Intermolecular Dipolar Interactions Research Articles

Detection and characterization of intermolecular multiple-quantum coherence NMR signals of IS ( I = 1/2; S = 3/2) spin systems

Intermolecular multiple-quantum coherences (iMQCs) have some intrinsic properties different from conventional single-quantum coherences in solution NMR. In this paper, we extended our study to heteronuclear iMQCs in IS ( I = 1/2, S = 3/2) spin systems. A sample of sodium chloride (NaCl) water solution was taken as an example. Heteronuclear COSY revamped by asymmetric Z-gradient echo detection (CRAZED) experiments were performed. One- and two-dimensional heteronuclear iMQC spectra were obtained. The quantum-mechanical treatment was used to deduce the signal expressions. Magic angle experiments validate that the signals are indeed from intermolecular dipolar interaction and insensitive to the imperfection of radio-frequency (RF) flip angles. Both experimental results and theoretical analysis indicate that heteronuclear CRAZED experiment allows coherence transfer from spin-3/2 nuclei to spin-1/2, and vice verse. Furthermore, the dependences of iMQC signal intensities on RF pulse flip angles follow the same rules as those for heteronuclear IS ( I = 1/2, S = 1/2 or 1) spin systems.

Alignment and Relaxation Dynamics of Dye Molecules in Host−Guest Inclusion Compounds As Probed by Dielectric Spectroscopy

The alignment and relaxation dynamics of a polar dye molecule, N,N-dimethyl-4(4-nitrophenylazo)aniline (DNAA), in zeolite L and perhydrotriphenylene (PHTP) channels were investigated by means of a combination of optical, dielectric, and quantum-chemical methods. Both the zeolite L and PHTP channels enable the dye molecules to align along the channel axis. An amplified net dipole moment of DNAA in PHTP is observed and attributed to enhanced 1D close alignment of dye molecules. In zeolite L channels, a concentration gradient is found with aggregation at the channel entrances. The dynamics of the dye in zeolite L channels reveals localized conical rotational fluctuation modes following Arrhenius-type activation with energy of 0.31 eV, which we assign to small noninteracting fluctuating polar units of the dyes being loosely aligned or isolated. Unlike zeolite L, relaxations in PHTP are characterized by cooperative wobbling motions interpreted as increased intermolecular dipole interaction due to a closely packed one-dimensional array. Temperature-dependent activation energies of 0.25 eV below 0 degrees C and 0.37 eV at ambient temperature reflect the role of the soft channel walls in the activation process.

Strong interfacial attrition developed by oleate/layered double hydroxide nanoplatelets dispersed into poly(butylene succinate)

Poly(butylene succinate) (PBS) nanocomposite structure was studied as a function of the filler percentage loading. The resulting state of dispersion was evaluated by XRD and TEM, and the interfacial attrition between PBS chain and lamellar platelets by the melt rheological properties. Hybrid organic inorganic (O/I) layered double hydroxide (LDH) organo-modified by oleate anions was used as filler. It was found that the confinement supplied by the LDH framework forces the interleaved organic molecule to be more distant from each other than in the case of oleate salt, this having as an effect to decrease strongly the homonuclear intermolecular 1H 1H dipolar interaction. An additional consequence of this relatively free molecular rotation, affecting the 13C CPMAS response as well, is to facilitate the delamination of the 2D-stacked layers during extrusion since an quasi-exfoliated PBS:Mg 2Al/oleate structure is observed for filler loading lower than 5% w/w. This is in association to a non-linear viscoelasticity in the low- ω region and the observed shear-thinning tendency compares better than other PBS:silicate nanocomposite derivatives and is here explained by the presence of a percolated LDH nanoparticle network. Indeed the plastic deformation in the low- ω region is found to be restricted by well-dispersed LDH tactoids in association with a rather strong attrition phenomenon between tethered oleate anions and PBS chains.

Characterization of cyclodextrin complexes of camostat mesylate by ESI mass spectrometry and NMR spectroscopy

Supramolecular interactions between camostat mesylate, a serine protease inhibitor ( 1), with α-, β-, and γ-cyclodextrin (CD) in water were investigated using electrospray ionization mass spectrometry (ESI-MS) and nuclear magnetic resonance (NMR) spectroscopy. ESI mass spectral analysis revealed that the 1:1 stoichiometry in all the complexes was formed. The binding constants ( K st) calculated by linear equations constructed from the ESI mass spectra of all the complexes indicated that γ-CD was most favorable complexing agent for the binding with 1 among the CDs. Pronounced changes in the 1H chemical shift upon complex formation with γ-CD were observed for the protons of the two aromatic rings of 1, with much larger chemical shift changes observed for the protons of the guanidinyl group-linked aromatic ring of 1. These results suggest that the cavity of γ-CD rather than that of α- or β-CD is large enough to accommodate the guanidine group of 1. Spatial geometry of 1 within the cavity of γ-CD was further identified with two-dimensional rotating frame nuclear Overhauser effect spectroscopy (2D ROESY) experiment. The observed ROESY cross peaks indicated intermolecular dipolar interactions between the two aromatic ring protons of 1 and the protons within the cavity of γ-CD. Based on the 1:1 stoichiometry of the complex, ROESY cross peaks suggest that two types of 1:1 complexes of γ-CD with 1 exist simultaneously in solution with different geometries.

Fast acquisition of high-resolution NMR spectra in inhomogeneous fields via intermolecular double-quantum coherences

A pulse sequence, IDEAL-II, is proposed based on the concept of intermolecular dipolar-interaction enhanced all lines [Z. Chen et al., J. Am. Chem. Soc. 126, 446 (2004)] for obtaining one-dimensional (1D) high-resolution liquid NMR spectra in inhomogeneous fields via two-dimensional acquisitions. With the new acquisition scheme, the range of magnetic field inhomogeneity rather than chemical shift is sampled in the indirect dimension. This enables a great reduction in acquisition time and amount of data, much improved over the original IDEAL implementation. It is applicable to both isolated and J-coupled spin systems in liquid. For the latter, apparent J coupling constants are magnified threefold in spectra obtained with this sequence. This allows a more accurate measurement of J coupling constants in the cases of small J coupling constants or large inhomogeneous fields. Analytical expression was derived based on intermolecular multiple-quantum coherence treatments. Solution samples that were purposely deshimmed and biological samples with intrinsic field inhomogeneities were tested. Experimental results demonstrate that this sequence retains useful structural information including chemical shifts, relative peak areas, and multiplet patterns of J coupling even when the field inhomogeneity is severe enough to almost erase all spectroscopic information with conventional 1D single-quantum coherence techniques. This sequence is more applicable to weakly coupled and uncoupled spin systems, potentially useful for studying metabolites in in vivo NMR spectroscopy and for characterizing technologically important new materials in combinatorial chemistry.

Insights into the Coal Extractive Solvent N-Methyl-2-pyrrolidone + Carbon Disulfide

A wide set of experimental and computational tools were used to characterize the N-methyl-2-pyrrolidone (NMP) + carbon disulfide mixed solvent in the full composition range. The interest in this solvent rose from its very efficient use for coal extraction through a mechanism still not fully understood. Thermophysical properties at ambient pressure together with pressure-volume-temperature (PVT) behavior were measured with the objective of providing the required data for the industrial use of the mixed fluid and to get insight into the fluid structure at the molecular level. NMR, FTIR, and solvatochromic studies were performed together with microwave dielectric relaxation spectroscopy (DRS) measurements, thus providing more information on the fluid's structure and allowing one to relate the molecular level behavior with the measured macroscopic properties. Moreover, density functional theory (DFT) and classical molecular dynamics simulations (MD) were used to obtain a detailed picture of the intermolecular interactions within the fluid, at short and long ranges, and of other relevant features leading to the structure of the studied system. The whole study leads to a fluid's picture in which carbon disulfide hinders the development of NMP/NMP intermolecular dipolar interactions, thus increasing the monomer population. We should remark that some properties reported in thismore » work are in remarkable disagreement with previously reported studies, the most important one being the positive excess molar volume in the whole pressure-temperature range studied, which contrasts with the negative values reported in the literature. Previously reported properties are hardly justified with a coherent molecular level picture, whereas the whole collection of properties reported in this work leads to a more reasonable fluid's structure. 56 refs., 17 figs., 2 tabs.« less

Radical pairs with rotational fluidity in the photochemical reaction of acetophenone and cyclohexane in the zeolite NAY: a 13C CPMAS NMR and product analysis study

The photochemical reaction of acetophenone and cyclohexane in the zeolite NaY occurs by combination of the geminate radical pairs to give products that reveal a significant amount of rotational fluidity, which was also documented by intermolecular nuclear dipolar interaction measurements using cross polarization (13)C NMR (CPMAS) experiments.

Quantum Phase Transition Between a Luttinger Liquid and a Gas of Cold Molecules

We consider cold polar molecules confined in a helical optical lattice similar to those used in holographic microfabrication. An external electric field polarizes molecules along the axis of the helix. The large-distance intermolecular dipolar interaction is attractive but the short-scale interaction is repulsive due to geometric constraints and thus prevents collapse. The interaction strength depends on the electric field. We show that a zero-temperature second-order liquid-gas transition occurs at a critical field. It can be observed under experimentally accessible conditions.

Enhanced nonlinear magnetic resonance signals via square wave dipolar fields.

This report introduces a new approach that enhances nonlinear solution magnetic resonance signals from intermolecular dipolar interactions. The resulting signals can theoretically be as large as the full equilibrium magnetization. Simple, readily implemented pulse sequences using square-wave magnetization modulation simultaneously refocus all even order intermolecular multiple quantum coherences, leading to a substantial net signal enhancement, complex nonlinear dynamics, and improved structural sensitivity under realistic conditions.

Mechanisms of Dipolar Ortho/Para-H2O Conversion in Ice

Abstract In this paper a possible explanation for an unexpected ortho/para-water ratio in the gas clouds of comets is given. The description is based on the quantum-mechanical density matrix formalism and the spin temperature concept. Only the nuclear spin system is treated quantum-mechanically. Employing the model of a four spin system, created by two nearest neighbour water molecules, spin eigenstates and their dynamics under the influence of their mutual dipolar interactions are studied. It is shown that a fast conversion between ortho- and para-states occurs on a msec time scale, caused by the intermolecular homonuclear magnetic dipolar interaction. Moreover the spin eigenstates of water in an ice crystal are determined by magnetic dipolar interactions and are not given by normal ortho- and para-H2O states of gaseous water. As a result of this the spin temperature of gaseous water evaporated from ice depends strongly on its evaporation history and the ortho/para-ratio of water molecules are only an indirect measure of the temperature of ice crystals from where they descend. This result could explain the unexpected experimentally observed ortho/para-ratios in the clouds of comets.

Assessment of the Anisotropy in the Molecule Mn19 with a High-Spin Ground State S = 83/2 by 35 GHz Electron Paramagnetic Resonance

35 GHz electron paramagnetic resonance experiments on a powder sample of the magnetic molecule Mn 19 with a high-spin ground state S = 83/2 are presented. At low temperatures, the data are well described by the simulated spectra for an isolated spin with a zero-field-splitting parameter D = 0.004 cm (-1), which is, in particular, positive. Hence, Mn 19 is not a single-molecule magnet; the previously observed magnetic hysteresis at ultralow temperatures is likely due to intermolecular dipolar interactions.

Physico-chemical investigation of asymmetrical peptidolipidyl-cyclodextrins

A new class of amphiphilic peptidolipidyl-cyclodextrins is reported. The derivatives are chiral due to the presence of an L-leucine in the spacer arm that links a saccharide moiety and a grafted, saturated hydrocarbon chain. Self-assembly properties of the peptidolipidyl-cyclodextrins are characterized by quasi-elastic light scattering, turbidity and UV–visible absorption measurements. NMR experiments give insight into the intermolecular dipolar interactions as a function of temperature and concentration. N-dodecyl- N α -(6 I-amidosuccinyl-6 1-deoxy-cyclomaltoheptaose)- L-leucine ( 1) is poorly soluble in aqueous media. N-dodecyl- N α -(6 I-amidosuccinyl-6 I-deoxy-2 I,3 I-di- O-methyl-hexakis-(2 II-VII,3 II-VII,6 II-VII-tri- O-methyl)-cyclomaltoheptaose)- L-leucine ( 2) is found to be more soluble and self-assembles into stable supramolecular colloidal aggregates with nanometric dimensions above a critical aggregation concentration (CAC). It has a propensity for solubilization of hydrophobic species revealing a micellar-like behavior, which is compared to that of the non-ionic detergent octyl glucoside. On the contrary, compound 1 precipitates in a crystalline phase beyond its water solubility limit, and it does not display any solubilizing capacity. The observed behavior corroborates at the molecular level with the NMR results.

Application of the forward linear prediction on high-resolution NMR spectra in inhomogeneous fields

In homogeneous fields, the advantages of forward linear prediction (LP) for processing 2D NMR data sets have long been recognized. In this paper, the forward LP method was employed to obtain high-resolution NMR spectra in inhomogeneous fields. Intermolecular multiple-quantum coherence (iMQC) signals are caused by intermolecular dipolar interactions and can be used to obtain 1D high-resolution NMR spectra from the 2D iMQC spectra acquired in inhomogeneous fields. However, when the 2D spectra are acquired with insufficient increments to save experimental time, wiggles around strong peaks and bad resolution will occur. Extending the data set by forward LP in the indirect dimension is a good way to improve spectral resolution. Compared to normal discrete Fourier transform, the forward LP method can shorten experimental time by a factor of four or more at the same level of sensitivity and resolution.

Longitudinal Nuclear Spin Relaxation of Ortho- and Para-Hydrogen Dissolved in Organic Solvents

The longitudinal relaxation time of ortho-hydrogen (the spin isomer directly observable by NMR) has been measured in various organic solvents as a function of temperature. Experimental data are perfectly interpreted by postulating two mechanisms, namely intramolecular dipolar interaction and spin-rotation, with activation energies specific to these two mechanisms and to the solvent in which hydrogen is dissolved. This permits a clear separation of the two contributions at any temperature. Contrary to the self-diffusion coefficients at a given temperature, the rotational correlation times extracted from the dipolar relaxation contribution do not exhibit any definite trend with respect to solvent viscosity. Likewise, the spin-rotation correlation time obeys Hubbard's relation only in the case of hydrogen dissolved in acetone-d6, yielding in that case a spin-rotation constant in agreement with literature data. Concerning para-hydrogen, which is NMR-silent, the only feasible approach is to dissolve para-enriched hydrogen in these solvents and to follow the back-conversion of the para-isomer into the ortho-isomer. Experimentally, this conversion has been observed to be exponential, with a time constant assumed to be the relaxation time of the singlet state (the spin state of the para-isomer). A theory, based on intermolecular dipolar interactions, has been worked out for explaining the very large values of these relaxation times which appear to be solvent-dependent.

Apparent longitudinal relaxation in solutions with intermolecular dipolar interactions and slow chemical exchange

A modified CRAZED sequence with selective inversion before excitation was designed to investigate the longitudinal relaxation under the effects of slow chemical exchange and distant dipolar interactions in highly polarized spin systems. Analytical expressions for the apparent longitudinal relaxation time of such systems were derived from a combination of the dipolar field theory and product operator formalism. The result shows that the signal intensity follows a multi-exponential function of the inversion recovery time. Experimental results support the theoretical predictions.

Molecular diffusion on a time scale between nano- and milliseconds probed by field-cycling NMR relaxometry of intermolecular dipolar interactions: Application to polymer melts

A formalism is presented permitting the evaluation of the relative mean-squared displacement of molecules from the intermolecular contribution to spin-lattice relaxation dispersion of dipolar coupled spins. The only condition for the applicability is the subdiffusive power law character of the time dependence of the mean-squared displacement as it is typical for the chain mode regime in polymer liquids. Using field-cycling NMR relaxometry, an effective diffusion time range from nano- to almost milliseconds can be probed. The intermolecular spin-lattice relaxation contribution can be determined with the aid of isotopic dilution, that is, mixtures of undeuterated and deuterated molecules. Experiments have been performed with melts of polyethyleneoxide and polybutadiene. The mean-squared segment displacements have been evaluated as a function of time over five decades. The data can be described by a power law. The extrapolation to the much longer time scale of ordinary field-gradient NMR diffusometry gives good coincidence with literature data. The total time range thus covers nine decades.

Hydrogen Bonding of Water Confined in Controlled-Pore Glass 10-75 Studied by ¹H-Solid State NMR

In this paper a possible explanation for an unexpected ortho/para-water ratio in the gas clouds of comets is given. The description is based on the quantum-mechanical density matrix formalism and the spin temperature concept. Only the nuclear spin system is treated quantum-mechanically. Employing the model of a four spin system, created by two nearest neighbour water molecules, spin eigenstates and their dynamics under the influence of their mutual dipolar interactions are studied. It is shown that a fast conversion between ortho- and para-states occurs on a msec time scale, caused by the intermolecular homonuclear magnetic dipolar interaction. Moreover the spin eigenstates of water in an ice crystal are determined by magnetic dipolar interactions and are not given by normal ortho- and para-H₂O states of gaseous water. As a result of this the spin temperature of gaseous water evaporated from ice depends strongly on its evaporation history and the ortho/para-ratio of water molecules are only an indirect measure of the temperature of ice crystals from where they descend. This result could explain the unexpected experimentally observed ortho/para-ratios in the clouds of comets.

Cover Picture: Synthesis and Characteristics of a Nonaggregating Tris(tetrathiafulvaleno)dodecadehydro[18]annulene (Chem. Eur. J. 33/2006)

A new dodecadehydro[18]annulene fused to three tetrathiafulvalene moieties shows a strong charge‐transfer transition of intramolecular rather than intermolecular origin. In the Full Paper on page 8451 ff., F. Diederich, M. Brøndsted Nielsen et al. show that the π‐stacking aggregation tendency is negligible and not an intrinsic property of these acetylenic macrocycles. Yet, aromatic π‐stacking self‐assembly can be promoted by additional intermolecular dipolar interactions between appropriate functional groups.

Nuclear Spin Relaxation Driven by Intermolecular Dipolar Interactions: The Role of Solute−Solvent Pair Correlations in the Modeling of Spectral Density Functions

Nuclear spin relaxation provides useful information related to the dynamics of molecular systems. When relaxation is driven by intermolecular dipolar interactions, the relevant spectral density functions (SDFs) also have significant contributions, in principle, from distant spins all over the dynamic range typically probed by NMR experiments such as NOESY. In this work, we investigate the intermolecular dipolar spin relaxation as driven by the relative diffusion of solvent and solute molecules taking place under a central force field, and we examine the relevant implications for (preferential) solvation studies. For this purpose, we evaluate the SDFs by employing a numerical approach based on spatial discretization of the time-propagation equation, and we supply an analytical solution for the simplest case of a steplike mean-field potential. Several situations related to different solute-solvent pair correlation functions are examined in terms of static/dynamic effects and relaxation modes, and some conclusions are drawn about the interpretation of NOE measurements. While we confirm previous results concerning the spoiling effect of long-range spins (Halle, B. J. Chem. Phys. 2003, 119, 12372), we also show that SDFs are sufficiently sensitive to pair correlation functions that useful, yet rather complicated, inferences can be made on the nature of the solvation shell.

HRTEM microstructures of PAN precursor fibers

Ultrathin sections prepared from both transverse and longitudinal PAN precursor fibers were examined by high-resolution transmission electron microscopy. Ordered and amorphous phases coexist in these sections, providing verification for the two-phase structure of PAN fibers, however, onion-like spheres and crystallites predominate in the transverse sections, and highly oriented structures are dominant in the longitudinal sections. The formation of the onion-like spheres can be ascribed to intramolecular and intermolecular dipolar nitrile group interactions. The larger area of oriented structures in the longitudinal sections is due to the coalescence of several ribbons with the approximative orientations. Some defects, such as pores and voids, have also been observed. These examinations provide not only some direct structural evidence but also some information for optimizing spinning technology and for deliberately controlling the microstructures to obtain PAN precursor fibers applicable to fabricating high strength carbon fibers.