Quadrupole Coupling Constants Research Articles

Transient chirality of anilides - The rotational spectrum of trans-benzanilide

The rotational spectrum of trans-benzanilide C13H11NO is reported for the most part of the cm-wave range (3–19 GHz). Heavy atom frame conversion between the two possible enantiomers surprisingly leads to significant tunnelling splittings in the observed spectrum. The large amplitude motion parameters for the occurring tunnelling as well as the nuclear quadrupole coupling constants for the 14N were determined in addition to the rotational constants and quartic centrifugal distortion coefficients. Contrary to other trans-anilides, calculations indicate the existence of a non planar amide group in trans-benzanilide.

The β‐Agostic Structure in (C5Me5)2Sc(CH2CH3): Solid‐State NMR Studies of (C5Me5)2Sc−R (R=Me, Ph, Et)

AbstractMultinuclear solid‐state NMR studies of Cp*2Sc−R (Cp*=pentamethylcyclopentadienyl; R=Me, Ph, Et) and DFT calculations show that the Sc−Et complex contains a β‐CH agostic interaction. The static central transition 45Sc NMR spectra show that the quadrupolar coupling constants (Cq) follow the trend of Ph≈Me>Et, indicating that the Sc−R bond is different in Cp*2Sc−Et compared to the methyl and phenyl complexes. Analysis of the chemical shift tensor (CST) shows that the deshielding experienced by Cβ in Sc−CH2CH3 is related to coupling between the filled σC‐C orbital and the vacant orbital.

The β-Agostic Structure in (C5 Me5 )2 Sc(CH2 CH3 ): Solid-State NMR Studies of (C5 Me5 )2 Sc-R (R=Me, Ph, Et).

Multinuclear solid-state NMR studies of Cp*2 Sc-R (Cp*=pentamethylcyclopentadienyl; R=Me, Ph, Et) and DFT calculations show that the Sc-Et complex contains a β-CH agostic interaction. The static central transition 45 Sc NMR spectra show that the quadrupolar coupling constants (Cq ) follow the trend of Ph≈Me>Et, indicating that the Sc-R bond is different in Cp*2 Sc-Et compared to the methyl and phenyl complexes. Analysis of the chemical shift tensor (CST) shows that the deshielding experienced by Cβ in Sc-CH2 CH3 is related to coupling between the filled σC-C orbital and the vacant πSc⋯HC* orbital.

Accurate rest frequencies for propargylamine in the ground and low-lying vibrational states

Context.To date, several complex organic molecules have been detected in the interstellar medium, and they have been suggested as precursors of biologically important species. Propargylamine (HC ≡C−CH2−NH2) is structurally similar to a number of other organic molecules which have already been identified by radioastronomy, making it a good candidate for astrophysical detection.Aims.This work provides accurate rest frequencies of propargylamine, from the centimeter-wave to the submillimeter-wave region, useful to facilitate the detection of this molecule in the interstellar medium.Methods.An extensive laboratory study of the rotational spectrum of propargylamine has been performed using a pulsed-jet Fourier Transform Microwave (FTMW) spectrometer (7–19 GHz frequency range) and a frequency modulation microwave spectrometer (75–560 GHz). Several hundred rotational transitions of propargylamine were recorded in the ground and three lowest excited vibrational states. The experiments were supported by high-level ab initio computations, mainly employed to characterize the vibrational state structure and to predict spectroscopic parameters unknown prior to this study.Results.The measured transition frequencies yielded accurate rotational constants and the complete sets of quartic and sextic centrifugal distortion constants for propargylamine in its vibrational ground state.14N-nuclear quadrupole coupling constants were also determined. Rotational and quartic centrifugal distortion constants were also obtained for the low-lying excited statesv13= 1 (A′),v20= 1 (A″), andv21= 1 (A″). Thea-type Coriolis resonance which couples thev13= 1 andv21= 1 levels was analyzed.Conclusions.The determined spectroscopic constants allowed for the compilation of a dataset of highly accurate rest frequencies for astrophysical purposes in the millimeter and submillimeter regions with 1σuncertainties that are smaller than 0.050 MHz, corresponding to 0.03 km s−1at 500 GHz in radial equivalent velocity.

Linear dicoordinate beryllium: a 9Be solid-state NMR study of a discrete zero-valent s-block beryllium complex

Beryllium-9 (9Be) quadrupolar coupling and chemical shift tensor data are reported for bis(1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidine-2-ylidene)beryllium (Be(CAAC)2). These are the first such data for beryllium in a linear dicoordinate environment. The 9Be quadrupolar coupling constant, 2.36(0.02) MHz, is the largest recorded in the solid state to date for this isotope. The span of the beryllium chemical shift tensor, 22(2) ppm, covers about half of the known 9Be chemical shift range, and the isotropic 9Be chemical shift, 32.0(0.3) ppm, is the largest reported in the solid state to our knowledge. DFT calculations reproduce the experimental data well. A natural localized molecular orbital approach has been used to explain the origins and orientation of the beryllium electric field gradient tensor. The single-crystal X-ray structure of a second polymorph of Be(CAAC)2 is also reported. Inspection of the powder X-ray diffraction data shows that the new crystal structure is part of the bulk product next to another crystalline phase. Therefore, experimental X-ray powder data for the microcrystalline powder sample and the SSNMR data do not fully match either the originally reported crystal structure (Arrowsmith et al. Nat. Chem. 2016, 8, 890–894) or the new polymorph. The ability of solid-state NMR and powder X-ray diffraction to characterize powdered samples was thus particularly useful in this work.

The structural and dynamical role of water in natural organic matter: A 2H NMR and XRD study

Natural organic matter (NOM) is an important component in many near-surface geochemical environments, and its properties are greatly affected by the incorporation of water. Because of its importance, the macroscopic behavior and effects of water in NOM and soil organic matter (SOM) have been extensively studied using a wide range of experimental and computational methods. The molecular scale structural and dynamical behavior of water in these materials, however, is less well understood. This paper presents a variable temperature 2H NMR and XRD study of water in Suwannee River NOM and its fulvic acid (FA) and humic acid (HA) fractions that provides new insight into the dynamical behavior of structurally different types of water and exchangeable hydrogen environments in NOM. The results provide a basis for future studies of more complex natural organic materials and the interaction of organic materials with mineral surfaces. Room temperature 2H NMR spectra of samples hydrated in 2H2O and then dehydrated, distinguish 2H2O molecules that are in rapid reorientational motion (correlation times, νc, > 105 Hz), 2H exchanged onto carboxylic sites of the NOM that do not undergo rapid reorientation at frequencies >∼103 Hz, and 2H exchanged onto phenolic and possibly other alcohol sites of the NOM that undergoes rapid, but anisotropic, dynamical reorientation. For samples exposed to water and not dried, the XRD results collected at temperatures from 173 to 298 K show the formation of ice-1h in samples exposed to 100% relative humidity (R.H.) but not in samples exposed to 43% R.H. 2H NMR of those samples collected at temperatures from 313 K to 173 K show the presence of multiple sites. Near room temperature, the spectra contain a narrow resonance for mobile water undergoing rapid isotropic motion, and a broader symmetrical resonance probably due to a combination of more dynamically restricted water molecules and 2H exchanged onto phenolic and alcohol functional groups undergoing rapid anisotropic motion. The 43% R.H. samples also yield a broader, quadrupole-dominated, resonance for 2H exchanged onto functional groups of the NOM. With decreasing temperature the resonances for dynamically restricted water molecules and 2H exchanged onto phenolic and alcohol functional groups become broader, reflecting a decreasing rate of exchange between the water molecules and functional groups and a decreasing rate of reorientation of the 2H2O molecules. The formation of ice-1h is directly reflected in the 2H spectra of the 100% R.H. samples as a resonance with a quadrupole coupling constant (QCC) of ∼180 kHz. For the 43% R.H. samples, there is also a broad, poorly resolved resonance with typical QCCs of ∼180 kHz for which the relative signal intensity increases with decreasing temperature. This signal represents 2H2O molecules that are not crystallized in ice-1h but have greatly reduced reorientation frequencies at low temperature and a hydrogen bonding network with hydrogen bond strengths similar to, but somewhat weaker than, ice-1h. Such molecules are also likely to be present in the 100% R.H. samples. At both R.H.s, some of the 2H2O molecules do not freeze and retain their isotropic motion down to 173 K, the lowest temperature investigated.

Study of Defect Chemistry in the System La2–xSrxNiO4+δ by 17O Solid-State NMR Spectroscopy and Ni K-Edge XANES

The properties of mixed ionic–electronic conductors (MIECs) are most conveniently controlled through site-specific aliovalent substitution, yet few techniques can report directly on the local structure and defect chemistry underpinning changes in ionic and electronic conductivity. In this work, we perform high-resolution 17O (I = 5/2) solid-state NMR spectroscopy of La2–xSrxNiO4+δ, an MIEC and prospective solid oxide fuel cell (SOFC) cathode material, showing the sensitivity of 17O hyperfine (Fermi contact) shifts and quadrupolar coupling constants due to local structural changes arising from Sr substitution (x). Previously, we resolved resonances from three distinct oxygen sites (interstitial, axial, and equatorial) in the unsubstituted x = 0 material (Halat et al., J. Am. Chem. Soc. 2016, 138, 11958). Here, substitution-induced changes in these three spectral features indirectly report on the ionic conductivity, local octahedral tilting, and electronic conductivity, respectively, of the (substituted) ma...

Uniform signal enhancement in MAS NMR of half-integer quadrupolar nuclei using quadruple-frequency sweeps

We introduce two MAS schemes that allow manipulating the satellite-transition (ST) populations of half-integer quadrupolar nuclei, and which both exhibit improved robustness to the quadrupolar coupling constant (CQ). These schemes, called quadruple frequency sweep (QFS) or quadruple WURST (QWURST) are the sums of two DFS or four WURST to efficiently invert the ST populations of nuclei subject to large or small quadrupole interactions, simultaneously. These quadruple sweeps methods only require 6% more rf-power than the double sweeps ones. We demonstrate, both numerically and experimentally, that the QFS and QWURST schemes benefit from robustness to CQ and rf amplitude and offset and hence achieve uniform enhancement of the CT signal for 27Al nuclei subject to different quadrupole interactions. Although the version of QFS with repetitive accumulation can achieve higher enhancement in the S/N of the 27Al MAS spectrum, the final sensitivity gains mainly depend on the longitudinal relaxation time of different 27Al sites. We also confirm that these schemes provide an improved acceleration of the 31P-{27Al} coherence transfer in PT-J-HMQC experiments.

The Nature of Chemical Bonding in Lewis Adducts as Reflected by 27Al NMR Quadrupolar Coupling Constant: Combined Solid-State NMR and Quantum Chemical Approach.

Lewis acids and Lewis adducts are widely used in the chemical industry because of their high catalytic activity. Their precise geometrical description and understanding of their electronic structure are a crucial step for targeted synthesis and specific use. Herein, we present an experimental/computational strategy based on a solid-state NMR crystallographic approach allowing for detailed structural characterization of a wide range of organoaluminum compounds considerably differing in their chemical constitution. In particular, we focus on the precise measurement and subsequent quantum-chemical analysis of many different 27Al NMR resonances in the extremely broad range of quadrupolar coupling constants from 1 to 50 MHz. In this regard, we have optimized an experimental strategy combining a range of static as well as magic angle spinning experiments allowing reliable detection of the entire set of aluminum sites present in trimesitylaluminum (AlMes3) reaction products. In this way, we have spectroscopically resolved six different products in the resulting polycrystalline mixture. All 27Al NMR resonances are precisely recorded and comprehensively analyzed by a quantum-chemical approach. Interestingly, in some cases the recorded 27Al solid-state NMR spectra show unexpected quadrupolar coupling constant values reaching up to ca. 30 MHz, which are attributed to tetra-coordinated aluminum species (Lewis adducts with trigonal pyramidal geometry). The cause of this unusual behavior is explored by analyzing the natural bond orbitals and complexation energies. The linear correlation between the quadrupolar coupling constant value and the nature of bonds in the Lewis adducts is revealed. Moreover, the 27Al NMR data are shown to be sensitive to the geometry of the tetra-coordinated organoaluminum species. Our findings thus provide a viable approach for the direct identification of Lewis acids and Lewis adducts, not only in the investigated multicomponent organoaluminum compounds but also in inorganic zeolites featuring catalytically active trigonal (AlIII) and strongly perturbed AlIV sites.

Pushing the limit of NMR-based distance measurements – retrieving dipolar couplings to spins with extensively large quadrupolar frequencies

Dipolar recoupling under magic-angle spinning allows to measure accurate inter-nuclear distances provided that the two interacting spins can be efficiently and uniformly excited. Alexander (Lex) Vega has shown that adiabatic transfers of populations in quadrupolar spins during the application of constant-wave (cw) radio-frequency pulses lead to efficient and quantifiable dipolar recoupling curves. Accurate distance determination within and beyond the adiabatic regime using cw pulses is limited by the size of the quadrupolar coupling constant. Here we show that using the approach of long-pulse phase modulation, dipolar recoupling and accurate distances can be obtained for nuclei having extensively large quadrupolar frequencies of 5–10 MHz. We demonstrate such results by obtaining a 31P-79/81Br distance in a compound for which bromine-79 (spin-3/2) has a quadrupolar coupling constant of 11.3 MHz, and a 13C-209Bi distance where the bismuth (spin-9/2) has a quadrupolar coupling constant of 256 MHz, equaling a quadrupolar frequency of 10.7 MHz. For Bromine, we demonstrate that an analytical curve based on the assumption of complete spin saturation fits the data. In the case of bismuth acetate, a C-Bi3 spin system must be used in order to match the correct saturation recoupling curve, and results are in agreement with the crystallographic structure.

Rotational and translational dynamics and their relation to hydrogen bond lifetimes in an ionic liquid by means of NMR relaxation time experiments and molecular dynamics simulation.

We report a concerted theoretical and experimental effort to determine the reorientational dynamics as well as hydrogen bond lifetimes for the doubly ionic hydrogen bond +OH⋯O- in the ionic liquid (2-hydroxyethyl)trimethylammonium bis(trifluoromethylsulfonyl)imide [Ch][NTf2] by using a combination of NMR relaxation time experiments, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations. Due to fast proton exchange, the determination of rotational correlation times is challenging. For molecular liquids, 17O-enhanced proton relaxation time experiments have been used to determine the rotational correlation times for the OH vectors in water or alcohols. As an alternative to those expensive isotopic substitution experiments, we employed a recently introduced approach which is providing access to the rotational dynamics from a single NMR deuteron quadrupolar relaxation time experiment. Here, the deuteron quadrupole coupling constants (DQCCs) are obtained from a relation between the DQCC and the δ 1H proton chemical shifts determined from a set of DFT calculated clusters in combination with experimentally determined proton chemical shifts. The NMR-obtained rotational correlation times were compared to those obtained from MD simulations and then related to viscosities for testing the applicability of popular hydrodynamic models. In addition, hydrogen bond lifetimes were derived, using hydrogen bond population correlation functions computed from MD simulations. Here, two different time domains were observed: The short-time contributions to the hydrogen lifetimes and the reorientational correlation times have roughly the same size and are located in the picosecond range, whereas the long-time contributions decay with relaxation times in the nanosecond regime and are related to rather slow diffusion processes. The computed average hydrogen bond lifetime is dominated by the long-time process, highlighting the importance and longevity of hydrogen-bonded ion pairs in these ionic liquids.

QUADRUPOLE COUPLING CONSTANT AND ELECTRONIC STRUCTURE OF COMPOUNDS ЭХ3

For the series of molecules type ЭХ 3 (Э = N, As, Sb; X = F, Cl, Br, J) obtained quantitative relations between the values of the quadrupole coupling constants e 2 Q q and parameters J and μ э3- , characterizing the relative magnitude of the spatial redistribution of the valence electrons of ions Э 3- at varying Э and X. The difference observed dependences e 2 Q q off the dipole moments of the ions μ э3- , to the molecules in the gas and solid phases explained due to the influence of the crystal lattice field. Explored the dependence values of the NQR frequencies ν off the dipole moments of the ions μ э3- .

The argon nuclear quadrupole moments

ABSTRACTNew standard values -116(2) mb and 76(3) mb are suggested for the nuclear quadrupole moments (Q) of the 39Ar and 37Ar nuclei, respectively. The Q values were obtained by combining optical measurements of the quadrupole coupling constant (B or eqQ/h) of the 3s23p54s[3/2]2 (3Po) and 3s23p54p[5/2]3 (3De) states of argon with large scale numerical complete active space self-consistent field and restricted active space self-consistent field calculations of the electric field gradient at the nucleus (q) using the LUCAS code, which is a finite-element based multiconfiguration Hartree–Fock program for atomic structure calculations.

Theoretical Study of the Correlation between 14N NQCC and Its Partial Atomic Charge in Amino Derivatives of Adamantane

The electronic structures and the electrostatic potential of some amino derivatives of adamantane have been studied using the density functional theory. The partial atomic charges and nuclear quadrupole coupling constants (NQCC) of 14N nucleus of the considered molecules have been reported. The partial atomic charges are calculated with two methods for 14N nucleus: Mulliken charges and natural bond orbital (NBO). All of the computations were carried out with the Gaussian 03 program and the B3LYP/6-311++G (d, p) level of theory. The results of both methods have reasonably good correlation with NQCC parameter, but the correlation between NBO charges and NQCC of 14N nucleus has been suggested is much better.

Ferroquadrupolar Ordering due to the Quasi-degenerate Quartet in the Trigonal Chiral Structure of DyNi3Ga9

To clarify the origin of the phase transition at TN = 10 K in the trigonal chiral compound DyNi3Ga9, we performed ultrasonic measurements on single-crystalline samples. We found a significant elastic softening of the transverse modulus C66 toward TN. By analyzing the data using the strain susceptibility, the positive sign of the quadrupole–quadrupole coupling constant for C66 was obtained. These results strongly suggest that ferroquadrupolar ordering will be dominant for the phase transition at TN. Below TN, crystal symmetry will lower from the trigonal structure. Elastic softening below TN occurs in the other elastic moduli, reflecting that some kind of instability is still alive below TN. Elastic anomalies detected below TN suggest successive phase transitions.

DFT computations on surface physical adsorption of hydrocarbons produced in the Fischer-Tropsch synthesis on a CNT/Co nanocatalyst

The physical sorption of several paraffin and olefin hydrocarbons produced in the Fischer-Tropsch (FT) synthesis on the surface of cobalt nanocatalyst supported on (4,4)-armchair carbon nanotube (CNT) was studied using density functional theory (DFT) computations accomplished at B3LYP level of theory. The most negative binding energy (ΔEbinding) was obtained for (4,4)-CNT-Co/C5H10 (compound 10) in which pentene was adsorbed on the CNT/Co surface reflecting this compound had the most retention time to participate in secondary reactions on the surface of the (4,4)-CNT-Co nanocatalyst. The adsorption of all olefin species on the surface of (4,4)-CNT-Co nanocatalyst was exergonic (ΔGadsorption < 0). The negative ΔHadsorption values for compounds 4, 6, 10 and 11 indicated their exothermic adsorption nature. The QTAIM data supported the covalent character of the CC bonds in the (4,4)-CNT and the electrostatic nature of CCo bonds in the (4,4)-CNT-Co nanocatalyst. The nuclear quadrupole coupling constants for the quadrupole 2H, 59Co and 60Co were measured about 180–220 kHz, 23–140 MHz and 26–155 MHz, respectively. According to greater ΔEbinding values for the olefin containing structures, it was inferred that adsorption of the olefins were preferred on the catalyst surface compared with their related paraffin hydrocarbons. Hence, it was established that the (4,4)-CNT-Co nanocatalyst had a superior performance to produce heavy hydrocarbons instead of light types which is very beneficial for the FT process.

The influence of like-charge attraction on the structure and dynamics of ionic liquids: NMR chemical shifts, quadrupole coupling constants, rotational correlation times and failure of Stokes-Einstein-Debye.

Ion pairing is one of the most fundamental atomic interactions in chemistry and biology. In contrast, pairing between like-charged ions remains an elusive concept. So far, this phenomenon was observed only for large-scaled structures, assemblies, stabilizing frameworks, or in aqueous solution wherein like-charge attraction is supported by mediating water molecules. Recently, we reported the formation of cationic clusters in pure ionic liquids (ILs) which all include hydroxyl groups (OH) for possible hydrogen bonding. In such structures like-charge repulsion is overcome by cooperative hydrogen bonds. The vibrational bands in the OH-stretch region of the infrared spectra can be clearly assigned to H-bonded ion pairs (c-a) or to H-bonded cationic clusters (c-c). The equilibrium between both types of ionic clusters can be controlled by using the same cation but differently strong interacting anions. In the present work, we study the influence of the cationic cluster formation on structural and dynamical NMR properties of ionic liquids, where we know that they form cationic clusters to different extent. First, we measure proton chemical shifts, δ1H, and determine deuteron quadrupole coupling constants, χD, from a calculated relation between both NMR properties. Reliable χD values for the liquid phase are a prerequisite for calculating reorientational correlation times, τOH, from measured deuteron relaxation times, T1. It is shown that the correlation times are significantly influenced by the amount of cationic clusters present in the IL. The Stokes-Einstein-Debye (SED) relation is valid for the ILs wherein H-bonded ion pairs (c-a) are the dominant species. With increasing cationic cluster (c-c) formation of e.g. cyclic tetramers, SED breaks down because of the structural heterogeneities.

Functionalization of (n, 0) CNTs (n = 3–16) by uracil: DFT studies

Density functional theory (DFT) calculations were performed to investigate stabilities and properties for uracil (U)-functionalized carbon nanotubes (CNTs). To this aim, the optimized molecular properties were evaluated for (n, 0) models of CNTs (n = 3–16) in the original and U-functionalized forms. The results indicated that the dipole moments and energy gaps were independent of tubular diameters whereas the binding energies showed that the U-functionalization could be better achieved for n = 8–11 curvatures of (n, 0) CNTs. Further studies based on the evaluated atomic-scale properties, including quadrupole coupling constants (C Q ), indicated that the electronic properties of atoms could detect the effects of diameters variations of (n, 0) CNTs, in which the effects were very much significant for the atoms around the U-functionalization regions. Finally, the achieved results of singular U, original CNTs, and CNT-U hybrids were compared to each other to demonstrate the stabilities and properties for the U-functionalized (n, 0) CNTs.

Solid-state 45Sc NMR studies of Cp*2Sc-X and Cp*2ScX(THF).

Cp*2Sc-X, where X is a halide, were synthesized and studied by solid-state 45Sc NMR to determine how the Sc-X bond affects quadrupolar NMR parameters. The experimental quadrupolar coupling constants (CQ) show that the fluoride has the largest coupling constant and that the iodide has the smallest coupling constant. DFT analysis of this data indicates that the CQ of these compounds is related to core scandium and halide orbitals, which is related to polarizability of the halide and the Sc-X distance. Cp*2ScX(THF) were also investigated by solid-state 45Sc NMR spectroscopy, and have much smaller CQ values than the base-free halides. This is related to the change in structure of the THF adduct and occupation of orbitals of π-symmetry that reduce CQ.

Substituent Effect on the Stability and 14N NQR Parameters of Linkage Isomers of Nitriles in a Rhodium Half‐Sandwich Metallacycle: A Theoretical Study

In this work, using the MPW1PW91 method, the substituent effect on the stability and on the 14N NQR parameters of linkage isomers of nitriles in a rhodium half‐sandwich metallacycle is illustrated. After determination of the corresponding isomerization transition state (TS), the substituent effect on the barrier energy and on the activation thermodynamic parameters (ΔG‡ and ΔH‡) of isomerization is explored. The electric field gradient tensor, nuclear quadrupole coupling constant, asymmetry parameter, and nuclear quadrupole resonance frequencies of the studied isomers are calculated. Also, linear correlations between these parameters and Hammett constant of the substituent are explored.