Francis Masin

Solid State NMR study of nanodiamond surface chemistry

Solid state NMR measurements using 13C, 1H and 19F nuclei (MAS, CP-MAS) underline the surface chemistry of nanodiamonds from different synthesis (detonation, high pressure high temperature and shock compression). The comparison of the spin-lattice relaxation times T1 and physicochemical characterization (spin densities of dangling bonds, specific surface area and Raman and infrared spectroscopies) for the various samples, as synthesized, chemically purified and fluorinated allows the nature and the location of the various groups, mainly C-OH, C-H and C-F to be investigated. C-OH groups are located only on the surface whereas C-H and dangling bonds seem to be distributed in the whole volume. Fluorination was studied as a chemical treatment for purification and change of the hydrophobicity through the conversion of the C-OH groups into covalent C-F bonds.

13C NMR studies of TTF (13C)-TCNQ

The 13C NMR relaxation time T1 and Knight shift K have been measured in the organic conductor TTF-TCNQ with 13C-enriched TTF molecules. A clear anomaly in T1 and K shows that the gap in the conducting band of the TTF stack opens only below 38K. No T1 or K anomalies have been observed either at 54 or at 49K. The authors propose a new decomposition of the total spin susceptibility into contributions coming from individual TTF and TCNQ bands, which is based on 13C NMR Knight shift data. The salient new information provided by this work is that the ratio chi Q/ chi F is larger than unity above 60K. The present results have been used to reanalyse the 1H NMR data in the literature. They have reached the conclusion that the TTF band is broader ( approximately 0.65 eV) than the TCNQ band ( approximately 0.40 eV). The observed large enhancement of the Korringa relation suggests that the Coulomb correlation energy U is of the order of the band width for both stacks.

13C NMR spectroscopy of chloroform-solvated-C60

Abstract By means of pulsed Nuclear Magnetic Resonance and Magic Angle Spinning we have studied crystalline C60 × 0.2 C70 × 1.5(CHCl3). We present data of the spin lattice relaxation (T1) of 13C nuclei as a function of temperature between 140 and 310 K. Their analysis has been made in terms of the chemical shift anisotropy model. Our results show that the high temperature free rotation of C60 molecules is not suppressed and that there is no structural phase transition near 260 K as observed in pure crystals of C60.

Spin-lattice relaxation by solitons in 13C enriched iodine doped trans-poly acetylene

Abstract An experimental study of NMR spin-lattice relaxation of 13 C in enriched, iodine doped, polyacetylene is presented together with a theoretical analysis of the results based on the existence of solitons in this system. The good agreement with this model and the numerical values obtained here and those measured by different experimental techniques support firmly the applicability of the soliton picture to I 2 doped polyacetylene.

Benzene-solvated C60:1H nuclear spin-lattice magnetic relaxation

Abstract Crystalline C 60 .4C 6 H 6 was studied by NMR. Using a saturation-recovery method, the spin-lattice relaxation time, T 1 , of the protons of benzene was measured as a function of temperature between 4 K and 300 K. The 1 H data were fitted by a model where four correlation times are present. They correspond to thermally activated processes which follow an Arrhenius law: the activation energies are, respectively, 60 K, 279 K, 1365 K and 2945 K. All constituents of our system are extremely mobile and no structural phase transitions are observed near 260 K and 90 K as in the case of pure C 60 crystals. We have also studied the temperature dependence of the 1 H NMR linewidth which narrows with increasing temperature.

Non-exponential nuclear magnetic relaxation by solitary-waves in K2Pt(CN)4Cl0.303.2H2O

Abstract Pulsed NMR technique is used to study the 195Pt nuclear spin relaxation in KCP (Cl) in the temperature range 44 ⩽ T ⩽ 112 K where solitary waves are expected to be the main excitations of this quasi one-dimensional system. A theoretical explanation of the strongly non-exponential magnetization recovery is presented based on hopping of solitons in connection with NMR relaxation.

Structural and motional features of benzene-solvated C60 as revealed by high-resolution solid state NMR

Abstract Structural and motional features of C 60 · 4C 6 H 6 solvate has been investigated by high-resolution 13 C solid-state NMR spectroscopy. 13 C line shape analysis of the cross-polarization spectrum for the static sample at 293 K shows an unusual motional restriction of benzene molecules in this solvate. The existence of highly anisotropic motion of benzene fraction is visualized also by the detection of important homo and heteronuclear dipolar interactions. 13 C magic-angle spinning spectra reveal a complex character of the C 60 NMR signal composed of three components with slightly different isotropic chemical shifts indicating the existence of magnetically nonequivalent positions or sites.

NMR spin-lattice relaxation of 1H in iodine doped trans-polyacetylene

Abstract A study of NMR spin-lattice relaxation of 1 H in iodine doped polyacetylene vs temperature is presented. It is shown the doped polymer differs from the undoped one essentially by the presence of a new mechanism of relaxation proportional to kT . The theoretical description of our experimental data vs temperature is in complete agreement with the RF field frequency study made by Nechtshein et al . at room temperature.

13C and 1H nuclear spin lattice magnetic relaxation in undoped polyacetylene

Abstract Pulsed NMR spin lattice relaxation measurements on 13 C and 1 H nuclei in undoped trans -polyacetylene have been carried out between 6 and 295 K. The results indicate that the spin lattice relaxation is due to equilibrium fluctuations of the orientational order parameter for the protons while the carbon relaxation can be attributed to their coupling to paramagnetic impurities. In this temperature range no contribution of solitons has been detected in the relaxation mechanisms.

NMR study of C60⋅C7H8 by cross polarization and polarization inversion of rare-spin magnetization

Solid state NMR spectroscopy has been used to study the molecular dynamics of the toluene-solvated C60 compound, C60⋅C7H8. From the 1H→13C cross-polarization experiments (CP), we have obtained the cross polarization time, TIS=12.6 ms for C60 and 1H spin-lattice relaxation time in the rotating frame, T1p=170 ms. By using the cross polarization inversion method (ICP), we have deduced that all toluene molecules rotate (as seen by NMR) around the axis perpendicular to the cycle and are strongly oriented at an angle of 65° between the rotation axis of the toluene molecules and the direction of the principal magnetic field.