Search

Phospholipid bicelles that have positive anistropy of the magnetic susceptibility and align with their normal parallel to an external magnetic field is presented. The (super 31)P chemical shifts indicate that the conformation of the polar headgroup in the bicelles may be different from that in common bicelles.

The use of nuclear magnetic resonance (NMR) to carry out quantum information processing (QIP) often requires the preparation, transformation, and detection of pseudopure states. In our previous work, it was shown that the use of pairs of pseudopure states (POPS) as a basis for QIP is very convenient because of the simplicity in experimental execution. It is now further demonstrated that the product of the NMR spectra corresponding to two sets of POPS that share a common pseudopure state has the same peak frequencies as those of the common (single) pseudopure state. Examples of applying two different quantum logic gates to a 5-qubit system are given. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have found that the application of a weak radio frequency (rf) pulse to a spin system with indirect spin-spin (J) couplings can produce a narrow inverted peak in spectral regions where there are many overlapping peaks. Examples of three compounds with 1H-1H J couplings and one with 1H-13C J couplings are given. The dependences of the signals on the frequency, duration, and amplitude of the weak rf irradiation have been studied. For an rf power of γB1/2π=1.0 Hz, pulse widths longer than 0.25 s consistently produce inverted narrow peaks in spectral regions with unresolved peaks. An interpretation of the origin of this unusual result of selective excitation in spin systems with homogeneous broadening is given: the inverted signal observed can be considered as a cumulative effect of the weak rf irradiation acting on many uncorrelated transitions that are slightly off resonance on both sides of the irradiating frequency; the only role of the J couplings (or dipolar couplings in liquid crystals) is to produce a large number of closely spaced peaks that overlap with each other. Computer simulations of both coupled and noninteracting spin systems have been carried out, and the results support this interpretation. [ABSTRACT FROM AUTHOR]

A procedure is described for storing a two-dimensional (2D) pattern consisting of 32×32 = 1024 bits in a spin state of a molecular system and then retrieving the stored information as a stack of nuclear magnetic resonance spectra. The system used is a nematic liquid crystal, the protons of which act as spin clusters with strong intramolecular interactions. The technique used is a programmable multifrequency irradiation with low amplitude. When it is applied to the liquid crystal, a large number of coherent long-lived [SUP1]H response signals can be excited, resulting in a spectrum showing many sharp peaks with controllable frequencies and amplitudes. The spectral resolution is enhanced by using a second weak pulse with a 90° phase shift, so that the 1024 bits of information can be retrieved as a set of well-resolved pseudo-2D spectra reproducing the input pattern. [ABSTRACT FROM AUTHOR]

The use of pairs of pseudopure states (POPS) as the basis for nuclear magnetic resonance (NMR) quantum computing is a method recently introduced. The advantage of this approach is its experimental simplicity: each POPS can be prepared in a single experiment with two steps, the first step having two pulses and the second having only one. The POPS method is now demonstrated for a 4-qubit and a 5-qubit system, for which the [sup 19]F and [sup 1]H spectra are first order and all the peaks are completely resolved. The characteristics of the POPS spectra are examined in detail, and the spectra of one of the five available sets of POPS for the 5-qubit system are shown as an illustration. The result of applying a c[sup 4]-NOT quantum logic gate (controlled-NOT gate conditional upon the state of four other qubits) to another set of POPS in the 5-qubit system is also shown. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies the nuclear magnetic resonance (NMR) of polycrystalline solids with fast spinning at or very near the magic angle spinning (MAS). MAS spectrum of L-alanine at two spinning rates; Dependence of the second-order shift on crystal orientation; Increase in the signal intensity with the use of soft pulses.

The smectic-A to smectic-C transition for two binary mixtures of liquid crystals has been studied. These systems are mixtures of a 4’-(2-chloroalkoxy)-4-heptyloxybiphenyl, designated B7 or C7, with 4-butyloxyphenyl-4’-decyloxybenzoate (10O4). Pure B7 and C7, in their racemic forms, display characteristics of a first-order smectic-A to smectic-C transition. With increasing mole fraction of 10O4, the behavior of the phase transition changes from first order to second order. The order parameter of the mesogenic core of B7 and C7 in these systems was determined by the use of 13C nuclear magnetic resonance. It was found that the order parameter of the liquid crystal systems at the transition temperature is discontinuous for a first-order smectic-A to smectic-C transition, and continuous for a second-order transition. The composition of the tricritical mixture (XTCP, where X is the mole fraction of 10O4) was determined to be XTCP=0.12±0.01 for the B 7/10O4 system and XTCP=0.13±0.01 for the C7/10O4 system. The former has not been reported before, and the latter agrees reasonably well with previous studies. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations have been performed for the nematic liquid crystal 4-n-pentyl-4’-cyanobiphenyl (5CB) using a realistic atom–atom potential as well as a pseudorealistic atom–atom potential. The latter treats the phenyl rings in the 5CB molecule as simple spheres. This method allows a considerable decrease in computational time while still retaining important aspects of the structural anisotropy. Molecular ordering has been studied using both approaches by calculating order parameters for the principle molecular axis as well as for individual molecular segments. Molecular conformations and dynamics have been examined using dihedral angle distribution functions for the biphenyl core and for the alkyl chain. Analysis of the data allows the inter and intra molecular contributions to the conformational equilibria of the chain to be separated. This information can be used to determine the conformational equilibria of the alkyl chain under the influence of the nematic field created by surrounding molecules. Transport properties have been examined by calculating diffusion coefficients in directions both parallel and perpendicular to the nematic director. All properties calculated using the pseudomolecular potential are compared with those obtained from the more complete atom–atom potential to assess the validity of the simplified approach. [ABSTRACT FROM AUTHOR]

A new method which uses carbon-13 NMR with rapidly decelerated sample spinning to study the reorientation of a nematic and a smectic liquid crystal is presented. When a macroscopically ordered liquid crystal is subjected to a new torque of sufficient magnitude, the direction of the macroscopic ordering changes in response to the new torque. In our experiments, the liquid crystal is spun rapidly at an angle slightly less than the magic angle, so that its director aligns along the spinning axis. The rapid spinning is suddenly stopped, and the liquid crystal molecules reorient so that the director would align along the magnetic field. The carbon-13 chemical shift, which is very sensitive to the angle that the molecular segment forms with respect to the magnetic field, is observed as a function of time following the stopping of the spinner. By fitting the chemical shift data as a function of time to a physically meaningful equation, a time constant can be calculated for various molecular segments of the liquid crystal molecule. This time constant provides information about the macroscopic reorientation of the liquid crystal from the alignment caused by the spinning torque to the alignment caused by the magnetic field. A room temperature nematic, 4-n-pentyl-4’-cyanobiphenyl (5CB), and a room temperature smectic, 4-n-octyl-4’-cyanobiphenyl (8CB), are studied by this technique. The reorientation time constant of 8CB is about 300 times larger than that of 5CB, and the reorientation time constant of the aromatic core is slightly smaller than the reorientation time constant of the aliphatic chain for both compounds. These results are among the first experimental data in the field of studying liquid reorientation on the level of the molecular segment, and a possible explanation for these results is presented. [ABSTRACT FROM AUTHOR]

Proton cross relaxation between liquid crystal and polymer in a polymer dispersed liquid crystal (PDLC) system has been studied by the use of magic angle spinning and polarization transfer. The cross relaxation rates have been determined as functions of temperature, irradiation power, and off-resonance frequency setting. The data at each temperature are treated by a new set of relaxation equations which are not limited by the assumption of complete saturation of the polymer protons. The cross relaxation rates for the aromatic and aliphatic protons are similar over the temperature range studied, indicating that the liquid crystal molecules anchor parallel to the polymer surface. Before the glass transition, the cross relaxation rates increase with temperature due to a reduction in the contact time. After the glass transition, at about 37 °C, the cross relaxation rates decrease rapidly with temperature due to an increase in the motions of the polymer molecules which cause a reduction in the efficiency of spin energy exchange. [ABSTRACT FROM AUTHOR]

The orientational ordering of four 4-n-alkyl-4’-cyanobiphenyls (kCB’s: 5CB, 6CB, 7CB, 8CB) and four 4-n-alkoxy-4’-cyanobiphenyls (kOCB’s: 5OCB, 6OCB, 7OCB, 8OCB) in their nematic phase was studied at different temperatures by C-13 NMR. The technique used was a combination of variable-angle spinning and a two-dimensional method called separated local-field spectroscopy. The C-13 chemical shifts for each carbon nucleus were measured with the sample spinning parallel to the magnetic field. For the carbon atoms in the aromatic rings and the cyano group as well as the carbon atoms in the in α and β positions of the aliphatic chain, the order parameters for all four compounds in each homologous series show a linear dependence on the corresponding chemical shifts. For the ω-CH3 group and the (ω-1)-CH2 group, a simple linear dependence of the C–H order parameter on the chemical shift was found for all eight compounds in the two homologous series. The slopes and intercepts of all the results are tabulated. The results offer a simple way to determine the order parameters of liquid crystals directly from C-13 chemical shifts. [ABSTRACT FROM AUTHOR]

The orientational ordering of three ferroelectric liquid crystals 4’-(3-methyl-2-chloropentanoyloxy)-4-alkyloxybiphenyls has been studied by 2D carbon-13 nuclear magnetic resonance (NMR) with variable angle sample spinning. The three homologs have hexyloxy (C6), heptyloxy (C7), and octyloxy (C8) chains, respectively, at the 4 position. They exhibit smectic A (SA) and chiral smectic C (S[ATOTHER]@B|[/ATOTHER]C; ferroelectric) phases and have unusually large spontaneous polarization of the electric dipoles. In the SA phase, macroscopic alignment of the molecular director along the spinning axis can be achieved by cooling the sample rapidly from the isotropic liquid. For the S*C phase, the molecular alignment is different for the three compounds. At a magnetic field of 7.05 T, the helical structures of C6 and C8 unwind and the molecular directors align along the spinning axis at a spinning rate of 1 kHz. However, under the same conditions, the helical structure of C7 is retained and its carbon-13 NMR spectrum shows a partial powder pattern. On the other hand, the racemate of C7 has a normal smectic C (SC) phase without a helical structure, and the molecular director can be easily aligned for the NMR study. The racemate of C8 was also synthesized and studied for comparison. The 2D NMR technique used was separated local field (SLF) spectroscopy with an efficient proton–proton dipolar decoupling sequence BLEW-48 in the evolution period. The carbon–proton dipolar coupling constants were determined and the order parameters for different molecular segments of the liquid crystal were calculated. The carbon-13 chemical shifts were also measured as functions of temperature and the data were used to probe the temperature dependence of order parameters. The results show that the C–H bond at the first chiral center in these compounds has an unusually large negative order parameter. This could be due to restricted rotation of the molecular segment... [ABSTRACT FROM AUTHOR]

The potential of mean torque for the flexible molecules comprising liquid crystals which was proposed by Emsley et al. [Proc. R. Soc. London Ser. A 381, 117 (1982)] contains interaction parameters for rigid fragments of the molecule. These are determined for four members of the series 4-n-alkyl-4′-cyanobiphenyls (NCB) with N=5, 6, 7, and 8, at TNI -T=4 K, by comparing calculated values of SCH, the local order parameter for C–H bonds at each position in the alkyl chain, with values determined from dipolar couplings between carbon-13 and protons. The potential of mean torque characterized in this way is then used to calculate vicinal C–H dipolar coupling constants for nuclei in the alkyl chain for 5CB and 6CB; these are found to be in reasonable agreement with experimental values. [ABSTRACT FROM AUTHOR]

A method called RHODIUM (removal of homonuclear dipolar coupling and the use of off-magic-angle spinning) can be applied effectively to study the carbon-13 NMR of liquid crystals in natural abundance. In this method, the sample is spun rapidly (∼1 kHz) at an angle (θ) slightly less than the magic angle, so that the nematic director aligns along the spinning axis and the dipolar couplings are reduced by a factor of (3 cos2 θ-1)/2. Proton–proton dipolar couplings are removed by a special decoupling sequence (BLEW-48) so that carbon–proton dipolar couplings can be readily identified. The coupling constants (DC–H) of individual carbons are obtained by using a two-dimensional technique of separated local field spectroscopy. The RHODIUM method has been used to study the molecular ordering in the nematic phase of 4-n-alkyl-4’-cyanobiphenyls (kCB). Three homologs (5CB, 6CB, and 7CB) were investigated at TNI-T =4 K. Values of DC–H of the aliphatic carbons give rich information on the energetics of chain conformations and those of the aromatic carbons can be used to calculate order parameters of the phenyl rings. It was found that the values of DC–H for the α, β, and γ carbons in 5CB, 6CB, and 7CB are almost the same, and so are the values of Szz and Sxx-Syy of the rings. The results for 5CB are in good agreement with those obtained from deuterium and proton NMR studies of specially deuterated compounds. [ABSTRACT FROM AUTHOR]

With magic-angle spinning, the carbon-13 NMR spectra of nematic liquid crystals having positive anisotropy of the magnetic susceptibility show sharp peaks. Solutes which have carbon-13 peaks in the ‘‘window’’ regions of the solvent spectra can be conveniently studied. The hindered rotation of the dialkylamino group in 6-(dimethylamino)fulvene, 6-(diethylamino)fulvene, and 6-(N-piperidyl)fulvene in a liquid crystal solvent (E8) has been investigated by carbon-13 NMR with magic-angle spinning. The results are compared with those obtained in isotropic solutions of the same compounds in CDCl3 and CDCl3+E8. The free energy of activation (ΔG≠) of each compound in the three solvents is the same within experimental error. [ABSTRACT FROM AUTHOR]

Carbon-13 NMR of liquid crystals with magic-angle spinning (MAS) has been studied. Nematic liquid crystals with Δχ>0 orient parallel to the field at low spinning rates, and orient along the spinning axis at high spinning rates (ν). At a field of 7.05 T, the C-13 peaks of EBBA are very sharp and there are no spinning sidebands at ν>=1000 Hz. Spin lattice relaxation time (T1) and nuclear Overhauser effect (NOE) of individual carbon atoms of EBBA have been measured and the results give valuable information on the motional behavior of EBBA in its nematic phase. Nematic liquid crystals with Δχ<0 show rather broad peaks even at high ν. Smectic liquid crystals do not orient along the spinning axis, but can be made to do so when they are cooled through the nematic phase while the sample is being spun. With MAS at high ν, cross polarization is ineffective for nematic liquid crystals, and is not very effective for smectic liquid crystals. [ABSTRACT FROM AUTHOR]