Quadrupole Coupling Constants Research Articles

Fast and Accurate Electric Field Gradient Calculations in Molecular Solids With Density Functional Theory.

Modern approaches for calculating electric field gradient (EFF) tensors in molecular solids rely upon plane-wave calculations employing periodic boundary conditions (PBC). In practice, models employing PBCs are limited to generalized gradient approximation (GGA) density functionals. Hybrid density functionals applied in the context of gauge-including atomic orbital (GIAO) calculations have been shown to substantially improve the accuracy of predicted NMR parameters. Here we propose an efficient method that effectively combines the benefits of both periodic calculations and single-molecule techniques for predicting electric field gradient tensors in molecular solids. Periodic calculations using plane-wave basis sets were used to model the crystalline environment. We then introduce a molecular correction to the periodic result obtained from a single-molecule calculation performed with a hybrid density functional. Single-molecule calculations performed using hybrid density functionals were found to significantly improve the agreement of predicted 17O quadrupolar coupling constants (C q ) with experiment. We demonstrate a 31% reduction in the RMS error for the predicted 17O C q values relative to standard plane-wave methods using a carefully constructed test set comprised of 22 oxygen-containing molecular crystals. We show comparable improvements in accuracy using five different hybrid density functionals and find predicted C q values to be relatively insensitive to the choice of basis set used in the single molecule calculation. Finally, the utility of high-accuracy 17O C q predictions is demonstrated by examining the disordered 4-Nitrobenzaldehyde crystal structure.

Delivery of tioguanine anticancer drug by Fe-doped fullerene cage: DFT evaluation of electronic and structural features

The advantage of employing a model of iron (Fe)-doped carbon-20 fullerene cage (C) for delivery of tioguanine (TG) anticancer drug was investigated in this work by performing density functional theory (DFT) calculations. The models were optimized and corresponding electronic and structural features were evaluated to analyze the model systems of this work. Based on examining relaxation of TG at the C surface, two configurations of TG@C1 and TG@C2 were obtained regarding the orientation of sulfur-mediated TG from six-member ring or five-member ring towards the Fe-doped region of C surface. The calculations indicated that both models could be stabilized with more favorable features in energy and molecular orbitals for TG@C2 model than TG@C1 model. Further analyses of molecular orbital features also indicated mechanism of bimolecular formations. Additionally, the calculated atomic quadrupole coupling constants (QCC) approved achievements of molecular features to recognize TG@C2 as more favorable model for proposing for further investigations.

Rovibrational spectroscopic constants and anharmonic force fields of CH3AsH2 and CH2AsH3: An ab inito study

The spectroscopic constants and anharmonic force fields of Methylarsine and its isomer are systematically studied by B3LYP, CAM-B3LYP, M06-2X methods utilizing the cc-PVTZ and cc-PVQZ basis sets. The calculated equilibrium geometries, fundamental frequencies, and nuclear quadrupole coupling constants, et al of CH3AsH2 are supported by experimental or theoretical data. The harmonic frequencies, anharmonic constants, et al of CH3AsH2 are predicted for the first time. The anharmonic force fields and spectroscopic constants of CH2AsH3 are also investigated with the same theoretical level. The IR spectra of CH3AsH2 and CH2AsH3 are predicted theoretically, and the red-shifts are found in their peaks.

Uncovering Halogen Mixing and Octahedral Dynamics in Cs2SnX6 by Multinuclear Magnetic Resonance Spectroscopy

Cs2SnX6 (X = Cl, Br, and I) compounds have emerged as promising lead-free and ambient-stable materials for photovoltaic and optoelectronic applications. To advance these promising materials, it is crucial to determine the correlations between physical properties and their local structure and dynamics. Solid-state NMR spectroscopy of multiple NMR-active nuclei (133Cs, 119Sn, and 35Cl) in these cesium tin(IV) halides has been used to reveal the atomic structure, which plays a key role in the materials’ optical properties. The 119Sn NMR chemical shifts span approximately 4000 ppm, and the 119Sn spin-lattice relaxation times span 3 orders of magnitude when the halogen goes from chlorine to iodine in these diamagnetic compounds. Moreover, ultrawideline 35Cl NMR spectroscopy of Cs2SnCl6 indicates an axially symmetric chlorine electric field gradient tensor with a large quadrupolar coupling constant of ca. 32 MHz, suggesting the presence of a chlorine moiety that is directly attached to Sn(IV) ions. Variable-temperature 119Sn spin-lattice relaxation time measurements support the presence of dynamics of octahedral SnI6 units in Cs2SnI6. We further show that complete mixed-halide solid solutions of Cs2SnClxBr6–x and Cs2SnBrxI6–x (0 ≤ x ≤ 6) form at any halogen compositional ratio. 119Sn and 133Cs NMR spectroscopy resolve the unique local SnClnBr6–n and SnBrnI6–n (n = 0–6) octahedral and CsBrmI12–m (m = 0–12) cuboctahedral environments in the mixed-halide samples. The experimentally observed 119Sn NMR results are consistent with magnetic shielding parameters obtained by density functional theory computations that were obtained to model the random halogen distribution in mixed-halide analogues. Finally, we demonstrate the difference in the local structures and the optical absorption properties of Cs2SnI6 samples prepared by solvent-assisted and solvent-free synthesis routes.

Neighborhood matters: Steric effects on methyl internal rotation and chlorine nuclear quadrupole coupling in 2-fluoro-4-chlorotoluene

The rotational spectrum of 2-fluoro-4-chlorotoluene was studied in the frequency range 4 to 24 GHz by Fourier transform microwave spectroscopy. Fine torsional splittings into doublets were found for all rotational transitions due to methyl internal rotation hindered with a barrier of about 230 cm−1. In combination with the quadrupole coupling hyperfine splittings on the same order of magnitude originating from the chlorine nucleus, rich spectral features were observed. Rotational and quadrupole coupling constants for the 35Cl and 37Cl isotopologues studied in natural abundance were precisely determined. Results from the experiment were supported by quantum chemical calculations at several levels of theory for the rotational constants and by Bailey's semi-experimental method for the quadrupole coupling constants. The χcc quadrupole coupling tensor element and the barrier to methyl internal rotation were compared with those of other halogen-substituted toluene derivatives.

Mössbauer Study of BaTh2Fe4As4(N0.7O0.3)2

A superconductive BaTh2Fe4As4(N0.7O0.3)2 oxypnictide sample is synthesized using a high‐temperature solid‐state reaction route. X‐ray diffraction followed by Rietveld analysis is used for checking the phase content. 57Fe Mössbauer spectroscopy in transmission geometry is used for characterizing the hyperfine parameters of the iron site. Owing to the slightly deformed tetrahedral As coordination around Fe, the quadrupole coupling constant of Fe is nonzero, but small. This is visible in particular below 80 K. Judging by the consistency two Debye temperatures of 352 and 369 K values obtained using the total absorption and the second‐order Doppler shift, a harmonic approximation for the phonon spectra can be assumed.

Determination of the "Privileged Structure" of 8-Hydroxyquinoline.

An accurate semi-experimental equilibrium structure of 8-hydroxyquinoline (8-HQ) has been determined combining experiment and theory. The cm-wave rotational spectrum of 8-HQ was recorded in a pulsed supersonic jet using broadband dual-path reflection and narrowband Fabry-Perot-type resonator Fourier-transform microwave spectrometers. Accurate rotational and quartic centrifugal distortion constants and 14 N quadrupole coupling constants are determined. Rotational constants of all 13 C, 18 O and 15 N singly substituted isotopologues in natural abundance and those of a chemically synthesized OD isotopologue were used to obtain geometric parameters for all the heavy atoms and the hydroxyl hydrogen from a number of structure determination models. Theoretical approaches allowed for the determination of a semi-experimental equilibrium structure, in which computed rovibrational and electronic corrections were utilized to convert vibrational ground state constants into equilibrium constants. Despite the molecule having only a horizontal plane of symmetry and possessing 11 individual heavy atoms, microwave spectroscopy has allowed for a reliable and accurate structure determination. A mass dependent, structure was determined and proved to be equally reliable by comparison with the B3LYP-D3(BJ)/aVTZ equilibrium structure.

Laboratory Detection of Cyanoacetic Acid: A Jet-Cooled Rotational Study.

Herein we present a laboratory rotational study of cyanoacetic acid (CH2(CN)C(O)OH), an organic acid as well as a -CN bearing molecule, that is a candidate molecular system to be detected in the interstellar medium (ISM). Our investigation aims to provide direct experimental frequencies of cyanoacetic acid to guide its eventual astronomical search in low-frequency surveys. Using different jet-cooled rotational spectroscopic techniques in the time domain, we have determined a precise set of the relevant rotational spectroscopic constants, including the 14N nuclear quadrupole coupling constants for the two distinct structures, cis- and gauche- cyanoacetic acid. We believe this work will potentially allow the detection of cyanoacetic acid in the interstellar medium, whose rotational features have remained unknown until now.

Structural Analyses of Vanillin Derivative Compounds and their Molecular Docking with MPro and RdRp Enzymes of COVID-19

In this work, structural analyses of vanillin (Vanl) and eleven of its derivatives based on the modification of the aldehyde group were investigated using density functional theory (DFT) calculations. In this regard, molecular orbital features and atomic-scale quadrupole coupling constants were evaluated for geometrically optimized structures to see the impact of structural modification on the whole structure. The results indicated that the main impact of such modification was significant only for the modification region, whereas the impact on the rest of the structure was almost negligible. However, electronic features indicated a different tendency for Vanl derivatives for involving in interaction with enzymatic targets. Because of the importance of innovating medication for COVID-19, main protease (MPro) and RNA-dependent RNA polymerase (RdRp) were chosen for the enzymatic target of Vanl ligands for the formation of ligand-target complexes through performing molecular docking (MD) simulations. The results indicated that among the complexes, Vanl 9 (–NHNH2) and 11 (–CH2Cl) could work as the best ligands for interacting with each of RdRp and MPro, respectively. Consequently, optimization of Vanl derivatives could help innovate new compounds for the possible medication of the COVID-19 pandemic.

Solid-state nuclear magnetic resonance of spin-9/2 nuclei 115 In and 209 Bi in functional inorganic complex oxides.

Indium and bismuth are technologically important elements, in particular as oxides for optoelectronic applications. 115 In and 209 Bi are both I = 9/2 nuclei with high natural abundances and moderately high frequencies but large nuclear electric quadrupole moments. Leveraging the quadrupolar interaction as a measure of local symmetry and polyhedral distortions for these nuclei could provide powerful insights on a range of applied materials. However, the absence of reported nuclear magnetic resonance (NMR) parameters on these nuclei, particularly in oxides, hinders their use by the broader materials community. In this contribution, solid-state 115 In and 209 Bi NMR of three recently discovered quaternary bismuth or indium oxides are reported, supported by density functional theory calculations, numerical simulations, diffraction and additional multinuclear (27 Al, 69,71 Ga, and 121 Sb) solid-state NMR measurements. The compounds LiIn2 SbO6 , BiAlTeO6 , and BiGaTeO6 are measured without special equipment at 9.4 T, demonstrating that wideline techniques such as the QCPMG pulse sequence and frequency-stepped acquisition can enable straightforward extraction of quadrupolar tensor information in I = 9/2 115 In and 209 Bi even in sites with large quadrupolar coupling constants. Relationships are described between the NMR observables and local site symmetry. These are amongst the first reports of the NMR parameters of 115 In, 121 Sb, and 209 Bi in oxides.

Proton Transfer in a Bare Superacid-Amine Complex: A Microwave and Computational Study of Trimethylammonium Triflate.

The complex formed from trimethylamine ((CH3)3N) and trifluoromethanesulfonic acid (triflic acid, CF3SO3H) has been observed by Fourier transform microwave spectroscopy in a supersonic jet. Spectroscopic data, most notably 14N nuclear quadrupole coupling constants, are combined with computational results at several levels of theory to unambiguously demonstrate complete or near-complete proton transfer from the triflic acid to the trimethylamine upon complexation. Thus, the system is best regarded as a trimethylammonium triflate ion pair in the gas phase. The formation of an isolated ion pair in a 1:1 complex of a Brønsted acid and base is unusual and likely arises due to the strong acidity of triflic acid. Simple energetic arguments based on proton affinities and the Coulomb interaction energy can be used to rationalize this result.

Facilitating Interstellar Searches for Simple Amino Alcohols with Accurate Rest Frequencies into the Millimeter-wave Regime: Alaninol, Valinol, and Leucinol

Abstract The detection of larger complex organic molecules, such as molecules consisting of several functional groups or those which show conformational flexibility, in the interstellar medium could lead to insights into the availability of biomolecules in space. We present the rotational spectroscopic study of three amino alcohols: alaninol, valinol, and leucinol. The spectra were recorded over the 2–110 GHz region, which included the utilization of a newly developed instrument operating between 18–26 GHz. We report accurately determined line lists, rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling constants for two conformers of alaninol, four conformers of valinol, and three conformers of leucinol, as well as for several singly substituted heavy-atom isotopologues, which also provide structural insights. Further, a number of spectra of vibrationally excited states were assigned, and the associated motions were revealed with anharmonic frequency calculations. Accurate predictions of rotational transitions into the millimeter-wave regime for all species were made, facilitating searches for these complex molecules by observational facilities such as ALMA. Their detection would establish a new family of interstellar molecules.

Distance measurements to quadrupolar nuclei: Evolution of the rotational echo double resonance technique.

Molecular structure determination is the basis for understanding chemical processes and the property of materials. The direct dependence of the magnetic dipolar interaction on the distance makes solid-state nuclear magnetic resonance (NMR) an excellent tool to study molecular structure when X-ray crystallography fails to provide atomic-resolution data. Although techniques to measure distances between pairs of isolated nuclear spin-1/2 pairs are routine and easy to implement using the rotational echo double resonance (REDOR) experiment (Gullion & Schaefer, 1989), the existence of a nucleus with a spin > 1/2, appearing in approximately 75% of the elements in the periodic table, poses a challenge due to difficulties stemming from the large nuclear quadrupolar coupling constant (QCC). This mini-review presents the existing solid-state magic-angle spinning NMR techniques aimed toward the efficient and accurate determination of internuclear distances between a spin-1/2 and a "quadrupolar" nucleus having a spin larger than one half. Analytical expressions are provided for the various recoupling curves stemming from different techniques, and a coherent nomenclature for these various techniques is suggested. Treatment of some special cases such as multiple spin effects and spins with close Larmor frequencies is also discussed. The most advanced methods can recouple spins with quadrupolar frequencies up to tens of megahertz and beyond, expanding the distance measurement capabilities of solid-state NMR to an increasingly growing number of applications and nuclear spin systems.

Barbituric Acid Tautomers: DFT Computations of Keto-Enol Conversions, Frontier Molecular Orbitals and Quadrupole Coupling Constants

In this work, density functional theory (DFT) computations were performed to investigate tautomeric formation processes of barbituric acid (BA). Ten tautomers were totally investigated for the purpose based on the movement of hydrogen atoms among nitrogen and oxygen atoms providing one pure keto form (BA1) and nine other keto-enol forms. The structures were optimized, and BA1 was found to be the most stable one, and both BA3 and BA7 were found to be the most unstable ones. The point was that the ring structure was broken for both BA3 and BA7, but the structure's stability was still approved. Indeed, such serious tautomeric conversion with breaking the structure warns for using such BA bio-organic molecules for further applications, especially in pharmacy-related ones, in which side effects or byproduct synthesis might appear. Further analyses of frontier molecular orbitals features indicated the effects of such tautomerism processes on all model systems, in which more details were obtained by atomic-scale quadrupole coupling constant (Qcc). All obtained results approved significant changes of tautomers regarding molecular and atomic scale features with more or less significant effects regarding the original BA1 reference model.

A DFT approach on tioguanine: Exploring tio-tiol tautomers, frontier molecular orbitals, IR and UV spectra, and quadrupole coupling constants

Tioguanine (TG) is a drug for medication of several types of cancers with favorable efficacy and unfavorable side effects. TG is a modification of guanine nucleobase including two fused heterocyclic rings easy for contributing to tautomerism mechanism, which was investigated in this work based on density functional theory (DFT) calculations. The tautomerism processes of tio-tiol conversion could show serious impacts on the expected function of TG, in which the results of this work indicated that such tautomers could be easily formed with small magnitude of switching energy between the tautomers. Frontier molecular orbitals analyses showed serious variations in the electronic features of TG molecules in tautomeric forms, in which the results were further described by the atomic scale quadrupole coupling constant (Qcc) evaluations. Although the switching energy was small enough to see all the tautomers in the same mode, but molecular and atomic scale features revealed different achievements for such systems as possible reasons of side effects arising during medications. As a final remark of this work, it could be mentioned that TG drug might be modified to show improved efficacies for the patients with lower side effect impacts; otherwise, the tautomeric processes could lead to existence of different side effects.

Microwave measurements of rotational transitions and nitrogen quadrupole coupling for 2-aminopyridine

The high-resolution microwave rotational spectrum was measured for 2-aminopyridine in the 4.5–13.2 GHz range using a pulsed beam Fourier transform microwave spectrometer. The measured transitions for the parent isotopologue in the 0+ vibrational state of the amino group inversion vibration were used to determine accurate rotational constants and quadrupole coupling constants for the two 14N atoms in the molecule. The molecular parameters determined for the 2-aminopyridine in 0+ vibrational state have the following values: A = 5780.374(1) MHz, B = 2733.5017(3) MHz, C = 1857.6768(3) MHz, 1.5χaa (14N1) = 3.5789(45) MHz, 0.25(χbb – χcc) (14N1) = 1.5033(24)MHz, 1.5χaa (14N2) = -0.0958(70) MHz, and 0.25(χbb – χcc) (14N2) = −1.1615(21) MHz. The measured quadrupole coupling constants are in excellent agreement with those calculated using different computational methods and basis sets. It is noted that the calculation using the MP2 functional with cc-pvDz basis set closely reproduces the experimental values for quadrupole coupling constants for the two 14N atoms.

Dynamics of uniformly labelled solid proteins between 100 and 300 K: A 2D 2H-13C MAS NMR approach

We describe a 2H based MAS nuclear magnetic resonance (NMR) method to obtain site-specific molecular dynamics of biomolecules. The method utilizes the use of deuterium nucleus as a spin label that is proven to be very useful in dynamics studies of solid biological and functional materials. The aim is to understand overall characteristics of protein backbone and side-chain motions for CD3, CD2 and CD groups, in terms of timescale, type and activation energy of the underlying processes. Variable temperature two-dimensional (2D) 2H-13C correlation MAS NMR spectra were recorded for the uniformly 2H,13C,15N labelled Alanine and microcrystalline SH3 at a broad temperature range, from 320 K down to 100 K. First, the deuterium quadrupolar-coupling constant from specific D-C sites is obtained with the 2D experiment by utilizing carbon chemical shifts. Second, the static quadrupolar patterns are obtained at 100 K. Third, variable temperature approach enabled the observation of quadrupolar pattern over different motional regimes; slow, intermediate and fast. And finally, the apparent activation energies for C-D sites are determined and compared, by evaluating the temperature induced signal intensities. This information led to the determination of the dynamic processes for different D-C sites at a broad range of temperature and motional timescales. This is a first representation of 2D 2H-13C MAS NMR approach applied to fully isotope labelled deuterated protein covering 220 K temperature range.

Synthesis of the scandium chloride hydrates ScCl3·3H2O and Sc2Cl4(OH)2·12H2O and their characterisation by X-ray diffraction, 45Sc NMR spectroscopy and DFT calculations

Abstract The compounds ScCl3·3H2O (SCTH) and [{Sc(H2O)5(μ-OH)}2]Cl4·2H2O (SCOH), have been synthesised and characterised by single-crystal XRD, 45Sc NMR spectroscopy and DFT calculations, with the crystal structure of SCTH reported here for the first time. From 45Sc NMR measurements under static and MAS conditions, both chemical shift and quadrupolar coupling parameters have been determined. The quadrupolar coupling constants χ for the octahedrally coordinated scandium sites in SCTH are 2.0 ± 0.1 MHz for Sc(1) and 3.81 ± 0.05 MHz for Sc(2). For SCOH, where the hepta-coordination of the single scandium site constitutes a less symmetric electronic environment, 14.68 ± 0.05 MHz was found. DFT calculations for the static SCTH structure consistently overestimate the quadrupolar coupling constants, indicating the possible presence of crystal water dynamics on the NMR time scale.

FeC19 cage vehicle for fluorouracil anticancer drug delivery: DFT approach

This work was performed by the importance of applications of nanostructures in drug delivery processes especially for anticancer consumption. Fluorouracil (FU) is a known anticancer drug, in which its loading at the surface of FeC19 cage vehicle was investigated by performing density functional theory (DFT) calculations. The employed cage was indeed an iron-doped model of C20 cage to provide surface interacting with FU. Seven starting positions (S1 to S7) were predefined for FU to participate in interaction with the cage to provide S1 to S7 complex formations. Optimized structures of six of them were achieved properly, but that of S5 was not achieved even by performing long-hour optimization processes. High strength of obtained complexes approved the complex formation of models and indicating possible role of such metal nanoparticle (MNP) for external filed conducting targeted drug delivery processes. Moreover, the complexes themselves were categorized by the strength level, in which S2 and S4 were introduced as the strongest and the weakest models respectively. Analyses of energies also approved such achievements in addition to confirming results obtained by the atomic scale quadrupole coupling constant (QCC) parameters. Based on such obtained features of detailed information, FU@FeC19 complexes were proposed for drug delivery process.

Hyperfine Coupling Constants in Cu-Based Crystalline Compounds: Solid-State NMR Spectroscopy and First-Principles Calculations with Isolated-Cluster and Extended Periodic-Lattice Models

NMR spectroscopy is a powerful tool for structural analysis of solids, especially if it is complemented by computations of NMR observables, such as chemical shifts and quadrupole coupling constants. In paramagnetic solids, chemical shifts can be greatly affected by hyperfine couplings among the unpaired electrons and atomic nuclei. In this study 13C MAS NMR spectra of three representative crystalline solids (a simple coordination compound of copper and alanine and two complex copper-based metal–organic framework materials) were measured, and the observed paramagnetic shifts were correlated with the hyperfine coupling constants, calculated along two different density functional theory-based approaches. The first approach employed an isolated-cluster model, and the second one used an extended-lattice model and periodic boundary conditions. For both approaches the calculated isotropic hyperfine coupling constants correlated very well with the measured paramagnetic shifts and thus allowed the assignment of the 13C NMR signals, spread over the region of 1000 ppm. This assignment to individual carbon crystallographic sites was in complete agreement with the experimentally derived assignment.