Transitions ortho-para

Increasing the number of acid sites, the C-methylation becomes largely preferred over the O-methylation (the QC-mcthylation ratio increased), while between the two ortho-C-methylated compounds, 2,5-DMP becomes more preferred than 2,3-DMP, in agreement with the calculated energies for the corresponding transition states. Disagreement between results obtained in gas-and liquid-phase methylation with the Mg/Fe/O catalyst (Table 2) concerns the ortho/para-C-methylation ratio (which in liquid phase was not varied when the Fe content was increased), and the 2,5-DMP/2,3-DMP ratio (which in gas phase... 

The transition state that leads to the intermediate anion, which is formed when the nucleophile adds, is stabilized by the electron-withdrawing nitro groups ortho-para to the leaving group. 

The effect of monofluorination on alkene or aromatic reactivity toward electrophiles is more difficult to predict Although a-fluonne stabilizes a carbocation relative to hydrogen, its opposing inductive effect makes olefins and aromatics more electron deficient. Fluorine therefore is activating only for electrophilic reactions with very late transition states where its resonance stabilization is maximized The faster rate of addition of trifluoroacetic acid and sulfuric acid to 2-fluoropropene vs propene is an example [775,116], but cases of such enhanced fluoroalkene reactivity in solution are quite rare [127] By contrast, there are many examples where the ortho-para-dueeting fluorine substituent is also activating in electrophilic aromatic substitutions [128]... 

The ethylation of toluene by diethylhalonium ions gives ethyltoluenes with ortho para isomer ratios between 0.60 and 0.96. The ortho para isomer ratios obtained for the alkylation of toluene in conventional Friedel-Crafts ethylations range from 1.17 to 1.84 (average 1.60). Such differences are considered to be due to the steric ortho effect caused by diethylhalonium ions, and are in accordance with the most probable displacement reaction on the bulky diethylhalonium ions by the aromatic substrate. This can be envisioned to proceed through an SN2-type transition state involving no free alkyl cations [Eq. (5.109)]. 

Fig. 11. Effect on the ortho— para transition of hydrogen. [Cf. Justi (18).] Ordinate magnetic field abscissa yield in %.

Some experiments were made recently in collaboration with Justi in order to ascertain the effect of exterior magnetic fields an effect on the ortho—> para transition of hydrogen was registered (18). Similar effects are obtained also in the case of substances with so-called electrical Curie... 

There is no doubt that in these. reactions the nitrogen atom of the pyridine ring is complexed with either the lithium alkyl or aryl or with the lithium bromide which is usually present in many preparations of organolithium compounds. It has been established that, either in the presence of an excess of lithium bromide or in the total absence of this salt, phenyllithium still gives the same ortho .para ratio on reaction with 3-picoline.229 To account for the predominant formation of the 2,3-isomer in the reaction of CH3Li with 3-alkylpyridines, it was suggested261 that the transition states for these reactions were similar... 

The inversion operation i which leads to the g/u classification of the electronic states is not a true symmetry operation because it does not commute with the Fermi contact hyperfine Hamiltonian. The operator i acts within the molecule-fixed axis system on electron orbital and vibrational coordinates only. It does not affect electron or nuclear spin coordinates and therefore cannot be used to classify the total wave function of the molecule. Since g and u are not exact labels, it was realised by Bunker and Moss [265] that electric dipole pure rotational transitions were possible in ll], the g/u symmetry breaking (and simultaneous ortho-para mixing) being relatively large for levels very close to the dissociation asymptote. The electric dipole transition moment for the 19,1 19,0 rotational transition in the ground electronic state was calculated... 

As the ortho-para conversion involves the forbidden triplet-singlet transition in the nuclear spin state, spontaneous transformation is very slow. Thus, samples of hydrogen that have been cooled exhibit the proportion of the two forms corresponding to the equilibrium composition at room temperature. The conversion of the ortho molecules to para molecules and vice versa requires the simultaneous breaking of both the spin and orbital symmetries, and the equilibrium ratio is therefore established slowly. 

The oldest example of the application of the transition state method is the reaction H + Hg = H2 + H and the similar reactions with deuterium. They have been measured partly directly, partly by the ortho-para-conversion. The calculation of the corresponding energy surface is the simplest example of its kind. Although in my opinion, the agreement between experiment and theory is not complete, the disagreement amounts to less than 50 per cent, with the most suitably chosen set of constants. A similar example is Cl -f- H2 = CIH H, where the agreement with the measurement of Rodebush is within... 

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