J.P Marchal

Selective saturation as an aid for carbon-13 CP/MAS n.m.r. analysis of coals at high frequencies

Abstract A selective saturation experiment is described and applied to selective elimination of 13 C high resolution solid state n.m.r. signals and of their spinning sidebands, as well for sharp lines as for broad bands. A proper adjustment of experimental conditions permits selective saturation of the aromatic carbon signals of coal samples and eliminates unwanted overlapping of aromatic sidebands with the aliphatic region of the spectrum at high working frequencies. The described procedure seems to be especially useful for qualitative and quantitative high frequency n.m.r. investigations of coals having a distribution of short T 2 relaxation times.

Nuclear magnetic resonance study of alkylammonium chlorides. 1. Proton longitudinal relaxation times and linewidths of isotropic micellar medium

Abstract The binary systems D2O-n-alkylammonium chloride (with 8, 10, 12, and 14 carbons in the aliphatic chain) have been studied as a function of concentration and temperature. Based on the evolution of proton longitudinal relaxation times (T1) and linewidths, two different aggregates have been characterized. The first one appears at the classical critical micellar concentration (CMC) which is shown to be almost independent of temperature. The concentration at which the second type (presumably rod-shaped micelles) is formed is, on the contrary, strongly temperature dependent. It is demonstrated that forces involved in the formation of these two aggregates are of a different nature. The curves representing T1 variations as a function of concentration are satisfactorily interpreted by assuming a simple mass action law model. It is further shown that the CMC value can be approximated by K (−1 n) , where K is the equilibrium constant and n the average number of molecules per micelle.

Selective pulses applied to multiplets in homonuclear-coupled NMR Spectra; Determination of mutual couplings

Abstract It is shown experimentally and theoretically, using the density matrix formalism, that a selective pulse applied to a given multiplet of a homonuclear-coupled spin system may yield intensity alterations in other multiplets, the NMR spectrum being obtained from the conventional Fourier transform technique used immediately after the selective pulse. These effects occur if J coupling exists between the multiplets involved and if the rf amplitude used for the selective pulse is of the same order of magnitude as the coupling constant. Some examples are given; possible extensions of this method are presented.

Nuclear magnetic resonance study of alkylammonium chlorides: 3. Micelles characterization and dynamic properties determined by proton and carbon-13 longitudinal relaxation

Abstract Alkylammonium chloride/D 2 O systems have been studied by NMR proton longitudinal relaxation, proton linewidth, carbon-13 longitudinal relaxation, and carbon-13 nuclear Overhauser enhancement at three different temperatures: 25, 40, and 70°C. The investigated surfactants include normal chains with n c = 6, 8, 10, and 14 carbon atoms (denoted C nc ), octyl-oxy-propylammonium chloride (denoted OC 12 ), and octyl-amino-propylammonium chloride (denoted NC 12 ). Proton T 1 of monomers can be simply interpreted on the basis of the Stokes-Einstein law. Proton T 1 profiles, as a function of concentration, allow determination of the critical micellar concentration, and discrimination between two types of micelles: (A) involving a well-defined packing of surfactant molecules inside the aggregate in such a way that their dynamic properties are independent of concentration and chain length and (B) ill-defined micelles whose dynamic properties continuously change with concentration and depend on chain length. Proton linewidth permits detection of an eventual transition from globular to elongated micelles. This occurs only for type (A) micelles. Carbon-13 relaxation data are analyzed in terms of (i) fast motions arising from rotational isomerism with which is associated a correlation time τ Gi at the i th carbon in the chain and (ii) a slower overall motion of the surfactant molecule. τ G profiles provide a detailed picture of mobility along the alkyl chain and are also shown to be characteristic of the micelle type ((A) and (B)) as defined above but independent of the aggregate shape for a given long chain surfactant at a given temperature. C 12 , C 14 at all temperatures, C 10 at 25 and 40°C, and OC 12 at 25°C are of type (A) whereas C 6 , C 8 , NC 12 at all temperatures, C 10 at 70°C, and 0C 12 at 40 and 70°C are of type (B).

Nuclear magnetic resonance studies of alkylammonium chlorides in aqueous solution. II. Carbon-13 longitudinal relaxation times and nuclear overhauser enhancements in isotropic micellar media and in the lamellar mesophase

Abstract Carbon-13 longitudinal relaxation times (T1), and nuclear Overhauser enhancements (η) of a series of n-alkylammonium chlorides (with 8, 10, and 12 carbons in the aliphatic chain) have been measured at two concentrations in D2O corresponding to globular and rod-shaped micelles. In addition, due to its macroscopic orientation, the lamellar lyotropic mesophase of dodecylammonium chloride could be investigated. Data have been analyzed at the local level in terms of a correlation time τGi, associated to fast internal motions, a correlation time τR representing the overall surfactant molecule reorientation, and an angle θi defining the position of an effective rotation axis with respect to each CiH vector. It has been observed that mobility is quasi-independent on the considered aggregate. In the case of decyl and dodecylammonium chlorides, the τGi correlation times exhibit a “plateau” (at value of the order of 10 ps) up to three carbons from the chain extremity and then decrease rapidly to reach ∼ 1 ps. On the contrary, for octylammonium chloride a monotonic decrease from 10 to 1 ps is observed for the polar head toward the chain extremity. The slow motion (τR ∼ 10−7 to 10−8 sec) is attributed to fluctuations of the surfactant molecule with respect to an average position inside the aggregate.

Indirect determination of proton T2 and T1 from partial nuclear overhauser enhancement of carbon-13 resonances

Abstract Two procedures leading to partial carbon-13 nuclear Overhauser enhancements are proposed in order to determine proton transverse and longitudinal relaxation times, one, with the irradiation frequency shifted away from the proton resonance, and the other, with on-resonance proton irradiation switched oft for a period of variable length prior to the carbon observing pulse. These experiments, checked with simple proton systems, appear especially interesting for measuring the proton relaxation parameters of molecules whose proton resonances overlap.

Indirect determination of proton relaxation times from partial nuclear overhauser enhancements of carbon-13 resonances. II

Abstract It is shown that the recently developed methodology which determines proton relaxation parameters via partial Overhauser enhancements effected from off-resonance proton irradiation can be applied to 13 CH 2 and 13 CH 3 spin groupings as well as to the 13 CH system. However, it is also shown that possible complications may arise unless the phenomenological proton relaxation rate constants, (1/ T 1H ) and (1/ T 2H ), are correctly identified.

Deuterium longitudinal nuclear magnetic relaxation of heavy water in alkylammonium chloride solutions. Effect of deuterium exchange between water and polar heads

The water deuterium longitudinal relaxation rate is shortened in systems comprising alkylammonium chlorides (with 8, 10, 12 and 14 carbon atoms in the aliphatic chain) and D2O upon addition of a small quantity of DCl, whose effect is to slow down exchange between water and the polar headgroups. On the other hand, experiments carried out at different isotopic ratios, [D2O]/([H2O]+[D2O]), exclude any contribution from polar heads to the observed water relaxation rate. This apparent paradox is removed by a separate determination of the deuterium relaxation rates of water (R1)w and the polar heads (R1)s in acidified solutions of octylammonium chloride: two distinct signals are observed. The signal corresponding to the polar heads exhibits a marked non-exponential recovery, thus allowing the determination of τw, the deuterium residence time on the water molecule, in addition to (R1)w and (R1)s. It is also shown that, for all the systems investigated here and under any pH conditions, the apparent water deuterium longitudinal relaxation rate is equal to (R1)w+ 1/τw. In a more general way, the conditions of validity of this result are discussed.