Intermolecular dynamics conformer stabilization

STABILIZATION OF DIFFERENT CONFORMERS OF WEAKLY BOUND COMPLEXES TO ACCESS VARYING EXCITED-STATE INTERMOLECULAR DYNAMICS... 

Stabilization of Different Conformers of Weakly Bound Complexes to Access Varying Excited-State Intermolecular Dynamics 375 By David S. Boucher and Richard A. Loomis... 

Discrepancies between different researchers derive from the character inter-or intramolecular of the interactions presumably controlling the reactive conformation. Thus, in most of the cases, the population of the different rotamers in the sulfinylated substrate (only governed by intramolecular interactions) is the only factor considered for explaining the observed 7r-facial selectivity. This explanation (static conformational polarization) was formulated by Koizumi and used by many authors to justify the behavior of vinyl sulfoxides acting as dienophiles and dipolarophiles. A second explanation assumes that the interactions of the two reagents in the transition states determine a different reactivity of the rotamers around the C-S bond. This intermolecular factor can become the most important one in the control of the 7r-facial selectivity of the cycloadditions, and therefore the tendency expected from conformational stability criteria was not observed in those cases where the most reactive conformation is not the most populated one. This dynamic conformational polarization has been used just to explain some of the results obtained for sulfinyl quinones and sulfinyl dienes (unexplainable with the above model) but it can be applied to many other cases. 

To end this overview, let me say a few words about the likely evolution of the calculations devoted to theoretically analyze the stability and reactivity of cyclodextrin complexes. Hybrid methods have recently opened the door for the most sophisticated ab initio and DFT methods to deal with molecular systems with hundreds or even thousands of atoms if the reactive center is well localized within a particular zone of the whole macromolecular system. As computers keep on growing in capacity and theoretical methods are progressively more well established, theoretical work on host guest complexes will be ready to make a jump from the qualitative predictions, already available now, to the quantitative results. This will provide a powerful tool for the experiments as theory will be helpful not just to explain reactions already known but to predict new reactions and effects of the supramolecular environment that have been until now out of reach of precise measurements (as intermolecular forces are quite tenuous, their theoretical evaluation is subject to noticeable relative errors). On the other hand, it is to be noted that electronic calculations alone are not able to disclose the rich molecular dynamics of such a large molecular systems. Up to now MC and MD procedures have been restricted to find the more stable conformations but they are able to tell us more details about the mechanism of the inclusion process and reactivity inside CD and other large guests. The applicability of these methods to such problems has also been... 

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