Rotational dynamics of methyl groups in m-xylene

Methyl group dynamics of m-xylene was investigated by using incoherent inelastic and quasi-elastic neutron scattering. Inelastic measurements were carried out at the high flux backscattering spectrometer HFBS at the National Institute of Standards, quasi-elastic measurements at the time-of-flight spectrometer NEAT at the Hahn-Meitner-Institute. Rotational potentials are derived which describe the tunnel splittings, first librational, and activation energies of the two inequivalent CH(3) groups. Indications for coupling of the methyl rotation to low-energy phonons have been found. The finite width of one tunneling transition at He temperature is described by direct methyl-methyl coupling. The combined results of the experiments and the calculations allow a unique assignment of rotor excitations to crystallographic sites.     

Reorientational dynamics of the dodecahydro-closo-dodecaborate anion in Cs2B12H12

Rapid reorientational motions of the B(12)H(12)(2-) icosahedral anion, a key intermediate in borohydride dehydrogenation, are revealed by quasielastic neutron scattering (QENS) measurements of Cs(2)B(12)H(12) between 430 and 530 K. At 430 K, over the range of momentum transfers collected, the elastic incoherent structure factor (EISF) is consistent with a model for reorientational jumps about a single molecular axis. At temperatures of 480 K and higher, however, the EISF suggests the emergence of multiaxis reorientation by dynamically similar, independent jumps about two axes, on average, preserving crystallographic order. Alternatively, if one assumes that the anions are undergoing temperature-dependent rotational trapping, then the EISF is also consistent with a jump model involving a temperature-dependent mobile fraction of anions statistically tumbling between discrete crystallographic sites. Although neutron vibrational spectra demonstrate that the anion torsional modes soften dramatically with increasing temperature, the QENS-derived activation energy of 333 meV for reorientation clearly shows that the anions are not undergoing isotropic rotational diffusion.     

The effect of low-temperature dynamics of the dimethylammonium group in [(CH3)2NH2]3Sb2Cl9 on proton spin-lattice relaxation and narrowing of the proton NMR line

This paper reports the temperature dependence of the relaxation time T1 (55.2 and 90 MHz) and the second moment of the NMR line for protons in a polycrystalline sample of [NH2(CH3)2]3Sb2Cl9 (DMACA). The fundamental aspects of molecular dynamics from quantum tunneling at low temperatures to thermally activated reorientation at elevated temperatures have been studied. The experimentally observed spin-lattice relaxation rate is a consequence of dipolar interactions between the spin pairs inside the methyl group (1/T(1AE) contribution) as well as the spins belonging to neighboring methyl groups and pairs, methyl spin-outer methyl spin (1/T(1EE) contribution). These contributions are considered separately. Two methyl groups in the dimethylammonium (DMA) cations are dynamically inequivalent. The values of the tunnel splitting of separate methyl groups are obtained from the T1 (55.2 MHz) experiment. The tunneling dynamics taking place below the characteristic temperatures 74 and 42 K for separate methyl groups are discussed in terms of the Schr?dinger equation. These temperatures point to the one at which thermal energy C(p)T and potential barrier take the same value. It is established that the second moment of the proton NMR line below 74 K up to liquid helium temperature is much lower than the rigid lattice value, which is due to a tunneling stochastic process of the methyl groups.     

On the possible manifestation of harmonic-anharmonic dynamical transition in glassy media in electron paramagnetic resonance of nitroxide spin probes

Continuous wave (cw) electron paramagnetic resonance (EPR) and echo-detected (ED) EPR were applied to study molecular motions of nitroxide spin probes in glassy glycerol and o-terphenyl. A linear decrease with increasing temperature of the total splitting in the cw EPR line shape was observed at low temperatures in both solvents. Above some temperature points the temperature dependencies become sharper. Within the model of molecular librations, this behavior is in qualitative and quantitative agreement with the numerical data on neutron scattering and Mossbauer absorption for molecular glasses and biomolecules, where temperature dependence of the mean-squared amplitude of the vibrational motion was obtained. In analogy with these data the departure from linear temperature dependence in cw EPR may be ascribed to the transition from harmonic to anharmonic motion (this transition is called dynamical transition). ED EPR spectra were found to change drastically above 195 K in glycerol and above 245 K in o-terphenyl, indicating the appearance of anisotropic transverse spin relaxation. This appearance may also be attributed to the dynamical transition as an estimation shows the anisotropic relaxation rates for harmonic and anharmonic librational motions and because these temperature points correspond well to those known from neutron scattering for these solvents. The low sensitivity of ED EPR to harmonic motion and its high sensitivity to the anharmonic one suggests that ED EPR may serve as a sensitive tool to detect dynamical transition in glasses and biomolecules.     

The dynamical properties of the aromatic hydrogen bond in NH4(C6H5)4B from quasielastic neutron scattering

NH(4)(C(6)H(5))(4)B represents a prototypical system for understanding aromatic H bonds. In NH(4)(C(6)H(5))(4)B an ammonium cation is trapped in an aromatic cage of four phenyl rings and each phenyl ring serves as a hydrogen bond acceptor for the ammonium ion as donor. Here the dynamical properties of the aromatic hydrogen bond in NH(4)(C(6)H(5))(4)B were studied by quasielastic incoherent neutron scattering in a broad temperature range (20< or =T< or =350 K). We show that in the temperature range from 67 to 350 K the ammonium ions perform rotational jumps around C(3) axes. The correlation time for this motion is the lifetime of the "transient" H bonds. It varies from 1.5 ps at T=350 K to 150 ps at T=67 K. The activation energy was found to be 3.14 kJ mol, which means only 1.05 kJ mol per single H bond for reorientations around the C(3) symmetry axis of the ammonium group. This result shows that the ammonium ions have to overcome an exceptionally low barrier to rotate and thereby break their H bonds. In addition, at temperatures above 200 K local diffusive reorientational motions of the phenyl rings, probably caused by interaction with ammonium-group reorientations, were found within the experimental observation time window. At room temperature a reorientation angle of 8.4 degrees +/-2 degrees and a correlation time of 22+/-8 ps were determined for the latter. The aromatic H bonds are extremely short lived due to the low potential barriers allowing for molecular motions with a reorientational character of the donors. The alternating rupture and formation of H bonds causes very strong damping of the librational motion of the acceptors, making the transient H bond appear rather flexible.     

Neutron inelastic scattering from ethylene: An unusual spectrum

Neutron inelastic scattering spectra of crystalline ethylene have been measured at two temperatures, 4 and 80 K, between 80 and 250 meV. The spectra are found to consist of weak and shifted peaks superimposed on a very broad background. This is accounted for by multi-phonon effects and Doppler broadening due to the small molecular mass of ethylene. A generalization to neutron scattering from adsorbed molecules is made.     

Rotational tunneling of methyl groups in low temperature phases of mesitylene: potentials and structural implications

Mesitylene can be stabilized at He temperature in three solid phases of so far unknown crystal structures. Rotational tunneling of methyl groups is based on rotational potentials and used to characterize structural aspects. In phase III found after the first fast cooling of the sample three nonequivalent methyl rotors with splittings of 2.7, 4.1 and 16.3 microeV are observed. Three other unresolved bands are identified by their librational modes. In the second phase II the metastability is emphasized by tunneling energies still changing at temperatures T< or = 12 K. Above this temperature tunneling bands at 6.6, 12.5, 15.0 and 18.3 microeV evolve in the manner characteristic of coupling to phonons. In the equilibrium phase I a single tunnel splitting of 10.2 microeV represents all methyl groups. A unit cell containing a single molecule at a site of threefold symmetry explains quantitatively this spectrum. Phases II and III most likely contain two nonequivalent molecules in the unit cell with no local symmetry in phase II and a mirror plane in phase III. The good moderator properties for neutrons are most likely not connected to the low energy tunneling bands but to a dense vibrational phonon density of states.     

Dielectric Relaxation and Quasielastic Neutron Scattering Study of Molecular Reorientation in the Nematic and Solid Phases of 4,4'-Di-N-Butyloxyazoxybenzene

Dielectric relaxation measurements carried out in the nematic phae of 4,4'-di-n-butyloxyazoxybenzene (BOAOB) reveal fast reorientational motions of the whole molecule around the long axis (τ₁ ∼ 60 ps) as well as slow reorientational motions of the whole molecule around the short axis (τ₁ ∼ 10^-8 s). Incoherent quasielastic neutron scattering spectra obtained for nematic BOAOB, with normal and deuteriated alkoxy terminals, are interpreted as dominated by reorientation (around the C-N bonds) of moieties consisting of benzene rings coupled with alkoxy terminal chains (τ₁ ∼ 4ps). In addition fast conformational changes occur in these chains. Dielectric relaxation measurements reveal librations of the whole molecule in the crystal phases CI and CII accompanied by reorientation of the terminal chains in these phases. The reorientations occur on the time scale amounting to 10^-8 s. Incoherent quasielastic neutron scattering spectra obtained for the CI and CII phases of BOAOB are interpreted as being dominated by overdamped librational motions of the moieties accompanied by fast conformational changes in the alkyl chains. The CIII phase corresponds to a normal molecular crystal.     

Intermolecular vibrations and fast relaxations in supercooled ionic liquids

Short-time dynamics of ionic liquids has been investigated by low-frequency Raman spectroscopy (4 < ω < 100 cm(-1)) within the supercooled liquid range. Raman spectra are reported for ionic liquids with the same anion, bis(trifluoromethylsulfonyl)imide, and different cations: 1-butyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium, 1-butyl-1-methylpiperidinium, trimethylbutylammonium, and tributylmethylammonium. It is shown that low-frequency Raman spectroscopy provides similar results as optical Kerr effect (OKE) spectroscopy, which has been used to study intermolecular vibrations in ionic liquids. The comparison of ionic liquids containing aromatic and non-aromatic cations identifies the characteristic feature in Raman spectra usually assigned to librational motion of the imidazolium ring. The strength of the fast relaxations (quasi-elastic scattering, QES) and the intermolecular vibrational contribution (boson peak) of ionic liquids with non-aromatic cations are significantly lower than imidazolium ionic liquids. A correlation length assigned to the boson peak vibrations was estimated from the frequency of the maximum of the boson peak and experimental data of sound velocity. The correlation length related to the boson peak (～19 A?) does not change with the length of the alkyl chain in imidazolium cations, in contrast to the position of the first-sharp diffraction peak observed in neutron and X-ray scattering measurements of ionic liquids. The rate of change of the QES intensity in the supercooled liquid range is compared with data of excess entropy, free volume, and mean-squared displacement recently reported for ionic liquids. The temperature dependence of the QES intensity in ionic liquids illustrates relationships between short-time dynamics and long-time structural relaxation that have been proposed for glass-forming liquids.     

Cobalt(II) citrate cubane single-molecule magnet

An investigation of the magnetic properties of the cobalt(II) citrate cubane [C(NH 2) 3] 8{Co 4(cit) 4}.4H 2O reveals that the cluster is a new cobalt(II) single-molecule magnet, with an energy barrier to reorientation of the magnetization, Delta E/ k B = 21 K, and tau 0 = 8 x 10 (-7) s. The compound displays distinct, frequency-dependent peaks in the out-of-phase (chi') component of the ac magnetic susceptibility and magnetization versus field hysteresis loops that are temperature and sweep rate dependent. The hysteresis loops collapse at zero field due to very fast quantum tunneling of the magnetization (QTM).     

X-ray diffraction and inelastic neutron scattering study of 1:1 tetramethylpyrazine chloranilic acid complex: temperature, isotope, and pressure effects

The x-ray diffraction studies of the title complex were carried out at room temperature and 14 K for H/D (in hydrogen bridge) isotopomers. At 82 K a phase transition takes place leading to a doubling of unit cells and alternation of the hydrogen bond lengths linking tetramethylpyrazine (TMP) and chloranilic acid molecules. A marked H/D isotope effect on these lengths was found at room temperature. The elongation is much smaller at 14 K. The infrared isotopic ratio for O-H(D)...N bands equals to 1.33. The four tunnel splittings of methyl librational ground states of the protonated complex required by the structure are determined at a temperature T=4.2 K up to pressures P=4.7 kbars by high resolution neutron spectroscopy. The tunnel mode at 20.6 microeV at ambient pressure shifts smoothly to 12.2 microeV at P=3.4 kbars. This is attributed to an increase of the strength of the rotational potential proportional to r(-5.6). The three other tunnel peaks show no or weak shifts only. The increasing interaction with diminishing intermolecular distances is assumed to be compensated by a charge transfer between the constituents of deltae/e approximately 0.02 kbar(-1). The phase transition observed between 3.4 and 4.7 kbars leads to increased symmetry with only two more intense tunneling bands. In the isotopomer with deuterated hydrogen bonds and P=1 bar all tunnel intensities become equal in consistency with the low temperature crystal structure. The effect of charge transfer is confirmed by a weakening of rotational potentials for those methyl groups whose tunnel splittings were independent of pressure. Density functional theory calculations for the model TMP.(HF)2 complex and fully ionized molecule TMP+ point out that the intramolecular rotational potential of methyl groups is weaker in the charged species. They do not allow for the unequivocal conclusions about the role of the intermolecular charge transfer effect on the torsional frequencies.     

Low-temperature dynamics of (S)-4-(1-methylheptyloxy)-4ʹ-cyanobiphenyl (8*OCB) and (S)-4-(2-methylbutyl)-4ʹ-cyanobiphenyl (5*CB) in disordered crystalline and glassy phases

The inelastic neutron scattering (INS) spectra were measured for two materials of chiral molecules: (S)-4-(1-methylheptyloxy)-4?-cyanobiphenyl (8*OCB) and (S)-4-(2-methylbutyl)-4?-cyanobiphenyl (5*CB), revealing solid state polymorphism with two partially disordered crystalline phases I and II and glassy state of liquid and of crystalline phase in each substance. The experiments were performed in the energy range up to 30 µeV in the temperature range from 4 to 35 K. For 8*OCB the elastic scans were measured as well up to 300 K illustrating well the phase diagram. For all solid phases of both substances in the µeV range of INS spectra, the existence of the excess density of vibrational states over that typical for fully ordered crystalline phases was evidenced. Contribution of this so-called boson peak occurred to be much larger in glass of isotropic phase than in the phase II and glass of phase I of 8*OCB, while for 5*CB it was larger in the phase I and glass of phase II than in glass of cholesteric phase. The quasi-elastic broadening of elastic peak corresponding to stochastic reorientations in the ns time scale was detected for both substances. Comparison of the results obtained for glassy and crystalline phases of 8*OCB and 5*CB compounds have been given and confronted with those obtained previously in meV energy range.     

Absorption spectra of crystalline thiophthene

The near ultraviolet absorption spectra of crystalline thiophthene have been measured in polarised light at temperatures down to 4–6 K. The spectra show that only the Davydov component located at higher energies remains broad at low temperature. The nature of this broadening is discussed in terms of downward inter-band phonon scattering from the upper optical level by a librational phonon. A comparison of experimental data and theoretical calculations of energy levels and polarisation ratios based on the dipole—dipole approximation has been carried out, and the direction of polarisation of the π* ← π electronic transition which lies in the thiophthene molecular plane has been defined.     

Rotational tunnelling of methane on the surface of graphitized carbon blacks studied by neutron scattering spectroscopy

Rotational tunnelling transitions of methane adsorbed on the surface of graphitized carbon black have been observed by incoherent neutron inelastic scattering. Two transitions have been observed, at 58 μeV and 108 μeV, which arise from the barrier to rotation about axes parallel to the surface. The methane molecules undergo rotational diffusion about an axis perpendicular to the surface. The tunnelling transitions are quite sharp at 6 K but become broader as the temperature is raised. At 30 K the motion is best described as isotropic rotational diffusion. The tunnelling spectrum has been observed on two different graphitized carbon blacks, Vulcan III (71 m² g^?1) and Sterling FT (11 m² g^?1). the latter has a more homogeneous surface and gives a sharper spectrum. Tunnelling has also been observed for methane adsorbed on Sterling FT preplated with a monolayer of xenon. When a second layer of methane is added to the first the tunnelling spectrum becomes broad and an inelastic feature appears at higher energy corresponding to a slightly hindered rotation. The value of using tunnelling spectra of adsorbed species to test models of surface forces is discussed with examples of different atom—atom potential parameters.     

Probing the interaction of hydrogen chloride with low-temperature water ice surfaces using thermal and electron-stimulated desorption

The interaction and autoionization of HCl on low-temperature (80-140 K) water ice surfaces has been studied using low-energy (5-250 eV) electron-stimulated desorption (ESD) and temperature programmed desorption (TPD). There is a reduction of H(+) and H(2)(+) and a concomitant increase in H(+)(H(2)O)(n=1-7) ESD yields due to the presence of submonolayer quantities of HCl. These changes are consistent with HCl induced reduction of dangling bonds required for H(+) and H(2)(+) ESD and increased hole localization necessary for H(+)(H(2)O)(n=1-7) ESD. For low coverages, this can involve nonactivated autoionization of HCl, even at temperatures as low as 80 K; well below those typical of polar stratospheric cloud particles. The uptake and autoionization of HCl is supported by TPD studies which show that for HCl doses ≤0.5 ± 0.2 ML (ML = monolayer) at 110 K, desorption of HCl begins at 115 K and peaks at 180 K. The former is associated with adsorption of a small amount of molecular HCl and is strongly dependent on the annealing history of the ice. The latter peak at 180 K is commensurate with desorption of HCl via recombinative desorption of solvated separated ion pairs. The activation energy for second-order desorption of HCl initially in the ionized state is 43 ± 2 kJ/mol. This is close to the zero-order activation energy for ice desorption.     

Temperature-dependent complex indices of refraction for crystalline (NH(4))(2)SO(4)

We present a significantly improved set of complex indices of refraction (optical constants) for crystalline (NH(4))(2)SO(4) at 298 K, determined from extinction spectra measured in an aerosol flow tube (AFT). The improved values provide more accurate reproductions of experimental extinction spectra when used in light scattering calculations (Mie, T-matrix, etc.). Optical constants were also derived from measurements using a cryogenic AFT at 243, 223, and 213 K, temperatures characteristic of the upper troposphere and stratosphere. Only minor changes in the optical constants were observed down to 223 K, the transition temperature to the ferroelectric phase, after which significant changes were observed. Here we report the first complex indices of refraction at reduced temperatures for both phases of crystalline (NH(4))(2)SO(4).     

Interlayer water molecules in vanadium pentoxide hydrate. 8. Dynamic properties by quasi-elastic neutron scattering

The dynamics of water molecules in the layered vanadium pentoxide hydrate, V(2)O(5).nH(2)O, were studied by quasi-elastic neutron scattering (QENS) measurements. Heterogeneity of the dynamic properties was confirmed by alpha-relaxation model analysis. Translational diffusion of monolayer and double-layer water molecules is by site-to-site diffusion and is reduced relative to that of bulk water. Water molecules lose their mobility markedly and solidify with decreasing temperature. However, mobile water remains at 253 K. Rotational diffusion coefficients are unaffected by confinement and are very similar to the bulk values determined at temperatures in the range 253-298 K. The dynamic speed characterized by QENS is much faster than that expected from the data determined by deuterium NMR (DNMR) measurements at low temperatures.     

The high resolution inelastic neutron scattering spectrum of ammonium fluoride

The measured high resolution (deltaE/E approximately 2-3%) incoherent inelastic neutron scattering spectrum of ammonium fluoride is presented and discussed with reference to the available optical spectra. In addition, a full set of dispersion curves have been obtained from a new ab initio lattice dynamics calculation and these have been used to produce a rigorous interpretation of the spectrum. The librational modes of the ammonium ion occur at 560 cm(-1) and the anharmonicity in these modes is estimated to be 4%, about half that observed in the other ammonium halides. The reduction in anharmonicity is attributed to stronger hydrogen bonding and a deeper potential well. The calculations agree well with the observed spectrum apart from the librational modes which are shifted upwards by around 40 cm(-1) from the measured values. Dispersion and LO-TO splitting are important in this system with modes changing frequency by up to 135 cm(-1). The nature of the calculated LO-TO splitting in the optic mode region is indicative of a pseudo-cubic system confirming that the site symmetry of the ammonium ion is very close to T(d). Because of LO-TO splitting the ammonium ion asymmetric stretch, nu3, has components calculated to be at higher frequencies than those of the symmetric stretch, nu1, which contradicts the assignment scheme produced from optical data.     

Comprehensive vacuum ultraviolet photoionization study of the CF3(●) trifluoromethyl radical using synchrotron radiation

The trifluoromethyl radical, CF(3)(●), is studied for the first time by means of threshold photoelectron spectroscopy (TPES). The radical is produced in the gas phase using the flash-pyrolysis technique from hexafluoroethane as a precursor. CF(3)(+) total ion yield and mass-selected TPES of the radical are recorded using a spectrometer based upon velocity map imaging and Wiley-McLaren time-of-flight coupled to the synchrotron radiation. The high resolution of the instrument and of the photons allows the observation of rich vibrational progressions in the TPES of CF(3)(●). By using Franck-Condon factors computed by Bowman and coworkers, we have been able to simulate the TPES. The initial vibrational temperature of the radical beam has been evaluated at 350 ± 70 K. The structures have been identified as transitions between (n(1),n(2)) and (n(1)(+),n(2)(+)) vibrational levels of CF(3) and CF(3)(+) with small excitation of the breathing mode, ν(1)(+) (,) and large excitation (n(2)(+) = 10-26) of the umbrella mode, ν(2)(+), in the cation. From the energy separation between the two resolved peaks of each band, a value of 994 ± 16 cm(-1) has been derived for the ν(1)(+) breathing frequency of CF(3)(+). For the high-lying n(2)(+) levels, the apparent ν(2)(+) umbrella spacing, 820 ± 14 cm(-1), is fairly constant. Taking into account the ν(2)(+) anharmonicity calculated by Bowman and coworkers, we have deduced ν(2)(+) = 809 ± 14 cm(-1), and semi-empirical estimations of the adiabatic ionization energy IE(ad.)(CF(3)(●)) are proposed in good agreement with most of previous works. A value of the vertical ionization potential, IE(vert.)(CF(3)(●)) = 11.02 eV, has been derived from the observation of a photoelectron spectrum recorded at a fixed photon energy of 12 eV.     

Nature of the Rh-H(2) Bond in a Dihydrogen Complex Stabilized Only by Nitrogen Donors. Inelastic Neutron Scattering Study of Tp(Me)2RhH(2)(eta(2)-H(2)) (Tp(Me)2 = Hydrotris(3,5-dimethylpyrazolyl)borate)

The tunnel splitting of the librational ground state and the torsional frequencies of the dihydrogen ligand in Tp(Me)()2RhH(2)(eta(2)-H(2)) (Tp(Me)()2 = hydrotris(3,5-dimethylpyrazolyl)borate) were measured using inelastic neutron scattering spectroscopy. The barrier for the rotation of the H(2) ligand and its H-H separation, calculated from these data, are 0.56(2) kcal/mol and 0.94 ?, respectively. These values indicate that pi-back-donation from the Tp(Me)()2RhH(2) fragment to H(2) is relatively weak and/or the interaction between the coordinated dihydrogen molecule and the two cis-hydride ligands significantly lowers the barrier for H(2) rotation.