**4. Conclusions**

This work is devoted to the influence of the substituent and solvent on the tautomeric preferences and intramolecular interactions of uracil. For this purpose, the four most stable uracil tautomers and two rotamers of the dienol form, substituted by nitro and amino groups at C5 and C6 positions in ten environments, were studied. In addition, changes in the properties of the substituents were also realized by rotating the NO2 group 90 degrees about the CN bond. The research was carried out using the DFT-D method and the polarizable continuum solvent model (PCM).

In uracil derivatives, the properties of the substituents depend primarily on their position with respect to endocyclic N atoms and less on the tautomeric form. Changing the =O to −OH group in the C2 or/and C4 position has less effect on the electronic properties of the substituent (and geometry), despite their opposite electronic properties. Therefore, the relationships between the relative position of endocyclic N atoms and the substituent on its electronic properties and geometry observed in simple monosubstituted N-heterocycles (pyridine, pyrimidine, pyrazine, etc.) [33] can be applied to more complex systems, such as uracil. Thus, the NH2 substituent at the 6 position of uracil has more than twice (in the cSAR scale) stronger electron-donating properties than at the C5 position. In contrast, the NO2 group has more electron-withdrawing power in position C5 than in position C6. The characteristic properties of both NO2 and NH2 groups are enhanced in polar solvents. The strength of the solvation effects on the substituent properties depends on

through-space *ortho* interactions. This has also been previously observed in purine and adenine derivatives [29,30].

Regarding the intramolecular interactions between non-covalently bonded atoms, both repulsive and attractive interactions, including hydrogen bonds, are observed. This is evidenced by the results of the NCI and AIM analyses and geometric parameters. Interesting hydrogen bonding interactions, NO···HO and H2N···HO (with NH2 rotated by 90◦), were found in **u6** 5-NO2 and 5-NH2 derivatives, respectively. The NO···HO interaction is strong and it highly stabilizes the **u6** tautomeric form of 5-NO2 derivative, with respect to other tautomers. The attractive interactions between the 6-NO2 group and endocyclic NH group (NO···HN) are visible on the NCI plots and in the geometry data, but do not have the bond critical point. Interaction between the NH2 group and endocyclic N atom (NH···N) is not detectable by any method.

The substitution of the uracil molecule, as well as the solvation effects, does not significantly alter its tautomeric preferences. This differs from what has been reported for purine and adenine derivatives, where substitution and solvation significantly affected the tautomeric equilibrium [17,19,29,30]. However, the observed decrease in the relative energy of **u6** and **u2** uracil tautomers due to the 5-NO2 and 6-NH2 substitution, respectively, may cause an increase in the amount of enol tautomers in the equilibrium mixture.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules27217240/s1, Table S1: Dipole moments; Tables S2–S21: Relative and solvation electronic energies and Gibbs energies of all systems; Tables S22 and S23: Statistical data on *G* and *G*solv vs. 1/ε correlations; Tables S24–S29: cSAR(X) values for nitro and amino substituents in all systems; Table S30: Geometry data in the gas phase and formamide; Table S31: Differences between CN bond length in formamide and the gas phase; Table S32: CO bond lengths of C=O/C-OH groups in 2 and 4 position of the uracil molecule; Table S33: cSAR(X) values of C=O/C-OH groups; Table S34: Ranges and average values of cSAR(X) of C=O/C-OH groups in 2 and 4 position.

**Author Contributions:** Conceptualization, H.S. and T.M.K.; methodology, H.S. and P.A.W.; validation, H.S. and P.A.W.; formal analysis, H.S. and P.A.W.; investigation, P.A.W.; data curation, P.A.W.; writing—original draft preparation, H.S and P.A.W.; writing—review and editing, T.M.K. and P.A.W.; visualization, P.A.W.; supervision, H.S.; funding acquisition, T.M.K. and H.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** H.S. and P.A.W. thank the Warsaw University of Technology for financial support. The APC was funded by MDPI.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available in the article and in the associated Supplementary Materials.

**Acknowledgments:** The authors would like to thank the Wrocław Center for Networking and Supercomputing and the Interdisciplinary Center for Mathematical and Computational Modeling (Warsaw, Poland) for providing computer time and facilities.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

