Proton NMR studies have been carried out as a function of temperature from 210 K to 300 K on water confined within single-walled carbon nanotubes. The NMR lineshape at and below the freezing point of bulk water is asymmetric and can be decomposed into a sum of two Lorentzians. The intensities of both the components decrease with the lowering of the temperature below 273 K, one component, L1, vanishing below 242 K and the other component, L2, below 217 K. Following the simulations of Koga et al. showing that the radial density profile of confined water in single-wall carbon nanotubes has a distribution peak at the center which disappears below the freezing temperature, the L1-component is associated with the protons of the water molecules at the center and the L2-component is associated with protons of water molecules at a distance of ∼ 3 Å from the walls of the nanotubes. In this scenario the complete freezing of the water at ∼ 212 K is preceded by the withdrawal of the water molecules from the center.Carbon nanotubes are unique in many ways not only for their unusual electric and mechanical properties resulting in many potential applications [1][2][3][4] but also for their quasi-onedimensional nanometric-sized hollow structures. Single-walled carbon nanotubes (SWNT) assemble themselves into a two-dimensional triangular lattice to form bundles of aligned nanotubes with a nearest intertubular gap of ∼ 0.3 nm [5]. Thus, the inside of the nanotubes and the interstitial gaps in the bundle provide very well characterized nanometer-sized volumes to show how liquids, in particular, water, behave on nanoscale [6,7]. These studies on confined water may also have relevance to understand the behavior of water on nanoscale in biological environments like in ion channels [8].SWNT can be wetted by liquids with surface tension less than 0.2 N/m and hence water (surface tension ∼ 0.07 N/m) is expected to wet nanotubes [9]. Recent computer simulations have brought out many new features in the behavior of water inside carbon nanotubes [6,[10][11][12]. It is shown that water inside cylindrical pores of SWNT of diameter (2R) of 1.42 nm is expected to show a first-order freezing (melting) transitions at about 230 K (260 K). The low-temperature phase is an ice-nanotube composed of orderly stacked 6-or 7-membered water rings. The simulations [6] have also shown a significant change in the radial density
Nuclear magnetic resonance spectroscopy has demonstrated significant experimental progress toward the development of quantum computations. The developments so far have taken place mainly through the use of spin 1 2 nuclei. In this paper we describe the use of a spin 3 2 nucleus, oriented in a liquid crystal matrix for the creation of pseudopure states and the implementation of a complete set of two-qubit reversible logic gates using single-quantum transition-selective pulses, extending the range of practice of NMR toward quantum computation.
Molecules exhibiting a thermotropic liquid-crystalline property have acquired significant importance due to their sensitivity to external stimuli such as temperature, mechanical forces, and electric and magnetic fields. As a result, several novel mesogens have been synthesized by the introduction of various functional groups in the vicinity of the aromatic core as well as in the side chains and their properties have been studied. In the present study, we report three-ring mesogens with hydroxyl groups at one terminal. These mesogens were synthesized by a multistep route, and structural characterization was accomplished by spectral techniques. The mesophase properties were studied by hot-stage optical polarizing microscopy, differential scanning calorimetry, and small-angle X-ray scattering. An enantiotropic nematic phase was noticed for lower homologues, while an additional smectic C phase was found for higher homologues. Solid-state high-resolution natural abundance (13)C NMR studies of a typical mesogen in the solid phase and in the mesophases have been carried out. The (13)C NMR spectrum of the mesogen in the smectic C and nematic phases indicated spontaneous alignment of the molecule in the magnetic field. By utilizing the two-dimensional separated local field (SLF) NMR experiment known as SAMPI4, (13)C-(1)H dipolar couplings have been obtained, which were utilized to determine the orientational order parameters of the mesogen.
Structural characterizations using XRD and (13)C NMR spectroscopy of two rodlike mesogens consisting of (i) three phenyl ring core with a polar cyano terminal and (ii) four phenyl ring core with flexible dodecyl terminal chain are presented. The three-ring-core mesogen with cyano terminal exhibits enantiotropic smectic A phase while the four-ring mesogen reveals polymesomorphism and shows enantiotropic nematic, smectic C, and tilted hexatic phases. The molecular organization in the three-ring mesogen is found to be partial bilayer smectic Ad type, and the interdigitation of the molecules in the neighboring layers is attributed to the presence of the polar terminal group. For the four-ring mesogen, the XRD results confirm the existence of the smectic C and the tilted hexatic mesophases. A thermal variation of the layer spacing across the smectic C phase followed by a discrete jump at the transition to the tilted hexatic phase is also observed. The tilt angles have been estimated to be about 45° in the smectic C phase and about 40° in tilted hexatic phase. (13)C NMR results indicate that in the mesophase the molecules are aligned parallel to the magnetic field. From the (13)C-(1)H dipolar couplings determined from the 2D experiments, the overall order parameter for the three-ring mesogen in its smectic A phase has been estimated to be 0.72 while values ranging from 0.88 to 0.44 have been obtained for the four-ring mesogen as it passes from the tilted hexatic to the nematic phase. The orientations of the different rings of the core unit with respect to each other and also with respect to the long axis of the molecule have also been obtained.
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