Scalar Coupling Constants Research Articles

Unambiguous and accurate measurement of scalar coupling constants through a selective refocusing NMR experiment

Scalar coupling plays an important role in the analysis of molecular structure and dynamics. A great number of nuclear magnetic resonance (NMR) selective refocusing experiments, such as 2D G-SERF and PSYCHEDELIC, were developed to extract scalar coupling constants involving a selected proton from overlapped spectra. However, intense axial peaks occur in this type of experiments, leading to possible ambiguity in the assignment of spectral peaks and subsequent accurate measurement of 1H–1H scalar coupling constants. Here, a method based on selective coherence transfer and PSYCHEDELIC module is designed to acquire absorption-mode selective refocusing spectrum while suppressing intense axial peaks. Therefore, unambiguous and accurate measurement of scalar coupling constants involving the selectively excited proton can be achieved. The performances of the proposed method are demonstrated on several samples.

A simple data post-processing method for axial peaks free 2D PSYCHEDELIC NMR spectra

Homonuclear scalar coupling plays an important role in the elucidation of molecular structure and dynamics. However, complex multiplets due to 1H–1H scalar coupling splittings complicate the assignment of peaks in overcrowded spectral regions. Although many methods focusing on disentangling couplings have been proposed in recent years, some defects like intense axial peaks and dispersive components still exist. Herein, a simple data post-processing method based on the interleaved acquisition mode PSYCHEDELIC (Pure Shift Yielded by CHirp Excitation to DELiver Individual Couplings) is designed to acquire absorption-mode 2D J spectrum while eradicating axial peaks. This approach provides a high resolution and pure absorptive spectrum, permitting unambiguous and accurate measurement of scalar coupling constants involving a given proton.

Development and Validation of a DFT-Based Force Field for a Hydrated Homoalanine Polypeptide.

A new force field has been created for simulating hydrated alanine polypeptides using the adaptive force matching (AFM) method. Only density functional theory calculations using the Perdew–Burke–Ernzerhof exchange–correlation functional and the D3 dispersion correction were used to fit the force field. The new force field, AFM2020, predicts NMR scalar coupling constants for hydrated homopolymeric alanine in better agreements with experimental data than several other models including those fitted directly to such data. For Ala7, the new force field shows about 15% helical conformations, 20% conformation in the β basin, and 65% polyproline II. The predicted helical population of short hydrated alanine is higher than previous estimates based on the same experimental data. Gas-phase simulations indicate that the force field developed by AFM solution-phase data is likely to produce a reasonable conformation distribution when hydration water is no longer present, such as the interior of a protein.

Backgrounds in gravitational effective field theory

Effective field theories describing gravity coupled to matter are investigated, allowing for operators of arbitrary mass dimension. Terms violating local Lorentz and diffeomorphism invariance while preserving internal gauge symmetries are included. The theoretical framework for violations of local Lorentz and diffeomorphism invariance and associated conceptual issues are discussed, including transformations in curved and approximately flat spacetimes, the treatment of various types of backgrounds, the implications of symmetry breaking, and the no-go constraints for explicit violation in Riemann geometry. Techniques are presented for the construction of effective operators, and the possible terms in the gravity, gauge, fermion, and scalar sectors are classified and enumerated. Explicit expressions are obtained for terms containing operators of mass dimension six or less in the effective action for General Relativity coupled to the Standard Model of particle physics. Special cases considered include Einstein-Maxwell effective field theories and the limit with only scalar coupling constants.

15N NMR studies provide insights into physico-chemical properties of room-temperature ionic liquids.

Ionic liquids (ILs) have gained a lot of attention as alternative solvents in many fields of science in the last two decades. It is known that the type of anion has a significant influence on the macroscopic properties of the IL. To gain insights into the molecular mechanisms responsible for these effects it is important to characterize these systems at the microscopic level. Such information can be obtained from nuclear spin-relaxation studies which for compounds with natural isotope abundance are typically performed using direct 1H or 13C measurements. Here we used direct 15N measurements to characterize spin relaxation of non-protonated nitrogens in imidazolium-based ILs which are liquid at ambient temperature. We report heteronuclear 1H-15N scalar coupling constants (nJHN) and 15N relaxation parameters for non-protonated nitrogens in ten 1-ethyl-3-methylimidazolium ([C2C1IM]+)-based ILs containing a broad range of anions. The 15N relaxation rates and steady-state heteronuclear 15N-{1H} NOEs were measured using direct 15N detection at 293.2 K and two magnetic field strengths, 9.4 T and 16.4 T. The experimental data were analyzed to determine hydrodynamic characteristics of ILs and to assess the contributions to 15N relaxation from 15N chemical shift anisotropy and from 1H-15N dipolar interactions with non-bonded protons. We found that the rotational correlation times of the [C2C1IM]+ cation determined from 15N relaxation measurements at room temperature correlate linearly with the macroscopic viscosity of the ILs. Depending on the selected anion, the 15N relaxation characteristics of [C2C1IM]+ differ considerably reflecting the influence of the anion on the physicochemical properties of the IL.

The bi-Lebedev scheme for the Maxwell eigenvalue problem with 3D bi-anisotropic complex media

This paper focuses on studying the eigenstructure of generalized eigenvalue problems (GEPs) arising in the three-dimensional source-free Maxwell equations for bi-anisotropic complex media with a 3-by-3 permittivity tensor ε>0, a permeability tensor μ>0, and scalar magnetoelectric coupling constants ξ=ζ̄=ıγ. The bi-Lebedev scheme is appealing because it preserves the symmetry inherent to the Maxwell eigenvalue problem exactly and because full degrees of freedom of electromagnetic fields at each grid point are taken into account in the discretization. The resulting GEP has eigenvalues appearing in quadruples {±ω,±ω̄}. We consider two main scenarios, where γ<γ∗ and γ>γ∗ with γ∗ as a critical value. In the former case, all the eigenvalues are real. In the latter case, the GEP has complex eigenvalues, and we particularly focus on the bifurcation of the eigenstructure of the GEPs. Numerical results demonstrate that the newborn ground state has occurred after γ=γ̃>γ∗, and the associated eigenvector has an exotic phenomenon of localization. Moreover, the Poynting vectors of the newborn eigenvector not only are concentrated in the material but also display exciting patterns.

NMR Calculations with Quantum Methods: Development of New Tools for Structural Elucidation and Beyond.

Structural elucidation is an important and challenging stage in the discovery of new organic molecules. Single-crystal X-ray analysis provides the most unquestionable results, though in practice the availability of suitable crystals limits its broad use. On the other hand, NMR spectroscopy has become the leading and universal technique to accomplish the task. Despite continuous advances in the field, the misinterpretation of NMR data is commonplace, evidenced by the large number of erroneous structures being published in top journals. Quantum calculations of NMR chemical shifts and scalar coupling constants emerged as ideal complements to facilitate the elucidation process when experimental NMR data is inconclusive. Since seminal reports demonstrated that affordable DFT methods provide NMR predictions accurate enough to differentiate among closely related isomers, the discipline has experienced substantial growth. The impact has been felt in different areas, and nowadays the results of such calculations are routinely seen in high impact literature.This Account describes our investigations in the field of quantum NMR calculations, focusing on the development of tools for structural elucidation and practical applications. We pioneered the use of artificial intelligence methods in the development of novel strategies of structural validation. Our first generation of trained artificial neural networks (ANNs) showed excellent ability to identify mistakes at the atom connectivity level, whereas the use of multidimensional pattern recognition pushed the performance to the stereochemical limit. In a conceptually different approach, we developed DP4+, an updated version of the DP4 probability used to determine the most likely structure among two or more candidates when one set of experimental data is available. Increasing the level of theory in NMR calculations and including unscaled data in the formalism improved the performance of the method, further validated to settle the configuration of challenging motifs such as spiroepoxides or Mosher's derivatives. One of the limitations of DP4+ is related to the relatively large computational cost involved in obtaining DFT-optimized geometries, which led to the development of a fast variant including the valuable information provided by coupling constants (J-DP4 method).These tools were explored to suggest the most probable structure of controversial natural or unnatural products originally misassigned, with some predictions further validated by synthesis (as in the case of pseudorubriflordilactone B). The possibility of predicting the structure of a natural product without requiring authentic sample was investigated in collaboration with Prof. Pilli (UNICAMP, Brazil) in the computer-guided total synthesis and stereochemical revisions of several natural products. Despite these advances, there remain considerable challenges, such as the case of configurational assessment of polar systems featuring multiple intramolecular hydrogen bonding interactions because of the poor energy predictions provided by most DFT methods. In our latest work, we tackle this problem by averaging the results provided by randomly generated ensembles, paving the way for a new paradigm in quantum NMR-assisted structural elucidation.

Scalegenesis and fermionic dark matters in the flatland scenario

We propose an extension of the standard model with Majorana-type fermionic dark matters based on the flatland scenario where all scalar coupling constants, including scalar mass terms, vanish at the Planck scale, i.e. the scalar potential is flat above the Planck scale. This scenario could be compatible with the asymptotic safety paradigm for quantum gravity. We search the parameter space so that the model reproduces the observed values such as the Higgs mass, the electroweak vacuum and the relic abundance of dark matter. We also investigate the spin-independent elastic cross section for the Majorana fermions and a nucleon. It is shown that the Majorana fermions as dark matter candidates could be tested by dark matter direct detection experiments such as XENON, LUX and PandaX-II. We demonstrate that within the minimal setup compatible with the flatland scenario at the Planck scale or asymptotically safe quantum gravity, the extended model could have a strong predictability.

Fully Exploiting the Power of 2D NMR J-Resolved Spectroscopy.

Nuclear magnetic resonance (NMR) spectroscopy is a powerful analytical tool that enables one to study molecular properties and interactions. Homonuclear couplings provide valuable structural information but are often difficult to disentangle in crowded 1H NMR spectra where complex multiplets and signal overlap commonly exist. Multidimensional NMR experiments push the power of NMR to a new level by providing better signal dispersion. Among them, 2D J-resolved spectroscopy is widely used for multiplet analysis and the measurement of scalar coupling constants. Here, we present a new 2D J-resolved method, CASCADE, through which easier multiplet analysis and unambiguous measurement of specific coupling constants can be achieved at the same time, fully exploiting the power of 2D J-resolved spectroscopy. It is expected that this method may replace a conventional 2D J experiment in many cases, facilitating structural and configurational studies as well as chemical and biological analyses.

Anomalies in the space of coupling constants and their dynamical applications I

It is customary to couple a quantum system to external classical fields. One application is to couple the global symmetries of the system (including the Poincaré symmetry) to background gauge fields (and a metric for the Poincaré symmetry). Failure of gauge invariance of the partition function under gauge transformations of these fields reflects ’t Hooft anomalies. It is also common to view the ordinary (scalar) coupling constants as background fields, i.e. to study the theory when they are spacetime dependent. We will show that the notion of ’t Hooft anomalies can be extended naturally to include these scalar background fields. Just as ordinary ’t Hooft anomalies allow us to deduce dynamical consequences about the phases of the theory and its defects, the same is true for these generalized ’t Hooft anomalies. Specifically, since the coupling constants vary, we can learn that certain phase transitions must be present. We will demonstrate these anomalies and their applications in simple pedagogical examples in one dimension (quantum mechanics) and in some two, three, and four-dimensional quantum field theories. An anomaly is an example of an invertible field theory, which can be described as an object in (generalized) differential cohomology. We give an introduction to this perspective. Also, we use Quillen’s superconnections to derive the anomaly for a free spinor field with variable mass. In a companion paper we will study four-dimensional gauge theories showing how our view unifies and extends many recently obtained results.

Scalar Coupling Constant Prediction Using Graph Embedding Local Attention Encoder

Scalar coupling constant (SCC) plays a key role in the analysis of three-dimensional structure of organic matter, however, the traditional SCC prediction using quantum mechanical calculations is very time-consuming. To calculate SCC efficiently and accurately, we proposed a graph embedding local self-attention encoder (GELAE) model, in which, a novel invariant structure representation of the coupling system in terms of bond length, bond angle and dihedral angle was presented firstly, and then a local self-attention module embedded with the adjacent matrix of a graph was designed to extract effectively the features of coupling systems, finally, with a modified classification loss function, the SCC was predicted. To validate the superiority of the proposed method, we conducted a series of comparison experiments using different structure representations, different attention modules, and different losses. The experimental results demonstrate that, compared to the traditional chemical bond structure representations, the rotation and translation invariant structure representations proposed in this work can improve the SCC prediction accuracy; with the graph embedded local self-attention, the mean absolute error (MAE) of the prediction model in the validation set decreases from 0.1603 Hz to 0.1067 Hz; using the classification based loss function instead of the scaled regression loss, the MAE of the predicted SCC can be decreased to 0.0963 HZ, which is close to the quantum chemistry standard on CHAMPS dataset.

Accurate measurement of effective Li-Li scalar coupling constants: the NMR missing link for alkyllithium aggregates.

Scalar coupling in organolithium systems can provide access to useful structural and dynamic informations. In this work, we propose a robust method for the accurate measurement of the effective 2JLi-Li coupling constant in tetramerics alkyllithium aggregates. This crucial information, unavalaible to date, gives a simple access to various structural factors, including the dynamics, solvation and the operative steric hindrance of alkyl chains.

Detectability of ultralight scalar field dark matter with gravitational-wave detectors

An ultralight scalar field is one of the dark matter candidates. If it couples with Standard Model particles, it oscillates mirrors in gravitational-wave detectors and generates detectable signals. We study the spectra of the signals taking into account the motion of the detectors due to the Earth's rotation/the detectors' orbital motion around the Sun and formulate a suitable data-analysis method to detect it. We find that our method can improve the existing constraints given by fifth-force experiments on one of the scalar field's coupling constants by a factor of $\sim 30$, $\sim 100$ and $\sim 350$ for $m_\phi = 2 \times 10^{-17}~\mathrm{eV},~10^{-14}~\mathrm{eV}$ and $10^{-12}~\mathrm{eV}$ respectively, where $m_\phi$ is the scalar field's mass. Our study demonstrates that experiments with gravitational-wave detectors play a complementary role to that Equivalence Principle tests do.

Prototropic behavior of cyclohexane substituted urethane and urea compounds. Observation of H-bond mediated 4HJH1H3 coupling constants across urea fragments

Two 1-aryl-3-cyclohexylurea and two 1-aryl-3-cyclohexylurethane with and without alkyl tail in aryl fragments were synthesized and their variable-temperature 1H NMR spectra in chloroform, DMSO and DMF were recorded. The temperature dependences of chemical shift of NH protons for all compounds have been observed. The type of dependence has been explained by the aggregation of molecules and formation of ionic structures. The formation of intermolecular complexes leads to possible formation of symmetrical N1 … … H ……N3 intramolecular hydrogen bond in urea fragment. As a result,4hJH1H3 trans-hydrogen bond scalar coupling constants have been observed for the first time in low molecular weight compounds using a simple one-dimensional 1H NMR experiment. The formation of strong intramolecular H-bond leads to π-conjugation not only with spacer but with phenyl ring too. It support the Gilli conception that RAHB formation is a result of π-electron delocalization. The presence in urea fragment of such kind interaction leads to formation of ionic structure, which has been detected by NMR and UV spectroscopies. The formation of ionic structure can explain the catalytic activity of such compounds and the mechanism of transformation in organic and bioorganic reactions in which involved the urea compounds.

How to measure long-range proton-carbon coupling constants from 1 H-selective HSQMBC experiments.

Heteronuclear long-range scalar coupling constants (n JCH ) are a valuable tool for solving problems in organic chemistry and are especially suited for stereochemical and configurational analyses of small molecules and natural products. This tutorial will focus on the step-by-step implementation of several 2D 1 H frequency selective HSQMBC experiments for the easy and accurate measurement of either the magnitude or both the magnitude and the sign of long-range n JCH couplings. The performance of these experiments will be showcased with several scenarios in a range of different experimental conditions.

Evaluating stereoelectronic properties of bulky dialkylterphenyl phosphine ligands

The stereoelectronic properties of a series of sterically hindered phosphines containing a terphenyl substituent, PR2Ar’ (R = alkyl; Ar’ = C6H3-2,6-Ar2), have been evaluated by various methods. Their σ-donating capacity has been assessed on the basis of the carbon monoxide stretching frequencies in benchmark iridium [IrCl(CO)2(PR2Ar’)] and rhodium [Rh(acac)(CO)2(PR2Ar’)] (acac = acetylacetonate) complexes, as well as by measuring 31P–77Se scalar coupling constants (1JSeP) for the corresponding phosphine selenides (Se=PR2Ar’). In turn, the steric profile of terphenyl phosphines has been gauged by calculating Tolman Cone Angle (TCA), ligand shielding (G) and percent buried volume (%VBur) parameters. These calculations have been carried out from both X-ray diffraction and DFT-optimized structures. We have also examined several of the widely used biaryl phosphines for comparative purposes.

Pushing resolution limits for extracting 1H-1H scalar coupling constants by a resolution-enhanced selective refocusing method.

Nuclear magnetic resonance (NMR) spectroscopy enables one to study molecular structure and dynamics in a noninvasive manner and has long served as a versatile and indispensable analytical tool in physics, chemistry, and biology. Scalar coupling, an essential feature in NMR spectroscopy, provides rich information regarding molecular structure and conformation. The measurement of scalar coupling constants, therefore, constitutes an important issue in NMR spectroscopy. Homonuclear 2D J-resolved spectroscopy is a powerful tool for multiplet analysis and coupling measurement. Recently, a number of phase-sensitive J-resolved methods and selective measuring methods have been developed to facilitate the extraction of coupling constants. However, resolution remains a crucial challenge when extracting small coupling constants or under inhomogeneous fields. In this paper, we present a resolution-enhanced selective refocusing (RESERF) method for the extraction of coupling constants. The effect of magnetic field inhomogeneity can be eliminated, resulting in very narrow linewidths. Therefore, samples with small coupling constants or under inhomogeneous fields can be well analyzed. The RESERF method may be of great value for structural and conformational studies in chemistry and biology.

Hawking radiation for scalar fields by Einstein-Gauss-Bonnet–de Sitter black holes

We study the greybody factor and power spectra of Hawking radiation for the minimally or nonminimally coupled scalar field with exact numerical method in spherically symmetric Einstein-Gauss-Bonnet-de Sitter black hole spacetime. The effects of scalar coupling constant, angular momentum number of scalar, spacetime dimension, cosmological constant and Gauss-Bonnet coupling constant on the Hawking radiation are studied in detail. Specifically, the Gauss-Bonnet coupling constant always increases the greybody factor in the entire energy regime. Different from the case of Schwarzschild-de Sitter black hole, the effects of the scalar coupling constant on the greybody factor are not monotonic but relevant to the values of Gauss-Bonnet coupling constant. Moreover, both these two coupling constants always suppress the power spectra of Hawking radiation in the whole energy regime.

Probing O-H Bonding through Proton Detected 1H-17O Double Resonance Solid-State NMR Spectroscopy.

The ubiquity of oxygen in organic, inorganic, and biological systems has stimulated the application and development of 17O solid-state NMR spectroscopy as a probe of molecular structure and dynamics. Unfortunately, 17O solid-state NMR experiments are often hindered by a combination of broad NMR signals and low sensitivity. Here, it is demonstrated that fast MAS and proton detection with the D-RINEPT pulse sequence can be generally applied to enhance the sensitivity and resolution of 17O solid-state NMR experiments. Complete 2D 17O → 1H D-RINEPT correlation NMR spectra were typically obtained in less than 10 h from less than 10 mg of material, with low to moderate 17O enrichment (less than 20%). Two-dimensional 1H-17O correlation solid-state NMR spectra allow overlapping oxygen sites to be resolved on the basis of proton chemical shifts or by varying the mixing time used for 1H-17O magnetization transfer. In addition, J-resolved or separated local field (SLF) blocks can be incorporated into the D-RINEPT pulse sequence to allow the direct measurement of one-bond 1H-17O scalar coupling constants (1 JOH) or 1H-17O dipolar couplings ( DOH), respectively, the latter of which can be used to infer 1H-17O bond lengths. 1 JOH and DOH calculated from plane-wave density functional theory (DFT) show very good agreement with experimental values. Therefore, the 2D 1H-17O correlation experiments, 1H-17O scalar and dipolar couplings, and plane-wave DFT calculations provide a method to precisely determine proton positions relative to oxygen atoms. This capability opens new opportunities to probe interactions between oxygen and hydrogen in a variety of chemical systems.

Neutron dark matter decays and correlation coefficients of neutron β−-decays

As we have pointed out in (arXiv:1806.10107 [hep-ph]), the existence of neutron dark matter decay modes n→χ+anything, where χ is a dark matter fermion, for the solution of the neutron lifetime problem changes priorities and demands to describe the neutron lifetime τn=888.0(2.0)s, measured in beam experiments and defined by the decay modes n→p+anything, in the Standard Model (SM). The latter requires the axial coupling constant λ to be equal to λ=−1.2690 (arXiv:1806.10107 [hep-ph]). Since such an axial coupling constant is excluded by experimental data reported by the PERKEO II and UCNA Collaborations, the neutron lifetime τn=888.0(2.0)s can be explained only by virtue of interactions beyond the SM, namely, by the Fierz interference term of order b∼−10−2 dependent on scalar and tensor coupling constants. We give a complete analysis of all correlation coefficients of the neutron β−-decays with polarized neutron, taking into account the contributions of scalar and tensor interactions beyond the SM with the Fierz interference term b∼−10−2. We show that the obtained results agree well with contemporary experimental data that does not prevent the neutron with the rate of the decay modes n→p+anything, measured in beam experiments, to have dark matter decay modes n→χ+anything.