α-methylation and α-fluorination electronic effects on the regioselectivity of carbonyl groups of uracil by H and triel bonds in the interaction of U, T and 5FU with HCl and TrH (Tr = B, Al).

Quantum chemical calculations at the ωB97XD/6-311++G(d,p) level of theory have been executed to investigate the effect of substituents via hydrogen-bonded and triel-bonded complexes between uracil (U), thymine (T) and 5-fluorouracil (5FU) with HCl for the former complexes, and with BH and AlH for the latter complexes. These calculations are supported by single-point energy calculations at MP2/6-311++G(d,p) and CCSD/6-31 + G(d,p) levels of theory, Natural Bond Orbital (NBO) and Molecular Electrostatic Potentials (MEPs) analyses, and global/local reactivity descriptors. The results reveal that triel-bonded complexes are strongly bounded than hydrogen-bonded ones, and Al-containing dimers stronger than B-containing ones. In addition, as the central triel atom grows in size, B-containing dimers (B-O triel bond) are accompanied by weak B-H⋯O unconventional H-bonds. According to local reactivity descriptors, the B-O triel bond is hard-hard interaction that indicates that the association is primarily charge controlled, while the Al-O triel bond is soft-soft interaction that is primarily orbital controlled. In both Hydrogen as well as triel-bonded complexes, the α-methylation slightly overestimates the binding strength of U, while the α-fluorination exerts the opposite role by underestimating the binding strength of U. In overall, the effect of substituents on the bond strength and thus on the regioselectivity is very small, suggesting a competition between the two carbonyl groups in terms of structures and binding energies.