Activation Energy For Reorientation Research Articles

Calculation of the spin-lattice relaxation time and the activation energy near the IV–III phase transition in pyridinium fluorosulfonate (C5NH6)FSO3

The spin-lattice relaxation time T1 H for protons nuclei is calculated in term of the pseudospin–phonon (PS) coupled and the energy fluctuation (EF) models close to the IV–III solid–solid phase transition of in (C5NH6)FSO3. This calculation was performed by associating the observed second moment of the 1H as the order parameter below T C and the disorder parameter above T C. Values of the activation energy for the cation reorientation in this crystal are also deduced by using both models. In addition, the observed dielectric permittivity of this crystal is analyzed within the framework of the Landau theory and values of the spontaneous polarization (P S) are determined as a function of temperature. The normalized values of P S are used in the PS and EF models to extract the activation energy for the reorientation of the dipole moment of this compound arising from cation–anion interaction. Our results show that the PS and EF models can describe the observed behavior of the spin-lattice relaxation time adequately for the IV–III solid–solid transition in (C5NH6)FSO3.

Reorientation of a titanium–vacancy complex in a vanadium alloy

ABSTRACT The reorientation of a titanium–vacancy complex in a vanadium alloy with and without the presence of a grain boundary (GB) is investigated by atomistic simulations. The results show that a second-nearest-neighbour complex is a common transition state during the reorientation. Also, the atomic composition of a GB may affect the activation energy for reorientations: atoms with a high potential energy at a ∑3{111} GB significantly affect the activation energy, while those at a ∑3{112} GB have a weaker effect.

C60 in a peptidic cage: a case of symmetry mismatch studied by crystallography and solid-state NMR.

A supramolecular complex, formed by encapsulation of C60 fullerene in a molecular container built from two resorcin[4]arene rims zipped together by peptidic arms hydrogen bonded into a cylindrical β-sheet, was studied by X-ray crystallography, solid-state and solution NMR, EPR spectroscopy and differential scanning calorimetry (DSC). The crystal structure, determined at 100 K, reveals that the complex occupies 422 site symmetry, which is compatible with the molecular symmetry of the container but not of the fullerene molecule, which has only 222 symmetry. The additional crystallographic symmetry leads to a complicated but discrete disorder, which could be resolved and modelled using advanced features of the existing refinement software. Solid-state NMR measurements at 184-333 K indicate that the thermal motion of C60 in this temperature range is fast but has different activation energies at different temperatures, which was attributed to a phase transition, which was confirmed by DSC. Intriguingly, the activation energy for reorientations of C60 in the solid state is very similar for the free and encaged molecules. Also, the rotational diffusion coefficients seem to be very similar or even slightly higher for the encaged fullerene compared to the free molecule. We also found that chemical shift anisotropy (CSA) is not the main relaxation mechanism for the 13C spins of C60 in the studied complex.

Quasielastic neutron scattering study of tetrahydroborate anion dynamical perturbations in sodium borohydride due to partial halide anion substitution

Equimolar NaBH4–NaX (X=Cl and I) solid solutions were synthesized to study, via quasielastic neutron scattering, the effect of partial halide anion substitution on the reorientational dynamics of tetrahydroborate (BH4−) anions in NaBH4. The BH4− reorientational mobility increased in the order of NaBH4–NaCl, NaBH4, and NaBH4–NaI, which corresponded with expanding face-centered-cubic lattices accommodating the respective increasing sizes of the Cl−, BH4−, and I− anions. The BH4− anions in NaBH4–NaCl were found (at least above 400K) to undergo ‘cubic’ tumbling motions with the four H atoms per anion visiting all eight corners of a cube, similar to what was previously observed for NaBH4. In contrast, the BH4− anions in NaBH4–NaI were found to undergo something more akin to ‘tetrahedral’ tumbling motions, where the H atoms visit all four corners of a tetrahedron. Despite a noticeable softening of the BH4− torsional energies with increasing lattice constant amongst NaBH4 and the two solid solutions, all three compounds exhibited similar activation energies for reorientation of about 11–12kJmol−1.

Anion Reorientations in the Superionic Conducting Phase of Na2B12H12

Quasielastic neutron scattering (QENS) methods were used to characterize the reorientational dynamics of the dodecahydro-closo-dodecaborate (B12H122–) anions in the high-temperature, superionic conducting phase of Na2B12H12. The icosahedral anions in this disordered cubic phase were found to undergo rapid reorientational motions, on the order of 1011 jumps s–1 above 530 K, consistent with previous NMR measurements and neutron elastic-scattering fixed-window scans. QENS measurements as a function of the neutron momentum transfer suggest a reorientational mechanism dominated by small-angle jumps around a single axis. The results show a relatively low activation energy for reorientation of 259 meV (25 kJ mol–1).

Rayleigh scattering of depolarized light from liquid pyridine

Liquid pyridine was investigated by Rayleigh light scattering at a normal pressure, in the temperature range 236–385K. The function of reorientation G(t), both the first K(t) and the second K(2)(t) memory functions [1], the autocorrelation time, the rotational diffusion coefficient and the activation energy for reorientation have been calculated from the measured depolarized Rayleigh wing. The results are summarized in tables and shown in figures. Molecular dynamics (MD) testing simulations were made to obtain more interesting data on pyridine.

Dynamical Perturbations of Tetrahydroborate Anions in LiBH4 due to Nanoconfinement in Controlled-Pore Carbon Scaffolds

Neutron vibrational spectroscopy and quasielastic neutron scattering (QENS) were used to probe the dynamical properties of BH4– anions in both bulk LiBH4 and LiBH4 confined in nanoporous carbons (NPCs) having ≤4-nm-diameter, hexagonally arranged, cylindrical pores. The BH4– torsional band of the confined LiBH4 is significantly broadened relative to that of bulk LiBH4, reflecting a disruption of the bulk crystal lattice and thus a broader distribution of BH4– rotational potentials. QENS measurements of bulk orthorhombic LiBH4 indicate a single quasielastic component yielding an activation energy for localized BH4– jump reorientation of 19.2 ± 0.8 kJ/mol, consistent with previous QENS and NMR results. At room temperature, the measurements are in good agreement with BH4– reorientational jumps about a single C2 or C3 tetrahedral symmetry axis, with evidence for multiaxis rotations emerging as the temperature increases. In contrast, the QENS spectra of the NPC-confined LiBH4 exhibit two quasielastic components, one an order of magnitude broader than the other. The narrower component is presumably associated with more slowly reorienting BH4– anions in the interior of the pores and the broader component with much more rapidly reorienting BH4– anions in the vicinity of the pore surfaces. For 4-nm pores, these components yield two corresponding activation energies for reorientation: 16 ± 1 and 10.6 ± 0.7 kJ/mol. The data suggest that both components undergo single C2- or C3-axis reorientational jumps below 330 K, albeit with one an order of magnitude faster than the other. By 400 K (which is above the bulk phase transition temperature), both reorient more diffusively around multiple axes. These results were found to be qualitatively consistent with comparative dynamical measurements of LiBH4 confined in a 13-nm-average-pore-size carbon aerogel, which exhibited a much broader pore size distribution.

Radiolabelling Measurements of Free Radicals Derived from Aromatic Volatile Organic Compounds Adsorbed in Zeolite Nanomaterials to and above Saturation Loadings

Benzene, para-xylene and mesitylene were adsorbed into silicalite, Na - morde- nite, Na - ZSM5 and Li - X zeolites, and studied using the longitudinal field muon spin relaxation (LF-μSRx) technique. The zeolites ZSM5/silicalite and mordenite contain microporous channels while zeolite X has only supercages. For cyclohex- adienyl radicals/benzene adsorbed in Na-ZSM5, silicalite and Na - mordenite, a fraction was detected with a common reorientational activation energy in the region of ca 5kJ mol−1; however, in all cases there appeared a second fraction with an activation energy of ca 12kJmol’. [In Li-X only a single fraction was observed with Ea = 8.1kJ mol−1, from molecules adsorbed in supercages]. Given that high loadings of benzene, beyond the saturation capacities of the zeolites were employed, we believe the two distinct motional distributions represent the channel intersection and channel/external-surface niche locations in ZSM5/silicalite, the latter having the higher activation energy. An extraneous fraction may also be present (possibly as a thin film coating the zeolite grains), which probably also contributes to the detected ca 5 kJ mol−1 component in which, as at the channel-intersections, the motion tends toward bulk behaviour. Mordenite has only a single accessible channel structure and is hence devoid of the relatively unrestricting channel intersecting pores that are present in ZSM5/silicalite. However, the channels are wider (ca 7 A diameter) and we may ascribe therefore, the ca 5 kJ mol−1 fraction to the formation of benzene clusters within these channels that exhibit bulk-type behaviour, while we assign, in analogy with the results for ZSM5/silicalite, the ca 12kJ mol−1 fraction to molecules adsorbed in niches on the external surface. It is thought that the essential difference between the two activation energies is that the ca 5 kJ mol -1 processes involve molecular motion within clusters of benzene molecules with properties similar to the bulk phase (6.6 kJ mol -1 was measured in pure benzene), while those characterised at ca 12kJ mol−1 reflect single benzene molecules or small, ordered molecular clusters interacting with a zeolite surface. The results for dimethylcyclohexadienyl radicals (derived from p-xylene) are rather similar, but indicate slightly reduced activation energies. The larger mesitylene molecules can penetrate zeolite X and (more slowly) mordenite, but are excluded from the internal pores of ZSM5/silicalite.

Low-temperature tunneling and rotational dynamics of the ammonium cations in (NH4)2B12H12

Low-temperature neutron scattering spectra of diammonium dodecahydro-closo-dodecaborate [(NH(4))(2)B(12)H(12)] reveal two NH(4)(+) rotational tunneling peaks (e.g., 18.5 μeV and 37 μeV at 4 K), consistent with the tetrahedral symmetry and environment of the cations. The tunneling peaks persist between 4 K and 40 K. An estimate was made for the tunnel splitting of the first NH(4)(+) librational state from a fit of the observed ground-state tunnel splitting as a function of temperature. At temperatures of 50 K-70 K, classical neutron quasi-elastic scattering appears to dominate the spectra and is attributed to NH(4)(+) cation jump reorientation about the four C(3) axes defined by the N-H bonds. A reorientational activation energy of 8.1 ± 0.6 meV (0.79 ± 0.06 kJ/mol) is determined from the behavior of the quasi-elastic linewidths in this temperature regime. This activation energy is in accord with a change in NH(4)(+) dynamical behavior above 70 K. A low-temperature inelastic neutron scattering feature at 7.8 meV is assigned to a NH(4)(+) librational mode. At increased temperatures, this feature drops in intensity, having shifted entirely to higher energies by 200 K, suggesting the onset of quasi-free NH(4)(+) rotation. This is consistent with neutron-diffraction-based model refinements, which derive very large thermal ellipsoids for the ammonium-ion hydrogen atoms at room temperature in the direction of reorientation.

Comprehensive NMR Study of Magnesium Borohydride

1H and 11B NMR measurements were performed to study BH4 reorientations and diffusion in four samples of Mg(BH4)2. These include the low (α) and high (β) temperature bulk solid phases, α-phase material with TiF3 and ScCl3 additives, and α-phase material incorporated into a carbon aerogel. All four samples show 1H T1 minima within our temperature range from reorientational motion of the BH4 tetrahedra. The β-phase 1H T1 minimum occurs at a lower temperature than that for the α-phase, indicating a lower activation energy for reorientations. The α-phase 1H spectra display the onset of broadening at low temperatures because of the slowing of reorientations but show no indication of line narrowing at high temperatures from translational diffusion. The β-phase 1H spectra are similar, but here T1D measurements were performed to determine the BH4 hopping rate. Above 150 °C, a significant narrow component does appear in the 1H spectra of the aerogel sample, indicating substantially increased diffusive motions in th...

Dielectric and molecular dynamic studies of sub-Tg by thermally stimulated current (TSC) analysis for liquid crystalline epoxy thermosets

Dielectric constant ( ε′), loss factor (tan δ) and molecular dynamic behavior of liquid crystalline epoxy cured with diamine were studied by thermally stimulated discharge current (TSC) technique and were compared with the conventional epoxy system. The relaxation time τ and the activation energy of reorientation of the dipoles for each TSC characteristic band were expressed as Arrhenius relationship. Liquid crystalline epoxy (LCE) monomer was diglycidyl ether of biphenyl (DGEBP) and the conventional epoxy was diglycidyl ether of bisphenol A (DGEBA). The conventional epoxy had two sub-Tg relaxations; γ-relaxation was related to the motion of glycidyl ether segment and β-relaxation was due to the crankshaft motion of the hydroxylether group. LCE thermoset exhibited another γ′-relaxation in addition to those two relaxations; the reorientation of mesogenic unit. The para-ether linkage of the biphenyl mesogenic unit allows it to rotate without disturbing neighborhood unit.

Dielectric relaxation and conductivity in barium salt of 12-tungstophosphoric acid hydrate

Barium salt of 12-tungstophosphoric acid (WPA) hydrate BaHPW 12O 40·7H 2O (BaHWPA·7H 2O) has been investigated both in the microwave (X-band) region and in the lower frequency (5 Hz–500 kHz) region. This work is a continuation of our previous investigations on the influence of monovalent and bivalent cations on the dielectric properties of WPA hydrate salts. The real and imaginary parts of the permittivity (ε′, ε″) as a function of frequency and temperature, were determined in both frequency regions. The observed relaxations were attributed to specific relaxation processes. In the lower frequency range, the relaxation time τ=1.0×10 −6 s was assigned to barium ion jumps. In the microwave region, the dielectric relaxation time τ=1.7×10 −11 s was attributed to polyatomic ions (H 3O +) and/or H 2O molecules reorientations. A phase transition was observed at 330 K. The low value of the activation energy for reorientation in the microwave frequency region is discussed. The d.c. conductivity and its temperature dependence were determined. Measurements were made in the temperature interval from 286 to 353 K. In the upper part of this temperature interval a PTCR effect was observed.

Azafullerene C 59N, a stable free radical substituent in crystalline C 60

Solid solutions of C 59N azafullerene in C 60 with concentrations of 10 −5 to 10 −4 were produced in large quantities in an electric gas discharge tube. C 59N is a stable monomeric substituent molecule in crystalline C 60. The isotropic 14N and 13C hyperfine coupling constants measured by electron spin resonance (ESR) are characteristic of the extent of delocalization of the charge over the cage and are a sensitive test of electronic structure calculations. The C 59N reorientational activation energy measured below the face centered cubic (fcc) to simple cubic (sc) transition is 2300 K. This value is similar to that of the matrix C 60 molecules, indicating that C 59N–C 60 intermolecular interactions are weak.

SHG properties of pure and doped incipient ferroelectrics KTaO3 and SrTiO3 under applied electric fields

The investigation of the incipient ferroelectric systems KTO and STO at low temperature with optical second harmonic generation (SHG) using a Nd:YAG-laser shows a similar behaviour for both crystal systems. The SHG intensity S increases drastically upon reduction and conversely decreases upon oxidation. Oxygen defect related inversion-symmetrybreaking-centers interacting with dipolar centers introduced by doping (f.i. in Li+ doped KTO) result in strong changes of the dipolar properties. The static and dynamic behaviour of the SHG intensity under applied electric field confirms the dipolar nature of the centers and allows us to estimate the activation energy for reorientation. The SHG signal S of nominally pure, but containing oxygen defect-related-dipole-centers, in KTO first is approximately proportional to E. For Li-doped KTO the proportionality-coefficient b increases strongly with increasing Lidoping. At moderately higher E-fields the SHG intensity S appears to be saturated in agreement with recent theoretical model calculations.

Orientational relaxation of liquid water molecules as an activated process

Femtosecond mid-infrared pump–probe spectroscopy is used to study the orientational relaxation of HDO molecules dissolved in liquid D2O. In this technique, the excitation of the O–H stretch vibration is used as a label in order to follow the orientational motion of the HDO molecules. The decay of the anisotropy is nonexponential with a typical time scale of 1 ps and can be described with a model in which the reorientation time depends on frequency and in which the previously observed spectral diffusion is incorporated. From the frequency and temperature dependence of the anisotropy decay, the activation energy for reorientation can be derived. This activation energy is found to increase with increasing hydrogen bond strength.

Molecular dynamics in crystalline C 60·2CHBr 3

The nature of the molecular motion of C 60 in crystalline C 60·2CHBr 3 has been determined by solid-state NMR. In the 193–295 K temperature range the C 60 molecules undergo fast isotropic reorientation, similar to that in pure C 60. The activation energy of reorientation of the C 60 cages is 65 meV, with a corresponding rotational diffusion constant at 298 K of 9.6×10 −11 s −1.

Proton magnetic resonance of CnH2n+2(n= 1–4) adsorbed in mordenite. Dynamic behaviour and host–guest interaction

Proton spin–lattice relaxation times, T1, for methane, ethane, propane and butane adsorbed in mordenite have been measured in situ over wide ranges of temperature and loading. The spin–lattice relaxation was found to be governed by the reorientation of the guest molecules in the low-temperature region. No phase transition was observed between 4.2 K and room temperature. The results also revealed that methane molecules are distributed between the main channel and the side-pocket of the host mordenite, whereas ethane, propane and butane are accommodated only in the main channel. For the molecules accommodated in the main channel the activation energy for reorientation (methane: 1.0 kJ mol–1, ethane: 1.0 kJ mol–1, propane: 3.5 kJ mol–1, butane: 5.0–5.5 kJ mol–1) and the temperature at which the T1 minimum appears increase systematically with the size of the guest molecule, suggesting that the potential barrier for reorientation originates from the steric host–guest interaction.

EPR of interstitial hydrogen in silicon: Uniaxial stress experiments

This paper deals with an electron paramagnetic resonance (EPR) study of the Si-AA9 EPR center, which has been previously identified as arising from a 〉111〈 bond-centered (BC) interstitial hydrogen in the neutral charge state (H0) and is a hydrogenic analog of the anomalous state of muonium (Mu*) in silicon. Hydrogen was implanted into a high purity silicon sample by means of a cyclotron at 80 K. A uniaxial stress of ∼ 2000 kg cm−2 was applied to the 〉110〈 axis of the sample in the dark in the temperature range from 170 K to 77 K, and the stress was then removed. It was found that a preferential alignment of the defect had been frozen in at 77 K for both the H+ and H0 states. Thermally activated recovery for alignment of the H+ in the BC position reveals an activation energy for reorientation of (0.43 ± 0.02) eV with a pre-exponential factor of 1/τ = 2.3 × 1012s−1.For equilibrium alignment of the H+ in the BC position (at 140 K) the component of the piezospectroscopic B tensor was also determined to be B ≈ −2 eV. The fact that uniaxial stress (in the dark) produces alignment of the nonparamagnetic state of the AA9 center is the first experimental evidence that H+ is indeed situated in the BC position on the 〉111〈 axis, while a negative sign of the B tensor is direct confirmation that nearest to hydrogen, silicon atoms are outwardly relaxed.

Quasielastic incoherent neutron scattering study of the rotational dynamics of the water molecules in analcime

The rotational motion of the water molecules in analcime has been investigated by quasielastic incoherent neutron scattering. Reorientational times vary from 65 ± 20 ps at 260 K to 26 ± 3 ps at 370 K, and the temperature dependence suggests an activation energy for reorientations of 780 ± 200 K. Measurements of the temperature dependence of the intensity of the elastic incoherent scattering indicate that there is no cooperative ordering at low temperatures, and that instead there is a gradual freezing-in of the motion of individual water molecules.

The 18-crown-6.2 chloroacetonitrile complex. X-ray crystal structure and solid phase motions of guest and host as studied by variable temperature 13C CPMAS NMR spectroscopy

The title complex has two distinct 18-crown-6 moieties of approximate D3d symmetry in the unit cell. The 45.3 MHz 13C CPMAS spectra have two regions of broadening for the crown ether carbons. At high temperatures a dipolar washout mechanism is operative, leading to broadening when molecular motion has a correlation time approximately equal to the inverse of the decoupling field. At lower temperatures broadening occurs when the motional correlation time is equal to the inverse of the chemical shift difference. The activation energy for 18-crown-6 reorientation in this solid is calculated to be ca. 48 kJ/mol. For the chloroacetonitrile, the CH2 carbon also shows dipolar washout, but at a temperature much lower than the 18-crown-6. The activation energy for reorientation for this "guest species" is ca. 42 kJ/mol.