The calculation of NMR parameters for natural products was pioneered by Bifulco and coworkers in 2002. Since then, modelling 1H and 13C chemical shifts and spin-spin coupling constants for this...

(-)-Centratherin is a bioactive sesquiterpenoid lactone, whose absolute configuration (AC) was not established, but has been proposed based on those of germacrane precursors. To verify this proposal, the experimental electronic circular dichroism (ECD), electronic dissymmetry factor (EDF), optical rotatory dispersion (ORD), vibrational circular dichroism (VCD), and vibrational dissymmetry factor (VDF) spectra of (-)-centratherin have been analyzed with the corresponding density functional theoretical predictions. These analyses suggest the AC of naturally occurring (-)-centratherin to be (6R,7R,8S,10R,2'Z).

In this paper, we present a GIAO‐HDFT scaling factor study, in which we intent to investigate the ability of GIAO‐HDFT/cc‐pVDZ to achieve a high cost‐effectiveness ratio. A set of 22 small molecules providing 27 different 13C chemical shifts determined in the gas phase was used. Geometry optimizations were performed for a single molecule in a gas phase using B3LYP/cc‐pVDZ, B3LYP/aug‐cc‐pVDZ, B3PW91/cc‐pVDZ and B3PW91/aug‐cc‐pVDZ level of theory. NMR chemical shifts were computed at the B3LYP and B3PW91 with cc‐pVnZ (n = 2–3) and augment aug‐cc‐pVnZ (n = 2–3) levels using the GIAO method and are given relative to that of TMS calculated at the same level of theory. Despite the calculation approximations the chemical shifts calculated at the B3PW91/cc‐pVDZ//B3PW91/cc‐pVDZ using a simple relationship (δscalc = 1.02 δcalc − 0.92, where δcalc and δscalc are the calculated and the linearly scaled values of the 13C chemical shifts, respectively) were able to yield MAD and RMS errors as small as those obtained with other GIAO‐HDFT (B3LYP, B3PW91 and OPBE) with bigger basis sets, such as 6‐311 + G(2d,p) and cc‐pVTZ. The used approach also proved successful in distinguishing regioisomers, but, unfortunately, it lacks the ability to distinguish between diastereoisomers, a task otherwise proved to be very difficult to accomplish. Copyright © 2010 John Wiley & Sons, Ltd.

Comprehensive work suggesting the amendment of helianane to the open ring curcudiol by means of computational spectroscopy and thermodynamic reasons.

Nuclear magnetic resonance (NMR) spectroscopy is one of the most important tools for determining the structures of organic molecules. Despite the advances made in this technique, revisions of erroneously established structures for natural products are still commonly published in the literature. In this context, the prediction of chemical shifts through ab initio and density functional theory (DFT) calculations has become a very powerful tool for assisting with the structural determination of complex organic molecules. In this work, we present the development of a protocol for (13)C chemical shift calculations of terpenes, a class of natural products that are widely distributed among plant species and are very important due to their biological and pharmacological activities. This protocol consists of GIAO-DFT calculations of chemical shifts and the application of a parameterized scaling factor in order to ensure accurate structural determination of this class of natural products. The application of this protocol to a set of five terpenes yielded accurate calculated chemical shifts, showing that this is a very attractive tool for the calculation of complex organic structures such as terpenes.

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