Molecular dynamics nucleophilic addition

Additional experimental, theoretical, and computational work is needed to acquire a complete understanding of the microscopic dynamics of gas-phase SN2 nucleophilic substitution reactions. Experimental measurements of the SN2 reaction rate versus excitation of specific vibrational modes of RY (equation 1) are needed, as are experimental studies of the dissociation and isomerization rates of the X--RY complex versus specific excitations of the complex s intermolecular and intramolecular modes. Experimental studies that probe the molecular dynamics of the [X-. r - Y]- central barrier region would also be extremely useful. 

Molecular dynamics simulations have been used to predict solvent and temperature effects in the nucleophilic addition of a-chiral carbonyl compounds.253 Prediction of diastereoselectivity break temperatures (i.e. inversion points) has been achieved with fair accuracy by comparison with experimental data on n-BuLi addition. Dramatic differences are seen for additions to 2-phenylpropanol in pentane solvent, compared with octane. 

Before going into details, let us make a brief statement that propagation in new controlled/living carbocationic systems has nearly the same mechanism as in the conventional systems discussed in Chapter 3, which consists of the electrophilic addition of carbenium ions to alkenes. The main difference is that carbenium ions are in dynamic equilibria with dormant species (covalent esters and onium ions). The correct choice of structures and concentrations of activators and nucleophilic additives as well as those of initiator allows for the preparation of polymers with predetermined molecular weights, low polydispersities, and controlled end functionality, including block copolymers (see Chapter 5). 

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