Mobility of CD4 molecules in nanoscale cages of zeolites as studied by deuteron NMR relaxation

Deuteron spin–lattice relaxation was applied to study mobility of CD₄ molecules trapped in the cages of zeolite NaY. There are two, interconnected sets of cages: α-cages and β-cages with 1.16 and 0.74 nm diameter, respectively. The relaxation temperature dependence, measured between 4 and 300 K, can be divided into four ranges with characteristic motional parameters. At higher temperatures exchange between cages dominates. Increasing rate of translational motion leads to a significant reduction of the relaxation rate. Features typical for quantum rotors were observed at low temperatures. Molecules in the α-cages exhibit reorientational freedom, while motion of these in β-cages is significantly restricted. Increasing abundance of molecules in β-cages indicates slow diffusion down to low temperatures.     

Rotational dynamics of a discotic liquid crystal studied by deuteron spin relaxation

We report on measurements of deuteron quadrupolar splitting and spin-lattice relaxation times T_1Q and T_1Z in the columnar phase of a ring-deuteriated hexaoctyloxyrufigallol at 46 MHz as a function of temperature. To describe small-step rotations of these molecules within each column in the columnar phase, a space-fixed frame is used to diagonalize the molecular diffusion tensor. The principal diffusion constants in this so called 'anisotropic viscosity' model D_alpha and D_beta are for rotations of a molecule around and perpendicular to the columnar axis, respectively. A global target analysis of the spectral densities at seven temperatures in a minimization procedure was carried out. We found that D_alpha > D_beta, which is consistent with the picture that the motion towards or away from the local director tends to disrupt the packing of molecules within the column.     

Anisotropic reorientation of 9-methylpurine and 7-methylpurine molecules in methanol solution studied by combining 13C and 14N nuclear spin relaxation data and quantum chemical calculations

Reorientation of 9-(trideuteromethyl)purine and 7-(trideuteromethyl)purine molecules in methanol-d4 solutions has been investigated on the basis of the interpretation of the nuclear spin relaxation rates of their 14N (or 1H) and 13C nuclei. The transverse quadrupole relaxation rates of 14N nuclei have been obtained from the line shape analysis of their 14N NMR spectra. Alternatively, the information on the longitudinal 14N relaxation rates has been obtained via the scalar relaxation of the second kind of protons coupled to 14N. The longitudinal dipolar relaxation rates of the protonated 13C nuclei in the investigated molecules have been determined by measuring their overall relaxation rates and NOE enhancement factors. The molecular geometries, scalar coupling constants, and EFG tensors needed for quantitative interpretation of the above data have been calculated theoretically [DFT B3LYP/6-311++G(2d,p) or B3PW91/6-311+G(df,pd)] including the impact of the solvent by using discrete solvation and the polarizable continuum model. The reorientation of the investigated purines has been described as rotational diffusion of an asymmetrical top. It has been found that to get a fully consistent interpretation of the relaxation data, effective C-H bond lengths being 3% longer than the calculated ones had to be used in analysis to compensate for the ground-state vibrations. The obtained rotational diffusion coefficients and orientations of the principal diffusion axes show that the investigated molecules reorient anisotropically and that the mode of their solvation is remarkably different, in spite of their structural similarity.     

Miscibility of nifedipine and hydrophilic polymers as measured by (1)H-NMR spin-lattice relaxation

The miscibility of a drug with excipients in solid dispersions is considered to be one of the most important factors for preparation of stable amorphous solid dispersions. The purpose of the present study was to elucidate the feasibility of (1)H-NMR spin-lattice relaxation measurements to assess the miscibility of a drug with excipients. Solid dispersions of nifedipine with the hydrophilic polymers poly(vinylpyrrolidone) (PVP), hydroxypropylmethylcellulose (HPMC) and alpha,beta-poly(N-5-hydroxypentyl)-L-aspartamide (PHPA) with various weight ratios were prepared by spray drying, and the spin-lattice relaxation decay of the solid dispersions in a laboratory frame (T(1) decay) and in a rotating frame (T(1rho) decay) were measured. T(1rho) decay of nifedipine-PVP solid dispersions (3 : 7, 5 : 5 and 7 : 3) was describable with a mono-exponential equation, whereas T(1rho) decay of nifedipine-PHPA solid dispersions (3 : 7, 4 : 6 and 5 : 5) was describable with a bi-exponential equation. Because a mono-exponential T(1rho) decay indicates that the domain sizes of nifedipine and polymer in solid dispersion are less than several nm, it is speculated that nifedipine is miscible with PVP but not miscible with PHPA. All the nifedipine-PVP solid dispersions studied showed a single glass transition temperature (T(g)), whereas two glass transitions were observed for the nifedipine-PHPA solid dispersion (3 : 7), thus supporting the above speculation. For nifedipine-HPMC solid dispersions (3 : 7 and 5 : 5), the miscibility of nifedipine and HPMC could not be determined by DSC measurements due to the lack of obviously evident T(g). In contrast, (1)H-NMR spin-lattice relaxation measurements showed that nifedipine and HPMC are miscible, since T(1rho) decay of the solid dispersions (3 : 7, 5 : 5 and 7 : 3) was describable with a mono-exponential equation. These results indicate that (1)H-NMR spin-lattice relaxation measurements are useful for assessing the miscibility of a drug and an excipient in solid dispersions.     

Molecular reorientation of CD(4) in gas-phase mixtures

Spin-lattice relaxation times were measured for the deuterons in CD(4) in pure gas and in mixtures with the following buffer gases: Ar, Kr, Xe, HCl, N(2), CO, CO(2), CF(4), and SF(6). Effective collision cross sections sigma(theta, 2) for the molecular reorientation of CD(4) in collisions with these ten molecules are obtained as a function of temperature. These cross sections are compared with the corresponding cross sections sigma(J) obtained from (1)H spin-rotation relaxation in mixtures of CH(4) with the same set of buffer gases. Various classical reorientation models typically applied in liquids predict different ratios of the reduced correlation times for the reorientation of spherical tops. The Langevin model comes closest to predicting the magnitude of the sigma(theta, 2)/sigma(J) ratio that we obtain for CD(4).     

Dynamic processes in polymeric networks as studied by NMR relaxation methods

By means of NMR pulse methods molecular motions of polymer subchains in crosslinked polystyrene gels are studied. The temperature dependence of the effective transverse relaxation timeT _{2 eff} is explained by a local reorientation process. The local configuration of the elastic chains of the network is shown to be mainly determined by polymer-solvent interaction. The results are compared with those of concentrated solutions. The influence of static-like contributions to the 2nd moments is discussed.     

Kinetics of relaxation and unmixing in polymer blends as studied by x-ray scattering

Concentration fluctuations in polystyrene/poly(styrene-co-bromostyrene) blends and changes induced by temperature jumps were studied by small-angle X-ray scattering. Structure factors in thermal equilibrium show the form given by the random phase approximation. Kinetics of structure changes after T-jumps within the homogeneous phase or jumps from the mixed state into the two-phase region are in accordance with the predictions of latest theories, being described by a simple equation of motion. Application of the theory enables a determination of relaxation times dependent upon the wave vectors. At temperatures near to the glass transition, relaxation and growth become nonexponential.     

Bond orbital analysis of the deuteron quadrupole coupling constants in MD4 molecules

The deuteron quadrupole coupling constants were used as constraints on the bond orbital functions of a series of MD₄ molecules (M=B, C, N, Al, Si, P). It was demonstrated that a minimal basis set may be sufficient to describe first row deuterates, but not for second row deuterates, where double zeta orbitals are needed.This work was started when the author was a post doctoral fellow at Battelle Memorial Institute, Columbus, Ohio.     

State of alkali azides in zeolites studied by thermal analysis

Thermal behaviour of pure LiN₃, NaN₃, CsN₃ and their mixture with the respective LiY-FAU, NaY-FAU, CsY-FAU zeolite was investigated by means of thermogravimetry and IR spectroscopy. Thermodesorption of CO₂ was applied to compare the basicity of the alkali ionexchanged Y zeolites. Two of the investigated systems, the NaN₃/NaY-FAU and the CsN₃/CsY-FAU gave single, well defined and reproducible azide decomposition features rendering these samples to apply as catalyst precursors for preparation of zeolite with basic character.     

NMR field-cycling study of proton and deuteron spin relaxation in the nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl

NMR field-cycling measurements of the deuteron spin relaxation dispersion T₁(v) for the fully deuteriated nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB-d₁₉) over a broader Larmor frequency range (v≈10 kHz to 30 MHz) than reported so far in the literature basically confirm the magnetic relaxation mechanisms previously observed by frequency dependent proton spin studies of various nematogenic molecules, namely collective nematic modes of the director field in the kilohertz regime, and anisotropic reorientations of individual molecules (mainly self-diffusion for the protons and mainly rotations about the long axis for the deuterons) in the megahertz range. Within the experimental error limits such a model allows a self-consistent interpretation of the available deuteron and proton T₁(v) results for deuteriated or protonated 5CB, respectively. In particular, the magnitudes of the measured order fluctuation contributions are in approximate accordance, i.e. within a factor of less than two, with theoretical estimates from NMR line splittings and the relevant material parameters. More exact and more extensive deuteron studies are needed to locate the origin of the observed minor inconsistency.     

Isotropic hyperfine interaction and spin-lattice relaxation for radicals in the solid state. Part 4

Spin-lattice relaxation is discussed for radicals in a solid with isotropic hyperfine interaction; various models are used for the interaction between the vibrations in the radical and the lattice vibrations. The formulas relate the relaxation time to temperature, magnetic field, and the parameters of the spin hamiltonian.     

Weak molecular complexes as studied by NMR spectroscopy of oriented molecules

Weak molecular interactions such as those in pyridine—iodine, benzene—iodine and benzene—chloroform systems oriented in thermotropic liquid crystals have been studied from the changes of the order parameters as a result of complex formation. The results indicate the formation of at least two types of charge transfer complexes in pyridine—iodine solutions. The pi-complexes in benzene—chloroform and benzene—iodine mixtures have also been detected. No detectable changes in the inter-proton distances in these systems were observed.     

Acceleration of carbon-13 spin-lattice relaxation times in amino acids by electrolytes

A series of amino acids and carboxylic acids were determined by 13C NMR spectroscopy.The results showed that addition of 3M MgCl2 led to the 13C NMR integral area of samples being well proportional to number of carbon atoms that produce the particular signal with reliability over 95%. Measurements of 13C spin-lattice relaxation times (T1's) are reported for a number of amino acids. T1's of all the carbons in amino acids generally tend to decrease with the increase of the concentration of electrolytes, and the presence of magnesium slats is of significant. Carboxylic carbons in amino acids are the most sensitive "acceptor" of the 13C spin-lattice relaxation accelerating effects in electrolytes, and the 13C spin-lattice relaxation accelerating ability of electrolytes is Mg(ClO4)2 ＞MgCl2 ＞CaCl2 ＞NaCl ＞KCl ＞LiClO4 ＞NaOH. In general, T1's of C1 carbons in nonpolar a-amino acids are higher than those in polar and basic a-amino acids both in aqueous and 3M MgCl2 medium. In aliphatic straight-chain amino acids, a-, a-, a-, ai- and a- amino acids, T1's of C1 carbons tend to reduce with the increase of inserted carbon numbers between amino and carboxylic groups compared with Gly. T1's can be decreased even more when amino acids are mixed in 3M MgCl2, but T1's of carbons in amino acids decrease slightly with increase of the concentration of amino acids in 3M MgCl2. The mechanisms of the observed phenomena are discussed in terms of intermolecular interaction and paramagnetic impurity in electrolytes, large contributions of intermolecular interaction which is enhanced in electrolytes concentrate on the incoming "unsaturation" of the primary solvation shell of cations with the increase of electrolytes concentration and complexes formation of amino acids with metal ions. In electrolytes, amino acids are "anchored" to cations and molecule tumbling is slowed down, molecular rigidity is increased and molecular size is "enlarged", all of these are helpful to accelerate the 13C spin-lattice relaxation. Atlast, MgCl2 is proposed as an efficient relaxation agent for analysis of amino acids and some carboxylic acids.Samples were dissolved with the aid of supersonic which has the effect of degassing, and they were degassed again with supersonic for 30 seconds right before determination. All of the 13C NMR was obtained with a Bruker DPX-300 NMR instrument, using NOE-suppressed inverse gated decoupling with a recycle delay 8.00s and a sweep width 30120.48Hz, experiment temperature is integral of the carbon with the smallest chemical shift is calibrated as 10.00. Spin-lattice relaxation times (T1's) were determined by using inversion recovery according to Bruker avance user's guide.The pulse sequence is (T-90.°-T-180°-o-90t°) n.     

Acceleration of carbon-13 spin-lattice relaxation times in amino acids by electrolytes

Measurements of the enhancement, by various electrolytes, of the spin-lattice relaxation time of carbon-13 at different locations in a number of amino acids are reported. Spin-lattice relaxation times T1 of all the carbons in amino acids generally tend to decrease with increase in the concentration of electrolytes, the largest effects often being observed for the charged carboxylate groups of the amino acids. Carboxylic carbons in amino acids are the sensitive 'acceptor' of the 13C spin-lattice relaxation accelerating effects offered by electrolytes, and the 13C spin-lattice relaxation accelerating ability of electrolytes decreases in the order Mg(ClO4)2 > MgCl2 > CaCl2 > NaCl > KCl > LiClO4 > NaOH. The mechanisms of the observed phenomena are discussed in terms of intermolecular interaction, paramagnetic impurities in electrolytes and other mechanisms; large contributions of intermolecular interactions with electrolytes are present on complex formation between amino acids and metal ions and the incoming 'unsaturation' of the primary solvation shell of cations with the increase in electrolyte concentration.     

Dynamics of superionic Rb3H(SeO4)2 single crystals studied by H and Rb spin-lattice relaxation

The ¹H and ⁸⁷Rb spin-lattice relaxation and spin-spin relaxation times in superionic Rb₃H(SeO₄)₂ single crystals grown by the slow evaporation method were measured over the temperature range 160-450 K. The temperature dependencies of the ¹H T₁, T_1ρ, and T₂ are measured. In the ferroelastic phase, T₁ differs from T_1ρ, which is in turn different from T₂, although these three relaxation times converge to similar values near 410 K. This transition seems to occur at temperature which is about 40 K lower than the superionic transition temperature. The observation of liquid-like values of the ¹H T₁, T_1ρ, and T₂ in the high temperature is compatible with the phase being superionic, indicating that the destruction and reconstruction of hydrogen bonds does indeed occur at high temperature. In addition, the ⁸⁷Rb T₁ and T₂ values at high temperature were similar (on the order of milliseconds), a trend that was also observed for ¹H T₁ and T₂. This behavior is expected for most hopping-type ionic conductors, and could be attributed to interactions between the mobile ions and the neighboring group ions within the crystal. The motion giving rise to this liquid-like behavior is related to the superionic motion.     

Surface molecular relaxation in smectic C* phase as studied by dielectric spectroscopy

The molecular relaxation process of a ferroelectric liquid crystal with a high tilt angle and a high spontaneous polarization in a homeotropically aligned cell has been studied by the dielectric relaxation method in the frequency range 10 Hz to 10 MHz. The measurements have been done using thin (3.5μm) cells with gold coated electrodes and samples aligned by a magnetic field. It has been observed that the molecular relaxation around the short axis of the molecule is detected in the chiral nematic and smectic C* phases. The surface molecular process is observed in the S*_c phase down to nearly 6 to 7 K below the transition temperature of the N* to the S*_c phase. The experimental results of the surface molecular process are analysed by theoretical calculations. The experimental results agree with the theoretical predictions.