Shielding Effects of Isopropyl Groups on forming Ionic Plastic Crystals of [NEt x Me(2- x )(i-Pr)2][BEt3R] ( x = 0–2. R = Me, Et.)

Eight salts of [NEtxMe(3-x)(i-Pr)][BEt(4-y)Mey] (x = 1-3, y = 0, 1) were prepared to reduce Coulombic interactions in the crystals. These novel ionic plastic crystals exhibited low activation energies for isotropic rotational motion (Ea rot) and ion diffusion (Ea diff). The lattice constant a in the cubic structure indexed to a CsCl-type was larger than that of the corresponding plastic crystals of [NEtxMe(4-x)][BEt(4-y)Mey] (nonbranched sample). Solid-state 1H and 13C nuclear magnetic resonance (NMR) spectra of the salts revealed that both cations and anions undergo isotropic reorientation motions in the plastic crystalline phase. This result is consistent with the differential scanning calorimetry data, which showed that the compounds have large entropy changes at the transition temperature between the ordinal and plastic crystalline phases, except for [NEt2Me(i-Pr)][BEt3Me] and [NEt3(i-Pr)][BEt3Me]. The Ea rot and Ea diff values were estimated using 1H NMR spin-lattice relaxation time (T1) and electrical conductivity measurements, respectively.

This study provides the first evidence that a Li ion can form ionic plastic crystals using crown ether with a bis-(trifluoromethanesulphonyl) amide (TFSA) anion. 1H, 7Li, and 13C nuclear-magnetic-resonance (NMR) measurements of the 15-crown-5-Li-TFSA complex revealed that the constituents underwent isotropic reorientation in the plastic crystalline phase. The NMR data of the 12-crown-4-Li-TFSA salt showed that the complex is a rotator crystal (the complexes are denoted as [Li 15C5] and [Li 12C4] in this paper). The X-ray diffraction (XRD) reflection patterns of the [Li 15C5] crystal recorded in the highest-temperature solid phase (plastic phase) could be indexed to a cubic structure. Conversely, [Li 12C4] could be fitted to a trigonal structure. In this study, [M (3n)Cn] (M = Li, Na, K; n = 4-6) complexes were also prepared, and NMR, DSC, XRD, and electrical conductivity measurements were performed. Based on these results, we additionally revealed that the [Na 15C5] and [K (15C5)2] complexes are also new rotator crystals. Single-crystal XRD measurements also revealed that the [Na 15C5] compound has two stable sites in the crystal. Activation energies of molecular motions in the [M (3n)Cn] crystals were estimated using 1H NMR relaxation time (T1 and T2) measurements. The electrical conductivity measurements of [Li 12C4], [Li 15C5], and [Na 15C5] showed high ionic conductivities (∼10-2 S cm-1).

Two new ionic plastic crystals of [NEtMe2Pr][BEt3Me] and [NEt2MePr][BEt3Me] were found. In contrast, the highest-temperature solid-phase of [NEtMe2Bu][BEt3Me] and [NEt2MeBu][BEt3Me] were assigned to rotator phases. Solid-state 1H and 13C nuclear magnetic resonance (NMR) measurements revealed that both the cations and anions perform isotropic reorientations in the plastic phase. Conversely, the cations of [NEtMe2Bu] and [NEt2MeBu] undergo rotation about an axis. Based on these results, it is revealed that ellipsoidal cations of [NEtMe2Pr]+ and [NEt2MePr]+ can form plastic crystalline phases with [BEt3Me]−. In the lower temperature solid-phase of the plastic phase, a rotator phase was also found in [NEtMe2Pr][BEt3Me] and [NEt2MePr][BEt3Me] salts. This is rarely reported in alkylammonium compounds with [BEt3Me]. 1H NMR spin-lattice relaxation time (T1) measurements showed that activation energies of isotropic reorientation were slightly large when compared to those reported in other ionic plastic crystals constructed with globular cations. This difference can be explained by assuming the aspect ratio. On differential scanning calorimetry (DSC) charts, small entropy changes were recorded at melting points of four compounds. These results support the observation that cations and anions have large degrees of freedom of motion in the highest-temperature solid-phases (plastic and rotator phases).

A new class of proton-conducting ionic plastic crystals based on organic cations and dihydrogen phosphate

A plastic crystalline phase of dimethylaminoalane has been discovered at T > 332 K. The phase transitions solid - plastic phase - liquid are fully reversible. The plastic crystalline phase exhibits a cubic unit cell, space group Pm3[combining macron]n, in which the dimethylaminoalane molecules rotate and adopt a structural arrangement reminiscent of the A15 phase.

In a compound exhibiting plastic (crystal B) and liquid crystalline (smectic A) phases, we report calorimetric and X-ray features that are drastically affected by confinement and orientation in Anopore membranes. Data on the untreated membranes in which the molecules aligned parallel to the pore axes, show a significant diminution in the correlation length corresponding to the positional order of the plastic phase suggesting that finite size effects can perhaps transform the plastic phase to a hexatic one. In the orthogonal case, having the molecules in the plane of the membrane, a new phase is induced, whose structural possibilities are discussed. Calorimetric measurements corroborate these results and also bring out the different thermal behaviors, not only between the bulk and the confined cases, but also between the situations wherein the orientations of the molecules are different. These findings are expected to open up a new path to understand the melting phenomenon, especially that which occurs in lower dimensions.

Salts of cationic sandwich complexes often exhibit an ionic plastic phase; however, only a few exhibit a plastic phase at room temperature. To explore the use of the CF 3 BF 3 anion to lower the transition temperature to the plastic phase, we prepared salts of CF 3 BF 3 with various ferrocene derivatives, [D][CF 3 BF 3 ] (D=FeCp* 2 , Fe(C 5 Me 4 H) 2 , Fe(C 5 H 4 Me) 2 , FeCp(C 5 H 4 Me), FeCp 2 ; Cp*=C 5 Me 5 , Cp=C 5 H 5 ). Although [FeCp* 2 ][CF 3 BF 3 ] exhibited a plastic phase above 417 K, the other salts formed room‐temperature ionic plastic crystals with a phase transition to the plastic phase in the range 266–291 K. The crystal structure and thermal properties of [FeCp 2 ][OTf] were elucidated for comparison. In addition, decamethylferrocenium salts with other anions were synthesized and structurally characterized: [FeCp* 2 ][X] (X=N(SO 2 F) 2 and B(CN) 4 ) exhibited a phase transition to the plastic phase above 400 K, whereas carborane‐containing salts [FeCp* 2 ] 2 [B 12 F 12 ] and [FeCp* 2 ][Co(C 2 B 9 H 11 ) 2 ] did not exhibit a plastic phase.

Ionic liquid and plastic crystalline phases of pyrazolium imide salts as electrolytes for rechargeable lithium-ion batteries

The infrared spectra of 2‐chloromethyl‐2‐methyl‐1,3‐dichloropropane in several phases, including the pure liquid, as a solute and as an amorphous and crystalline solid at 80 K and under pressure (2–30 kbar), have been recorded. Raman spectra of the neat liquid and of the plastic and anisotropic crystalline phases were obtained at various temperatures.A large number of IR and Raman bands present in the neat liquid, in solution and in the plastic crystalline phase vanished in the low‐temperature and in the high‐pressure crystal spectra. Among seven possible conformers, three (having symmetries C1, C8 and C3) have no 1,3‐parallel CCI bonds and are present in the liquid, in solution and in the plastic phase. In the anisotropic crystal, only the C1 conformer was present. A phase transition between the plastic and anisotropic crystal occurred at 253 K. ΔH0 values between the most stable conformers Cs and C1 were 5±1 and 3±1 kJ mol−1 K−1 in the plastic phase and in the liquid phase, respectively.A force constant calculation for the C1, Cs and C3 conformers and for various conformers of other halogenated neopentanes (a total of 11 conformer molecules) was carried out with the overlay technique. A 41‐parameter force field was adjusted to more than 280 observed frequencies, giving very satisfactory agreement.The spectra were interpreted in terms of conformers of C1, Cs and C3 symmetry.

Temperature dependence of 13C 1H one-bond coupling constants of methyl groups in plastic crystals

A plastic crystal (PC) has a crystalline phase characterized by disordered molecular orientations while maintaining a regular lattice of molecular positions. Recent studies have reported heterogeneous dynamics within the PC phase of several ionic PCs (IPCs). This study investigates whether similar dynamics exist in a molecular PC (MPC) by examining the temperature dependence of the spin-lattice relaxation time (T1) and the spin-spin relaxation time (T2) of succinonitrile (NC-CH2-CH2-CN) using pulsed low-frequency nuclear magnetic resonance. The results reveal a splitting of T2 values, which are sensitive to translational motion, within the PC phase, indicating the presence of heterogeneous translational dynamics in both MPCs and IPCs. These findings suggest that such heterogeneous dynamics are an intrinsic, yet previously overlooked, property common to plastic crystals regardless of their ionic or molecular nature.

The dynamics of norbornane molecules is studied over the 120 K-348 K temperature range in its ordered (T – 10–12 s), an elastic incoherent structure factor is extracted in the two plastic phases, which corresponds to isotropic rotations of the molecules about their centre of gravity, with apparently no change at the transition between the two phases. A description in terms of an isotropic rotational diffusion model yields a unique Arrhenius law for the diffusion constant through all the temperature range of these two phases. A more sophisticated description based on reorientations about lattice and molecules axes gives a continuous evolution of the correlation times across the transition. A comparison is made with recent NMR results. At the lowest temperatures in the h.c.p. plastic phase, an extra amount of elastic scattering is evidenced. A possible interpretation in terms of either a local ordering of the molecules or a distribution of the correlation times is proposed.

The relaxation kinetics of succinonitrile in the plastic crystalline phase has been investigated between 250 and 320 K by transient optical Kerr effect with femtosecond pulses. Three different noninstantaneous contributions to the signal time profile have been identified. The fastest one is a subpicosecond decay, attributed to the relaxation of the damped librational and torsional vibrations of the molecules. The intermediate decay time, ranging from 4 ps at 323 K to 30 ps at 250 K, is interpreted as due to rotations of the trans molecules about the shortest inertia axis which bring the molecule from one cube diagonal to another. The slowest decay time ranges from 28 ps at 323 K to 190 ps at 250 K and agrees very well with previous measurements with different experimental techniques. This decay is interpreted as due mostly to rotational motions of the gauche molecules jumping from one equilibrium position to another in the unit cell. The activation energy for the two processes is 3.7 and 4.3 kcal/mol, respectively, for the intermediate and slow kinetics.

Spectroscopic and thermodynamic studies of molecular crystals—I: The internal vibrations of adamantane in the plastic phase

Phase-dependent dielectric properties and proton conduction of neopentyl glycol (NPG), which is an organic molecular plastic crystal, were studied via variable-temperature broadband dielectric spectroscopy (BDS). Permittivity and conductivity data show the phase transformations of NPG from the crystalline state to the plastic crystalline state at 315 K and then to the molten state at 402 K across the temperature range of 293–413 K. The Vogel temperatures (Tv) fitted from the Vogel–Fulcher–Tammann (VFT) equation agree well with the values extrapolated by the Stickel plot (linearized Vogel plot). Impedance and modulus data display a separation of the −Z′′ (the imaginary part of the complex impedance) and M′′ (the imaginary part of the complex electric modulus) peaks in the crystalline phase. However, they overlap in both the plastic crystalline phase and the molten phase, indicating long-range proton conduction. In both the molten phase and the plastic crystalline phase, the temperature dependence of direct current conductivity (σdc) obeys the VFT equation very well. While the vehicle mechanism (translational diffusion) is an intrinsic mechanism for ionic or protonic conduction in the molten phase, it is speculated that the Grotthuss mechanism also works due to the self-dissociation of NPG molecules, which are similar to water molecules. In the plastic crystalline phase, the proton hopping mechanism is most likely the underlying ion-conducting mechanism because of the rotational disorder and intrinsic defects (vacancies) of the NPG molecules. In the ordered crystalline phase, the proton conduction is presumed to follow the proton hopping mechanism as determined from the localized relaxation and the temperature dependence of σdc (Arrhenius behavior).