Fullerenes Under Pressure Studied by 13C-NMR

1994 ◽  
Vol 359 ◽  
Author(s):  
Pascale Auban-Senzier ◽  
R. Kerkoud ◽  
D. Jerome ◽  
F. Rachdi ◽  
P. Bernier
Keyword(s):  
Ambient Pressure ◽  
Lattice Relaxation ◽  
Spin Lattice Relaxation ◽  
Spin Lattice ◽  
Doped Fullerenes ◽  
Jahn Teller ◽  
Exponential Behaviour ◽  
Superconducting Compound ◽  
Coulomb Correlations ◽  
C Nmr

ABSTRACTHigh pressure is an important parameter for the study of C60 and doped fullerenes as these molecular crystals are very compressible. 13C-NMR experiments under pressure in K3C60 have given access to the determination of the 13C Knight shift and the chemical shift of this superconducting compound. These NMR data do not reveal significant effects of Coulomb correlations in K3C60 and support a pairing mechanism for superconductivity mediated by intramolecular vibrations.We report also a 13C-NMR investigation of Rb4C60 under pressure and temperature. The temperature dependence of the spin-lattice relaxation rate clearly shows, under pressure, the increase of a linear contribution which gradually substitutes to the exponential behaviour present at ambient pressure. The activated relaxation is attributed to intrinsic spin excitations through the direct Jahn-Teller gap whereas the closing of a small indirect gap under pressure gives rise to a semimetal and a Korringa like relaxation.

Conformation of ring A in triterpenoid ketones. Carbon-13 chemical shifts and spin-lattice relaxation times of lupane and 18α-oleanane derivatives

1989 ◽  
Vol 54 (7) ◽  
pp. 1928-1939 ◽  
Author(s):  
Miloš Buděšínský ◽  
Jiří Klinot
Keyword(s):  
Chemical Shifts ◽  
Relaxation Times ◽  
Lattice Relaxation ◽  
Chair Conformation ◽  
Spin Lattice Relaxation ◽  
Spin Lattice ◽  
Structural Assignment ◽  
Boat Form ◽  
The One ◽  
C Nmr

13C NMR spectra of sixteen lupane and 19β,28-epoxy-18α-oleanane triterpenoids I-XVI were measured and a complete structural assignment of chemical shifts was made. For most compounds also the carbon spin-lattice relaxation times T1 were obtained. Characteristic differences in chemical shifts of some carbon atom signals were found between 2α-methyl-3-oxo and 2α-methyl-1-oxo derivatives II, V and VIII with chair conformation of the ring A on the one hand and their 2β-isomers III, VI and IX (boat form) on the other. Using these 2-methyl ketones as models, the chair-boat population in allobetulone (I), 3-oxo-28-lupanenitrile (IV) and 1-oxo derivative VII was determined. The results agree well with the data obtained by other physical methods.

Theory of Tunneling Assisted Electron Spin–Lattice Relaxation

1971 ◽  
Vol 49 (12) ◽  
pp. 1620-1629 ◽  
Author(s):  
K. P. Lee ◽  
D. Walsh
Keyword(s):  
Lattice Relaxation ◽  
Zeeman Splitting ◽  
Spin Lattice Relaxation ◽  
Phonon Coupling ◽  
Orbital State ◽  
Strongly Coupled ◽  
Practical Applications ◽  
Spin Lattice ◽  
Jahn Teller ◽  
Temperature Relaxation

It is shown for an Eg orbital state and a tunneling splitting which is small compared with the Zeeman splitting that a strong Jahn–Teller coupling can lead to an enhancement of the direct spin–phonon coupling by several orders of magnitude. By comparing the theory with low temperature relaxation measurements on Cu2+ in a double nitrate the magnitude of several of the significant parameters associated with the Jahn–Teller problem is derived. A T2g orbital state strongly coupled to t2g modes of vibration can also have a strong spin–phonon coupling; the corresponding situation is briefly discussed.The strong coupling of the vibronic states to the lattice and the considerable range in the strength of this coupling have a number of practical applications.

Spin–lattice relaxation and hydrodynamical rotation of triphenylene

1977 ◽  
Vol 55 (20) ◽  
pp. 3602-3608 ◽  
Author(s):  
Roderick E. Wasylishen ◽  
Brian A. Pettitt ◽  
Werner Danchura
Keyword(s):  
Nmr Relaxation ◽  
Lattice Relaxation ◽  
Spin Lattice Relaxation ◽  
Relaxation Rates ◽  
Hindered Rotation ◽  
Spin Lattice ◽  
Slip Boundary ◽  
The Times ◽  
Dynamic Rotation ◽  
C Nmr

The viscosity and temperature dependence of 1H, 2H, and 13C nmr relaxation rates of triphenylene were investigated. The observed η/T dependence of the reorientation correlation time, τc, is compared with that derived from the work of Hu and Zwanzig based on hydro–dynamic rotation with 'stick' and 'slip' boundary conditions. In the hydrodynamic regime,τ = Cη/T. The observed slope, Cexperimental, is 6.2 ± 2 ns K/cP and the calculated values are Cslip = 4.1 ± 0.6 ns K/cP and Cstick = 31.0 ± 3.1 ns K/cP. These data imply that the reorientation of triphenylene obeys a near-slip condition. That is, rotation in the plane of the molecule encounters little resistance, but rotation of the plane of the molecule is resisted by shear forces in the solvent displaced during rotation. Expressions are given for Boltzmann-averaged free rotation times and comparisons are made with the η = 0 intercept and the times for slightly hindered rotation.

Transport, AC-Susceptibility and PQR Measurements of Strongly Electron Correlated Compound YbInCu4 at High Pressures

2003 ◽  
Vol 17 (18n20) ◽  
pp. 3661-3663 ◽  
Author(s):  
S. Wada ◽  
T. Mito ◽  
T. Koyama ◽  
T. C. Kobayashi ◽  
T. Muramatsu ◽  
...  
Keyword(s):  
High Pressures ◽  
Ambient Pressure ◽  
Ac Susceptibility ◽  
Lattice Relaxation ◽  
Lattice Relaxation Time ◽  
Spin Lattice Relaxation ◽  
Kondo Temperature ◽  
Quadrupole Frequency ◽  
Susceptibility Data ◽  
Spin Lattice

We have investigated physical properties of the valence fluctuating compound YbInCu 4 at high pressures and low temperatures. The first-order valence transition temperature TV ~ 42 K at ambient pressure is completely suppressed for pressures above 2.49 GPa. Present electrical resistivity data in the pressure-stabilized high-temperature (HT) phase exhibit successive shoulders at around 20 and 2.4 K. The former can reasonably be compared with the Kondo temperature TK ~ 25 K estimated from the static-susceptibility data at ambient pressure. For pressures above 2.27 GPa, the ac-susceptibility showed a clear peak at 2.4 K, which can be explained as the onset of a long-range ferromagnetic ordering. 63 Cu pure-quadrupole-resonance in the HT phase showed that the value of spin-lattice relaxation time T1 and the almost T-independent behavior are hardly affected with pressure up to 1.3 GPa, in contrast to the large increase in quadrupole frequency.

The conformational equilibrium in [13C-1-methyl]-cis-1,4-dimethylcyclohexane

1980 ◽  
Vol 58 (23) ◽  
pp. 2709-2713 ◽  
Author(s):  
Harold Booth ◽  
Jeremy Ramsey Everett
Keyword(s):  
Conformational Equilibrium ◽  
Chemical Shifts ◽  
Relaxation Times ◽  
Lattice Relaxation ◽  
Coupling Constants ◽  
Spin Lattice Relaxation ◽  
Spin Lattice ◽  
Nuclear Overhauser ◽  
In Cis ◽  
C Nmr

The conformational equilibrium in [13C-1-methyl]-cis-1,4-dimethylcyclohexane has been assessed by (a) direct integration of signals due to equatorial and axial methyl carbons in the 13C nmr spectrum at 172 K and (b) by measurement of the 13C chemical shifts of C-1 and C-4 in the spectrum at 300 K. It is concluded that a 13C isotope effect on the position of the degenerate equilibrium in cis-1,4-dimethylcyclohexane is either nonexistent, or is too small to be detected by methods of analyses employed. The 13C nmr data incidental to the study (chemical shifts, coupling constants, spin–lattice relaxation times, nuclear Overhauser enhancements, and 1-bond isotope shifts) are recorded for the title compound and its trans-isomer.

Keto-Enol Tautomerism of Dimedone Studied by Dynamic NMR

1996 ◽  
Vol 50 (11) ◽  
pp. 1408-1412 ◽  
Author(s):  
Antonio Martínez-Richa ◽  
Guillermo Mendoza-Díaz ◽  
Pedro Joseph-Nathan
Keyword(s):  
High Resolution ◽  
Chemical Shifts ◽  
Relaxation Times ◽  
Lattice Relaxation ◽  
Tautomeric Equilibrium ◽  
Spin Lattice Relaxation ◽  
Exponential Factor ◽  
Spin Lattice ◽  
C Nmr ◽  
Coalescence Temperature

The keto-enol tautomeric equilibrium of dimedone has been investigated by high-resolution 13C NMR spectroscopy. Kinetic information of the solution keto-enol tautomerism for dimedone in DMSO, in the temperature range of 25–85 °C, was derived from line shape measurements in a 75-MHz spectrometer. A value of 3.43 Kcal/mol was found for the Arrhenius activation energy Ea and of 1.07 × 106 s−1 for the pre-exponential factor A. With the use of the observed chemical shifts in the high-resolution 13C-NMR spectra of dimedone in the solid state, an estimate coalescence temperature of 240 K for dimedone in DMSO was obtained by extrapolation of the experimental curve. The estimated free energy of activation at the coalescence temperature, Δ Gc≠, is 10.8 Kcal/mol. Finally, the 13C spin-lattice relaxation times, T1(13C), in solid dimedone were measured as a function of temperature in the range of 25 to 90 °C. The data are discussed in terms of the different motional environments that result from the geometric restrictions imposed by hydrogen bonding in the crystal structure.

Activation Energies for Fluxional Behavior in Aryl(pentachlorocyclopentadienyl)mercurials, η1 -C5Cl5 HgR, from 35Cl NQR Relaxation Times

1990 ◽  
Vol 45 (3-4) ◽  
pp. 503-510 ◽  
Author(s):  
Norbert Weiden ◽  
Alarich Weiss ◽  
Gary Wulfsberg ◽  
William Ilsley ◽  
William Ilsley ◽  
...  
Keyword(s):  
Nmr Spectra ◽  
Relaxation Times ◽  
Lattice Relaxation ◽  
Lattice Relaxation Time ◽  
Activation Energies ◽  
Spin Lattice Relaxation ◽  
Spin Lattice ◽  
35Cl Nqr ◽  
C Nmr ◽  
Phenyl Mercury

Abstract Solid-state activation energies for fluxional behavior in three aryl-subsituted (pentachlorocyclo-pentadienyl)phenylmercury compounds RHgC5 Cl5 : (pentachlorocyclopentadienyl)(pentamethyl-phenyl)mercury (I, R = C6 (CH3)5 , Eact = 19.3 kJ mol-1); (pentachlorocyclopentadienyl)(2,4,6-tris-(terf-butyl)phenyl)mercury (II, R = 2,4,6-C 6 H 2 (C(CH3)3)3 , Eact = 59.5 kJ mol-1); and (pentachloro-cyclopentadienyl)(phenyl)mercury (III, R = C6H5 , E act = 62.8 kJ mol-1) have beeb obtained from 35Cl NQR spin-lattice relaxation-time measurements. II has also been shown to be fluxional in solution by 13C NMR spectra. II was prepared by an exchange reaction between Hg(C5Cl5)2 and Hg(2,4,6-C6H2(C(CH3)3)3)2 , which reacted readily despite the great steric hindrance present in the latter reagent.

scholarly journals Order–disorder transitions in adamantane derivatives: vibrational spectroscopic and 13C NMR studies of 1-chloroadamantane

1993 ◽  
Vol 71 (11) ◽  
pp. 1890-1897 ◽  
Author(s):  
Yining Huang ◽  
Ralph M. Paroli ◽  
D.F.R. Gilson ◽  
I.S. Butler
Keyword(s):  
Variable Temperature ◽  
Relaxation Times ◽  
Lattice Relaxation ◽  
Spin Lattice Relaxation ◽  
Adamantane Derivatives ◽  
Scanning Calorimetry ◽  
Nmr Studies ◽  
Spin Lattice ◽  
Two Phases ◽  
C Nmr

The order–disorder behaviour of 1-chloroadamantane (1-C10H15Cl) has been investigated by differential scanning calorimetry, and variable-temperature vibrational and 13C NMR spectroscopy. The factor group splittings in the vibrational spectra are in accord with the known crystal structures of the two phases. The 13C spin-lattice relaxation times and dipolar dephasing measurements have been analysed to give the barriers to rotation in both phases and to determine the nature of the rotations in each phase. In the ordered phase, the motion is limited to rotation about the molecular axis. In the disordered phase, additional motions occur about axes through the tertiary carbon atoms.

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