On aT1andT1ρStudy of Molecular Motion and Phase Transitions in the Tetramethylammonium Halides

NMR analysis of structural geometry and molecular dynamics in perovskite-type N(CH3)4CdBr3 crystal near high-temperature phase transition

The NMR chemical shifts, linewidths, spin-lattice relaxation times in the rotating system T1ρ, and spin-lattice relaxation times in the laboratory system T1 were evaluated for the perovskite-type N(CH3)4CdBr3 crystal, aiming to understand the changes in the structural geometry and molecular dynamics from phase I to phase II. From the temperature-dependence of the 1H, 13C, 14N, and 113Cd NMR chemical shifts, the structural geometry underwent a continuous change, without anomalous changes around (TC = 390 K). However, the linewidths in phase I were narrower than those in phase II, indicating that the motional averaging effects were caused by the rapid rotation of the N(CH3)4 group. Sudden changes in T1 and T1ρ were observed near TC, for which the activation energy Ea in phase I was approximately 12 times larger than that in phase II; the small Ea values in phase II indicate a large degree of freedom for the methyl group and CdBr6 octahedra, whereas the large Ea in phase I was primarily attributed to the overall N(CH3)4 and the 113Cd in the CdBr6 groups. Consequently, the phase transition mechanisms of N(CH3)4CdBr3 are related to reorientation of the N(CH3)4 group and the arrangement of the CdBr6 groups.

Structural phase transitions coupled with prominent dielectric anomalies and dielectric relaxation in [(CH3)3NH]2[KCo(CN)6] and mixed [(CH3)3NH]2[KFexCo1-x(CN)6] double perovskite hybrids

The crystals of pure [(CH₃)₃NH]₂[KFe(CN)₆] (TrMAFe) and [(CH₃)₃NH]₂[KCo(CN)₆] (TrMACo) as well as their mixed crystals (TrMAFe_xCo_1-x), with different ratios of x = 0, 0.12, 0.18, 0.49, 0.56, 0.73, 0.81, 1.0, have been grown from aqueous solutions. The structure of TrMACo has been determined at 360 K to be cubic (Fm3[combining macron]m). In phase II (100 K), the TrMACo crystal is monoclinic (C2/c). The thermal stability of the pure and mixed crystals has been determined by using both DTA and TGA. Based on the DSC results, we have found a single phase transition (PT) for both pure and mixed crystals. The Fe(iii) concentration was estimated by using the SEM technique. We have found a linear relationship between the PT temperature (T_c) and the molar concentration of Fe(iii). Based on the obtained results, a phase diagram has been constructed. The mechanism of the structural PT has been discussed based on the results of dielectric relaxation and ¹H NMR and X-ray spectroscopy measurements. The results confirmed that the PT mechanism of both pure and mixed crystals is related to the change in the dynamic state of the TrMA⁺ cations. The dielectric activation energy changes linearly with the mole fraction ranking from 35 to 38 kJ mol^-1, for the mixed crystals.

Solid state ¹H spin-lattice relaxation and isolated-molecule and cluster electronic structure calculations in organic molecular solids: the relationship between structure and methyl group and t-butyl group rotation

We report ab initio density functional theory electronic structure calculations of rotational barriers for t-butyl groups and their constituent methyl groups both in the isolated molecules and in central molecules in clusters built from the X-ray structure in four t-butyl aromatic compounds. The X-ray structures have been reported previously. We also report and interpret the temperature dependence of the solid state (1)H nuclear magnetic resonance spin-lattice relaxation rate at 8.50, 22.5, and 53.0 MHz in one of the four compounds. Such experiments for the other three have been reported previously. We compare the computed barriers for methyl group and t-butyl group rotation in a central target molecule in the cluster with the activation energies determined from fitting the (1)H NMR spin-lattice relaxation data. We formulate a dynamical model for the superposition of t-butyl group rotation and the rotation of the t-butyl group's constituent methyl groups. The four compounds are 2,7-di-t-butylpyrene, 1,4-di-t-butylbenzene, 2,6-di-t-butylnaphthalene, and 3-t-butylchrysene. We comment on the unusual ground state orientation of the t-butyl groups in the crystal of the pyrene and we comment on the unusually high rotational barrier of these t-butyl groups.

Comprehensive solid-state NMR analysis reveals the effects of N-methylation on the molecular dynamics of glycine

Molecular dynamics of metabolites are important for their interactions and functions. To understand the structural dependence of molecular dynamics for N-methylated glycines, we comprehensively measured the (13)C and (1)H spin-lattice relaxation times for sarcosine, N,N-dimethylglycine, betaine, and betaine hydrochloride over a temperature range of 178-460 K. We found that the reorientations of methyl groups were observed for all these molecules, whereas reorientations of whole trimethylamine groups were detected in betaines. While similar rotational properties were observed for methyl groups in N,N-dimethylglycine and those in betaine, three methyl groups in betaine hydrochloride had different motional properties (E(a) ≈ 20.5 kJ/mol, τ(0) ≈ 1.85 × 10(-13) s; E(a) ≈ 13.9 kJ/mol, τ(0) ≈ 2.1 × 10(-12) s; E(a) ≈ 15.8 kJ/mol, τ(0) ≈ 1.1 × 10(-12) s). N,N-Dimethylglycine showed a phase transition at 348.5 K with changed relaxation behavior for methyl groups showing distinct E(a) and τ(0) values. The DIPSHIFT experiments showed that CH(3) and CH(2) moieties in these molecules had dipolar-dephasing curves similar to these moieties in alanine and glycine. The activation energies for CH(3) rotations positively correlated with the number of substituted methyl groups. These findings provided useful information for the structural dependence of molecular dynamics for N-methylated glycines and demonstrated solid-state NMR as a useful tool for probing the structure-dynamics relationships.

Complex molecular dynamics of (CH3NH3)5Bi2Br11 (MAPBB) protons from NMR relaxation and second moment of NMR spectrum

Molecular dynamics of a polycrystalline sample of (CH(3)NH(3))(5)Bi(2)Br(11) (MAPBB) is studied on the basis of the proton T(1) (55.2 MHz) relaxation time and the proton second moment of NMR line. The T(1) (55.2 MHz) was measured for temperatures from 20K to 330 K, while the second moment M(2) for those from 40K to 330 K. The proton spin pairs of the methyl and ammonium groups perform a complex stochastic motion being a resultant of four components characterised by the correlation times τ(3)(T), τ(3)(H), τ(2), and τ(iso), referring to the tunnelling and over the barrier jumps in a triple potential, jumps between two equilibrium sites and isotropic rotation. The theoretical expressions for the spectral densities in the cases of the complex motion considered were derived. For τ(3)(H), τ(2), and τ(iso) the Arrhenius temperature dependence was assumed, while for τ(3)(T)-the Schrödinger one. The correlation times τ(3)(H) for CH(3) and NH(3) groups differ, which indicates the uncorrelated motion of these groups. The stochastic tunnelling jumps are not present above the temperature T(tun) at which the thermal energy is higher than the activation energy of jumps over the barrier attributed to the hindered rotation of the CH(3) and NH(3) groups. The T(tun) temperature is 54.6 K for NH(3) group and 46.5 K for CH(3) group in MAPBB crystal. The tunnelling jumps of the methyl and ammonium protons are responsible for the flattening of T(1) temperature dependence at low temperatures. The isotropic tumbling is detectable only from the M(2) temperature dependence. The isotropic tumbling reduces the second moment to 4 G(2) which is the value of the intermolecular part of the second moment. The motion characterised by the correlation time τ(2) is well detectable from both T(1) and M(2) temperature dependences. This motion causes the appearance of T(1) minimum at 130 K and reduction of the second moment to the 7.7 G(2) value. The small tunnelling splitting ω(T) of the same value for the methyl and ammonium groups was estimated as 226 MHz from the Haupt equation or 80 MHz from the corrected by us Haupt equation. These frequencies correspond to 0.93 μeV and 0.34 μeV tunnel splitting energy.

Study of molecular dynamics and cross relaxation in tetramethylammonium hexafluorophosphate (CH(3))(4)NPF(6) by (1)H and (19)F NMR

(CH(3))(4)NPF(6) is studied by NMR measurements to understand the internal motions and cross relaxation mechanism between the heterogeneous nuclei. The spin lattice relaxation times (T(1)) are measured for (1)H and (19)F nuclei, at three (11.4, 16.1 and 21.34 MHz) Larmor frequencies in the temperature range 350-50K and (1)H NMR second moment measurements at 7 MHz in the temperature range 300-100K employing home made pulsed and wide-line NMR spectrometers. (1)H NMR results are attributed to the simultaneous reorientations of both methyl and tetramethylammonium groups and motional parameters are evaluated. (19)F NMR results are attributed to cross relaxation between proton and fluorine and motional parameters for the PF(6) group reorientation are evaluated.

1H NMR study of internal motions and quantum rotational tunneling in (CH3)4NGeCl3

(CH3)4NGeCl3 is prepared, characterized and studied using 1H NMR spin lattice relaxation time and second moment to understand the internal motions and quantum rotational tunneling. Proton second moment is measured at 7 MHz as function of temperature in the range 300-77 K and spin lattice relaxation time (T1) is measured at two Larmor frequencies, as a function of temperature in the range 270-17 K employing a homemade wide-line/pulsed NMR spectrometers. T1 data are analyzed in two temperature regions using relevant theoretical models. The relaxation in the higher temperatures (270-115 K) is attributed to the hindered reorientations of symmetric groups (CH3 and (CH3)4N). Broad asymmetric T1 minima observed below 115 K down to 17 K are attributed to quantum rotational tunneling of the inequivalent methyl groups.

Study of molecular reorientation and quantum rotational tunneling in tetramethylammonium selenate by 1H NMR

1H NMR spin-lattice relaxation time measurements have been carried out in [(CH3)4N]2SeO4 in the temperature range 389-6.6 K to understand the possible phase transitions, internal motions and quantum rotational tunneling. A broad T1 minimum observed around 280 K is attributed to the simultaneous motions of CH3 and (CH3)4N groups. Magnetization recovery is found to be stretched exponential below 72 K with varying stretched exponent. Low-temperature T1 behavior is interpreted in terms of methyl groups undergoing quantum rotational tunneling.

Rotating frame analog of spin-locking and spin-lattice relaxation in the doubly rotating frame

The present paper reports the achievement of the rotating-frame analog of spin-locking and its application to the precise measurements of the spin-lattice relaxation time T(1DR) in the doubly rotating frame. After the magnetization is aligned along the resonant RF field H(1), a pulse sequence of a low-frequency oscillating magnetic field at exact resonance is applied perpendicular to H(1). We have overcome several technical difficulties arising from the fact that the rotating-wave approximation is not valid for the low-frequency field. We have theoretically derived an expression of T(-1)(1DR) due to fluctuating magnetic dipole interactions in the weak collision case and found an important relation among the spin-lattice relaxation rates T(-1)(1), T(-1)(1rho), and T(-1)(1DR). This relation can be used to ascertain whether the relaxation is only due to the fluctuating magnetic dipole interactions between like spins. The experiment was carried out on (1)H nuclei in tetramethylammonium iodide (CH(3))(4)NI and the temperature dependence of T(-1)(1DR) was measured together with that of T(-1)(1) and T(-1)(1rho). The activation energies and the preexponential factors of Arrhenius expressions of the correlation times are newly determined.

1H NMR study of N(CH3)4H(ClF2CCOO)2

The proton spin-lattice relaxation times and 1H NMR second moments were measured over a wide range of temperature. The results were compared with those of the 19F NMR relaxation that we obtained earlier. For both nuclear species, the evolution of the longitudinal magnetizations with time is observed to be strongly bi-exponential and were in good quantitative agreement with the cross-relaxation theory.