THAÍS FOREST GIACOMELLO
Abstract:
Chalcone is a class of natural products that has a lot of interest, mainly pharmaceutical because of its biological actions. They are several classes and in addition they are non rigid and complex molecules making their structural characterizations become difficult task in experimental techniques. Spectroscopic techniques, in the last years, have developed very fast, greatly aiding the elucidation of natural products. However, several cases of review of natural product structures have been found in the literature due to erroneous elucidations in analytical techniques of experimental routines. Thus, it is extremely important to develop protocols that can assist in determining the correct structures of these molecules. In this work aimed to develop a parameterized protocol for NMR 13C chemical shift calculations with the purpose of assisting in the correct determination of polyphenol type molecules. Thus, a group of polyphenols, specifically a subclass of these, chalcones, having varied substituents and experimental structural elucidation were selected in the literature. This base of chalcones was submitted to randomized conformational searches using Monte Carlo method and MMFF force field. In addition, the configurators with energy of up to 3 kcal/mol of each chalcone were calculations optimization of geometry and frequency. The chemical shift of 13C was calculated after assuming Boltzmann statics. All of these calculations were performed using the mPW1PW91 / 6-31G (d) level. After that, the scaled chemical shifts (δesc) was defined. This was obtained using the expression δ𝑒𝑠𝑐 = 𝑎 𝑥 δ𝑐𝑎𝑙𝑐 + 𝑏, where a and b are the coefficients of linear regressions obtained between calculated (δcalc) versus experimental chemical shift. In order to validate the protocol, the scaling factor were used to obtain δesc values for chalcones different from those used in the base. The result shows that the level of theory applied reproduced excellently the experimental data. Calculations performed with a scaling factor lead to a better result than when there is no use of this factor. In addition, the applicability of the scaling factor allows the cancellation of systematic errors, which make δesc are closer to the experimental ones. Thus, the parameterized protocol was shown to be an important tool for the structural elucidation of polyphenols through theorotical calculations of ¹³C NMR chemical shifts.
