Comparison of experimental and DFT-calculated NMR chemical shifts of 2-amino and 2-hydroxyl substituted phenyl benzimidazoles, benzoxazoles and benzothiazoles in four solvents using the IEF-PCM solvation model

DFT calculations of 1H- and 13C-NMR chemical shifts of 3-methyl-1-phenyl-4-(phenyldiazenyl)-1H-pyrazol-5-amine in solution

Geometries of the 3-methyl-1-phenyl-4-(phenyldiazenyl)-1H-pyrazol-5-amine azo-dye compound and its tautomer were optimized using B3LYP and M06-2X functionals in coupling with TZVP and 6-311 + G(d,p) basis sets. The ¹H- and ¹³C-NMR chemical shifts of all species were predicted using 13 density functional theory (DFT) approaches in coupling with TZVP and 6-311 + G(d,p) basis sets at the different optimized geometries by applying the using GIAO method using the eight geometries. The selected functionals are characterized by having different amount of Hartree-Fock exchange. The selected DFT methods were B3LYP, M06-2X, BP86, B97XD, TPSSTPSS, PBE1PBE, CAM-B3LYP, wB97XD, LSDA, HSEH1PBE, PW91PW91, LC-WPBE, and B3PW91. The results obtained were compared with the available experimental data using different statistical descriptors such as root mean square error (RMSE) and maximum absolute error (MAE). Results revealed that the prediction of the ¹H-NMR chemical shifts has more significant dependence on the applied geometry than that of the prediction of the ¹³C-NMR chemical shifts. Among all the examined functionals, B97D and TPSSTPSS functionals were found to be the most accurate ones, while the M06-2X functional is the least accurate one. Results also revealed that the prediction of NMR chemical shifts using TZVP basis sets results is more accurate results than 6-311 + G(2d,p) basis set.

Decumbic anhydride from the stem barks of Swintonia floribunda (Anacardiaceae)

From an EtOAc-soluble fraction of the stem barks of Swintonia floribunda (Anacardiaceae), decumbic anhydride (1) and four known compounds 2-5 were isolated. Their chemical structures were elucidated based on the spectroscopic data interpretation. The GIAO-DFT calculation of ¹³C NMR chemical shifts was carried out to clarify the structure of 1. The absolute configuration of 1 was assigned based on the Cotton effects in its ECD spectrum. Compound 1 showed potent tyrosinase inhibitory activity with an IC₅₀ value of 52.2 μM.

NMR and DFT investigations of structure of colchicine in various solvents including density functional theory calculations

A detailed NMR investigation of the chemical shifts of hydrogen and carbon atoms associated with the structure of the naturally occurring alkaloid colchicine was conducted using high field NMR. Initially, the experimental chemical shifts for colchicine in chloroform and DMSO were compared to the values calculated using density functional theory (DFT). There were significant deviations observed for the chloroform solvent, but these were only slight in the DMSO solution. Dilution of the chloroform solution changed the experimental chemical shifts and improved agreement with the DFT calculations, suggesting self-aggregation at higher concentrations. A dimeric model was proposed for which agreement with the DFT calculated chemical shifts was better than for corresponding monomeric structures. Three further solvents were studied to evaluate changes in chemical shift values at different dilutions. Chloroform, benzene and water showed significant chemical shift changes implying self-aggregation, whereas DMSO and acetone did not show significant change upon dilution.

Computational (DFT and MP2) and spectral interpretations, normal coordinate analysis, force constants and barriers to internal rotations of Trimethylacetonitrile

The Raman (3500[Formula: see text]cm[Formula: see text]100[Formula: see text]cm[Formula: see text] and IR spectra (4000[Formula: see text]cm[Formula: see text]400[Formula: see text]cm[Formula: see text] of liquid trimethylacetonitrile (C5H9N, TMA) have been obtained. In addition, the 1H and [Formula: see text]C NMR spectra of TMA were obtained in DMSO-[Formula: see text] and CDCl3. The staggered conformer (C[Formula: see text] was favored using MP2 and DFT(B3LYP/[Formula: see text]B97XD) quantum mechanical calculations utilizing a 6-311[Formula: see text]G(d,p) basis set. High energy difference estimates of 4534[Formula: see text]cm[Formula: see text]5338[Formula: see text]cm[Formula: see text] (12.96[Formula: see text]kcal/mol[Formula: see text]15.26[Formula: see text]kcal/mol) were predicted, along with three imaginary torsion frequencies for the eclipsed conformer, therefore considered a transition state. The 1H and [Formula: see text]C NMR chemical shifts were predicted with B3LYP and [Formula: see text]B97XD methods using the GIAO approximation and 6-311[Formula: see text]G(d,p) basis set. B3LYP frequencies calculation is favored herein owing to the relatively good compilation with the experimental measurements. The computed structural parameters are well correlated to those reported from electron diffraction and microwave studies. Moreover, the [Formula: see text]C[Formula: see text]H coupling constant was estimated and found consistent with that observed for the sample dissolved in DMSO-[Formula: see text]/CDCl3 solvents. Using the observed methyl torsion at 266[Formula: see text]cm[Formula: see text] in gas phase and the kinetic parameter F number, a potential function (V[Formula: see text] of [Formula: see text][Formula: see text]cm[Formula: see text]([Formula: see text][Formula: see text]kcal/mol) was obtained, this barrier to internal rotation is well correlated to 1527[Formula: see text]cm[Formula: see text] (4.37[Formula: see text]kcal/mol) predicted from MP2/6-311[Formula: see text]G(d,p) potential surface scan. Aided by the predicted wavenumbers and their IR intensity/Raman activity, the observed IR/Raman bands were intensively discussed and therefore assigned to their corresponding fundamentals, in agreement with novel normal coordinate analysis and potential energy distributions (PEDs).