Charge transfer structural variation

More recently we have turned our attention to the influence of dynamic variations in stacking on base-base CT. Here again Ap is particularly useful given its demonstrated charge transfer chemistry in DNA, and its ability to report on the structure and dynamics of the DNA environment. Using spe-... 

The extent to which steric effects adversely affect the attainment of such intimate ion-pair structures would be reflected in an increase in the work term and concomitant diminution of the inner-sphere rate. This qualitative conclusion accords with the reactivity trend in Figure 16. However, Marcus theory does not provide a quantitative basis for evaluating the variation in the work term of such ion pairs. To obtain the latter we now turn to the Mulliken theory of charge transfer in which the energetics of ion-pair formation evolve directly, and provide quantitative informa-... 

Further evidence of the importance of intermolecular donor-acceptor interactions can be obtained by deleting these interactions from the variational calculation, and recalculating the optimized geometries with charge transfer (CT) omitted. The structures resulting from such CT-deleted species are shown in Fig. 5.2. Energetic and structural properties of CT-deleted species are summarized in Table 5.4 for direct comparison with the actual H-bonded species in Table 5.1. 

The results show only a modest variation when the van der Waals radii are changed within reasonable bounds (Table 6.2). As the data were not refined with the aspherical atom formalism, the scale of the observed structure factors may be biased, an effect estimated on the basis of other studies (Stevens and Coppens 1975) to correspond to a maximal lowering of the scale by 2%. Values corrected for this effect are listed in the last two columns of Table 6.2. Since neutral TTF and TCNQ have, respectively, 72 and 52 valence electrons, the results imply a charge transfer close to 0.60 e. 

Thermal fluctuations are known to affect considerably the structure and other properties of biomolecules [30]. Recently, it was recognized that conformational changes in DNA can produce significant variations in the t-stacking of base pairs and thereby modulate the efficiency of charge transfer [31-33]. Thus, one has to employ a combination of molecular dynamics... 

If the substituent is situated on the central carbon the model fails to make a clear-cut prediction. This is because the effect of substitution on the gap and slope factor /are opposed. For example, an electron-withdrawing substituent (e.g. CH2C12 compared to CH3C1) is expected to decrease the gap and increase the slope factor, so that without more extensive informaton on the effect of the substituent on the above parameters, no definitive prediction is possible. A second limitation of the model is that it does not readily lend itself to treating mechanistic variations. Since its more detailed structure favours its application with just two state curves, in this case ground and charge transfer states, it is less readily extended to situations in which mechanistic variations are explicitly considered (e.g. the SN2-SN1 spectrum). 

---