NMR Spin-spin Coupling Constants Research Articles

Probing the Electric Field Response of a Water Molecule Confined in Small Carbon Nanocages: A Density Functional Theory Investigation.

We consider a water molecule under tight confinement in the small-sized fullerenes (C28, C30, C32) within the density functional theory (DFT) calculations with suitable exchange-correlation functionals. Such nanoscopic molecular cages provide an ideal setup to study their characteristic properties not present in the condensed phase. The water molecule entirely loses its feature of typical water when it is confined in small fullerenes of size equal to C30or smaller, in which the asymmetric O-H stretching vibration occurs at a lower wavenumber than the symmetric stretching. We study the response of the confined water molecule to the applied electric fields in terms of change in geometrical parameters, NMR spin-spin coupling constants, dipole moment, HOMO-LUMO (HL) gap, and vibrational frequency shift. The electric field shielding property of small-sized fullerene cages is explored and found to be strongly correlated with the HL gap. Since the electric field modulates the gap to decrease generally, shielding efficiency varies with field strength, thereby making large fields better shielded than small fields for the small penetration factor at large fields. The results that hold significance for technological applications are discussed.

Reducing Exact Two-Component Theory for NMR Couplings to a One-Component Approach: Efficiency and Accuracy.

The self-consistent and complex spin-orbit exact two-component (X2C) formalism for NMR spin-spin coupling constants [ J. Chem. Theory Comput. 17, 2021, 3874-3994] is reduced to a scalar one-component ansatz. This way, the first-order response term can be partitioned into the Fermi-contact (FC) and spin-dipole (SD) interactions as well as the paramagnetic spin-orbit (PSO) contribution. The FC+SD terms are real and symmetric, while the PSO term is purely imaginary and antisymmetric. The relativistic one-component approach is combined with a modern density functional treatment up to local hybrid functionals including the response of the current density. Computational demands are reduced by factors of 8-24 as shown for a large tin compound consisting of 137 atoms. Limitations of the current ansatz are critically assessed for Sn, Pb, Pd, and Pt compounds, i.e. the one-component treatment is not sufficient for tin compounds featuring a few heavy halogen atoms.

Vibrational Corrections to NMR Spin-Spin Coupling Constants from Relativistic Four-Component DFT Calculations.

Zero-point vibrational (ZPV) corrections to the nuclear spin–spin coupling constants have been calculated using four-component Dirac–Kohn–Sham DFT for H2X (where X = O, S, Se, Te, Po), XH3 (where X = N, P, As, Sb, Bi), and XH4 (where X = C, Si, Ge, Sn, and Pb) molecules and for HC≡CPbH3. The main goal was to study the influence of relativistic effects on the ZPV corrections and thus results calculated at relativistic and nonrelativistic approaches have been compared. The effects of relativity become notable for the ZPV corrections to the spin–spin coupling constants for compounds with lighter elements (selenium and germanium) than for the spin–spin coupling constants themselves. In the case of molecules containing heavier atoms, for instance BiH3 and PbH4, relativistic effects play a crucial role on the results and approximating ZPV corrections by the nonrelativistic results may lead to larger errors than omitting ZPV corrections altogether.

O-Methylation in Carbohydrates: An NMR and MD Simulation Study with Application to Methylcellulose.

Methylated carbohydrates are important from both biological and technical perspectives. Specifically, methylcellulose is an interesting cellulose derivative that has applications in foods, materials, cosmetics, and many other fields. While the molecular dynamics simulation technique has the potential for both advancing the fundamental understanding of this polymer and aiding in the development of specific applications, a general drawback is the lack of experimentally validated interaction potentials for the methylated moieties. In the present study, simulations using the GROMOS 56 carbohydrate force field are compared to NMR spin–spin coupling constants related to the conformation of the exocyclic torsion angle ω in d-glucopyranose and derivatives containing a 6-O-methyl substituent and a 13C-isotopologue thereof. A 3JCC Karplus-type relationship is proposed for the C5–C6–O6–CMe torsion angle. Moreover, solvation free energies are compared to experimental data for small model compounds. Alkylation in the form of 6-O-methylation affects exocyclic torsion only marginally. Computed solvation free energies between nonmethylated and methylated molecules were internally consistent, which validates the application of these interaction potentials for more specialized purposes.

Conformational analysis of halobenzaldehydes: A theoretical and spectroscopic study

Rotation of the formyl group in halobenzaldehydes (halo = F, Cl, and Br) has been studied through high-level density functional theory (DFT) calculations and natural bond orbital (NBO) analysis. The isomeric and conformational energies are dictated by non-Lewis and Lewis-type interactions. The 4-substituted benzaldehydes the most stable isomer due especially to an effective electron resonance and small dipole moment, while destabilizing Lewis-type interactions slightly override the non-Lewis electron delocalization both in the syn and anti conformers of meta isomer. No significant halogen effect is observed on stabilities, excepting for the least stable ortho isomer, where the halogen interacts with the formyl group. Such interaction is more repulsive in the syn conformer (where the halogen and oxygen atoms face one another), but the nature of this interaction changes from predominantly electrostatic to steric contribution on going from F to Br. Infrared stretching vibrations, as well as NMR chemical shifts and spin-spin coupling constants, provided valuable insight into through-bond and through-space effects influencing the stabilities of these compounds.

Assessing the Accuracy of Local Hybrid Density Functional Approximations for Molecular Response Properties.

A comprehensive overview of the performance of local hybrid functionals for molecular properties like excited states, ionization potentials within the GW framework, polarizabilities, magnetizabilities, NMR chemical shifts, and NMR spin-spin coupling constants is presented. We apply the generalization of the kinetic energy, τ, with the paramagnetic current density to all magnetic properties and the excitation energies from time-dependent density functional theory. This restores gauge invariance for these properties. Different ansätze for local mixing functions such as the iso-orbital indicator, the correlation length, the Görling-Levy second-order limit, and the spin polarization are compared. For the latter, we propose a modified version of the corresponding hyper-generalized gradient approximation functional of Perdew, Staroverov, Tao, and Scuseria (PSTS) [Phys. Rev. A 2008, 78, 052513] to allow for a numerically stable evaluation of the exchange-correlation kernel and hyperkernel. The PSTS functional leads to a very consistent improvement compared to the related TPSSh functional. It is further shown that the "best" choice of the local mixing function depends on the studied property and molecular class. While functionals based on the iso-orbital indicator lead to rather accurate excitation energies and ionization energies, the results are less impressive for NMR properties, for which a considerable dependence on the considered molecular test set and nuclei is observed. Johnson's local hybrid functional based on the correlation length yields remarkable results for NMR shifts of compounds featuring heavy elements and also for the excitation energies of organic compounds.

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.

Solvent effect on the 195Pt NMR properties in pyridonate-bridged PtIII dinuclear complex derivatives investigated by ab initio molecular dynamics and localized orbital analysis.

An ab initio molecular dynamics investigation of the solvent effect (water) on the structural parameters, 195Pt NMR spin-spin coupling constants (SSCCs) and chemical shifts of a series of pyridonate-bridged PtIII dinuclear complexes is performed using Kohn-Sham (KS) Car-Parrinello molecular dynamics (CPMD) and relativistic hybrid KS NMR calculations. The indirect solvent effect (via structural changes) has a dramatic effect on the 1JPtPt SSCCs. The complexes exhibit a strong trans influence in solution, where the Pt-Pt bond lengthens with increasing axial ligand σ-donor strength. In the diaqua complex, where the solvent effect is more pronounced, the SSCCs averaged for CPMD configurations with explicit plus implicit solvation agree much better with the experimental data, while the calculations for static geometry and CPMD unsolvated configurations show large deviations with respect to experiment. The combination of CPMD with hybrid KS NMR calculations provides a much more realistic computational model that reproduces the large magnitudes of 1JPtPt and 195Pt chemical shifts. An analysis of 1JPtPt in terms of localized and canonical orbitals shows that the SSCCs are driven by changes in the s-character of the natural atomic orbitals of Pt atoms, which affect the 'Fermi contact' mechanism.

Dimeric and monomeric dihydrostilbenes from the roots of Stemona mairei together with their hypoglycemic activities

A carefully performed phytochemical investigation of the roots of Stemona mairei (Stemonaceae) led to the discovery of six so far undescribed dihydrostilbene dimers maistilbene A – F and four monomers maistilbene G – J. This is the first report of stilbenoids of this plant species and the first report of dimeric dihydrostilbenes from this plant family. The absolute configuration of these compounds was determined by a combination of Mo 2 (AcO) 4 -induced ECD spectra and quantum chemical calculation of NMR chemical shift, ECD and spin-spin coupling constants. Maistilbene G could increase the glucose consumption in L6 myotubes, disclosing a new hypoglycemic candidate. • Six undescribed dihydrostilbene dimers and four monomers were isolated from roots of Stemona mairei. • The absolute configuration was determined by a quantum chemical calculation of NMR, ECD and coupling constants. • Maistilbene G could increase the glucose consumption in L6 myotubes model.

Experimental and computational spectroscopic studies of 3,5-disubstituted 4,5-dihydro-1,2,4-oxadiazoles

Some 3,5-disubstituted 4,5-dihydro-1,2,4-oxadiazoles (DHOZs) were synthesized, and steric and electronic effects of the substituents on the heterocyclic ring C3- and C5-positions were investigated using the NMR and UV–Vis spectroscopies, and (TD)DFT/6-311++G(d,p) computations. The NMR and UV–Vis spectroscopic characteristics of the synthesized compounds could be explained in terms of the π- and σ-electronic effects of the C3- and C5-aryl substitutions on the heterocyclic ring, respectively. Computational NMR chemical shifts and spin-spin coupling constants of the C5–H and N4–H protons display a Karplus correlation with corresponding H–C5–N4–H dihedral angle. The experimental and computational 1H NMR chemical shifts of the C5–H proton show very good compatibility. Conformational analysis on the o-nitro and o-methoxyphenyl C5-substituted DHOZs showed intramolecular interactions affecting their structural and spectroscopic features.

On the development of the exact two-component relativistic method for calculating indirect NMR spin-spin coupling constants

Recently, to obtain the proper basis functions of small component spinors from the basis functions of large component spinors, the restricted magnetic balance (RMB) has been used in relativistic calculations of molecular magnetic properties, instead of the restricted kinetic balance (RKB). In this study, we attempt to develop the RMB-based exact two-component relativistic method, i.e., RMB-based normalized elimination of the small component (RMB-NESC) method for calculating indirect NMR spin-spin coupling constants (SSCCs), as well as the RKB-NESC method. The RMB-NESC SSCC formulas require complicated one-electron integrals, unlike the RKB-NESC SSCC formulas. However, the standard integral evaluation method for the complicated integrals has not yet been established, while we calculate them by using the finite-size nuclear model and partial integration. By comparison with the RKB-NESC and experimental SSCC values, we discuss whether the RMB-NESC results are valid. Finally, we conclude that our calculation for the complicated integrals must have some problems.

Spectroscopic studies of aryl substituted 1-phenyl-2-pyrazolines: Steric and electronic substitution effects

A series of aryl substituted 1-phenyl-2-pyrazolines containing electron-donating and electron-withdrawing substituents on different positions of the C3 or C5-aryl groups were synthesized and their steric and electronic effects on characteristic spectral data were investigated by experimental spectroscopic methods (UV–Vis, IR and NMR) and DFT computations. Whereas the C5-aryl group of the heterocyclic ring behaves as σ-donor/acceptor substituent, the π-donor/acceptor ability of the C3-aryl group depends on the co-planarity of the substituent with the aryl ring and also the extent of the orientation of this C3-aryl ring towards the CN double bond of the heterocyclic ring. A significant through conjugation of the lone pair on the N1-atom towards the C3-aryl ring is observed when the electron-withdrawing nitro group is located on the para-position of this aryl ring. The experimental spectroscopic results for the substitution effects are also supported by (TD)DFT/6-311++G(d,p) computed UV–Vis spectra. Experimental and theoretical NMR chemical shifts and spin-spin coupling constants obtained for these 2-pyrazolines validated by Karplus diagram approves the structures predicted for these compounds.

Exploring the pnicogen bond non-covalent interactions in 4-XPhNH2:PFnH3-n complexes (n = 1–3, X = H, F, CN, CHO, NH2, CH3, NO2 and OCH3)

Quantum chemical calculations were carried out to explore the effects of substituents in para position of aniline as well as phosphine on the strength of interactions in pnicogen-bonded (PB) complexes of the 4-XPhNH2:PFnH3-n (n=1–3 and X=H, F, CN, CHO, NH2, CH3, NO2 and OCH3). The P⋯N pnicogen bond strength in these complexes was examined at M06-2X/6–311++G(d,p), MP2/6–311++G(d,p), MP2/cc-pVDZ, CCSD/cc-pVDZ and MP2/aug-cc-pVTZ levels of theory. The dispersion corrected M06-2X-GD3, B2PLYP-GD3 and mPW2PLYP-GD2 functionals were also used to calculate the binding energies. The changes in the pnicogen bond strength due to the change of the substituents were well monitored by the changes in the interaction energy, structural parameters, natural charge, charge transfer, electron density topology, NMR chemical shielding and spin-spin coupling constants. The BSSE-corrected BEs change from 14.5 to 21.4kJmol−1 for the 4-XPhNH2:PF3, 21.5 to 30.5kJmol−1 for the 4-XPhNH2:PF2H and 24.2 to 33.1kJmol−1 for the XPhNH2:PFH2 complex at MP2/aug-cc-pVTZ level and 12.5 to 18.9kJmol−1 for the 4-XPhNH2:PF3, 17.4 to 26.5kJmol−1 for the 4-XPhNH2:PF2H and 18.9 to 26.5kJmol−1 for the XPhNH2:PFH2 complexes at MP2/6–311++G(d,p) level. For each PFnH3-n monomer, results demonstrated that the strength of the pnicogen bonds increases by introducimg electron-donating substituents in 4-XPhNH2 molecule, whereas a reverse situation was found upon introducing electron-accepting substituents. Natural bond orbital analysis confirmed that the charge transfer takes place from 4-XPhNH2 to PFnH3-n. Electron density properties based on atoms in molecules theory were also utilized to characterize the pnicogen bonds.

Application of spectroscopic methods (FT-IR, Raman, ECD and NMR) in studies of identification and optical purity of radezolid

In the presented study, N-{[(5S)-3-(2-fluoro-4′-{[(1H-1,2,3-triazol-5-ylmethyl)amino]methyl}biphenyl-4-yl)-2-oxo-1,3-oxazolidin-5-yl]methyl}acetamide (radezolid) was synthesized and characterized using FT-IR, Raman, ECD and NMR. The aim of this work was to assess the possibility of applying classical spectral methods such as FT-IR, Raman, ECD and NMR spectroscopy for studies on the identification and optical purity of radezolid. The experimental interpretation of FT-IR and Raman spectra of radezolid was conducted in combination with theoretical studies. Density functional theory (DFT) with the B3LYP hybrid functional was used for obtaining radezolid spectra. Full identification was carried out by COSY, 1H {13C} HSQC and 1H {13C} HMBC experiments. The experimental NMR chemical shifts and spin-spin coupling constants were compared with theoretical calculations using the DFT method and B3LYP functional employing the 6-311++G(d,p) basis set and the solvent polarizable continuum model (PCM). The experimental ECD spectra of synthesized radezolid were compared with experimental spectra of the reference standard of radezolid. Theoretical calculations enabled us to conduct HOMO and LUMO analysis and molecular electrostatic potential maps were used to determine the active sites of microbiologically active form of radezolid enantiomer. The relationship between results of ab initio calculations and knowledge about chemical-biological properties of S-radezolid and other oxazolidinone derivatives are also discussed.

Conformational Populations of β-(1→4) O-Glycosidic Linkages Using Redundant NMR J-Couplings and Circular Statistics.

Twelve disaccharides containing β-(1→4) linkages and displaying systematic structural variations in the vicinity of these linkages were selectively labeled with 13C to facilitate measurements of multiple NMR spin-spin (scalar; J) coupling constants (JCH and JCC values) across their O-glycosidic linkages. Ensembles of spin-couplings (2JCOC, 3JCOCH, 3JCOCC) sensitive to the two linkage torsion angles, phi (ϕ) and psi (ψ), were analyzed by using parametrized equations obtained from density functional theory (DFT) calculations, Fredholm theory, and circular statistics to calculate experiment-based rotamer populations for ϕ and ψ in each disaccharide. With the statistical program MA'AT, torsion angles ϕ and ψ were modeled as a single von Mises distribution, which yielded two parameters, the mean position and the circular standard deviation (CSD) for each angle. The NMR-derived rotamer populations were compared to those obtained from 1 μs aqueous molecular dynamics (MD) simulations and crystallographic database statistical analyses. Conformer populations obtained exclusively from the MA'AT treatment of redundant J-couplings were in very good agreement with those obtained from the MD simulations, providing evidence that conformational populations can be determined by NMR for mobile molecular elements such as O-glycosidic linkages with minimal input from theory. The approach also provides an experimental means to validate the conformational preferences predicted from MD simulations. The conformational behaviors of ϕ in the 12 disaccharides were very similar, but those of ψ varied significantly, allowing a classification of the 12 disaccharides based on preferred linkage conformation in solution.

Electronic Structure, NMR, Spin-Spin Coupling, and Noncovalent Interactions in Aromatic Amino Acid Based Ionic Liquids.

Noncovalent interactions accompanying phenylalanine (Phe), tryptophan (Trp), and tyrosine (Tyr) amino acids based ionic liquids (AAILs) composed of 1-methyl-3-butyl-imidazole and its methyl-substituted derivative as cations have been analyzed employing the dispersion corrected density functional theory. It has been shown that cation-anion binding in these bioionic ILs is primarily facilitated through hydrogen bonding in addition to lp---π and CH---π interactions those arising from aromatic moieties which can be probed through (1)H and (13)C NMR spectra calculated from the gauge independent atomic orbital method. Characteristic NMR spin-spin coupling constants across hydrogen bonds of ion pair structures viz., Fermi contact, spin-orbit and spin-dipole terms show strong dependence on mutual orientation of cation with the amino acid anion. The spin-spin coupling mechanism transmits spin polarization via electric field effect originating from lp---π interactions whereas the electron delocalization from lone pair on the carbonyl oxygen to antibonding C-H orbital is facilitated by hydrogen bonding. It has been demonstrated that indirect spin-spin coupling constants across the hydrogen bonds correlate linearly with hydrogen bond distances. The binding energies and dissected nucleus independent chemical shifts (NICS) document mutual reduction of aromaticity of hydrogen bonded ion pairs consequent to localization of π-character. Moreover the nature and type of such noncovalent interactions governing the in-plane and out-of-plane NICS components provide a measure of diatropic and paratropic currents for the aromatic rings of varying size in AAILs. Besides the direction of frequency shifts of characteristic C═O and NH stretching vibrations in the calculated vibrational spectra has been rationalized.

5-Fluoro-5-halo- and 5-fluoro-5-nitro-substituted uracil derivatives. Synthesis and structure

5-Bromo-5-fluoro- and 5-chloro-5-fluoro-6-hydroxy-5,6-dihydrouracils were obtained in high yields by oxidative halogenation of 5-fluorouracil. Nitration of 5-fluorouracil gave 5-fluoro-6-hydroxy-5-nitro-5,6-dihydrouracil. Theoretical calculations in B3LYP/6-311+G(d,p) // B3LYP/6-311G(d,p) + IEFPCM approximation and GIAO simulation of 13 C NMR spectra and spin-spin coupling constants agree with the structure of the compounds obtained, which manifest an equatorial orientation of the fluorine atom and an axial orientation of the hydroxy group at position 6 of the dihydrouracil ring. The principal possibility of oxidative iodination of 5-halouracils was studied in B3LYP/CEP-121G approximation. It was found that reversible elimination of iodine by a nucleophilic agent to give the original compounds is the main transformation pathway of the intermediate in this process. How to Cite Chernikova, I. B.; Khursan, S. L.; Spirikhin, L. V.; Yunusov, M. S. Chem. Heterocycl. Compd. 201 5 , 51 , 568. [ Khim. Geterotsikl. Soedin. 2015 , 51 , 568.] For this article in the English edition see DOI 10.1007/s10593-015-1737-y

Informing saccharide structural NMR studies with density functional theory calculations.

Density functional theory (DFT) is a powerful computational tool to enable structural interpretations of NMR spin-spin coupling constants ( J-couplings) in saccharides, including the abundant (1)H-(1)H ( JHH), (13)C-(1)H ( JCH), and (13)C-(13)C ( JCC) values that exist for coupling pathways comprised of 1-4 bonds. The multiple hydroxyl groups in saccharides, with their attendant lone-pair orbitals, exert significant effects on J-couplings that can be difficult to decipher and quantify without input from theory. Oxygen substituent effects are configurational and conformational in origin (e.g., axial/equatorial orientation of an OH group in an aldopyranosyl ring; C-O bond conformation involving an exocyclic OH group). DFT studies shed light on these effects, and if conducted properly, yield quantitative relationships between a specific J-coupling and one or more conformational elements in the target molecule. These relationships assist studies of saccharide structure and conformation in solution, which are often challenged by the presence of conformational averaging. Redundant J-couplings, defined as an ensemble of J-couplings sensitive to the same conformational element, are particularly helpful when the element is flexible in solution (i.e., samples multiple conformational states on the NMR time scale), provided that algorithms are available to convert redundant J-values into meaningful conformational models. If the latter conversion is achievable, the data can serve as a means of testing, validating, and refining theoretical methods like molecular dynamics (MD) simulations, which are currently relied upon heavily to assign conformational models of saccharides in solution despite a paucity of experimental data needed to independently validate the method.

Communication: Localized molecular orbital analysis of the effect of electron correlation on the anomalous isotope effect in the NMR spin-spin coupling constant in methane

We discuss the effect of electron correlation on the unexpected differential sensitivity (UDS) in the (1)J(C-H) coupling constant of CH4 using a decomposition into contributions from localized molecular orbitals and compare with the (1)J(N-H) coupling constant in NH3. In particular, we discuss the well known fact that uncorrelated coupled Hartree-Fock (CHF) calculations are not able to reproduce the UDS in methane. For this purpose we have implemented for the first time a localized molecular orbital analysis for the second order polarization propagator approximation with coupled cluster singles and doubles amplitudes--SOPPA(CCSD) in the DALTON program. Comparing the changes in the localized orbital contributions at the correlated SOPPA and SOPPA(CCSD) levels and at the uncorrelated CHF level, we find that the latter overestimates the effect of stretching the bond between the coupled atoms on the contribution to the coupling from the localized bonding orbital between these atoms. This disturbs the subtle balance between the molecular orbital contributions, which lead to the UDS in methane.

Basis set error estimation for DFT calculations of electronic g-tensors for transition metal complexes.

We present a detailed study of the basis set dependence of electronic g-tensors for transition metal complexes calculated using Kohn-Sham density functional theory. Focus is on the use of locally dense basis set schemes where the metal is treated using either the same or a more flexible basis set than used for the ligand sphere. The performance of all basis set schemes is compared to the extrapolated complete basis set limit results. Furthermore, we test the performance of the aug-cc-pVTZ-J basis set developed for calculations of NMR spin-spin and electron paramagnetic resonance hyperfine coupling constants. Our results show that reasonable results can be obtain when using small basis sets for the ligand sphere, and very accurate results are obtained when an aug-cc-pVTZ basis set or similar is used for all atoms in the complex.