Proton spin–lattice relaxation study of a partial bilayer smectic A phase

Abstract

Measurements of proton spin-lattice relaxation rates for the partial bilayer smectic A phase of 4-((4′-n-hexadecyloxybenzylidene)-amino) benzonitrile obtained at different Larmor frequencies and temperatures show that the essential relaxation mechanisms in the MHz frequency region are translational self-diffusion and local molecular reorientations similar to those in monolayer smectics. The values of the diffusion constant obtained from the fit of the theory to the experimental data show a range from 2.6 × 10^?11 m² s^?1 at 95°C to 1.7 × 10^?11 m² s^?1 at 75°C. A dynamic process specific to the partial bilayer smectic A phase seems to influence relaxation below 10 MHz. It can be associated either with the dimerization of molecules in the layers or with a higher value of the low cut-off frequency of order director fluctuations than that found in monolayer smectic A phases.     

31P and 17O nuclear spin—lattice relaxation in some phosphorylated molecules. Determination of 31P spin—rotation coupon constants, 17O quadrupolar coupling constants and chemical shielding anisotropy of the 31P nucleus

The spin—lattice relaxation time of the ³¹P nucleus was measured for 11 phosphorylated molecules (phosphine oxides, trialkylphosphates and phosphoramides) dissolved in nitromethane at three different frequencies and as a function of the temperature for three compounds. The different contributions to the relaxation rate due to dipolar, chemical shift anisotropy and spin—rotation interactions were determined and the reorientational correlation times of the molecules were deduced when the anisotropy of the chemical shift tensor of the ³¹P nucleus could be (re)determined. The quadrupolar coupling constant of the ¹⁷O nucleus was also determined from the linewidth of the nuclear magnetic resonance signals, for phosphine oxides and triphenylphosphate, giving some information on the electronic distribution into the phosphoryl bond. The spin—rotation coupling constants for trimethylphosphine oxide and triphenylphosphine oxide were deduced and the chemical shift anisotropy Δσ of trialkylphosphates estimated.     

NMR chemical shielding and spin–spin coupling constants across hydrogen bonds in uracil–α-hydroxy-N-nitrosamine complexes

One- and two bond spin–spin coupling constants, ¹ J, ^1h J , and ^2h J across X–H?O hydrogen bonds and shielding constants of bridging hydrogens have been computed for complexes formed from interaction between the α-hydroxy-N-nitrosamine (NP) and four preferential binding sites of the uracil (U) at B3LYP/6-311++G(2d,2p)//MP2/6-311++G(2d,2p) level of theory. All complexes are stabilized by two H_U?O_NP and H_NP?O_U hydrogen bonds. Very good correlations were found between NMR spin–spin coupling constant as well as isotropic shielding constant and the binding energy, H-bond distance, red-shift of vibration frequency, charge transfer energy, and electron density at H-bond critical point.     

Inertia-corrected budó treatment of dielectric relaxation in polar molecules: Application to the fir spectrum of acetonttrile and hexanone-2

The theory of dielectric relaxation of an assembly of molecules with neareit-neighbour dipole-dipole coupling, originally developed by Budó, is extended to include inertial effects. This yields an orientational correlation function that gives rise to Debye-type absorption in the microwave region and resonance absorption in the FIR region. The results are formally similar to the itinerant-oscillator model when the outer cage in that model carries a dipole moment. The results compare favourably with experimental observations on acetonitrile and hexanone-2.     

Synthesis and characterization of some pyrimidine–quinoline clubbed molecules and their microbicidal efficacy

The required chalcones 1a–h were prepared by reaction of aryl acetophenone with different aryl aldehydes, which in turn treated with guanidine nitrate, yielded 4-(substituted phenyl)-6-(substituted phenyl)-2-pyrimidinamines 2a–h. Novel pyrimidine–quinoline clubbed molecules 3a–h have been prepared by reaction of 2a–h with 4-chloro-2,6-dimethylquinoline. All the compounds were characterized by elemental analysis and spectral studies. The newly synthesized compounds were evaluated for their antibacterial and antifungal activities.     

Evidence of ZrO2 sol–gel transition by gelation time and viscosity

Journal of Sol-Gel Science and Technology - Sol–gel transition is a stage of the sol that evolves towards the formation of a gel-like network and that exhibits gradually a mobility loss....     

High resolution NMR study of T(1) magnetic relaxation dispersion. III. Influence of spin 1∕2 hetero-nuclei on spin relaxation and polarization transfer among strongly coupled protons

Effects of spin-spin interactions on the nuclear magnetic relaxation dispersion (NMRD) of protons were studied in a situation where spin [fraction one-half] hetero-nuclei are present in the molecule. As in earlier works [K. L. Ivanov, A. V. Yurkovskaya, and H.-M. Vieth, J. Chem. Phys. 129, 234513 (2008); S. E. Korchak, K. L. Ivanov, A. V. Yurkovskaya, and H.-M. Vieth, ibid. 133, 194502 (2010)], spin-spin interactions have a pronounced effect on the relaxivity tending to equalize the longitudinal relaxation times once the spins become strongly coupled at a sufficiently low magnetic field. In addition, we have found influence of (19)F nuclei on the proton NMRD, although in the whole field range, studied protons and fluorine spins were only weakly coupled. In particular, pronounced features in the proton NMRD were found; but each feature was predominantly observed only for particular spin states of the hetero-nuclei. The features are explained theoretically; it is shown that hetero-nuclei can affect the proton NMRD even in the limit of weak coupling when (i) protons are coupled strongly and (ii) have spin-spin interactions of different strengths with the hetero-nuclei. We also show that by choosing the proper magnetic field strength, one can selectively transfer proton spin magnetization between spectral components of choice.     

Thermodynamic evaluation of viscosity in In–Zn and Sn–Zn liquid alloys

A theoretical formalism that links thermodynamic properties to transport properties has been used to study the viscosity of Sn–Zn and In–Zn liquid alloys at various temperatures. The formalism was successful at describing the thermodynamic properties of these alloys and showed a better estimation of the viscosity of the Sn–Zn alloy than that of the In–Zn alloy.     

Slow relaxation mode in mixtures of water and organic molecules: supramolecular structures or nanobubbles?

Aqueous solutions of tetrahydrofuran, ethanol, urea, and alpha-cyclodextrin were studied by a combination of static and dynamic laser light scattering (LLS). In textbooks, these small organic molecules are soluble in water so that there should be no observable large structures or density fluctuation in either static or dynamic LLS. However, a slow mode has been consistently observed in these aqueous solutions in dynamic LLS. Such a slow mode was previously attributed to some large complexes or supramolecular structures formed between water and these small organic molecules. Our current study reveals that it is actually due to the existence of small bubbles ( approximately 100 nm in diameter) formed inside these solutions. Our direct evidence comes from the fact that it can be removed by repeated filtration and regenerated by air injection. Our results also indicate that the formation of such nanobubbles in small organic molecule aqueous solutions is a universal phenomenon. Such formed nanobubbles are rather stable. The measurement of isothermal compressibility confirms the existence of a low density microphase, presumably nanobubbles, in these aqueous solutions. Using a proposed structural model, that is, each bubble is stabilized by small organic molecules adsorbed at the gas/water interface, we have, for the first time, estimated the pressure inside these nanobubbles.     

Formation and NMR study of some cyclic β-ketodithioacetals

An efficient method has been found for the preparation of cyclic monodithioacetats of 1,3-dicarbonyl compounds using boron trifluoride etherate as the acid catalyst. Asymmetric 1,3-diketones react regioselectively in most of the cases tried. A study of the ¹³C and ¹H nmr characteristics of these compounds has been carried out.     

NMR and Excess Volumes Studies in DMF–Alcohol Mixtures

Chemical shifts of the alcohol and DMF protons in DMF–alcohol mixtures with the mole fraction of alcohol are reported in order to study the hydrogen bond interaction present in the mixtures. The densities of DMF–methanol mixture at 22°C are also measured. Excess volumes and excess chemical shifts are correlated by the Redlich–Kister equation. The relation between excess volumes and excess chemical shifts in the mixtures is discussed. It is found that the maximum excess chemical shifts ^E(CHO-OH) and ^E(CH₃-OH) are positioned at about mole fraction methanol = 0.57 for the DMF–methanol system, as is V ^E. The results show that the NMR spectral method offers a valuable approach to similar future studies of interactions in mixtures.     

Energy–time uncertainty relations and time operators

It is proved that for any Hamiltonian in a separable Hilbert space, having a non-empty absolutely continuous spectrum, there exists a time operator densely defined in the subspace of absolutely continuous vectors. This result is obtained by using the Carbó-Dorca parameterized vector spaces and the spectral representation theorem for self-adjoint operators in Hilbert spaces. The restriction of the Hamiltonian to the absolutely continuous subspace and its time operator are incompatible. These results bring a completely new light on the energy–time uncertainty relations. The possible physical interpretations and related facts are also examined.     

Structural parameter relaxation and pucker mode—internal mode coupling in the four-ring molecules cyclobutanol,cyclobutylfluoride and cyclobutylchloride

The present work is directed towards the investigation of the pucker—internal rotation modes of cyclobutanol and cyclobutanol-OD by the aid of a semirigid two-dimensional model. Based on spherical four-ring coordinates and general geometrical relations expressing wagging, twisting and rocking coordinates for CH₂ and CHX groups, experimental rotational constants of cyclobutanol, cyclobutylfluoride and cyclobutylchloride in the ground and excited pucker states are analyzed in terms of a two-dimensional four- ring pucker model with structural relaxation of CH₂ and CHX groups concerted with the ring puckering angles. The rotational data are found to be satisfactorily described by a one-dimensional ring pucker model including structural relaxation of the rocking angles of the CHX and the CH₂ groups. Then results of extended SCF computations (4-31G level) of the puckering—internal rotation potential of cyclobutanol are presented. These emphasize again the importance of structural relaxation of bond length and rocking type coordinates of the corner groups concerted with the puckering angle and serve as a basis for numerical solution of the two-dimensional pucker—internal rotation problem within the framework of a semirigid model. Predicted transitions and energy eigenstates will be correlated with far i.r. spectral data of cyclobutanol and cyclobutanol-OD observed in the 30–300 cm⁻¹ range and will be interpreted in terms of an alternative assignment, whose relation to an earlier analysis will finally be discussed.     

Thermodynamic scaling of α-relaxation time and viscosity stems from the Johari-Goldstein β-relaxation or the primitive relaxation of the coupling model

By now it is well established that the structural α-relaxation time, τ(α), of non-associated small molecular and polymeric glass-formers obey thermodynamic scaling. In other words, τ(α) is a function Φ of the product variable, ρ(γ)/T, where ρ is the density and T the temperature. The constant γ as well as the function, τ(α) = Φ(ρ(γ)/T), is material dependent. Actually this dependence of τ(α) on ρ(γ)/T originates from the dependence on the same product variable of the Johari-Goldstein β-relaxation time, τ(β), or the primitive relaxation time, τ(0), of the coupling model. To support this assertion, we give evidences from various sources itemized as follows. (1) The invariance of the relation between τ(α) and τ(β) or τ(0) to widely different combinations of pressure and temperature. (2) Experimental dielectric and viscosity data of glass-forming van der Waals liquids and polymer. (3) Molecular dynamics simulations of binary Lennard-Jones (LJ) models, the Lewis-Wahnstr?m model of ortho-terphenyl, 1,4 polybutadiene, a room temperature ionic liquid, 1-ethyl-3-methylimidazolium nitrate, and a molten salt 2Ca(NO(3))(2)·3KNO(3) (CKN). (4) Both diffusivity and structural relaxation time, as well as the breakdown of Stokes-Einstein relation in CKN obey thermodynamic scaling by ρ(γ)/T with the same γ. (5) In polymers, the chain normal mode relaxation time, τ(N), is another function of ρ(γ)/T with the same γ as segmental relaxation time τ(α). (6) While the data of τ(α) from simulations for the full LJ binary mixture obey very well the thermodynamic scaling, it is strongly violated when the LJ interaction potential is truncated beyond typical inter-particle distance, although in both cases the repulsive pair potentials coincide for some distances.     

Solvent–solute and solute–solute interactions from NMR in nematic phases

Abstract

The use of NMR spectroscopy of molecules oriented in liquid-crystalline media to study solvent–solute and solute–solute interactions in π-systems such as benzene–chloroform and in charge transfer complexes, for example pyridineiodine, is illustrated. Changes in molecular order and chemical shifts as a result of complexation are employed in such studies. The extraordinary symmetry of C₆₀ has also been investigated by using a mixture of liquid crystals of opposite diamagnetic anisotropies indicating, thereby, negligible solvent–solute/solute–solute interactions.     

Theory of chemical bonds in metalloenzymes XII: Electronic and spin structures of metallo–oxo and isoelectronic species and spin crossover phenomena in oxygenation reactions

The broken-symmetry (BS) and multideterminant approaches to atomic oxygen (O), molecular oxygen (O₂) and iron–oxo (Fe(IV)O) core in P450 have elucidated electronic structures of the ground triplet and excited singlet states, which indicate isoelectronic characteristics of the species. The dissociation processes of the O–O and Fe–O double bonds are also examined to clarify the radical character, namely O-atom property responsible for radical mechanism of hydroxylations of alkanes and epoxidation of alkenes. This isolobal analogy has indeed enabled us to propose possible reaction mechanisms of oxygenation reactions by the Fe(IV)O species on the basis of available theoretical and experimental results for O and O₂. Similarly, an isolobal analogy of the σ^* bond among Fe(IV)O, dioxirane, peracids, etc. indicates the common electrophilic property for the oxygenation reactions. The small energy gaps between the high- and low-spin states of the transition structures and intermediates generated in the oxygenation reactions are found to be origins for spin crossover phenomena along the reaction pathways of these reactions.     

Dielectric relaxation study of formamide–propylene glycol using time domain reflectometry

Using picoseconds time domain reflectometry, dielectric relaxation studies have been carried out for formamide (FMD)–propylene glycol (PLG) mixtures over the frequency range from 10?MHz to 20?GHz at various temperatures. The dielectric parameters, i.e. static dielectric constant (ε ₀) and relaxation time (τ), have been obtained by Fourier transform and least squares fit methods. The excess dielectric properties and Kirkwood correlation factor of the mixtures have also been determined. The Kirkwood angular correlation factor is greater than one (g ^eff?>?1) in FMD-rich region and less than one (g ^eff?<?1) in PLG-rich region, which indicates that in the mixture the dipole pairs have been formed in such a way that their orientation is parallel in FMD-rich region and antiparallel in the PLG-rich region.