AGNES JALOWITZKI SILVA

Abstract:

Chalcones exhibit a wide variety of beneficial biological activities. In addition, these compounds include the prevention of diseases related to oxidative stress. The structural characterization of these molecules by means of analytical techniques can become a difficult task due to the complexity of some structures. However, cases of erroneously established natural product structure review are still found in the literature despite recent advances in spectroscopic techniques. Therefore, it is necessary to develop quantum calculation protocols that can aid in the correct structural determination of these compounds. Thus, in this work, we tried to develop a parameterized protocol for calculations of chemical displacements of 13C NMR, in order to ensure a correct structural determination of polyphenols, with a focus on chalcones. For this, a series of molecules belonging to this class, with complex and varied structural skeletons, reliably elucidated in the literature, was selected and subjected to stochastic conformational searches using the Monte Carlo method and the MMFF field of forces. The lower energy conformations of each molecule were selected for the geometry optimization step, performed at the mPW1PW91/6-31G(d) level. The chemical displacements of 13C were Calculated at the same level of theory, taking into account the population distribution of Boltzmann. The calculations were performed in both liquid phase, using the Polarizable Continuous Model (PCM) as an implicit solvation model. The stepwise chemical displacements (δesc) were obtained using the expression δ𝑒𝑠𝑐 = 𝑎.δ𝑐𝑎𝑙𝑐 + 𝑏, where 𝑎 and 𝑏 are the coefficients of linear regressions obtained between the calculated (δcalc) and experimental chemical shifts. The results show that the level of theory applied in the liquid phase allows a good reproduction of the experimental data. The application of the scaling factor allows the cancellation of systematic errors, which means that the values of δesc are closer to the experimental ones. Thus, the parameterized protocol proved to be an important tool for the structural elucidation of polyphenols by calculations of 13C NMR chemical shifts.

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