*3.4. Tautomer Stability*

The last section is devoted to the effects of substitution and solvation on the stability of uracil tautomers. Table 5 presents electronic energies of each system relative to the **u1** tautomer. In all cases, this tautomer remains the most stable, irrespective of substitution and solvation. Considering the 5-NO2 substitution, the **u6** 5-NO2 derivative is a particularly interesting case. Formation of a strong NO···HO H-bond results in a large stabilization relative to the unsubstituted **u6** tautomer (by 12.5 kcal/mol). Consequently, among the 5-NO2 tautomers, **u6** becomes the second most stable tautomer after **u1**, despite the fact that **u6** is the least stable tautomer for unsubstituted uracil. Rotating the 5-NO2 group

by 90 degrees and breaking the hydrogen bond increases the relative energy of **u6** by 10.4 kcal/mol and in 5-NO2 (90◦), **u6** is again the least stable tautomer.

**Table 5.** Energies (in kcal/mol) relative to the **u1** tautomer. Δ indicates a difference in relative energies between the aqueous phase and the gas phase, Δ = *E*rel(aq) − *E*rel(gas).


In the case of 5-NH2 substitution, the energy difference between the **u1** and **u2** tautomers decreases compared to the unsubstituted systems, while between **u1** and others it increases. In 6-NH2, the relative energies are smaller than for unsubstituted systems. A noteworthy increase in stability relative to **u1** is observed for **u2**, **u4**, **u5** and **u6** tautomers (between 5 and 6 kcal/mol), while much less for **u3** (1.1 kcal/mol). In the case 6-NO2 tautomers, apart from **u3**, the relative energies decrease slightly, but not as much as in 6-NH2. In all cases, the relative energies between the **u1** tautomer and the second most stable one are above 5.4 kcal/mol; therefore, it is unlikely that substitution with NH2 or NO2 groups can significantly affect the tautomeric equilibrium. Solvation. in most cases, further increases the difference between **u1** and the other forms, as evidenced by the positive values of Δ (apart of two cases) in Table 5. The only cases where Δ is negative are the two NH2 derivatives of the **u3** tautomer: **u3** 5-NH2 (Δ = −2.2 kcal/mol) and **u3** 6-NH2 (Δ = −0.6 kcal/mol).

Similarly to the cSAR (X), electronic energy can be plotted against 1/ε and relations approximated with straight lines can be obtained (Table 6). In this case, the slopes (*a*) inform about the sensitivity of the energy of a given system to the solvent effect. In most cases, the **u1** and **u3** tautomers are the most sensitive, these two tautomers have an endo NH group in the 1 position of the uracil ring. The only exception is the 6-NO2 substitution, where the **u2** and **u6** tautomers are most sensitive to the solvent effect. The **u4** and **u5** tautomers are in all but one case (H-bond forming **u6** 5-NO2) the least sensitive. In amino derivatives, the sensitivity to the solvent effect seems to be correlated with the dipole moments of the molecules, i.e., a large dipole moment is associated with a large value of *a*. However, no such relation can be observed in the case of nitro derivatives.

**Table 6.** Slopes, *<sup>a</sup>*, of *<sup>E</sup>*rel <sup>=</sup> *<sup>a</sup>* · (1/ε) + *<sup>b</sup>* linear regressions (in all cases *<sup>R</sup>*<sup>2</sup> > 0.97) and molecular dipole moments in the gas phase, *μ* (*E*rel in kcal/mol, *μ* in Debye).


Plotting the relative energy, *E*rel, against the cSAR(X) for all systems in all solvents (Figure 10) reveals linearly correlated groups of points for each tautomer. The linearity comes from the fact that both *E*rel and cSAR change linearly with 1/ε (see Tables 3 and 6). The ranges on the y and x axes for particular tautomers are a visual representation of the

strength of the solvent effect on *E*rel and cSAR, respectively. It is clearly visible that, in general, the greatest changes in both parameters occur for the 5-NO2 and 6-NH2 derivatives.

**Figure 10.** Plots of relative energy of tautomer against the cSAR(X) (in *e*) for (**a**) 5-NO2, (**b**) 5-NH2, (**c**) 6-NO2, (**d**) 6-NH2 systems in all considered solvents.
