Theoretical calculations of carbon-hydrogen spin-spin coupling constants

Recent advances in liquid-phase NMR of the coal-derived products

Present review focuses on the most recent advances in a liquid-phase nuclear magnetic resonance (NMR) of the coal-derived products-coal tar pitches, asphaltenes, and humic and fulvic acids, covering exclusively the results in the liquid-phase NMR studies leaving apart an overwhelming amount of publications dealing with the solid-state NMR investigations in this field (which are comprehensively reviewed elsewhere). Owing to the complexity of the coal-derived products, their 1H and 13C NMR spectra consist of a number of overlapping signals belonging to different hydrocarbon types. Comprehensive studies of coal tar pitches, asphaltenes, and humic and fulvic acids by means of NMR over the past several decades revealed characteristic functional groups of those fractions together with spectral regions in which they resonate. Quantitative 1H and 13C NMR spectra characterize aromatic and saturated carbons spread over many structural moieties, which provides a solid guideline into molecular structure of the coal-derived products. Nowadays, quantitative 13C NMR measurements yield information about a variety of structural parameters such as functional group distribution, aromaticity, degree of condensation of aromatic rings, and medium chain lengths together with many other more specific parameters. The structural NMR studies of coal and coal-derived products are developing on a backdrop of a marked progress in computational NMR. At present, we are witnessing an unprecedentedly fast development of theoretical and computational methods in the field of NMR spectroscopy. Discussed in the present review are the most recent advances in the NMR studies of the processing products of peat, lignite or brown coal, anthracite or hard coal, and graphite in solution, like coal tar pitches, asphaltenes, and humic and fulvic acids.

Conformational Sampling in Computational Studies of Natural Products: Why Is It Important?

Conformational sampling is a vital component of a reliable computational chemistry investigation. With the aim of illustrating the importance of conformational sampling, and building awareness among new practitioners, we present a series of case studies that show how the quality and reliability of computational studies depend on undertaking a thorough conformer search. The examples are drawn from the most common types of research questions in natural products chemistry, but the fundamental principles apply more generally to computational studies of molecular structure and behavior in any field of chemistry.

Gas-Phase Studies of NMR Shielding and Indirect Spin-Spin Coupling in 13C-Enriched Ethane and Ethylene

13C and 1H NMR spectra were observed as the function of density in 1,2-13C-enriched ethane and ethylene for the pure gaseous compounds and their binary mixtures with xenon and carbon dioxide gases as the solvents. All the chemical shifts and indirect spin-spin couplings were linearly dependent on the solvent density. The appropriate NMR parameters (σ and nJ) in isolated 13C2H6 and 13C2H4 molecules and the coefficients responsible for the binary molecular interactions were determined and compared with previous similar measurements and selected calculated shielding data. The newly obtained 13C shielding values in the isolated ethane and ethylene molecules suggest visible secondary isotope effects due to the additional carbon-13 atom. All the investigated shielding parameters depend on intermolecular interactions, and the dependence of 13C shielding is much more marked. In contrast, the indirect spin-spin couplings in 13C2H6 and 13C2H4 molecules are almost independent of solvent molecules. Their nJ values determined in liquids over sixty years ago are generally consistent with the same nJ parameters in isolated 13C2H6 and 13C2H4 molecules.

Liquid-phase NMR of asphaltenes

The present review focuses on the most recent advances in liquid-phase NMR of asphaltenes, leaving apart an overwhelming amount of publications dealing with solid-state NMR investigations in this field. Owing to the complexity of the coal-derived products, and in particular, asphaltenes, their 1H and 13C NMR spectra consist of a number of overlapping signals belonging to different hydrocarbon types. Comprehensive studies of asphaltenes by means of NMR reveal the characteristic functional groups of their fractions together with the spectral regions in which they resonate. NMR studies of asphaltenes provide a straightforward guideline for their chemical composition and that of the related coal-derived products.

DFT investigation of coupling constant anomalies in substituted β-lactams

β-lactams are a chemically diverse group of molecules with a wide range of biological activities. Having recently observed curious trends in 2JHH coupling values in studies on this structural class, we sought to obtain a more comprehensive understanding of these diagnostic NMR parameters, specifically interrogating 1JCH, 2JCH, and 2JHH, to differentiate 3- and 4-monosubstituted β-lactams. Further investigation using computational chemistry methods was employed to explore the geometric and electronic origins for the observed and calculated differences between the two substitution patterns.

17 O nuclear magnetic resonance: Recent advances and applications

The present review is focused on the most recent achievements in the application of liquid phase 17 O nuclear magnetic resonance (NMR) to inorganic, organic, and biochemical molecules focusing on their structure, conformations, and (bio)chemical behavior. The review is composed of four basic parts, namely, (1) simple molecules; (2) water and hydrogen bonding; (3) metal oxides, clusters, and complexes; and (4) biological molecules. Experimental 17 O NMR chemical shifts are thoroughly tabulated. They span a range of as much as almost 650 ppm (from -35.6 to +610.0 ppm) for inorganic and organic molecules, whereas this range is much wider for biological species being of about 1350 ppm (from -12 to +1332 ppm), and in the case of hemoproteins and heme-model compounds, isotropic chemical shifts of up to 2500 ppm were observed. The general prospects and caveats in the modern development of the liquid phase 17 O NMR in chemistry and biochemistry are critically discussed and briefly outlined in view of their future applications.

Computational 19 F NMR of trifluoromethyl derivatives of alkenes, pyrimidines, and indenes

The ¹⁹ F NMR chemical shifts of 13 trifluoromethyl derivatives of alkenes, pyrimidines, and indenes were calculated at the DFT level using the BhandHLYP, BHandH, PBE, PBE0, O3LYP, B3LYP, KT2, and KT3 functionals in combination with the pcS-2 basis set. Best result was documented for the BHandHLYP functional: The mean absolute error (MAE) of 0.66 ppm for the scaled values was achieved for the range of about 20 ppm. Solvent, vibrational, and relativistic corrections were found to be rather small, especially when taken in combination, generally demonstrating a slight decrease in the difference between calculated and experimental fluorine chemical shifts. As a measure of the practical importance of these compounds, one should recall that the growing number of life science products that contain trifluoromethyl groups provides a continuing driving force for the development of an effective methodology that enables both regio- and stereoselective introduction of trifluoromethyl groups into both aliphatic and aromatic systems.

Tritium NMR: A compilation of data and a practical guide

The present review is focused on experimental methods and structural applications of tritium NMR. It consists of five parts covering accordingly, introduction, brief overview, early (based on the papers appearing before 2000), more recent (based on the papers appeared in the interim of 2000 to 2015), and recent (based on the papers that appeared after 2015) reports. A special interest in this review is focused on practical aspects of tritium NMR spectroscopy, which is thoroughly illustrated by its numerous applications in chemistry and biochemistry.

Recent advances in the liquid-phase 6,7 Li nuclear magnetic resonance

The present review is focused on experimental methods and structural applications, including computational aspects, of classical lithium liquid-phase nuclear magnetic resonance (NMR). It consists of four parts covering accordingly a brief overview, early experimental reports (papers of up to about 2015) and more recent (papers appearing in the interim of about 2015 until 2022) results, together with very few but highly prospective computational results.

Searching for the Best Values of NMR Shielding and Spin-Spin Coupling Parameters: CH4-nFn Series of Molecules as the Example

Attempts at the theoretical interpretation of NMR spectra have a very long and fascinating history. Present quantum chemical calculations of shielding and indirect spin-spin couplings permit modeling NMR spectra when small, isolated molecules are studied. Similar data are also available from NMR experiments if investigations are performed in the gas phase. An interesting set of molecules is formed when a methane molecule is sequentially substituted by fluorine atoms-CH4-nFn, where n = 0, 1, 2, 3, or 4. The small molecules contain up to three magnetic nuclei, each with a one-half spin number. The spectral parameters of CH4-nFn can be easily observed in the gas phase and calculated with high accuracy using the most advanced ab initio methods of quantum chemistry. However, the presence of fluorine atoms makes the calculations of shielding and spin-spin coupling constants extremely demanding. Appropriate experimental 19F NMR parameters are good but also require some further improvements. Therefore, there is a real need for the comparison of existing NMR measurements with available state-of-the-art theoretical results for a better understanding of actual limits in the determination of the best shielding and spin-spin coupling values, and CH4-nFn molecules are used here as the exceptionally important case.

Fluorine spin-spin coupling constants of pentafluorobenzene revisited at the ab initio correlated levels

All possible spin-spin coupling constants, ¹⁹ F-¹⁹ F, ¹⁹ F-¹³ C, and ¹⁹ F-¹ H, of pentafluorobenzene were calculated at five different levels of theory, HF, DFT, SOPPA (CCSD), CCSD, and the SOPPA (CCSD)-based composite scheme with taking into account solvent, vibrational, relativistic, and correlation corrections. Most corrections were next to negligible for the long-range couplings but quite essential for the one-bond carbon-fluorine coupling constants. Hartree-Fock calculations were found to be entirely unreliable, while DFT results were comparable in accuracy with the data obtained using the wave function-based methods.

Computational 1 H and 13 C NMR in structural and stereochemical studies

Present review outlines the advances and perspectives of computational ¹ H and ¹³ C NMR applied to the stereochemical studies of inorganic, organic, and bioorganic compounds, involving in particular natural products, carbohydrates, and carbonium ions. The first part of the review briefly outlines theoretical background of the modern computational methods applied to the calculation of chemical shifts and spin-spin coupling constants at the DFT and the non-empirical levels. The second part of the review deals with the achievements of the computational ¹ H and ¹³ C NMR in the stereochemical investigation of a variety of inorganic, organic, and bioorganic compounds, providing in an abridged form the material partly discussed by the author in a series of parent reviews. Major attention is focused herewith on the publications of the recent years, which were not reviewed elsewhere.

Quantum Chemical Approaches to the Calculation of NMR Parameters: From Fundamentals to Recent Advances

Quantum chemical methods for the calculation of indirect NMR spin–spin coupling constants and chemical shifts are always in progress. They never stay the same due to permanently developing computational facilities, which open new perspectives and create new challenges every now and then. This review starts from the fundamentals of the nonrelativistic and relativistic theory of nuclear magnetic resonance parameters, and gradually moves towards the discussion of the most popular common and newly developed methodologies for quantum chemical modeling of NMR spectra.

Stereoelectronic interactions: A booster for 4 JHF transmission

Long-range proton-fluorine coupling constants (ⁿ J_HF ) are helpful for the structure elucidation of fluorinated molecules. However, their magnitude and sign can change with the relative position of coupled nuclei and the presence of substituents. Here, trans-4-tert-butyl-2-fluorocyclohexanone was used as a model compound for the study of the transmission of ⁴ J_HF . In this compound, the ⁴ J_H6axF was measured to be +5.1 Hz, which is five times larger than the remaining ⁴ J_HF in the same molecule (⁴ J_H4F  = +1.0 Hz and ⁴ J_H6eqF  = +1.0 Hz). Through a combination of experimental data, natural bond orbital (NBO) and natural J-coupling (NJC) analyses, we observed that stereoelectronic interactions involving the π system of the carbonyl group are involved in the transmission pathway for the ⁴ J_H6axF . Interactions containing the π system as an electron acceptor (e.g., σ_C6H6ax  → π^*_C═O and σ_CF  → π^*_C═O ) increase the value of the ⁴ J_H6axF , while the interaction of the π system as an electron donor (e.g., π_C═O  → σ^*_CF ) decreases it. Additionally, the carbonyl group was shown not to be part of the transmission pathway of the diequatorial ⁴ J_H6eqF coupling in cis-4-tert-butyl-2-fluorocyclohexanone, revealing that there is a crucial symmetry requirement that must be fulfilled for the π system to influence the value of the ⁴ J_HF in these systems.

Probing Halogen Bonds by Scalar Couplings

As halogen bonding is a weak, transient interaction, its description in solution is challenging. We demonstrate that scalar coupling constants (J) are modulated by halogen bonding. The binding-induced magnitude change of one-bond couplings, even up to five bonds from the interaction site, correlates to the interaction strength. We demonstrate this using the NMR data of 42 halogen-bonded complexes in dichloromethane solution and by quantum chemical calculations. Our observation puts scalar couplings into the toolbox of methods for characterization of halogen bond complexes in solution and paves the way for their applicability for other types of weak σ-hole interactions.

Computational 1 H and 13 C NMR of the trimeric monoterpenoid indole alkaloid strychnohexamine: Selected spectral updates

Very large trimeric indole alkaloid strychnohexamine, with empirical formula C₅₉ H₆₀ N₆ O (66 second-row atoms and 60 protons), has been subjected to the state-of-the-art computation of the ¹ H and ¹³ C nuclear magnetic resonance (NMR) chemical shifts of its configurational isomers at each of the 14 asymmetric centers. Several spectral reassignments and corrections of ¹ H and ¹³ C NMR spectra of this alkaloid were suggested based on the PBE0/pcSseg-2//pcseg-2 calculation of its NMR chemical shifts. Thus, all pairs of diastereotopic protons were assigned together with four aromatic carbon resonances of C-9 and C-11, C-9″, and C-11″. In addition, the unassigned chemical shifts of carbon C-23″ and proton at C-3' in, accordingly, ¹³ C and ¹ H NMR spectra were predicted.

Computational NMR of Carbohydrates: Theoretical Background, Applications, and Perspectives

This review is written amid a marked progress in the calculation of NMR parameters of carbohydrates substantiated by a vast amount of experimental data coming from several laboratories worldwide. By no means are we trying to cover in the present compilation a huge amount of all available data. The main idea of the present review was only to outline general trends and perspectives in this dynamically developing area on the background of a marked progress in theoretical and computational NMR. Presented material is arranged in three basic sections: (1)-a brief theoretical introduction; (2)-applications and perspectives in computational NMR of monosaccharides; and (3)-calculation of NMR chemical shifts and spin-spin coupling constants of di- and polysaccharides.

Recent advances in computational liquid-phase 77Se NMR

This review aims to highlight significant progress in the calculation of 77Se NMR chemical shifts and spin – spin coupling constants involving selenium substantiated with a vast amount of experimental data. The material is arranged in two basic sections: the first one dealing with the calculation of 77Se NMR chemical shifts and the second one dealing with the computation of spin – spin coupling constants involving 77Se nucleus, namely 77Se–1H, 77Se–13C and 77Se–77Se together with some more exotic types of couplings, 77Se – 15N, 77Se–19F, 77Se–29Si and 77Se–31P. A special attention is focused on the stereoelectronic effects involving selenium atom and their manifestation in the 77Se NMR spectra of organoselenium compounds studied with the aid of the modern calculation of 77Se NMR parametres in combination with experimental results.

The bibliography includes 114 references.

Selective Nuclear Magnetic Resonance Experiments for Sign-Sensitive Determination of Heteronuclear Couplings: Expanding the Analysis of Crude Reaction Mixtures

State-of-the-art nuclear magnetic resonance (NMR) selective experiments are capable of directly analyzing crude reaction mixtures. A new experiment named HD-HAPPY-FESTA yields ultrahigh-resolution total correlation subspectra, which are suitable for sign-sensitive determination of heteronuclear couplings, as demonstrated here by measuring the sign and magnitude for proton-fluorine couplings (J_HF) from major and minor isomer products of a two-step reaction without any purification. Proton-fluorine couplings ranging from 51.5 to -2.6 Hz could be measured using HD-HAPPY-FESTA, with the smallest measured magnitude of 0.8 Hz. Experimental J_HF values were used to identify the two fluoroketone intermediates and the four fluoroalcohol products. Results were rationalized and compared with the density functional theory (DFT) calculations. Experimental data were further compared with the couplings reported in the literature, where pure samples were analyzed.

One-bond 1 J(15 N,H) coupling constants at sp2 -hybridized nitrogen of Schiff bases, enaminones and similar compounds: A theoretical study

¹ J(¹⁵ N,H) coupling constants for enaminones and NH-forms of intramolecularly hydrogen-bonded Schiff bases as model compounds for sp² -hybridized nitrogen atoms are evaluated using density functional theory (DFT) to find the optimal functionals and basis sets. Ammonia is used as a test molecule and its one-bond coupling constant is compared with experiment. A methylamine Schiff base of a truncated molecule of gossypol is used for checking the performance of selected B3LYP, O3LYP, PBE, BHandH, and APFD density functionals and standard, modified, and dedicated basis sets for coupling constants. Both in vacuum and in chloroform, modeled by the simple continuum model of solvent, the modified basis sets predict significantly better the ¹ J(¹⁵ N,H) value in ammonia and in the methylamine Schiff base of a truncated molecule of gossypol than the standard basis sets. This procure is then used on a broad set of intramolecularly hydrogen-bonded molecules, and a good correlation between calculated and experimental one-bond NH coupling constants is obtained. The ¹ J(¹⁵ N,H) couplings are slightly overestimated. The calculated data show for hydrogen-bonded NH interatomic distances that the calculated values depend on the NH bond lengths. The shorter the bond lengths, the larger the ¹ J(¹⁵ N,H). A useful correlation between ¹ J(¹⁵ N,H) and NH bond length is derived that enables realistic predictions of one-bond NH coupling constants. The calculations reproduce experimentally observed trends for the studied molecules.

Recent advances in computational 31 P NMR: Part 1. Chemical shifts

This is the first part of two closely related reviews dealing with the computation of phosphorus-31 nuclear magnetic resonance chemical shifts in a wide series of organophosphorus compounds including complexes, clusters, and bioorganic phosphorus compounds. In particular, the analysis of the accuracy factors, such as substitution effects, solvent effects, vibrational corrections, and relativistic effects, is presented. This review is dedicated to the Full Member of the Russian Academy of Sciences Professor Boris A. Trofimov in view of his invaluable contribution to the field of synthesis, nuclear magnetic resonance, and computation studies of organophosphorus compounds.

One-bond 1 J(15 N─19 F) spin-spin coupling constants of cationic fluorinating reagents: Insights from DFT calculations

We have investigated, by means of density functional theory protocols, the one-bond ¹ J(¹⁵ N─¹⁹ F) spin-spin coupling constants in a series of fluorinating reagents, containing the N─F bond, recently studied experimentally. The results of the calculations show a very good linear relationship with the experimental values, even though only the M06-2X(PCM)/pcJ-2//B3LYP/6-311G(d,p) level affords a very low mean absolute error. The calculations allow to analyze the various molecular orbitals contributions to the J coupling and to rationalize the observed positive sign, corresponding to a negative sign of the reduced spin-pin coupling constant K(N─F). Moreover, of the four Ramsey contributions, only the diamagnetic spin orbit is negligible, whereas the paramagnetic spin orbit and spin dipole terms decrease the magnitude of the Fermi contact (FC) term by an amount that goes from a minimum of 35% up to more than 60% of the FC term itself. Several effects have been investigated, namely, the contribution of the long-range solvent reaction field, relativistic corrections, and conformational and vibrational effects.

Recent advances in computational 31 P NMR: Part 2. Spin-spin coupling constants

This is the second part of two closely related reviews dealing with the computation of ³¹ P nuclear magnetic resonance (NMR) parameters in a wide range of phosphorous containing compounds. The first part of this review concentrated primarily on the computation of ³¹ P NMR chemical shifts, whereas the second part concerns the calculation of spin-spin coupling constants involving phosphorus nucleus, focusing primarily on their stereochemical dependencies and stereodynamic behavior in particular classes of organophosphorus compounds. This review is dedicated to the Full Member of the Russian Academy of Sciences Professor Boris A. Trofimov in view of his invaluable contribution to the field of synthesis, NMR, and computation studies of organophosphorus compounds.

The 1 H and 13 C NMR chemical shifts of Strychnos alkaloids revisited at the DFT level

The density functional theory calculation of ¹ H and ¹³ C NMR chemical shifts in a series of ten 10 classically known Strychnos alkaloids with a strychnine skeleton was performed at the PBE0/pcSseg-2//pcseg-2 level. It was found that calculated ¹ H and ¹³ C NMR chemical shifts provided a markedly good correlation with experiment characterized by a mean absolute error of 0.08 ppm in the range of 7 ppm for protons and 1.67 ppm in the range of 150 ppm for carbons, so that a mean absolute percentage error was as small as ~1% in both cases.

Unusual behaviour of the spin-spin coupling constant 1JCH upon formation of CHX hydrogen bond

One-bond coupling constants ¹J_XY are usually used as a measure of the corresponding XY interatomic distances. However, the physical nature of this correlation is not well understood and, in some cases, a counterintuitive behaviour of ¹J_XY upon hydrogen bonded complex formation has been reported. In this work, the behavior of ¹J_CH upon formation and strengthening of complexes with CHX hydrogen bonds and upon a proton transfer process is investigated by means of ¹H NMR spectroscopy and quantum chemical calculations. ¹H NMR spectra of 1,1-dinitroethane solution at room temperature in various solvents (carbon tetrachloride, chloroform, dichloromethane, acetone, dimethylformamide and dimethyl sulfoxide) illustrate the increase of ¹J_CH by several Hz upon an increase of the complex strength. Computational results (MP2/aug-cc-pVDZ) reproduce this observation and allow one to conclude that the increase of ¹J_CH is mainly caused by the change of the carbon hybridization (an increase of s-character), rather than by the change in interatomic distance r_CH. The behavior of ¹J_CH was also examined computationally for a wide range of CHX hydrogen bond energies and geometries. For this purpose, quantum-chemical modeling of the partial proton transfer process for complexes formed by 1,1-dinitroethane and trinitromethane as hydrogen bond donors with acetone, pyridine and fluoride anion as hydrogen bond acceptors was performed. The obtained results have confirmed the above-mentioned idea - for rather weak complexes, the dominant impact on the change of ¹J_CH magnitude is the increase of the s-character of carbon atom hybridization, while for complexes with a significantly transferred proton, the exponential decrease of the Fermi-contact term dominates.

DFT computational schemes for 1 H and 13 C NMR chemical shifts of natural products, exemplified by strychnine

A number of computational schemes based on different Density Functional Theory (DFT) functionals in combination with a number of basis sets were tested in the calculation of ¹ H and ¹³ C NMR chemical shifts of strychnine, as a typical representative of the vitally important natural products, and used as a challenging benchmark and a rigorous test for such calculations. It was found that the most accurate computational scheme, as compared with experiment, was PBE0/pcSseg-4//pcseg-3 characterized by a mean absolute error of 0.07 ppm for the range of about 7 ppm for ¹ H NMR chemical shifts and that of only 1.13 ppm for ¹³ C NMR chemical shifts spread over the range of about 150 ppm. For more practical purposes, including investigation of larger molecules from this series, a much more economical computational scheme, PBE0/pcSseg-2//pcseg-2, characterized by almost the same accuracy and much less computational demand, was recommended.

Computational 1 H NMR: Part 3. Biochemical studies

This is the third and the last part of three closely interrelated reviews dealing with computation of ¹ H nuclear magnetic resonance chemical shifts and ¹ H-¹ H spin-spin coupling constants. Present review deals with the computation of these parameters in biologically active natural products, carbohydrates, and other molecules of biological origin focusing on stereochemical applications of computational ¹ H nuclear magnetic resonance to these objects.

Computational 1 H NMR: Part 2. Chemical applications

This is the second one of three closely interrelated reviews dealing with computation of ¹ H NMR chemical shifts and ¹ H-¹ H spin-spin coupling constants prepared for Magnetic Resonance in Chemistry. Presented in this review are some basic notes and illustrative examples of how modern computational ¹ H NMR could be used for structural elucidation and stereoelectronic studies of the medium-sized organic molecules involving saturated, unsaturated, aromatic, and heteroaromatic compounds together with their functional derivatives and coordination complexes to get deeper insight into their stereochemical structure and stereodynamic behavior.

On the heavy atom on light atom relativistic effect in the NMR shielding constants of phosphine tellurides

The relativistic HALA effect has been shown to depend on the spatial deformation of the lone electron pairs of a heavy atom, as demonstrated for alkyl and alkene phosphine tellurides. It was found that HALA effect on phosphorous nuclear magnetic resonance shielding constant is strongly dependent on the spatial arrangements of light substituents on phosphorus, resulting in the deformation of the lone electron pairs of tellurium.

Computational 1 H NMR: Part 1. Theoretical background

This is the first one of the three closely interrelated reviews to be published in Magnetic Resonance in Chemistry dealing with accordingly theoretical background, chemical applications, and biochemical studies of and by means of computational ¹ H NMR. Presented in the first part of the review is a general outline of the modern theoretical methods and accuracy factors of computational ¹ H NMR involving locally dense basis set schemes, solvent effects, vibrational corrections, and relativistic effects performed at the density functional theory and/or nonempirical levels. This review is dedicated to Prof. Stephan Sauer in view of his invaluable contribution to the field of computational nuclear magnetic resonance.

Characterization by Empirical and Computational Methods of Dictyospiromide, an Intriguing Antioxidant Alkaloid from the Marine Alga Dictyota coriacea

The challenging structural motif of dictyospiromide (1), a spirosuccinimide alkaloid with antioxidant properties that are associated with activation of the Nrf2/ARE signaling pathway, was assigned using contemporary NMR experiments complemented with anisotropic NMR, chiroptical, and computational methodologies. Anisotropic NMR parameters provided critical orthogonal verification of the configuration of the difficult to assign spiro carbon and the other stereogenic centers in 1.

Computational protocols for calculating 13C NMR chemical shifts

The most recent results dealing with the computation of ¹³C NMR chemical shifts in chemistry (small molecules, saturated, unsaturated and aromatic compounds, heterocycles, functional derivatives, coordination complexes, carbocations, and natural products) are reviewed, paying special attention to theoretical background and accuracy, the latter involving solvent effects, vibrational corrections, and relativistic effects.

Isolation of the Metalated Ylides [Ph3 P-C-CN]M (M=Li, Na, K): Influence of the Metal Ion on the Structure and Bonding Situation

The isolation and structural characterization of the cyanido-substituted metalated ylides [Ph3 P-C-CN]M (1-M; M=Li, Na, K) are reported with lithium, sodium, and potassium as metal cations. In the solid-state, most different aggregates could be determined depending on the metal and additional Lewis bases. The crown-ether complexes of sodium (1-Na) and potassium (1-K) exhibited different structures, with sodium preferring coordination to the nitrogen end, whereas potassium binds in an unusual η2 -coordination mode to the two central carbon atoms. The formation of the yldiide was accompanied by structural changes leading to shorter C-C and longer C-N bonds. This could be attributed to the delocalization of the free electron pairs at the carbon atom into the antibonding orbitals of the CN moiety, which was confirmed by IR spectroscopy and computational studies. Detailed density functional theory calculations show that the changes in the structure and the bonding situation were most pronounced in the lithium compounds due to the higher covalency.

Enhancing the utility of 1JCH coupling constants in structural studies through optimized DFT analysis

High accuracy for DFT-predicted ¹J_CH-couplings can be achieved without ad hoc corrections or empirical scaling by careful selection of the DFT method utilized for geometry optimization and J-coupling calculations.