2 research outputs found
Effect of the H-Bonding on Aromaticity of Purine Tautomers
Four tautomers of purine (1-H, 3-H, 7-H, and 9-H) and
their equilibrium
H-bonded complexes with F<sup>–</sup> and HF for acidic and
basic centers, respectively, were optimized by means of the B3LYP/6-311++G(d,p)
level of theory. Purine tautomer stability increases in the following
series: 1-H < 3-H < 7-H < 9-H, consistent with increasing
aromaticity. Furthermore, the presence of a hydrogen bond with HF
does not change this order. For neutral H-bonded complexes, the strongest
and the weakest intermolecular interactions occur (−14.12 and
−10.49 kcal/mol) for less stable purine tautomers when the
proton acceptor is located in the five- and six-membered rings, respectively.
For 9-H and 7-H tautomers the order is reversed. The H-bond energy
for the imidazole complex with HF amounts to −14.03 kcal/mol;
hence, in the latter case, the fusion of imidazole to pyrimidine decreases
its basicity. The ionic H-bonds of N<sup>–</sup>···HF
type are stronger by ∼10 kcal/mol than the neutral N···HF
intermolecular interactions.
The hydrogen bond N<sup>–</sup>···HF energies
in pyrrole and imidazole are −32.28
and −30.03 kcal/mol, respectively, and are substantially stronger
than those observed in purine complexes. The aromaticity of each individual
ring and of the whole molecule for all tautomers in ionic complexes
is very similar to that observed for the anion of purine. This is
not the case for neutral complexes and purine as a reference. The
N···HF bonds
perturb much more the π-electron structure of five-membered
rings than that of the six-membered ones. The H-bonding complexes
for 7-H and 9-H tautomers are characterized by higher aromaticity
and a much lower range of HOMA variability
Hydrogen Bonding as a Modulator of Aromaticity and Electronic Structure of Selected <i>ortho</i>-Hydroxybenzaldehyde Derivatives
Properties of hydrogen bonds can induce changes in geometric or electronic structure parameters in the vicinity of the bridge. Here, we focused primarily on the influence of intramolecular H-bonding on the molecular properties in selected <i>ortho</i>-hydroxybenzaldehydes, with additional restricted insight into substituent effects. Static models were obtained in the framework of density functional theory at B3LYP/6-311+G(d,p) level. The electronic structure parameters evolution was analyzed on the basis of Atoms In Molecules (AIM) and Natural Bond Orbitals methods. The aromaticity changes related to the variable proton position and presence of substituents were studied using Harmonic Oscillator Model of Aromaticity (HOMA), Nucleus-Independent Chemical Shift (NICS) and AIM-based parameter of Matta and Hernández-Trujillo. Finally, Car–Parrinello molecular dynamics was applied to study variability of the hydrogen bridge dynamics. The interplay between effects of the substitution and variable position of the bridged proton was discussed. It was found that the hydrogen bond energies are ca. 9–10 kcal/mol, and the bridged proton exhibits some degree of penetration into the acceptor region. The covalent character of the studied hydrogen bond was most observable when the bridged proton reached the middle position between the donor and acceptor regions. The aromaticity indexes showed that the aromaticity of the central phenyl ring is strongly dependent on the bridged proton position. Correlations between these parameters were found and discussed. In the applied time-scale, the analysis of time evolution of geometric parameters showed that the resonance strengthening does not play a crucial role in the studied compounds