Uso dos métodos GIAO, CSGT e IGAIM na elaboração de fatores de escalonamento de deslocamento de RMN de 13C para terpenos baseados em regressões lineares

Abstract

Terpenes are natural products that have several biological and pharmacological properties that are directly related to their chemical structures. In the structural determination of organic molecules, Nuclear Magnetic Resonance (NMR) is used on a large scale. The chemical shift (δ) being the most important parameter. The present study aims to develop and test δ scaling factors from 13C to terpenes, based on linear regressions. 10 complex sesquiterpene molecules were selected with the unmistakably determined structures (confirmed with X-ray crystallography). The geometries were optimized at the B3LYP / 6-311 + G (d, p) level, in the gaseous phase, and the δ will be obtained at the PBE0 / aug-cc-pvdz level with three different approaches GIAO, CSGT, and IGAIM, in phase gaseous and liquid, where the PCM model (Polarized Continuum Model) was used. The TMS (tetramethylsilane) was used as a reference and the experimental data of 13C were obtained in chloroform. The results of scaled RMS for the terpenes used to generate the scaling factors show that when the effects of the solvent are taken into account, even implicitly, there is an improvement in the reproduction of experimental data. However, the difference in scaled RMS values is not large enough to justify taking into account interactions with the solvent, at least with the PCM model. It is interesting to note that with the level of theory PBE0 / aug-cc-pvdz, the GIAO method presented a lower performance than the other 2 used. Another interesting point is that its calculation time, according to the simulations generated in this work, was, on average, 30% greater than the CSGT and IGAIM. Thus, for studies with terpenes, with this level of theory, the use of the GIAO method is not indicated.