Molecular structures, observed phenomena

Symmetry is a common phenomenon in tlie world around us. IT Nature abhors a vacuum, it certainly seems to love symmetry It is difficult to overestimate the importance of symmetry in many aspects of science, not only chemistry. Just as the principle known as Occam s razor suggests that the simplest explanation for an observation is scientifically the best, so it is true that other tilings being equal, frequently the most symmetrical molecular structure is the preferable one. More important, die methods of analysis of symmetry allow simplified treatment of complex problems related to molecular structure. 

Such polymers adopt, when affected by a mechanical field, an optically uniaxial homeotropic structure polymers B.1.2, B.1.7, B.1.8 (Table 8) have positive birefringence polymers B.1.1, B.1.8. (Table 9) have negative birefringence, which has not been reported to our knowledge, for low-molecular nematic liquid crystals. Although the authors do not comment on the cause for the observed phenomenon, the fact in itself is sufficiently uncommon. 

As the phenomenon of crystallographic shear appears in transition metal oxides with anisotropic lattices, pronounced structure-sensitivity of catalytic properties is observed and the habit of crystallites of the catalyst may have strong influence on the selectivity of the reaction [39,59-61]. Multiple examples of the dependence of catalytic properties on the type of exposed crystal plane have been described in literature, but the only attempt to explain this phenomenon in terms of the molecular structure of different crystal planes was undertaken in the crystallogrphic model of active sites [62], The question awaits more dedicated experiments and deeper theoretical analysis. The first quantum-chemical approach addressing this question has been quite recently published [63]. 

In a paper in 1979, Carl Ballhausen [1] expressed the belief that today we realize that the whole of chemistry is one huge manifestation of quantum phenomena, but he was perfectly well aware of the care that had to be taken to express the relevant quantum theory appropriately. So in an earlier review [2] that he had undertaken with Aage Hansen, he scorned the usual habit of chemists in naming an experimental observation as if it was caused by the theory that was used to account for it. Thus in the review they remark that a particular phenomenon observed in molecular vibration spectra is presently refered to as the Duchinsky effect. The effect is, of course, just as fictitious as the Jahn-Teller effect. Their aim in the review was to make a start towards rationalization of the nomenclature and to specify the form of the molecular Hamiltonian implicit in any nomenclature. In an article that Jonathan Tennyson and I published in the festschrift to celebrate his sixtieth birthday in 1987 [3], we tried to present a clear account of a molecular Hamiltonian suitable for treating the vibration rotation spectrum of a triatomic molecule. In an article that I wrote that appeared in 1990 [4], I discussed the difficulty of deciding just how far the basic chemical idea of molecular structure could really be fitted into quantum mechanics. 

That is, in order for the phenomenon to be observed, both reactants must show inherent diastereoface selectivity in their reactions with achiral partners. If one of the reactants shows no inherent diastereoface selectivity in its reactions with achiral reactants, then mutual kinetic resolution will not be observed regardless of the stereoselectivity of the other reactant. For an example, consider the case of an enzyme which mediates some reaction, say reduction of the carbonyl group. We can let the enzyme be A and assume that, because of its uniquely-evolved molecular structure, it shows very high inherent diastereoface selectivity (thus, it will reduce prochiral carbonyl compounds to chiral alcohols with very high enantiomeric excess). For B, let us take a chiral aldehyde that shows no inherent diastereoface selectivity in its... 

Certain similarities observed between adsorption and solution have led some to believe that both may be different aspects of the same fundamental phenomenon.1 The amount of a gas that is adsorbed is a function of the pressure, and this is also true of the amount of a gas that dissolves in a liquid. The way in which a solute distributes itself quantitatively between the adsorbent and the solution recalls a similar distribution that occurs when a solute is placed in a mixture of two immiscible solvents—for instance, acetic acid in water and benzene. With solutions in which the solute has the same molecular structure in both solvents, the equation for the distribution may be written ... 

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