G. Lahajnar

High‐Temperature Phase Transition in KH2PO4

The high‐temperature phase transition in KH2PO4 has been investigated by differential thermal analysis, thermogravimetric analysis, proton magnetic resonance and relaxation, infrared spectroscopy, and x‐ray and quasielastic cold‐neutron scattering. The transition is not connected with a static breaking up of the hydrogen‐bond network but rather with the onset of disordered hindered rotation of the H2PO4 groups around all three axes. It seems that rotation of H2PO4 groups is as well rate determining for the proton conductivity of this crystal.

Magnetic Resonance Study of Molecular Motion in Cubic (NH4)2SiF6

Proton, fluorine, and deuteron magnetic resonance absorption line shapes, as well as 1H and 19F spin—lattice relaxation times, have been measured in polycrystalline cubic (NH4)2SiF6 and (ND4)2SiF6 from −170° to +100°C. The results demonstrate that the threefold proton disorder in the NH4 groups is indeed dynamic and not static, and suggest the following mode of reorientation: The NH4 tetrahedron is first excited from the ground torsional—vibrational state to nearly continuous levels above the potential barrier, and after a short interval of nearly free rotation again drops to one of the three possible equilibrium orientations in the ground state. On the average, each proton spends the same amount of time in each of the 12 equilibrium sites. Large‐amplitude rotational oscilations in the ground state are also consistent with the experimental data. The existence of a 19F linewidth transition, as well as spin—lattice relaxation data, demonstrate the onset of hindered rotation of the SiF6 ions above room tempe...

Surface-induced order and diffusion in 5CB liquid crystal confined to porous glass

Liquid crystals confined into small cavities are known to have a weak orientational order even above the nematic-isotropic transition temperature. The surface-induced order and molecular dynamics in this temperature range are studied with the aid of deuteron NMR spectra, spin relaxation times T(1) and T(2,) proton dipolar-correlation effect, and direct measurements of the effective diffusion coefficient for the liquid crystal 5CB confined to controlled-pore glasses. Our results show that an arrangement of molecules parallel to the wall is induced by local molecular interactions between the liquid crystal and solid, resulting in a weak and temperature independent surface order parameter, S(0) approximately 0.02 +/- 0.01. There is no indication of a significant slowing-down of molecular diffusion at the wall, neither rotational nor translational. In cavities of nanometer size, where the nematic order evolves gradually upon cooling, a broadening of the NMR linewidths due to dynamic effects should be taken into account.

Proton spin–lattice relaxation in nematic microdroplets

The frequency and temperature dependences of the proton spin–lattice relaxation rate have been studied in nematic droplets embedded in a solid polymer matrix. The comparison of these data to those for bulk nematic and pure polymer samples shows that cross‐relaxation between liquid crystal and polymer protons dominates the liquid crystal relaxation in the MHz frequency region, while at low frequencies the translationally induced molecular rotation becomes important. The cross‐relaxation rate is found to be ≈103 s−1 giving an order of magnitude estimate 10−4–10−3 s for the time for which a liquid crystal molecule is bonded to the surface.

On the use of pulse NMR techniques for the study of cement hydration

Abstract The application of pulse NMR to the study of hydration of cement and its constituents is discussed. The quantity of adsorbed water in hydrated samples can be most easily determined by measuring the proton free induction decay signal, whereas the rates of hardening and hydration can be best followed by measuring the proton spin-lattice and spin-spin relaxation times. The use of multiple pulse high resolution NMR in solids techniques is helpful in separating the H 2 O and OH-group signals whereas 27 Al quadrupole coupling and spin-lattice relaxation may as well contribute to our understanding of the structure and hydration of cement.

NMR Study of the Ferroelectric Transitions in Diglycine Nitrate and Trissarcosine Calcium Chloride

A proton magnetic resonance spin–lattice relaxation and second moment study of the ferroelectric transitions in diglycine nitrate (DGN), trissarcosine calcium chloride (TSCC), and its deuterated analog has been performed. The results support the order–disorder model of the ferroelectric transition in these compounds and demonstrate the freezing in of a “flipping” mode on going to the ferroelectric phase in analogy with triglycine sulphate. Deuteration results in a shift of the Curie temperature of TSCC from 127 to 149°K.

Hydration of white cement by spin grouping NMR

Abstract The hydrogen NMR parameters T 1 , T 2 and M oi of a white cement were monitored as a function of hydration time. From the nature and time evolution of the same 20 observables that eventually developed, a partial description of the dynamics of cement hydration is proposed. The results depict the behaviour of the calcium hydroxide, fluid gel, ettringite, solid C-S-H, Fe-relaxed C-S-H, Fe-relaxed pore water, and tentatively differentiated micropore and capillary pore water phases.

NMR study of the fast protonic conductor Cs5H3(SO4)4 · H2O

Abstract The appearance of a “liquid-like” proton T 2 and the large value of the proton self-diffusion coefficient D = (2.8-4.4) × 10 −8 cm 2 /s above T c = 414 K demonstrate the existence of fast protonic conductivity in Cs 5 H 3 (SO 4 ) 4 · H 2 O in this phase. Proton NMR linewidth transitions occur around 300 and 240 K demonstrating the slowing down of the motion of acid protons and the freeze-out of crystal water reorientations.

Proton NMR relaxation of adsorbed water in gelatin and collagen

The dependence of the water self-diffusion coefficients as well as of the proton spin-lattice and spin-spin relaxation rates on the concentration have been studied in the gelatin-water system and in hydrated native collagen. The bound and free water fractions and the corresponding spin-spin and spin-lattice relaxation rates have been determined within the multi-phase water proton exchange model. Various theoretical models for the water proton cross-relaxation to the biopolymer have been studied and the results compared with the observed Larmor frequency dependence of the water proton spin-lattice relaxation rate.

Proton NMR study of a discotic columnar mesophase

The molecular ordering and the dynamics of the isotropic and the discotic columnar liquid crystalline phase of hexapentoxy‐triphenylene have been studied by high resolution proton NMR, self‐diffusion measurements and spin–lattice relaxation in the laboratory (T1) and the rotating frame (T1ρ). The onset of the columnar ordering is accompanied by the gradual appearance of a short ’’liquid crystalline’’ component in the proton free induction decay (FID) in addition to the long component characteristic of the isotropic phase. The proton T1 is frequency dependent in the isotropic and in the columnar phase. It changes discontinuously at the clearing point at low frequencies where as it is continuous at 270 MHz. An analysis of the possible relaxation mechanisms is presented.