ABSTRACT This work presents examples of the effect of the nitroxide ring structure, the bridge composition, and the solvent nature on EPR spectra of short biradicals with similar structures, on the value of the electron spin exchange integral | J |. The attenuation coefficients γ NH ≈ 4.1 and γ O ≈ 3.3 which characterize the decrease of the spin density on −NH− and −O− groups were determined for the first time. They do not depend on other groups forming the linker. The biradical O=C (NHR 6 ) 2 was found to be structurally rigid in all polar and nonpolar solvents studied, over the temperature range of 200–410 K. The suggested approach allows the prediction of the exchange integral value, | J/a | or | J |, without electronic structure computation based on the bridge composition only.
ABSTRACT This review summarizes how DQCC values in deuterated TMHC can be experimentally determined in solutions and the solid state applying 2 H T 1 relaxation measurements and a line shape analysis of 2 H NMR spectra. Advantages and limitations of these methods are discussed including errors in DQCC calculations. The considered concepts in interpretations of quadrupolar interactions demonstrate that DQCCs in TMHC characterize and distinguish different metal–hydrogen bonding modes providing determinations of metal–deuterium bond lengths and the metal–deuterium bond ionicity. The aim of this review is also to show perspectives in further studies of TMHC including more complex molecular hydride systems such as metal nanoparticles and immobilized metal hydrides interacting with D 2 .
ABSTRACT The identification of bioactive compounds in Ugni molinae Turcz. extracts has gained interest due to the well‐known biological properties of the plant. Previous research has identified two main types of bioactive compounds: polyphenols and pentacyclic triterpenes. In this study, an ethyl acetate extract from the leaves of the 14‐4 genotype of U. molinae was separated by column chromatography (CC), and one fraction exhibited an antioxidant capacity 11 times higher than the crude extract, evaluated through DPPH assay. The presence of the pentacyclic triterpene asiatic acid (AA) was identified through NMR experiments. The AA concentration, estimated by standard addition NMR experiments was 2.18 mg/mL, which corresponds to 14.5% of the active fraction (AF). Two polyphenolic compounds were confirmed by HPLC‐MS and NMR experiments: gallic acid and a stable conformer of quercetin. The structure was elucidated by coordinated analysis of HPLC‐MS, along with several NMR experiments. These included 1 H NMR, 13 C NMR, HSQC, HMBC, SELCOSY, SELTOCSY, variable temperature, and standard addition experiments. Variable‐temperature NMR experiments provide evidence of no isomer exchange over a broad temperature range. Consequently, the isomer previously described as a conformer was reclassified as E conformer. These findings expand current knowledge of the chemical composition of U. molinae extracts and polyphenols.
ABSTRACT Chemical investigation of the aerial parts of Salvia polystachya Ortega afforded a new neo ‐clerodane‐type diterpenoid named polystachyne I ( 1 ) and the previously known diterpenoid microphyllandiolide ( 2 ). Their structures were elucidated by 1D and 2D NMR experiments. The absolute configuration of 1 was established by single‐crystal X‐ray diffraction. Compound 1 was tested for cytotoxicity against three human cancer cell lines, showing moderate activity.
19F NMR spectra were recorded in gaseous CFCl3/Xe mixtures on the NMR spectrometer, operated with a standard 2H(D) lock system. These results and shielding values obtained from the best relativistic quantum chemical calculations for single isolated CFCl3 were used to establish the 19F nuclear magnetic moment: mu(19F) = 2.6283258(27) mu N and a new absolute shielding scale based on liquid trichlorofluoromethane. The fluorine gas-to-liquid shift for trichlorofluoromethane was measured as Delta sigma gl = -5.28(1) ppm. Next, the 1H, 19F, and 3He NMR spectra were measured in CH3F/3He and CF4/3He gaseous mixtures as density functions. Extrapolating linear relationships to the zero-pressure limit yields frequency parameters for isolated CH3F, CF4, and helium atoms. These results have verified shielding constants for both fluorinated simple molecules. New exact results for the absolute 19F shielding of 16 chemical compounds measured in the gas phase were reported. Based on the new 19F shielding constant of the reference liquid CFCl3, a large set of previously measured shielding constants for fluorine compounds in the gas phase was obtained.
