure involved is a substituted cyclohexadienyl cation. The electrophile has localized one pair of electrons to form the new a bond. The Hiickel orbitals are those shown for the pentadienyl system in Fig. 10.1. A substituent can stabilize the cation by electron donation. The LUMO is 1/13. This orbital has its highest coefficients at carbons 1, 3, and 5 of the pentadienyl system. These are the positions which are ortho and para to the position occupied by the electrophile. Electron-donor substituents at the 2- and 4-positions will stabilize the system much less because of the nodes at these carbons in the LUMO. 

These fragments either combine intramolecularly to form the ortho and para nitro compounds or dissociate completely and then undergo an intermolecular reaction to form the same products. The theory was not developed to include a detailed transition state and no mention was made of how the para isomer was formed. Reduction of the cation-radical could give the amine (which was observed experimentally76), but one would expect the concurrent formation of nitrogen dioxide and hence nitrite and nitrate ions however, the latter has never been... 

The X-ray structure of the dibromine complex with toluene (measured at 123 K) is more complicated, and shows multiple crystallographically independent donor/acceptor moieties [68]. Most important, however, is the fact that in all cases the acceptor shows an over-the-rim location that is similar to that in the benzene complex. In both systems, the acceptor is shifted by 1.4 A from the main symmetry axis, the shortest Br C distances of 3.1 A being significantly less than the sum of the van der Waals radii of 3.55 A [20]. Furthermore, the calculated hapticity in the benzene/Br2 complex (x] = 1.52) is midway between the over-atom (rj = 1.0) and over-bond (rj = 2.0) coordination. In the toluene complex, the latter varies from rj = 1.70 to 1.86 (in four non-equivalent coordination modes) and thus lies closer to the over-bond coordination model. Moreover, the over-bond bromine is remarkably shifted toward the ortho- and para-carbons that correspond to the positions of highest electron density (and lead to the transition states for electrophilic aromatic bromination [12]). Such an experimental location of bromine is in good agreement with the results of high level theoretical... 

The startling observation made in this work was that meta derivatives were more reactive toward photosubstitution than ortho and para isomers, as shown below for the photoreaction of dimethoxynitrobenzene and methyl amine, in contradistinction to the rules of classical ground state nucleophilic substitution ... 

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