The molecular dynamics of fluxional organometallic molecules in the solid state: tetracarbonylbis(cyclooctatetraene)triruthenium(0)

The “wide line” NMR spectrum of tetracarbonylbis(cyclooctatetraene)triruthenium(0) is markedly temperature dependent, indicating that the cyclooctatetraene rings are reorienting in the solide state. An energy of activation of 5.15 kcal/mol for ring rearrangement is found from T₁ measurements. A comparison of the experimental second moment, 0.72 G² at 304 K, with theoretical second moments calculated for several different modes of reorientation supports a rearrangement process through a series of 1,2 shifts. The results are discussed in relation to the known crystal structure data for the complex.     

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 E_a for NH⁺₄ ion reorientation is estimated to be 1.4 kcal mol⁻¹.     

A pulsed NMR study of molecular motion in solid 5,5-dimethyl-5,6,11,12-tetrahydro-5 H-dibenzo[b,f]silocin

A pulsed NMR study of relaxation times, T₁ and T_1D, and second moments for solid 5,5-dimethyl-5,6,11,12-tetrahydro-5 H-dibenzo[b,f]silocin is reported. The compound was studied over the temperature range ?175 to 50°C. Evidence was obtained for three motions characterized by activation energies, 2.70, 1.2, and ?4 kcal/mole, respectively. The first motion is methyl reorientation. The second motion is not assigned but is probably a relatively small amplitude flexing of the central ring. The third motion is also unassigned and may be large amplitude flexing of the central ring, libration, or anisotropic molecular reorientation. The motions was assigned by comparison of X-ray crystallographic data with NMR second moment data.     

Phase transitions and molecular motions in [Cd(H2O)6](BF4)2 studied by DSC, H and F NMR and FT-MIR

Two solid phase transitions of [Cd(H₂O)₆](BF₄)₂ occurring on heating at T_C2=183.3 K and T_C1=325.3 K, with 2 K and 5 K hysteresis, respectively, were detected by differential scanning calorimetry (DSC). High value of entropy changes indicated large orientational disorder of the high temperature and intermediate phase. Nuclear magnetic resonance (¹H NMR and ¹⁹F NMR) relaxation measurements revealed that the phase transitions at T_C1 and T_C2 were associated with a drastic and small change, respectively, of the both spin-lattice relaxation times: T₁(¹H) and T₁(¹⁹F). These relaxation processes were connected with the “tumbling” motions of the [Cd(H₂O)₆]²⁺, reorientational motions of the H₂O ligands, and with the iso- and anisotropic reorientation of the BF₄⁻ anions. The cross-relaxation effect was observed in phase III. The line width and the second moment of the ¹H and ¹⁹F NMR line measurements revealed that the H₂O reorientate in all three phases of the title compound. On heating the onset of the reorientation of 3 H₂O in the [Cd(H₂O)₆]⁺², around the three-fold symmetry axis of these octahedron, causes the isotropic reorientation of the whole cation. The BF₄⁻ reorientate isotropically in the phases I and II, but in the phase III they perform slow reorientation only about three- or two-fold axes. A small distortion in the structure of BF₄⁻ as well as of [Cd(H₂O)₆]²⁺ is postulated. The temperature dependence of the bandwidth of the O-H stretching mode measured by Fourier transform middle infrared spectroscopy (FT-MIR) indicated that the activation energy for the reorientation of the H₂O did not change much at the T_C2 phase transition.     

Polymorphism and dynamics of MBBA as studied by NMR

We report proton NMR experiments on the liquid crystal material N-(p-methoxybenzylidene) p-n-butylaniline (MBBA) at 100MHz in the temperature range 110-350 K. The phase diagram was investigated by means of second moment and spin-lattice relaxation measurements in order to establish connections between dynamics and phase transitions. The results show that in a slow cooling experiment, two processes contribute to the relaxation, a slow ethyl group motion together with reorientation of the methyl groups. For the glassy nematic state, as well as for the phases observed after reheating a quenched sample, only methyl rotation is observed. The correlation times of these various mechanisms were determined and the results compared with those obtained by previous NMR and dielectric analysis.     

Phase transitions and molecular motions in [Ni(ND3)6](ClO4)2

[Ni(ND₃)₆](ClO₄)₂ has three solid phases between 100 and 300 K. The phase transitions temperatures at heating (T_C1^h=164.1 K and T_C2^h=145.1 K) are shifted, as compared to the non-deuterated compound, towards the lower temperature of ca. 8 and 5 K, respectively. The ClO₄⁻ anions perform fast, picosecond, isotropic reorientation with the activation energy of 6.6 kJ mol⁻¹, which abruptly slow down at T_C1^c phase transition, during sample cooling. The ND₃ ligands perform fast uniaxial reorientation around the Ni-N bond in all three detected phases, with the effective activation energy of 2.9 kJ mol⁻¹. The reorientational motion of ND₃ is only slightly distorted at the T_C1 phase transition due to the dynamical orientational order-disorder process of anions. The low value of the activation energy for the ND₃ reorientation suggests that this reorientation undergoes the translation-rotation coupling, which makes the barrier to the rotation of the ammonia ligands not constant but fluctuating. The phase polymorphism and the dynamics of the molecular reorientations of the title compound are similar but not quite identical with these of the [Ni(NH₃)₆](ClO₄)₂.     

The characterization of calcium iridium oxides

Several calcium iridium oxides were prepared by solid state reactions and by a hydroxide precipitation technique. These phases were characterized by X-ray powder diffraction data, electrical and magnetic properties. The calcium-rich phase, Ca₄IrO₆, was an insulator exhibiting Curie-Weiss behavior above 77°K with a Weiss constant of ?47°K and a magnetic moment of 1.76 μ_B.Ca₂IrO₄ is a semiconductor and the temperature-dependent susceptibility above 77°K was analyzed in terms of one-dimensional linear chains of octahedrally coordinated Ir⁴⁺ ions.Two CaIrO₃ phases were isolated, but only the orthorhombic perovskite modification was sufficiently pure for magnetic studies. It was Pauli paramagnetic with metallic conductivity.     

NMR study of 3,4-dimethoxy benzoic acid

The computed value of the rigid lattice second moment of 3,4-dimethoxy benzoic acid (15.31 gauss²) agrees fairly well with the experimental value of the second moment (17.01 ± 1 gauss²) indicating that the lattice is rigid at 77 K and confirming the proposed model. From the temperature dependence of the second moment, a transition is observed at room temperature which is attributed to the rotation of methoxy groups.     

Phase diagram and crystal phases of trans-1,3 butadiene probed by FTIR and Raman spectroscopy

In the present work the first spectroscopic data concerning solid butadiene are presented. FTIR and Raman spectra of solid trans-1,3 butadiene in the ordered phase I were recorded at atmospheric pressure down to 12 K. The FTIR technique was used to characterize the phase diagram of butadiene in the 0–7 GPa and 150–300 K pressure–temperature range. Besides phase I another solid phase (phase II), orientationally disordered, was found to be stable at pressures above 0.5 GPa between the liquid and the ordered phase I. A monoclinic C_2h⁵ crystal structure with two molecules per cell sitting on C_i sites is proposed for phase I.     

123Sb NQR and 19F NMR study of potassium,rubidium, and cesium heptafluorodiantimonates(iii)

The dynamics of crystal lattices of potassium, rubidium, and cesium heptafluorodiantimonates(iii) and specific features of internal rotations of the Sb₂F₇ fluoride groups in these compounds were studied using ¹²³Sb NQR in the temperature interval from 77 to 325 K and ¹⁹F NMR in the temperature interval from 240 to 470 K in combination with X-ray diffraction and thermogravimetric analyses. The distinctions in the dynamic behavior of the fluoride ions with changing the size (polarizability) of outer-sphere cations are discussed. The structural phase transition in CsSb₂F₇ was revealed at 425—430 K accompanied by the appearance of a high ion conductivity ( 1.3·10^–3 S cm^–1 at 450 K). A second type phase change can exist at 220—270 K.     

Nuclear quadrupole and magnetic resonance studies of structure and the molecular motion in SYMC6Cl3F3

³⁵Cl and ¹⁹F relaxation time measurements were carried out on symC₆Cl₃F₃. The analyses of the ¹⁹F high-resolution NMR spectra and the ³⁵Cl spin-lattice relaxation time showed that the crystal belongs to a trigonal or a hexagonal crystal system in which the molecules undergo three-fold reorientation about the molecular figure axis with the correlation time τ_c/s=3.42·10^?7 exp(18.2 kJ mol^?1/RT). Temperature dependences of the ³⁵Cl NQR frequency and ¹⁹F spin-lattice relaxation times are such that only a minor structural change is associated with the phase transition at 296 K.     

Collision dynamics of excited NaK. I. Reactive KNaK and elastic HeNaK collisions

The collision dynamics of excited NaK have been studied using the technique of circularly polarised laser fluorescence. A very unusual m-dependent rate is observed for KNaK* reactive or quenching collisions which is different depending on whether the initial excitation is via Q or P, R transitions. A model is presented attributing this to the spatial distribution of the ¹Π Λ-doublet components and the influence of the nuclear function. Elastic HeNaK collisions are observed and a reorientation cross section for f = 30 was evaluated. This was found to have an upper limit of 0.3 Ų. Comparison of this with HeLi₂ reorientation cross sections indicate that the presence of a permanent dipole moment has little effect on the reorientation rate in rare gas-alkali dimer collisions.     

35Cl NQR and 1H NMR studies of [C(NH2)3]AuCl4, [C(NH2)3]2PtCl4, and [C(NH2)3]2PdCl4

The temperature dependence measurements of ³⁵Cl NQR frequencies and ¹H NMR spin-lattice relaxation time T₁ were carried out for guanidinium tetrachloro-aurate(III), -platinate(II), and -palladate(II). The gold(III) complex showed four NQR lines at various temperatures between 77 and 344 K, while the platinum-(II) and palladium(II) complexes gave two NQR lines in the temperature ranges 77–169 K and 77–220 K, respectively. An unusual phase transition was located at 363 K for the gold(III) complex. The high-temperature phase was easily supercooled. All the complexes studied yielded a T₁ minimum attributable to the reorientation of the planar cation about its C₃ axis. The motional parameters were evaluated. The Zeeman-quadrupole cross relaxation between protons and chlorine nuclei was observed for the platinum(II) and palladium(II) complexes at various temperatures below room temperature, while it was also detected for the high-temperature phase of the gold(III) complex.     

The methyl and methoxyl torsional modes and the lattice vibration in the low-frequency Raman spectrum of 1,4-dimethoxybenzene

The low-frequency (10–450 cm^?1) Raman spectra of solid (at 300 K and 130 K) and liquid (at 335 K) 1,4-dimethoxybenzene-d₀ and 1,4-dimethoxybenzene-d₅ have been measured. The methyl nad methoxyl torsional transitions have been identified and the corresponding torsional barriers calculated. Upon deuleration the methyl torsional barrier is reduced by 450 cm^?1, implying a coupling between the methyl torsion and a low-frequency ring mode. As far as the torsions are considered, the internal dynamic situation in 1,4-dimethoxybezene resembles that in amisole. A tentative assignment of the observed lattice bands in given. Certain changes in the spectrum when going from the solid to the melt are attributed to the coexistence of both cis and trans conformers in the liquid state.     

Motion of cations and phase situation in solid tetrabutylammonium octacyanomolybdate(V)

Summary Studies of the i.r. spectra, lattice parameters and the proton second moment in [n-Bu₄N]₃[Mo(CN)₈] over the 77K to 383 K region are reported.¹Hn.m.r. data provide evidence for different rotational processes which are discussed in some detail. Atca. 260 K and 295 K the compound exibits anomalous changes in i.r. spectra and lattice parameters. The results reveal the existence of polymorphism in solid tetrabutylammonium octacyanomolybdate(V).     

A Raman spectroscopic study of the low-frequency vibrations in anisole and anisole-d3

Low-frequency Raman spectra of solid anisole and of solid anisole-d₃ have been recorded at 130 K. The phenyl torsion observed at 148 cm^?1 is shifted to 133 cm^?1 upon deuteration of the methyl group. The twofold torsional barriers calculated from these frequencies are 4033 ± 110 cm^?1 and 4094 ± 123 cm^?1 indicating that coupling to other low-frequency modes in both cases is of the same order of magnitude. The methyl torsional mode was observed at 285 cm^?1 in the spectrum of solid anisole and at 183 cm^?1 in the spectrum of anisole-d₃. The threefold barriers calculated using these frequencies are 1847 ± 20 cm^?1 and 1465 ± 18 cm^?1 respectively. These barrier values indicate that the methyl torsion is coupled to another low-frequency mode. A doublet centered at 230 cm^?1 in anisole is shifted to 245 cm^?1 in anisole-d₃; it is proposed that this is due to a ring mode coupled to the methyl torsion. The splitting is interpreted as an example of Davydov splitting.     

Adsorption-based characterization of hierarchical metal–organic frameworks

Nitrogen adsorption at 77 K on metal–organic framework (MOF) is investigated by means of molecular simulations. We consider both regular Cu–BTC crystal and a MOF-based hierarchical porous solid consisting of a mesopore carved out of a Cu–BTC crystal. The t-plot method is applied to these solids by using a non-porous Cu–BTC surface as the reference sample. The values of the mesoporous and external surface areas are determined from the t-plot, and the validity of the method for this type of hierarchical solid is discussed.     

Temperature dependence of trapped electrons in crystalline D2O ice from 77 to 6 k

The temperature dependence of the abundance of trapped electrons which absorb in the visible (e^?_vis) and infrared (e^?_IR) in crystalline D₂O ice has been studied by pulse radiolysis between 77 and 6 k. The yield (G) and decay of e^?_vis show little dependence on temperature or doping with NH₄F. At 6 K G(e^?_vis) is 0.54 and the electron decays by half within 5 μs. These observations are consistent with e^?_vis being mainly located in spurs. The yield and decay of e^?_IR, on the other hand, show a more marked dependence on temperature. In the pure crystal G(e^?_IR) increases more than tenfold from ≈ 0.1 at 77 K to 1.3 at 6 K and its decay rate is greatly decreased at the lower temperature. Doping with NH₄F increases G(e^?_IR) to 0.85 to 77 K and to 1.8 at 6 K and some decay is observed at 6 K but not at 77 K. These results are interpreted on the basis that geminate recombination between electrons and holes is very fast at 77 K but becomes sufficiently slow for the electrons to be observed at 6 K. It is also inferred that the hole is more mobile than e^?_IR. The mechanisms causing the decays of e^?_vis and e^?_IR are discussed.     

Tricritical point in ferroelastic ammonium titanyl fluoride: NMR study

Ionic mobility and phase transitions in ammonium titanyl pentafluoride (NH₄)₃TiOF₅ were studied using the ¹⁹F and ¹H NMR data. The high-temperature phase (I) is characterized by spherically symmetric (isotropic) reorientation of [TiOF₅]³⁻ anions and by uniaxial reorientation of these anions in the ferroelastic phase II. A previously unknown second-order phase transition to the low-temperature modification (NH₄)₃TiOF₅(III) was found at 205 K. The transition is accompanied by hindering of uniaxial rotations of [TiOF₅]^3- anions and by noticeable change of ¹⁹F magnetic shielding tensor associated with the influence of pseudo-Jahn-Teller effect. A pressure-induced tricritical point with coordinates p_TCR≈2 kbar and T_TCR≈170 K is estimated on the base of ¹⁹F NMR chemical shift data, and previously studied p-T diagram of (NH₄)₃TiOF₅.     

A study of molecular motion in solid cyclopropane and cyclopropane clathrate deuterate by nuclear magnetic resonance absorption and relaxation methods

Proton magnetic resonance absorption and spin-lattice relaxation measurements have been carried out for cyclopropane clathrate deuterate from 77 to 290 K together with spin—lattice relaxation measurements on solid cyclopropane from 90 to 146 K. The absorption measurement for the type I structure deuterate indicates the presence of an isotropic rotation of the cyclopropane molecule from about 230 K, while in the type II structure deuterate isotropic rotation of the enclathrated cyclopropane is present over all of the range of stability of the clathrate (~250 to 278 K). The spin-lattice relaxation measurements give an activation energy of 0.83 ± 0.03 kcal mole^?1 for the barrier to reorientation (not assigned) of the cyclopropane molecules inside the clathrate deuterate cavities. In solid cyclopropane the barrier associated with the threefold axis rotation is found to be 4.8 ± 0.2 kcal mole^?1.