V.S.S. Sastry

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.

Study of internal motions and phase transitions in (NH4)2ZnCl4 and [(N(CH3)4]2 ZnCl4 by proton magnetic resonance and relaxation

Proton second moment (M2) and spin-lattice relaxation time (T1 at 10 MHz have been measured in polycrystalline ammonium tetrachlorozincate (ATC-Zn) and tetramethyl ammonium tetrachlorozincate (TMATC-Zn) as a function of temperature, covering the phase transitions. In ATC-Zn, the motion of NH+4 ion is fast in all phases as seen by a small and temperature independent proton second moment. Two types of chemically inequivalent NH+4 ions have been identified from the observed double minima in proton T1. Proton T1 shows a discontinuous jump and change of slope at 319 K, probably due to a phase transition. In addition, T1 shows a maximum around the reported ferroelectric phase transition (270 K), indicating the presence of spin-rotation interaction. In TMATC-Zn, the lowest temperature transition seems to be related to the slowing down of CH3 groups as revealed by the proton second moment. In this system also, two kinds of [N(CH3)4] ions have been found to be present from the proton T1 studies. Proton T1 shows ch...

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.

Pulsed NMR study of molecular dynamics in liquid crystal 40.7

Abstract Molecular dynamics governing the nuclear magnetic relaxation times in a liquid crystal, butyloxybenzylidene heptylaniline (40.7), is studied using proton magnetic relaxation spectroscopy. These results are compared with those of similar studies on other compounds belonging to the same homologues series (viz. 40.8, 40.6, and 40.5). Earlier studies indicate that the frequency dispersion of the spin-lattice relaxation time in typical NMR frequency ranges is governed by order director fluctuations (ODF) in 40.8, self-diffusion (SD) in 40.5, whereas by both ODF and SD in 40.6. Molecular reorientations contribute to a frequency-independent component in all these compounds. The present studies show that SD and molecular reorientations effectively mediate the relaxation mechanism in 40.7 in the nematic and the smectic A phases and their relative contributions are quantified. The relaxation data and the line width measurements at magic angle orientation are used to obtain dynamical information on the smectic B phase. These results are supported by dipolar relaxation time measurements. From the temperature-dependence studies the molecular parameters associated with these dynamical processes are obtained.

NMR study of phase transitions in dicalcium barium propionate and dicalcium lead propionate

Dicalcium Barium Propionate (DBP) undergoes a first order phase transition at 267 K and a second order one at 204 K. Dicalcium lead propionate (DLP) also shows two phase transitions—a second-order one at 333 K and a first-order one at 191.5 K. The proton second moment

Phase transitions and cation dynamics in [(CH3)2NH2]3Bi2Cl9 (DMACB)—Proton spin relaxation studies

(M_2)