Chas. G. Wade

NMR pulsed gradient diffusion measurements. II. Residual gradients and lineshape distortions

Abstract In NMR diffusion measurements, the application of pulsed magnetic field gradients can produce additional magnetic field gradients, residual gradients, which decay slowly after the pulse. Results indicate that at least two types of residual gradients are present. These are quantitatively measured and analyzed. The consequences of their presence have been carefully examined with significant results for Fourier transform experiments and pulsed gradient calibration. For example, multicomponent systems may mistakenly exhibit different diffusion rates for each component when Fourier transform techniques are used. The residual gradient, since it distorts the background magnetic field gradient, may increase the sensitivity to mismatch of the pulsed gradients as well as affect the calibration of the pulsed gradient. It is demonstrated that the lineshape can be an accurate indicator of the magnetic field gradient. Several factors which may distort the lineshape are analyzed and found not to be critical. As a consequence of these findings, an optimal procedure is described for the measurement of diffusion rates using the pulsed gradient method.

Determination of the lateral diffusion coefficient of potassium oleate in the lamellar phase

Constant gradient and pulsed gradient NMR spin echo methods have been used to measure the diffusion constant of potassium oleate in the lamellar phase. Oriented samples were employed and yielded a mean value of ?1×10−6 cm2/sec at 29 °C for the diffusion constant parallel to the lamellar surface.

Deuteron line shape study of molecular order and conformation in the nematogens PAA and MBBA

The deuteron fast Fourier transform (FFT) spectra of the ring deuterated nematogens para‐azoxyanisole (PAA‐d8) and p‐methoxybenzylidene p‐n‐butylaniline (MBBA‐d8) have been studied as a function of temperature. An interpretation of the spectra is presented. It is based on the assumption of nonequivalent deuterons, a twist angle φ between the planes of the benzene rings, and an asymmetry order parameter δ in addition to the usual nematic order parameter S. The temperature dependences of δ and φ have been obtained.

Deuteron relaxation in liquid crystals: Di‐n‐alkoxyazoxybenzenes, n=1,7

Deuteron spin lattice relaxation T1Q has been measured in the liquid crystal and liquid phases of di‐methoxyazoxybenzene deuterated at the aromatic positions (PAA‐d8) and di‐n‐heptyloxyazoxybenzene similarly deuterated (HOAB‐d8). T1Q of PAA‐d8 was measured at 4.5 and 10.5 MHz over the entire liquid crystal range and also at 15.35 MHz at 133 °C, just below the clearing point. No frequency dependence was observed. In contrast to proton relaxation, T1Q is strongly temperature dependent in the mesophases of both compounds. The results indicate that relaxation in this frequency range is more strongly influenced by rapid molecular reorientation and internal rotations than by collective mode mechanisms.

A proton and deuteron spin–lattice relaxation study of the partially deuterated nematogens PAA and MBBA

Proton spin–lattice relaxation times (T1) have been measured in methyl deuterated para‐azoxyanisole (PAA‐d6) and in ring deuterated para‐azoxyanisole (PAA‐d8) samples as functions of frequency and temperature. When compared with recent order fluctuation theory, the agreement between theory and experiment is not as satisfactory for the intermolecular contribution to the spin–lattice relaxation as for the intramolecular contribution. Deuteron spin–lattice relaxation times (T1Q) have also been measured in a ring deuterated p‐methoxybenzylidene p‐n‐butylaniline (MBBA‐d8) sample. The above order fluctuation theory fails to predict the observed temperature dependence and the observed frequency independence of T1Q if the correlation time τc for local motions is taken to be one‐third the dielectric relaxation time τD. However, a fair agreement is obtained if one uses τc= (1/100) τD, a value consistent with Ref. 6.

Frequency dependence of proton relaxation in liquid crystals

The proton spin–lattice relaxation time (T1) of liquid crystalline methyl‐deuterated p‐azoxyanisole (PAA‐d6) was measured at four frequencies as a function of its concentration in perdeuterated PAA. Analysis of the isotope dilution data showed that the intermolecular contribution to the relaxation rate becomes more important at lower frequencies, and in intramolecular contribution has a less pronounced frequency dependence than the intermolecular part.

Deuterium NMR study of molecular order and conformation in the nematic phase of p‐azoxyanisole

A single orientation order parameter tensor is shown to be sufficient for the entire molecule in the nematic phase of perdeuterated p‐azoxyanisole by examining all the deuteron line splittings in the manner suggested by Bos and Doane. Two order parameters describing the molecular orientation order and the time averaged conformation of the molecule are measured. The relative conformation parameters are shown to be temperature independent within experimental error.

Lipid lateral diffusion in oriented lipid/D2O multilayers by pulsed NMR

Abstract The temperature and hydration dependences of lipid lateral diffusion in model membrane/D2O multilayers of dipalmitoyl (DPL), dilauryl (DLL) and egg yolk (EYPC) lecithins were measured using pulsed gradient proton nuclear magnetic resonance (NMR) spin echo techniques. Oriented samples were used to minimize anisotropic dipolar interactions and permit formation of a spin echo. Significantly lipid lateral diffusion is hydration dependent over the range studied (15–40% D2O w/w), varying in DPL over this range for example by a factor of 2. For the saturated lipids at the same hydration and temperature, diffusion decreases monotonically as the chain length increases. The results tend to be larger, by factors of 2–5, than the earlier electron spin resonance (ESR) spin label results, the differences being attributable in part to the differences in hydration and to the absence of probe effects in this work. The addition of cholesterol (28.6 mol%) decreases diffusion of the lipids. Comparisons with other methods of lateral diffusion measurements are made.

NMR free induction decay and spin echoes in oriented model membrane bilayers

A method is demonstrated which produces Carr-Purcell NMR spin echoes for protons in the lamellar potassium oleate-D2O system oriented between glass plates. When the hydrocarbon chains are oriented at an angle of cos−1(3−12) with the magnetic field, the dipolar interactions are removed and the free induction decay is inhomogeneity limited. The Carr-Purcell spin echo can be obtained only at this angle. It is proposed to use this method to measure diffusion in oriented membrane systems.

NMR Relaxation in Cholesterol and Cholesteric Liquid Crystals

The proton nuclear spin lattice relaxation has been measured in cholesterol and its esters (n‐propionate, n‐decanoate, myristate, and oleate) in the solid, mesophase, and liquid regions using conventional pulsed NMR techniques at a frequency of 30 MHz. All solid phases and the mesophases of all the esters except the oleate exhibited relaxation which could be fit within experimental error with a single exponential. Nonexponential relaxation, which was analyzed assuming two relaxation times, was observed in all liquid phases and throughout the mesophase of the oleate ester. Relaxation in the mesophases of all the esters had very little temperature dependence and this, plus its behavior at the phase transitions, indicated that the relaxation mechanism might be similar to that in nematic liquid crystals. It was not possible to determine the primary source of the nonexponential relaxation behavior but it did not require the presence of an ester group.