Organic experimental remarks

In summary, the combination of experimental results and theoretical advances described in this work suggests that chalcogen dications, which have been considered for a long time as hypothetical or ephemeral intermediates can be used in a variety of chemical transformations. Unusual structures of these compounds impart them with reactivities that are remarkably different from other organic intermediates. Harnessing this reactivity should lead to interesting practical applications and we foresee rapid development of this field in the future. 

The level of accuracy that can be achieved by these different methods may be viewed as somewhat remarkable, given the approximations that are involved. For relatively small organic molecules, for instance, the calculated AGsoivation is now usually within less than 1 kcal/mole of the experimental value, often considerably less. Appropriate parametrization is of key importance. Applications to biological systems pose greater problems, due to the size and complexity of the molecules,66 156 159 161 and require the use of semiempirical rather than ab initio quantum-mechanical methods. In terms of computational expense, continuum models have the advantage over discrete molecular ones, but the latter are better able to describe solvent structure and handle first-solvation-shell effects. 

Mutual conjugation of, as we describe it nowadays, + M and — M substituents is equivalent to an extra stabilization of the system. Thus, we can interpret this statement as the first formulation of the captodative effect even though the term was coined much later. The difficulty of organic chemists to comprehend the rather mathematically formulated theorem must have hindered its wider recognition and seems to be the reason that the phenomenon of interaction of -l-M and — M substituents has been reinvented several times since then. It is remarkable that these rediscoveries were always initiated by experimental studies in free radical chemistry. 

Before describing dye sensitized electron transfer reactions at the surface of organic crystals a few remarks shall be made concerning experimental peculiarities of the system. 

There are many organic compounds stabilized by resonance. As a first example we may cite benzene, in which there is resonance between the two Kekule structures. In this case, the resonance energy could be calculated to give good agreement with the experimental data on the heat of formation of the molecule. Perhaps even more remarkable is the compound KC5H5, which is formed when potassium reacts with ryr/opentadiene. 

The self-organization of both thermotropic and lyotropic liquid crystals make these ordered fluids remarkable media for the dispersion and organization (alignment) of CNTs. This subject has been the focus of a recent excellent review by Scalia [231], theoretical work on anchoring at the liquid crystal/CNT interface by Popa-Nita and Kralj [458], and a number of earlier experimental reports on liquid crystal/CNT composites demonstrating that liquid crystal orientational order can be transferred to dispersed CNTs, which is commonly illustrated using polarized Raman spectroscopy [459 -62]. 

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