NMR Spin-spin Research Articles

Low-density polymer gel dosimeters for 3D radiation dosimetry in the thoracic region: A preliminary study

Different low-density polymer gel dosimeters have been constructed that can be used to acquire the radiation dose distribution of IMRT treatments in the thoracic region. A heterogeneous phantom consisting of a low density polymer gel dosimeter sandwiched between two layers of soft tissue equivalent gel has been constructed. As a proof-of-principle, the phantom has been irradiated with a square 4 cm × 4 cm beam. The dose distribution is read out by use of both quantitative NMR spin-spin (R2) and magnetization transfer (MT) imaging. Sources of error in the dose readout have been assessed and are discussed.

Evaluation of Approximate Exchange-Correlation Functionals in Predicting One-Bond (31)P-(1)H NMR Indirect Spin-Spin Coupling Constants.

This work benchmarks density functional theory, with several different exchange-correlation functionals, for prediction of isotropic one-bond phosphorus-hydrogen NMR spin-spin coupling constants (SSCCs). Our test set consists of experimental SSCCs from 30 diverse molecules representing multiple phosphorus bonding environments. The results suggest the importance of a balance between the choice of correlation functional and the admixture of nonlocal exchange. Overall, standard DFT methods appear to suffice for usefully accurate predictions of (31)P-(1)H SSCCs.

Weak Te,Te Interactions through the Looking Glass of NMR Spin-Spin Coupling

Across the bay: J((125)Te, (125)Te) spin-spin coupling is a highly sensitive probe into the electronic and geometric structure of 1,8-peri-substituted naphthalene tellurium derivatives. The coupling is related to the onset of multicenter bonding in these systems.

NMR Spin-Spin Coupling Constants in Polymethine Dyes as Polarity Indicators

Herein, we explore the use of spin-spin coupling constants (SSCCs) in merocyanine (MCYNE) dyes as indicators of polarity. For this purpose, we use Car-Parrinello hybrid quantum mechanics/molecular mechanics (QM/MM) to determine the structures of MCYNE in solvents of different polarity, followed by computations of the SSCCs by using QM/MM linear-response theory. The molecular geometry of MCYNE switches between neutral, cyanine-like, and zwitterionic depending on the polarity of the solvent. This structural variation is clearly reflected in the proton SSCCs in the polymethine backbone, which are highly sensitive to the dielectric nature of the environment; this mechanism can be used as a "polarity indicator" for different microenvironments. This result is highlighted by computing the SSCCs of the MCYNE probe in the cavity of the beta-lactoglobulin protein. The computed SSCCs clearly indicate a non-polar hydrophobic dielectric nature of this cavity.

NMR vicinal JH-H from DFT theory as reliable tools for stereochemical analysis of 6-heptenyl-5,6-dihydro-2H-pyran-2-ones

A protocol for stereochemical analysis, based on the systematic comparison between theoretical and experimental vicinal 1H-1H NMR coupling constants, was developed and applied to a series of flexible compounds derived from the 6-heptenyl-5,6-dihydro-2H-pyran-2-one framework. The method included a broad conformational search, followed by geometry optimization at the DFT B3LYP/DGDZVP level, calculation of the vibrational frequencies, thermochemical parameters, magnetic shielding tensors, and the total NMR spin-spin coupling constants. The stereochemistry of natural and unnatural 6-heptenyl-5,6-dihydro-2H-pyran-2-ones containing diverse functional groups in the heptenyl side chain was analyzed by application of the correlation between the theoretical (Jpre) and experimental 1H-1H NMR (Jexp). A remarkable correlation demonstrated the predictive value of this approach for the stereochemical assignment of highly flexible compounds containing multiple chiral centers.

NMR compared to band structure calculations of the quaternary superconductor La3Ni2B2N3−x

We measured susceptibility and NMR spectra, spin-lattice, and spin-spin relaxation of ${}^{11}$B and ${}^{139}$La in the quaternary superconductor La${}_{3}$Ni${}_{2}$B${}_{2}$N${}_{3\ensuremath{-}x}$ and compare these experimental results to full potential density functional theory band structure calculations. The electric field gradients (EFGs) at B and La sites clearly show that the N vacancies are restricted to the N(2$b$) sublattice. Temperature-dependent susceptibility, Knight shift, and nuclear spin-lattice relaxation ${T}_{1}$ are successfully described within the Fermi-liquid model. The Hebel-Slichter peak is suppressed in the mixed state and in the presence of paramagnetic impurities.

Stereochemical Dependence of 3JCH Coupling Constants in 2-Substituted 4-t-Butyl-cyclohexanone and Their Alcohol Derivatives

Theoretical and experimental studies on (3)J(C2H6eq) NMR spin-spin coupling constants in both the 2-X-4-t-butyl-cyclohexanone (X = H, CH(3), F, Cl, and Br) and in their alcohol derivatives series are reported. Results thus found are rationalized in terms of the transmission of the Fermi contact contribution to such couplings. To this end, dependencies of (3)J(C2H6eq) couplings versus the C(2)-C(1)-C(6) angle are compared in both series for equatorial and axial X orientations. The main trend is described in terms of the rear lobes interaction. Besides, for X = halogen atom in equatorial orientation a rather strong interaction between oxygen and halogen lone pairs is observed, and its influence on (3)J(C2H6eq) couplings is discussed and rationalized in terms of different Fermi contact transmission pathways.

Palladium Complexes of Constrained Polyphosphines - Discovery and Investigation of “Through-Space” NMR Spin-Spin Couplings in Organometallic Compounds

Palladium Complexes of Constrained Polyphosphines - Discovery and Investigation of “Through-Space” NMR Spin-Spin Couplings in Organometallic Compounds

As75NMR of Ba(Fe0.93Co0.07)2As2in high magnetic field

The superconducting state of an optimally doped single crystal of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$ was investigated by $^{75}$As NMR in high magnetic fields from 6.4 T to 28 T. It was found that the Knight shift is least affected by vortex supercurrents in high magnetic fields, $H>11$ T, revealing slow, possibly higher order than linear, increase with temperature at $T \lesssim 0.5 \, T_c$, with $T_c \approx 23 \, K$. This is consistent with the extended s-wave state with $A_{1g}$ symmetry but the precise details of the gap structure are harder to resolve. Measurements of the NMR spin-spin relaxation time, $T_2$, indicate a strong indirect exchange interaction at all temperatures. Below the superconducting transition temperature vortex dynamics lead to an anomalous dip in $T_2$ at the vortex freezing transition from which we obtain the vortex phase diagram up to $H = 28$ T.

Thermotropic Phase Behavior of Choline Soaps

Choline carboxylates (ChCm with m = 12-18) are simple biocompatible anionic surfactants with very low Krafft temperatures, possessing a rich aqueous phase behavior. In the present work, we have investigated the thermotropic mesomorphism of anhydrous ChCm salts for m = 12-18. Transition temperatures and enthalpies determined by differential scanning calorimetry reveal that all investigated compounds exhibit three different phases between -20 and 95 °C. The phases were further characterized by optical polarizing microscopy, NMR spin-spin relaxation, and X-ray scattering measurements. The nature of the phases was identified with increasing temperature as crystalline, semicrystalline, and liquid-crystalline lamellar. Even long-chain choline carboxylates (m = 18) were found to melt into a lamellar liquid-crystalline phase below 100 °C. Accordingly, with choline as counterion in simple fatty acid soaps, not only the water solubility is considerably enhanced but also the melting points are substantially reduced, hence facilitating thermotropic mesomorphism at temperatures between 35 and 95 °C. Thus, simple choline soaps with m = 12-18 may be classified as ionic liquids.

Effect of Temperature on Quality-Related Changes in Cod (Gadus morhua) During Short- and Long-Term Frozen Storage

Cod (Gadus morhua) was stored at eight temperatures (−10 to −80°C) for 1 to 18 months, after which quality indicators were measured—including drip loss, water holding capacity, low field NMR spin-spin relaxation, color, amount of thiobarbituric acid reactive substances, and sarcoplasmic reticulum Ca2+-ATPase and lysosomal Cathepsin D activities. Results from samples stored up to 12 months showed no significant differences between −30°C and lower temperatures. The NMR measurements, however, indicated some changes in the water distribution of samples stored at −30°C for 12 or more months compared to storage at −40°C or lower.

From Molecular Vibrations to Bonding, Chemical Reactions, and Reaction Mechanism

The vibrational motions of a molecule in its equilibrium or during a chemical reaction provide a wealth of information about its structure, stability, and reactivity. This information is hidden in measured vibrational frequencies and intensities, however can be unraveled by utilizing quantum chemical tools and applying the Cal-X methods in form Vib-Cal-X. Vib-Cal-X uses the measured frequencies, complements them to a complete set of 3N-L values (N number of atoms; L number of translations and rotations), derives experimentally based force constants, and converts them into local mode stretching, bending, and torsional force constants associated with the internal coordinates describing the geometry of the molecule. This is done by utilizing the adiabatic vibrational mode concept, which is based on a decomposition of delocalized normal vibrational modes into adiabatic internal coordinate modes (AICoMs) needed to describe bonding or changes in bonding. AICoM force constants relate to the intrinsic bond dissociation energy (IBDE) of a bond and, accordingly, are excellent descriptors for bond order and bond strength. It is shown that bond dissociation energies, bond lengths, or bond densities are not directly related to the bond strength because they also depend on other quantities than just the bond strength: the bond dissociation energy on the stabilization energies of the fragments, the bond length on the compressibility limit distance between the atoms, the bond stretching frequency on the atom masses, etc. The bond stretching force constants however lead directly to bond order and bond strength as has been demonstrated for the bonds in typical organic molecules. Using this insight, the generalized vibrational frequencies of reacting molecules are used to obtain insight into the chemical processes of bond breaking and forming. An elementary chemical reaction based on these processes is characterized by a curved reaction path. Path curvature is a prerequisite for chemical change and directly related to the changes in the stretching force constants as they respond to the bond polarizing power of a reaction partner. The features of the path curvature can be used to partition the reaction path and by this the reaction mechanism in terms of reaction phases. A reaction phase is characterized by an elementary structural change of the reaction complex leading to a chemically meaningful transient structure that can convert into a real transition state or intermediate upon changing the environmental conditions or the electronic structure (substituents, etc.) of the reaction complex. A unified approach to the study of reaction mechanism (URVA: Unified Reaction Valley Approach) is discussed that is based extensively on the analysis of vibrational modes and that is aimed at detailed understanding of chemical reactions with the goal of controlling them. Keywords: Cal-X methods, vibrational frequency, bond strength, chemical bond, adiabatic internal Coordinate mode (AICoM), unified reaction valley approach (URVA), reaction path curvature, reaction path, Reaction Mechanism, Vib-Cal-X, Quantum chemical tools, Vibrational Force Constants, Fermi resonance, Mossbauer spectroscopy, Overtone Spectroscopy, Bond Energy (BE), Bond Dissociation Energy (BDE), NMR Spin-Spin Coupling Constant, Reaction Path Hamiltonian, Electron Density Analysis, Analysis of Coriolis Couplings, Vibrational spectroscopy, Scalar curvature K(s), Diels-Alder reaction, Woodward-Hoffmann rules

Quantum criticality in a uranium heavy fermion system revealed with NMR spin-spin relaxation

In order to clarify quantum criticality in heavy fermion systems, we propose to use the Gaussian spin-echo decay rate 1/T2G of the NMR spin-spin relaxation. In contrast to the spin-lattice relaxation rate 1/T1, which is connected with the dynamical susceptibility Imχ(q,ω), I/T2G is driven by the static susceptibility Reχ(q,0) at finite q for an antiferromagnet. The combined quantity T1T/T22G is found to provide key information in determining the nature of quantum criticality. The T-dependence of T1T/T22G for the Sn site in the heavy fermion compound USn3 is presented as an example.

Relativistic Zeroth-Order Regular Approximation Combined with Nonhybrid and Hybrid Density Functional Theory: Performance for NMR Indirect Nuclear Spin-Spin Coupling in Heavy Metal Compounds.

A benchmark study for relativistic density functional calculations of NMR spin-spin coupling constants has been performed. The test set contained 47 complexes with heavy metal atoms (W, Pt, Hg, Tl, Pb) with a total of 88 coupling constants involving one or two heavy metal atoms. One-, two-, three-, and four-bond spin-spin couplings have been computed at different levels of theory (nonhybrid vs hybrid DFT, scalar vs two-component relativistic). The computational model was based on geometries fully optimized at the BP/TZP scalar relativistic zeroth-order regular approximation (ZORA) and the conductor-like screening model (COSMO) to include solvent effects. The NMR computations also employed the continuum solvent model. Computations in the gas phase were performed in order to assess the importance of the solvation model. The relative median deviations between various computational models and experiment were found to range between 13% and 21%, with the highest-level computational model (hybrid density functional computations including scalar plus spin-orbit relativistic effects, the COSMO solvent model, and a Gaussian finite-nucleus model) performing best.

Proton-irradiation-induced anomaly in the electrical conductivity of a hydrogen-bonded ferroelastic system

An anomalous abrupt drop in the electrical conductivity has been observed at the ferroelastic phase transition of a proton-irradiated system of hydrogen-bonded TlH{sub 2}PO{sub 4}. As a result of the high-resolution {sup 31}P NMR chemical-shift measurements, distinct changes in the atomic displacements due to the irradiation were identified in the ferroelastic and paraelastic phases. Besides, {sup 1}H NMR spin-spin relaxation measurements revealed a change due to the irradiation in the proton dynamics at the ferroelastic phase transition, apparently accounting for the much-reduced electrical conductivity in the paraelastic phase of the irradiated system.

NMR Studies of Quantum Rotors Confined in Zeolite

We report the results of NMR studies of methane trapped in zeolite at low temperatures. Samples were prepared to contain 1.0±0.2 molecules per α-sodalite cage of zeolite-13X. The NMR spin-spin and spin-lattice relaxation times were measured for 4<T<95 K to determine the rotational dynamics of the molecules and the dependence on the concentration of the A, T and E-molecular species. The results are discussed relative to recent Monte Carlo calculations that show that the molecules are localized but free to tumble in the large α-cages at low temperatures. At higher temperatures there is an effective melting of the translational degrees of freedom for the lattice formed by the centers of the supercages. A sharp definitive jump in the NMR spin-spin relaxation is seen at this “melting” transition.

Conformational analysis of 2,4,5-trimethyl-1,3,2-dioxaborinane individual stereoisomers

The study of the conformational isomerization of cis- and trans-isomers of 2,4,5-trimethyl-1,3-dioxaborinane I by means of RHF//STO-3G, 3-21G and 6-31G(d) quantumchemical methods led to the conclusion that its route includes equilibrium between sofa conformers with a different steric orientation of substituents at the C-4 and C-5 ring atoms. These conformers are interconverted through the maxima, the conformations of equatorial and axial 2,5-twist-forms. A comparison between experimental 1H NMR and theoretical vicinal spin-spin coupling constants was used to determine the quantitative conformational composition of stereoisomers and a value of ΔG0 for conformational equilibrium.

Swelling and pressure-volume relationships in the dermis measured by osmotic-stress technique

Water transfer across the extracellular matrix (ECM) involves interstitial osmotic forces in as yet unclear ways. In particular, the traditional values of Starling forces cannot adequately explain fluid transfer rates. Here, we reassess these forces by analyzing fluid transfer in live pig and human dermal explants. Pressure potentials were controlled with inert polymers adjusted by membrane osmometry (range = 3-219 mmHg), and fluid transfer in and out of the explants was followed by sequential precision weighing. Water motional freedom in the dermis was examined by NMR. In pigs, mean hydration pressure (HP; the pressure at which volume did not change) was 107 +/- 22 and 47 +/- 12 (SE) mmHg at 4 degrees C and 37 degrees C (P = 0.012, paired t-test, n = 7). Volume changes observed in response to pressure potential were reversible. The equation, Volume change = V(max)/[1+(time/T(1/2))(d)], where V(max) is maximal volume change; T(1/2), time at volume = 1/2 V(max); and d, a rate parameter, was fitted to experimental progression curves (r(2) > 0.9), yielding V(max) values linearly related to pressure, with mean slopes -3.5 +/- 0.28 and -2.6 +/- 0.21(SE) mul.g(-1).mmHg(-1) at 4 degrees C and 37 degrees C. NMR spin-spin relaxation times (T(2)) varied within 200- to 400-mum distances in directions perpendicular to the epidermis, with slopes reaching 0.03 ms/mum. Results support a mechanism in which fluid transport across the ECM is locally regulated at micrometer scales by cell- and fiber-gel-dependent osmomechanical forces. The large HP helps to explain the fast interstitial in/out flow rates observed clinically.

Cationic Clathrate I Si46-xPxTey (6.6(1) ≤ y ≤ 7.5(1), x ≤ 2y): Crystal Structure, Homogeneity Range, and Physical Properties

A new cationic clathrate I Si(46-x)P(x)Te(y) (6.6(1) < or = y < or = 7.5(1), x < or = 2y at 1375 K) was synthesized from the elements and characterized by X-ray powder diffraction, thermal analysis, scanning electron microscopy, wavelength dispersive X-ray spectroscopy (WDXS), neutron powder diffraction, and (31)P NMR spectroscopy. The thermal behaviors of the magnetic susceptibility and resistivity were investigated as well. Si(46-x)P(x)Te(y) reveals a wide homogeneity range due to the presence of vacancies in the tellurium guest positions inside the smaller cage of the clathrate I structure. The vacancy ordering in the structure of Si(46-x)P(x)Te(y) causes the change of space group from Pm3n (ideal clathrate I) to Pm3 accompanied by the redistribution of P and Si atoms over different framework positions. Neutron powder diffraction confirmed that P atoms preferably form a cage around the vacancy-containing tellurium guest position. Additionally, (31)P NMR spin-spin relaxation experiments revealed the presence of sites with different coordination of phosphorus atoms. Precise determination of the composition of Si(46-x)P(x)Te(y) by WDXS showed slight but noticeable deviation (x < or = 2y) of phosphorus content from the Zintl counting scheme (x = 2y). The compound is diamagnetic while resistivity measurements show activated behavior or that of heavily doped semiconductors. Thermal analysis revealed high stability of the investigated clathrate: Si(46-x)P(x)Te(y) melts incongruently at approximately 1460 K in vacuum and is stable in air against oxidation up to 1295 K.

Analyzing NMR shielding tensors calculated with two-component relativistic methods using spin-free localized molecular orbitals

A recently developed analysis method [J. Chem. Phys. 127, 124106 (2007)] for NMR spin-spin coupling constants employing two-component (spin-orbit) relativistic density functional theory along with scalar relativistic natural localized molecular orbitals (NLMOs) and natural bond orbitals (NBOs) has been extended for analyzing NMR shielding tensors. Contributions from a field-dependent basis set (gauge-including atomic orbitals) have been included in the formalism. The spin-orbit NLMO/NBO nuclear magnetic shielding analysis has been applied to methane, plumbane, hydrogen iodide, tetracholoplatinate(II), and hexachloroplatinate(IV).