Magnetic Resonance in Chemistry

doi: 10.1002/mrc.5177pmid: N/A

No abstract is available for this article.

Rusakova, Irina L.; Rusakov, Yuriy Yu.; Krivdin, Leonid B.

doi: 10.1002/mrc.5296pmid: 35737297

Theoretical background and fundamental results dealing with the computation of mercury chemical shifts and spin–spin coupling constants are reviewed with a special emphasis on their stereochemical behavior and applications.

Kulikovskaya, Natalia S.; Denisova, Ekaterina A.; Ananikov, Valentine P.

doi: 10.1002/mrc.5295pmid: 35727217

Investigation of catalytic reactions using nuclear magnetic resonance (NMR) is a crucial task, which is often challenging to perform due to rather complex transformations at the metal center. In this work, it was shown that electrophoretic NMR can be a suitable method for studying catalytic reactions and for observing the changes in the catalyst nature. As an important example involving palladium catalysts with N‐heterocyclic carbine ligands (NHCs), the breakage of the Pd‐NHC bond can occur during the catalytic process. Electrophoretic NMR allows the distinction of compounds in the spectra depending on the charge, thus bringing new opportunities to mechanistic studies. Here, we present independent evidence of R‐NHC product formation in the Pd‐catalyzed Mizoroki–Heck reaction—the key process for catalyst change from the molecular to nano‐scale type.

Cohen, Ryan D.; Wang, Xiao; Sherer, Edward C.; Martin, Gary E.

doi: 10.1002/mrc.5297pmid: 35781893

Prior to the development of sensitive proton‐detected 2D NMR experiments, assigning 13C signals could be a significant challenge, and mistakes have occurred even for prominent compound classes. In this study, 1,1‐ADEQUATE data were used to unambiguously reassign the 13C chemical shifts for the β‐lactam carbonyl at the C‐7 position and the proximal carboxylate at the C‐10 position of the carbapenems, meropenem and imipenem. Density functional theory (DFT) was then investigated to provide sufficiently accurate 13C chemical shift predictions, allowing for the carbonyl signal reassignment of thienamycin.

Ahmed, Raheel; Siskos, Michael G.; Siddiqui, Hina; Gerothanassis, Ioannis P.

doi: 10.1002/mrc.5298pmid: 35830967

Density functional theory (DFT) calculations of δ(13C) and δ(1H) chemical shifts and 3J(13COO1H) coupling constants of three model hydroperoxides of the naturally occurring cis‐11‐OOH and trans‐9‐OOH isomers of oleate and 9‐cis, 11‐trans‐16‐OOH endo hydroperoxide of methyl linolenate are reported. The computational δ(OOH) for various functionals and basis sets were found to be nearly identical for the cis/trans geometric isomers. The chemical shifts of the methine CHOOH protons and carbons, on the contrary, are highly diagnostic for the identification of cis/trans geometric isomerism. The chemical shifts of the olefinic protons and carbons strongly depend on the orientation of the hydroperoxide unit relative to the double bond and, thus, of importance in conformational analysis. The results are in very good agreement with the available experimental data. For the various diastereomeric pairs of the model endo‐hydroperoxide, the strongly deshielded OOH resonances, due to the presence of an intramolecular hydrogen bond between the hydroperoxide proton and an oxygen of the endo‐peroxide ring, along with the δ(CHOOH), are highly diagnostic for identification and structure elucidation of complex erythro‐ and threo‐ diastereomeric pairs of endo‐hydroperoxides; the computational results are in very good agreement with the available experimental data. The 3J(13COO1H) coupling constants were found to be <  2 Hz for the cis–trans geometric models and <  0.5 Hz for the endo‐hydroperoxide and, thus, unimportant in stereochemical analysis. Sharp resonances of the hydroperoxide protons, with Δν1/2 < 3 Hz, are required for the successful implementation of the 1H13C heteronuclear multiple bond correlation (HMBC) technique.

Karpov, Valerii V.; Antonov, Alexander S.; Tupikina, Elena Yu.

doi: 10.1002/mrc.5299pmid: 35881390

In this work, we tested various computational schemes for calculations of 1JCLi constants with a high accuracy. On the example of six organolithium reagents (phenyllithium monomer and dimer, monomer s‐butyllithium, monomers of 1‐ and 2‐lithionaphthalenes, and a methyllithium tetramer), the following aspects are discussed: (i) the role of a model system geometry, (ii) influence of solvent effects, and (iii) the choice of a functional and basis set. Practical recommendations for calculation of 1JCLi with an accuracy ±2 Hz are formulated.

Morgan, Vinícius G.; Sad, Cristina M. S.; Leite, Juliete S. D.; Castro, Eustáquio R. V.; Barbosa, Lúcio L.

doi: 10.1002/mrc.5301pmid: 35899432

During the oil production and processing stages, the asphaltene precipitation is one of the great operation problems of oil industry. It can precipitate in the formation, tubing, or surface, causing operating problems, such as reduction in oil recovery by changing the reservoir permeability and wettability, clogging of the pipelines, and difficulty in separations process. The quantification of asphaltenes in petroleum by ASTM D6560 standard method is very laborious and use of a larger solvent volume than necessary. The present work proposes the use of time domain nuclear magnetic resonance (TD‐NMR) as new methodology to quantify the asphaltene precipitated in crude oil. Three (light, medium, and heavy) crude oils with asphaltenes content of 0.97, 1.88, and 7.00 wt% were mixed with n‐heptane in different R (ml of solvent/g of oil) values and analyzed by means of transverse relaxation time (T2). According NMR results, the R values enough for complete asphaltene precipitation for the oils A, B, and C were, respectively, equal to 16.50, 23.00, and 39.50 ml g−1. These outcomes represent a reduction of 58.75%, 42.50%, and 1.25% in the solvent volume per mass of oil for the oil A, B, and C, respectively, compared to the ASTM D6560 method, which imposes 40 ml g−1. Therefore, it has been shown that TD‐NMR can be applied to estimate the amount of asphaltene precipitated in petroleum and have potential to be applied in routine analysis with advantages of saving time and costs.

Umegawa, Yuichi; Shimonishi, Takeshi; Tsuchikawa, Hiroshi; Murata, Michio

doi: 10.1002/mrc.5303pmid: 35938541

2H solid‐state nuclear magnetic resonance (NMR) is a method for examining the mobility and orientation of molecules in the field of biophysics. In studies on lipid bilayer membranes, 2H NMR is often adopted to detect a phase transition from the gel to the liquid‐crystal phase, which is observed as a change in spectral shape, and to evaluate the ordering of lipid alkyl chains using quadrupole coupling values. Because the mobility of membrane lipids is highly temperature dependent, precise temperature control is a prerequisite for evaluating the physical properties of membranes. Generally, NMR instruments monitor the temperature of the variable temperature (VT) gas. The temperature inside the sample tube and the VT gas match only when the heat generated by the radio frequency (rf) pulse emitted from the coil or magic angle spinning is significantly lower than the cooling capacity of the VT gas. In other words, the sample temperature inside the tube depends on the measurement method. Therefore, in this study, we took advantage of temperature‐dependent changes in the chemical shift of a paramagnetic metal–ligand complex. We designed and synthesized a deuterated ligand complex and evaluated its temperature dependence as a thermometer for 2H solid‐state NMR spectroscopy. We chose Tb, Dy, Ho, and Er as the paramagnetic central metals. We then measured the 2H NMR spectrum of each metal complex and confirmed the 2H chemical shift to be temperature dependent. Furthermore, with the use of the thermometer molecule with Er, we succeeded in accurately evaluating the segmental melting of an alkyl chain in lipid bilayers with 0.1°C accuracy.