K. Venu

NMR studies of molecular dynamics in tetramethylammonium tetrabromo cadmate

Internal motions in a A2BX4 compound (tetramethylammonium tetrabromo cadmate) have been investigated using proton spin—lattice relaxation time (T1) and second moment (M2) measurements in the temperature range 77 to 400 K. T1 measurements at three Larmor frequencies (10, 20 and 30 MHz) show isotropic tumbling of the tetramethylammonium group, random reorientation of methyl groups and spin—rotation interaction, and the corresponding parameters have been computed. The cw spectrum is narrow throughout the temperature range and shows side bands at the lowest temperature. This observation, along with the free-induction-decay behavior at these temperatures, is interpreted as the onset of a coherent motion, e.g. methyl group quantum tunnelling.

Molecular motions in liquid crystal BBBA (4O.4): QENS study

Abstract Molecular rotational motions in butyloxybenzylidine butylanilene as studied by quasielastic neutron scattering technique in the smectic B and smectic A phases are reported. The end chain (butyl group) and the core of the liquid crystal are found to undergo rotational motions, independent of each other. Both of these are best described by the jump rotation, with N =12 equivalent sites on a circle or uniaxial rotational diffusion. Reorientation times, τ 1 , equal to 6.1×10 −11 ±1.6×10 −11 and 1.8×10 −11 ±0.6×10 −11 s are obtained for the core group and end chain, respectively, in the smectic B phase. The geometry of rotation has been found to be unchanged in the smectic A phase and the corresponding reorientation times equal to 4.1×10 −11 ±1.2×10 −11 and 1.6×10 −11 ±0.5×10 −11 s, respectively, are obtained.

Order-disorder phase transition and dimethyl ammonium group dynamics in [(CH3)2NH2]3Sb2Cl9-Proton NMR study

Abstract Cation dynamics in tris(dimethylammonium) nonachloro-diantimonate, which exhibits paraelectric-ferroelectric phase transition, is studied through proton spin lattice relaxation and second moments studies. Results show a clear signature of this phase transition. Analysis of the data based on a recent model indicates a freezing of isotropic motion of the dimethylammoiun (DMA) cation at the phase transition, as well as the existence of dynamically inequivalent DMA and methyl groups, both in the ratio of 1 : 1 in the low temperature phase. Activation energies associated with the dynamics of different inequivalent cations and methyl groups are calculated. These results are compared with similar studies on related compounds comprising of tetramethyl and trimethyl ammonium groups.

A nuclear magnetic resonance study of tetramethylammonium tribromocadmate

Proton spin lattice relaxation (T1) in (CH3)4NCdBr3 at different Larmor frequencies (10, 20 and 30 MHz) has been studied in the temperature range 77 to 400 K. The variations in T1 at high temperature are independent of frequency and show a maximum due to spin rotation- interaction. The other features are interpreted as being due to isotropic tumbling of the tetramethylammonium ion and random reorientation of the CH3 group. The CW spectrum remained narrow up to 77 K and develops a wing structure at low temperatures. This observation is attributed to a possible tunnelling motion of the CH3 group, which has rather low activation energy as demonstrated by the study of T1.

PMRD investigations of molecular dynamics and phase transitions in the liquid crystal 6CHBT

Proton Magnetic Relaxation Dispersion (PMRD) measurements were carried out over a wide Larmor frequency range near the isotropic-nematic transition temperature (T NI) and in the mid nematic phase of a low viscous liquid crystal 4-(trans-4′-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT), with a view to examining the pre-transitional effects just above isotropic-nematic transition (T > T NI) and the role of director fluctuations (DF) on the nematic phase stability of this system. The results near the transition indicate critical slowing down of the short-range nematic order modes just above T NI with a mean field critical exponent characterizing the coherence length (ξ). The observed temperature independence of the correlation time associated with the reorientations (τR) and the extension of DF mode spectrum, in the mid-nematic phase, to relatively higher resonance frequencies seem to suggest fairly low hindering barriers for the tumbling of the molecules about their short axes.

Proton magnetic relaxation study of cation dynamics in TMACA

Abstract Cation dynamics in tris(trimethylammonium) nonachlorodiantimonate, which shows different phases as a function of temperature, is studied through proton spin lattice relaxation measurements at five Larmor frequencies. Results show a clear signature of the para-ferroelectric phase transition at 363 K and the existence of dynamically inequivalent trimethylammonium cations and methyl groups, both in the ratio of 1:2. Various activation energies associated with the dynamics of the cations and methyl groups are calculated.

Cation dynamics in [N(CH3)4]3Sb2Cl9 (TEMACA) - NMR study

Abstract Molecular motions in TEMACA, which shows structural phase transitions at 223 and 156 K, are studied through proton spin-lattice relaxation and linewidth measurements at two Larmor frequencies. The results show that tetramethylammonium groups undergo isotropic tumbling with more hindrance in the phase below 223 K compared to the high temperature phase. The results also show that the methyl groups exist in dynamically inequivalent sites. There is no signature of a 156 K transition in the data. Various activation energies associated with the dynamics of the cation groups and the threefold reorientations of the methyl groups below the phase transition are calculated.

Proton NMR study of molecular dynamics in ammonium tribromo stannate (NH4SnBr3)

The proton second moment M2 and spin-lattice relaxation time T1 have been measured in ammonium tribromo stannate (NH4SnBr3) in the temperature range 77–300 K, to determine the ammonium dynamics. The continuous wave signal is strong and narrow at 77 and 300 K but has revealed an interesting intensity anomaly between 210 and 125 K. T1 shows a maximum (13 s) around 220 K. No minimum in the T1 vs 1000/T plot was observed down to 77 K. M2 and T1 results are interpreted in terms of NH+4 ion dynamics. The activation energy Ea for NH+4 ion reorientation is estimated to be 1.4 kcal mol−1.

Proton NMR investigation of molecular dynamics in [N(CH3)4]3 Sb2Br9 (TEMABA)

Abstract Cation dynamics in tris (tetra-methylammonium) nonabromodiantimonate, which shows structural phase transitions, are studied through proton spin lattice relaxation measurements (at two Larmor frequencies) and line width measurements. Results show a clear signature of a phase transition at 189 K and the existence of dynamically inequivalent methyl groups. The dynamics of the cation as a whole are found to be less hindered above this temperature. Activation energies associated with the dynamics of the cations and methyl groups are calculated.

Molecular dynamics in the liquid crystal 40.6 A nuclear magnetic relaxation study

Abstract A nuclear spin lattice relaxation study of the liquid crystal butoxybenzylidene hexylaniline (40.6) has been carried out as a function of Larmor frequency (4 to 30 MHz) and temperature covering all its mesophases (N, SA and SB), with a view to obtain information on different dynamic processes at the molecular level. These studies are also supplemented by a nuclear dipolar relaxation study as well as anisotropy of these relaxation rates with respect to the static magnetic field. The analysis of this data in nematic and smectic A phases based on a composite model, including the contributions from self diffusion (SD), order director fluctuations (ODF), and molecular reorientations (R), indicate that the nuclear spin relaxation is effectively mediated by diffusion and reorientation processes at frequencies above 10 MHz while at lower frequencies ODF also seem to contribute appreciably. The relative contribution from these mechanisms at different temperatures covering nematic and SA phases are evaluat...