Dielectric parameter, linear variation

Figure 7. Linear variation of dielectric parameter, K, with n at low water contents (24).

In the first DNMR studies of push-pull ethylenes, a strong effect of solvent polarity on the C=C barriers was noted. Thus Kende et al. (64) found AG = 18.0 kcal/mol for 46a in N,/V-dimethylformamide (dielectric constant e = 38) and 19.4 kcal/mol in Ph20 ( = 4). Similar observations have been made by many other workers, and they have been seen as a strong support for a zwitterionic transition state. Kessler et al. (140) observed reasonably linear correlations between AG for two ketene aminals and the solvent polarity parameter T (141) with variations in AG of ca. 2.5 kcal/mol over T values between 25 and 46. Similarly, Shvo et al. (78) found linear correlations between log km and the polarity parameter Z (141) for three compounds from Table 12. 

No experimental results are available for the nucleic acids, with or without methyl substitution, to test the theories, but we can compare the results for thymine to three theoretical estimates based on the linearized Poisson-Boltzmann equation. The AM1-SM2 and PM3-SM3 values are —16.5 and -20.1 kcal/mol, respectively. Using charges and force field parameters from the AMBER,347 CHARMM, and OPLS molecular mechanics force fields and a solute dielectric constant of 1, Mohan et al.i calculated solvation energies of -19.1, -10.4, and -8.4 kcal/mol. The wide variation is disconcerting. In light of such wide variations with off-the-shelf parameters, the SMx approach based on parameters specifically adjusted to solvation energies appears to be more reliable. 

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