Theoretical insights into the host–guest interactions between [6]cycloparaphenyleneacetylene and its anthracene‐containing derivative and fullerene C70

The host–guest complexes formed with [6]cycloparaphenyleneacetylene ([6]CPPA) and its anthracene‐containing derivative ([6]CPPAs) hosts and fullerene C₇₀ guest were explored by density functional calculations. Besides two previously reported configurations in which C₇₀ guest is standing or lying in the cavity of the host, we found a new kind of configuration in which C₇₀ guest is half‐lying in the cavity of the host. More interestingly, the calculated results revealed that the fine‐tuning deformations occur readily during the formations of the complexes, suggesting that both [6]CPPA and [6]CPPAs are highly elastic host molecules. The large host–guest binding energies indicate that both two host molecules, [6]CPPA and [6]CPPAs, have excellent encapsulation ability for C₇₀ guest, and the [6]CPPAs even has much better encapsulation ability for C₇₀ than [6]CPPA. Furthermore, the host–guest interactions regions were detected and visualized in real space based on the electron density and reduced density gradient. Additionally, ¹H NMR spectra of those three different kinds of configurations mentioned earlier have been calculated with gage‐independent atomic orbital method, which may be helpful for further experimental characterizations in future. Copyright © 2014 John Wiley & Sons, Ltd.     

<Superscript>27</Superscript>Al NMR Study of the Structure and Dynamics of Natrolite

The temperature dependences of nuclear magnetic resonance and magic angle spinning nuclear magnetic resonance spectra of ²⁷Al nuclei in natrolite (Na₂Al₂Si₃O₁₀· 2H₂O) have been studied. The influence of water molecules and sodium ions mobility on the shape of the ²⁷Al NMR spectrum and framework dynamics have been discussed The temperature dependences of the spin–lattice relaxation times T₁ of ²⁷Al nuclei in natrolite have also been studied. It has been shown that the spin–lattice relaxation of the ²⁷Al is governed by the electric quadrupole interaction with the crystal electric field gradients modulated by translational motion of H₂O molecules in the natrolite pores. The dipolar interactions with paramagnetic impurities become significant as a relaxation mechanism of the ²⁷Al nuclei only at low temperatures (<270 K).     

Proton Detection of Heteronuclear Dipolar Couplings

A method is proposed for the calculation of heteronuclear dipolar coupling between two 1/2 nuclei, X and Y, by measuring the spin-lattice relaxation rates of the abundant Y nucleus and of the satellite peaks (¹H, ³¹P, ¹⁹F) due to the scalar coupling of Y with the less abundant X nucleus. The ¹H-¹³C dipolar interaction has been evaluated from the proton spin-lattice relaxation rates of tyrosine in water solution and the effective correlation times of the aromatic moiety have been calculated.     

Non-secular part of nuclear dipolar broadening detected by zero-field spin relaxation of positive muon

The zero-field spin relaxation function of μ⁺ observed in ZrH₂ has revealed that the second moment of the nuclear dipolar broadening is five times larger than the high-field value. This experiment clearly demonstrates the recovery of the non-secular part of dipolar interaction between unlike spins. A general expression of zero-field relaxation function is presented to account for a slow modulation of random fields on μ⁺ found at room temperature.     

Solid-State NMR Studies on the Guest Ordering and Dynamics in α,ω-Dibromoalkane/Urea Inclusion Compounds

Solid-state nuclear magnetic resonance (NMR) spectroscopy is utilized to study the molecular behavior of 1,10-dibromodecane and 1,11-dibromoundecane in their urea inclusion compounds. The guest dynamics and conformational order are explored by ¹³C cross polarization magic-angle spinning (CP/MAS) and ¹H MAS NMR spectroscopy which confirm an all-trans conformation of the guest chains. Dynamic ²H NMR experiments are carried out on two guest molecules selectively deuterated at both end groups. A quantitative analysis of the experimental data, obtained from variable-temperature line shape, spin–spin and spin–lattice relaxation measurements, shows that both guest molecules undergo similar motions within the investigated temperature range between 100 and 298 K. The combination of nondegenerate 6-site (or 3-site) rotational jumps and small-angle overall chain wobbling provides an appropriate motional model for the guest motions in these compounds. It is found that the populations of the jump sites exhibit a characteristic temperature dependence, although a discontinuity is missing at the solid–solid phase transition. The same holds for the guest motions which also remain unaffected by the change of the urea lattice structure. Rather, a discontinuity of the guest dynamics at about 30 and 10 degrees above the corresponding solid–solid phase transition is observed for 1,10-dibromodecane and 1,11-dibromoundecane in urea, respectively. Likewise, there is no clear evidence for an odd–even effect due to the change of the guest chain length on the molecular properties of the present inclusion compounds. As a general result, it is concluded that the intermolecular interactions in the present materials are stronger than in n-alkane/urea inclusion compounds. Authors' address: Klaus Müller, Institut für Physikalische Chemie, Universit?t Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany     

The influence of the stoichiometry on the Eu nuclear spin-spin relaxation in EuO single crystals

The ¹⁵³Eu spin-spin relaxation for two spherical EuO single crystals of different composition has been measured for two saturating field values of 2 and 6 T. The relaxation can be described by two time constants, a short one increasing with the magnetic field, arising from the Suhl-Nakamura coupling and a long one, due to the dipolar coupling, which is field independent. It is shown that the number of nuclei which are relaxed due to the dipolar coupling increases at increasing magnetic fields, in agreement with the Suhl-Nakamura theory. For the sample which is nearly stoichiometric the relative number of nuclei which is relaxed due to the SN coupling is much larger than for the sample which contains an excess of Eu atoms.     

Proton spin-lattice relaxation in liquid water and liquid ammonia

The proton spin-lattice relaxation time has been measured at 20·8 Mc/s for a series of solutions of water in heavy water and solutions of ammonia in heavy ammonia for the temperature range from the melting point to the liquid-vapour critical temperature. Measurements have also been made for water over limited temperature ranges at several fixed densities.

The contributions to the spin-lattice relaxation time from direct dipolar and spin-rotation interactions have been separated. The spin-rotation interaction contribution appears to be the same for H₂O as for HDO and also as between NH₃, NH₂D and NHD₂ and this result is justified. The correlation times for molecular re-orientation, τ_d, and for molecular angular velocity, τ_sr, are derived from the results and in so doing some support for the Hubbard [12] relation betweent τ_sr and τ_d is adduced. It is found that at the critical temperature τ_sr?τ_d which contrasts with other liquids for which it is usually found that τ_sr??τ_d. The spin-rotation interaction constants in the water and ammonia molecules are found to be approximately 120 kc/s and 80 kc/s, respectively.

An attempt to separate the inter- and intra-molecular contributions to the dipolar spin-lattice relaxation time is possible in principle, in spite of the rapid proton exchange, but is frustrated by the fact that the equilibrium constants are little different from their statistical values. Nevertheless there is evidence that the two interactions vary in much the same way with temperature.

The correlation times deduced from the dipolar relaxation time show close relationship with dielectric, self diffusion and deuteron relaxation time data.

It is suggested that the re-orientation of both water and ammonia molecules may be by a small angle Brownian diffusion even near the critical temperature.     

Nuclear magnetic resonance studies of surface interactions between trifluoromethanesulfonic acid and silica

The nature of the interaction between trifluoromethanesulfonic acid (triflic acid) and the surface groups of silica as a support has been investigated by¹H and¹⁹F-NMR studies. The¹⁹F-NMR spin-lattice relaxation time (T ₁) dependence on temperature suggests that the dominant relaxation mechanism for¹⁹F is a combination of spin-rotation (SR) interactions, one of which seems to be quenched by cage-like structures. This mechanism is activated by the treatment followed to stabilize the acid on silica. Hydrogen proton relaxation seems instead to be driven by dipolar interactions and the formation of hydrogen bridges. Activation energies have been estimated for various cases, assuming an Arrhenius-type temperature dependence for the correlation times of molecular motion. A linear dependence of¹H chemical shift on inverse temperature was also found, which is interpreted as thermal weakening of hydrogen bonding. The results support the conclusion that the stability of triflic acid on silica is due to the formation of strong hydrogen bridges between the sulfonic groups of triflic acid and the silica surface hydroxyls, provided a network structure similar to that observed for pure acid is obtained in which strong interactions between acid molecules occur. The acid seems to be organized as solid-like “drops” on the silica surface, which accounts for the long residence time on the silica surface.     

Proton magnetic relaxation in solutions of E. coli ribosomal RNA containing Mn2+ ions

A study has been made over a range of temperatures and magnetic field strengths of the spin relaxation of water protons in aqueous solutions of E. coli ribosomal RNA containing Mn²⁺ ions. The effects of the paramagnetic ions are enhanced in the presence of the RNA. As the temperature falls T ₁ passes through a minimum value, the magnitude of which is field dependent, and this is attributed to a change in dipolar relaxation mechanism from rotation of the aquocomplex to electron spin relaxation. The relevance of this work is assessed in relation to other work on proton relaxation enhancement in Mn²⁺-containing solutions of biopolymers.     

Laurdan and Prodan as Polarity-Sensitive Fluorescent Membrane Probes

The steady-state and dynamic fluorescence spectral properties of 2-dimethylamino-6-lauroylnaphthalene (LAURDAN) and several other naphthalene derivatives are summarized to illustrate their sensitivity to the polarity of the environment. Results obtained both in solvents of different polarity and in phospholipid vesicles in two phase states are presented. The emission red shift observed in polar solvents and in the phospholipid liquid–crystalline phase is explained on the basis of dipolar relaxation of solvent molecules surrounding the fluorescent naphthalene moiety of these probes. In phospholipid environments, experimental evidence is shown that excludes the intramolecular relative reorientation of the dimethylamino and carbonyl groups in the naphthalene and the reorientation of the entire fluorescent moiety. The solvent dipolar relaxation observed for LAURDAN and PRODAN in phospholipid bilayers has been attributed to a small number of water molecules present at the membrane interface. A comparison between LAURDAN emission in phospholipid vesicles prepared in D₂O and in H₂O is also presented. The definition and the derivation of the generalized polarization function are also discussed.     

MOMENT EXPANSIONS OF PROTON DIPOLAR SPINNING-SIDEBAND INTENSITIES IN A 1H MAS NMR SPECTRUM

对于包含分子和分子基团绕至少一个轴高速运动的固体体系，本文推导出其质子核磁共振谱的偶极魔角旋转边带强度的理论计算表达式，建立了用其静态粉末谱的矩的展开的计算方法，计算出旋转边带强度按三十阶矩展开的系数，它可以处理包含高达十五阶边带的谱.     

On the dynamical coupling between the dipolar and tunneling system of CH3-groups in solids at high temperatures

For methyl groups a dynamical coupling has been predicted between the dipolar and tunnelling systems, also at high temperatures where the observable tunnelling frequency at thermal equilibrium ω_t is zero. This is experimentally confirmed by observed non-exponential dipolar relaxation for ω₀²τ_c² ? 1.     

Phase transitions and ordering in urea inclusion compounds with n-paraffins

Urea inclusion compounds consist of a pseudo-hexagonal framework of urea with “infinite” channels parallel to c, into which paraffins are embedded. The compound with hexadecane exhibits phase transitions at ≅ 365, ≅ 148 and ≅ 120 K (phases I–IV), which have been investigated by X-ray and neutron scattering. Phase I is hexagonal with a (longitudinal and orientational) random distribution of the included molecules. In phase II, which uniquely occurs in the adduct with C₁₆H₃₄, a doubling of the lattice constant c₀ of the host structure indicates partial longitudinal ordering. This is assigned to the approximately equal length of the hexadecane molecule and 2c_o leading to mutual deformations of both constituents. The transition II→ III is connected with a lateral orientational ordering of the guest molecules in adjacent channels and a simultaneous orthorhombic deformation of the host. Correlated rotational motions of the included molecules around their long axes are deduced from the increasing critical scattering above T_c In phase IV a crystal-like ordering tendency of the paraffins plays the dominant role, while the host breaks into a heavily disordered domain structure. Experimental results indicate a transformation process as no thermal equilibrium was reached during the measurements. The different ordering mechanisms are discussed in terms of interactions between host and guest, guest and guest along each channel, and guest and guest in adjacent channels.     

A Comparative Multinuclear Relaxation Study of Protein-DMSO and Protein-Water Interactions

A comparative multinuclear relaxation study of DMSO-protein and water-protein interactions has been undertaken to discover whether cross relaxation between water and protein protons proceeds mainly by proton-exchange or direct dipolar interactions. The analysis suggests that the proton-exchange mechanism dominates the cross relaxation from water. In contrast, the high efficiency of dipolar cross relaxation in DMSO-protein gels, which lack exchangeable protons, arises from an unusually long lifetime of DMSO molecules at the protein interface. These results are important for understanding the origin of image contrast in clinical nuclear magnetic resonance imaging and the nature of protein-solvent interactions in nonaqueous systems.     

Intermolecular interactions of benzene and carbon tetrachloride with selected free radicals in solution as studied by 13C and 1H dynamic nuclear polarization

Intermolecular hyperfine coupling between ¹³C nuclei and free-radical unpaired electron spins has been examined for six combinations of three free radicals and two solvent molecules. From magnetic field and temperature-dependent dynamic nuclear polarization measurements, scalar and dipolar coupling contributions have been separated. Interpretation of these results in terms of a modified diffusion model revealed scalar spectral density functions with two or three frequency components, each characterized by a distinct contact time. Collisions with the shortest time constant were found to be nearly independent of the system and accounted for approximately 90 per cent of all radical-receptor collisions and 30 per cent of the scalar relaxation rate. These have been related to random elastic collisions. In contrast, collisions with long contact times were found to be highly system dependent. Although few in number, they accounted for a relatively large fraction of the scalar relaxation rate because of their long duration. These have been interpreted as stereospecific collisions which reflect the tendency of the colliding partners to form weak transient complexes. Lastly, relative scalar coupling energies were obtained and related to the availability of the unpaired electron and to the structure of both colliding molecules.     

Detection and classification of host–guest interactions using β-cyclodextrin-decorated carbon nanotube-based chemiresistors

Carbon nanotubes (CNTs) in a chemiresistor setup have been widely explored in bio/chemical sensing. Detection of certain molecules with environmental and health related importance such as 9-anthracenecarboxylic acid, diclofenac sodium, and curcumin using electrochemical methods/unfunctionalized CNTs suffer from lack of response, high limit of detection (LOD) and poor selectivity. The key to overcome these issues is to decorate CNTs with host (receptor) molecules like β-cyclodextrin (β-CD) that interact with guest (target) molecules by host–guest complex formation. To improve guest recognition, and consequently, the sensor performance, effective immobilization of β-CD on the CNT surface using a non-covalent bridging molecule such as 3, 4, 9, 10-perylene tetracarboxylic acid (PTCA) is required. Furthermore, the selectivity can be assessed using the conductance correlation patterns of different host–guest systems in conjunction with a pattern classification tool. Our results indicate that PTCA linked β-CD-decorated CNT chemiresistors showed a good linear detection range (∼100 pM–100 nM), sensitivity (∼3 × 10⁻³–9 × 10⁻² nM⁻¹) and LOD (∼62 pM–101 nM), compared to devices without PTCA, in the detection of the guest molecules. The distinction in correlation patterns of different host–guest systems was corroborated by pattern classification yielding a classification accuracy, sensitivity, and specificity of ∼91.83%, ∼90.13%, and ∼85.39%, respectively.     

A Study of Dipolar Interactions and Dynamic Processes of Water Molecules in Tendon byH andH Homonuclear and Heteronuclear Multiple-Quantum-Filtered NMR Spectroscopy

The effect of proton exchange on the measurement of¹H–¹H,¹H–²H, and²H–²H residual dipolar interactions in water molecules in bovine Achilles tendons was investigated using double-quantum-filtered (DQF) NMR and new pulse sequences based on heteronuclear and homonuclear multiple-quantum filtering (MQF). Derivation of theoretical expressions for these techniques allowed evaluation of the¹H–¹H and¹H–²H residual dipolar interactions and the proton exchange rate at a temperature of 24°C and above, where no dipolar splitting is evident. The values obtained for these parameters at 24°C were 300 and 50 Hz and 3000 s⁻¹, respectively. The results for the residual dipolar interactions were verified by repeating the above measurements at a temperature of 1.5°C, where the spectra of the H₂O molecules were well resolved, so that the¹H–¹H dipolar interaction could be determined directly from the observed splitting. Analysis of the MQF experiments at 1.5°C, where the proton exchange was in the intermediate regime for the¹H–²H dipolar interaction, confirmed the result obtained at 24°C for this interaction. A strong dependence of the intensities of the MQF signals on the proton exchange rate, in the intermediate and the fast exchange regimes, was observed and theoretically interpreted. This leads to the conclusion that the MQF techniques are mostly useful for tissues where the residual dipolar interaction is not significantly smaller than the proton exchange rate. Dependence of the relaxation times and signal intensities of the MQF experiments on the orientation of the tendon with respect to the magnetic field was observed and analyzed. One of the results of the theoretical analysis is that, in the fast exchange regime, the signal decay rates in the MQF experiments as well as in the spin echo or CPMG pulse sequences (T₂) depend on the orientation as the square of the second-rank Legendre polynomial.     

NMR relaxation in multipolar AMX systems under spin-locking conditions

A relaxation network has been calculated for multipolar AMX systems under application of a spin-locking RF field. Systems of this type are of interest in the study of proteins with fractional ²H enrichment. All possible auto- and cross-correlation terms involving dipolar, quadrupolar, and CSA interactions have been taken into account. The results show the presence of spectral densities at zero frequency for interactions associated with the locked nuclei, which are nonvanishing in the absence of fast motions. In addition, the application of a spin-locking field blocks certain cross-correlation interactions, thereby considerably simplifying the relaxation network.     

1H relaxation and dynamics of triphenylbismuth in deuterated solvents

ABSTRACT

¹H spin–lattice relaxation experiments have been performed for triphenylbismuth dissolved in fully deuterated glycerol and tetrahydrofuran. The experiments have been carried out in a broad frequency range, from 10?kHz to 40?MHz, versus temperature. The data have been analysed in terms of a relaxation model including two relaxation pathways: ¹H-¹H dipole–dipole interactions between intrinsic protons of triphenylbismuth molecule and ¹H-²H dipole–dipole interactions between the solvent and solute molecules. As a result of the analysis, rotational correlation times of triphenylbismuth molecules in the solutions and relative translational diffusion coefficient between the solvent and solute molecules have been determined. Moreover, the role of the intramolecular ¹H-¹H relaxation contribution has been revealed, depending on the motional parameters, as a result of decomposing the overall relaxation dispersion profile into contributions associated with the ¹H-¹H and ¹H-²H relaxation pathways. The possibility of accessing the contribution of the relaxation of the intrinsic protons is important from the perspective of exploiting Quadrupole Relaxation Enhancement effects as possible contrast mechanisms for Magnetic Resonance Imaging.     

<Superscript>23</Superscript>Na and <Superscript>27</Superscript>Al NMR Study of Structure and Dynamics in Mordenite

Temperature dependencies of ²⁷Al and ²³Na nuclear magnetic resonance spectra and spin–lattice relaxations in mordenite have been studied in static and magic angle spinning regimes. Our data show that the spin–lattice relaxations of the ²³Na and ²⁷Al nuclei are mainly governed by interaction of nuclear quadrupole moments with electric field gradients of the crystal, modulated by translational motion of water molecules in the mordenite channels. At temperatures below 200 K, the dipolar interaction of nuclear spins with paramagnetic impurities becomes an important relaxation mechanism of the ²³Na and ²⁷Al nuclei.