Charles L. Mayne

Nuclear relaxation in coupled spin systems. A heteronuclear two pulse experiment and a general numerical method for analyzing spin dynamics

A convenient new heteronuclear two pulse nuclear spin relaxation experiment is described. When combined with relaxation experiments already in the literature, each of the interlevel transition probabilities for a system of weakly coupled nuclear spins may be determined. Previous work in coupled spin relaxation has yielded only certain linear combinations of these transition probabilities. The well understood AX spin system was selected to illustrate the method, and sodium formate in D2O solution is used as the specific case. Values for all four interlevel transition probabilities characterizing the spin lattice relaxation of the 1H–13C spin system of the formate ion are obtained. These data yield the first experimental verification of the extreme narrowing condition commonly assumed for small molecules in nonviscous liquids. A general numerical method is described for estimating the relaxation parameters from measurements of the time evolution of nonequilibrium magnetization. The method involves numerical...

Spin–lattice relaxation in coupled three spin systems of the AIS type

Relaxation in coupled three spin systems of the AIS type is described by an extension of the two spin case treated by Solomon. The proper linear combinations of spin level populations are presented as normal mode variables in a system of coupled differential equations so that maximum reduction in the number of differential equations is achieved. Decoupling experiments further reduce the AIS problem to a set of equations similar to those encountered in the two and one spin cases. The formalism includes in a natural way dipolar relaxation effects due to cross‐correlated motion of internuclear vectors. A theoretical expression is derived to describe these effects. In the normal mode variables these terms group together and relate total magnetization variables with a multiplet relaxation mode. Experimental results on the three spin system 13CHFCl2 are presented in terms of the above formalism.

Structure and dynamics of methylene iodide in solution from nuclear spin–lattice relaxation studies

The nuclear spin–lattice relaxation of carbon‐13 enriched methylene iodide (diiodomethane) dissolved in benzene‐d6 has been studied both with and without proton decoupling and using various pulse techniques to perturb the AX2(13CH2) spin system from thermal equilibrium. The return of the spin system to steady state was monitored using carbon‐13 Fourier transform nuclear magnetic resonance techniques. It is shown that the equation of motion of the spin density matrix reduces in general to the master equation for populations in terms of interlevel transition rates, and that a linear transformation based on the complete set of irreducible spherical tensor operators which span the spin space further simplifies the equation of motion. The relaxation was modeled as intramolecular dipole–dipole interactions modulated by rotational reorientation of the molecule plus other mechanisms which can be treated collectively as external random magnetic fields interacting with the nuclear spins of interest. Extreme narrowi...

Nuclear relaxation in coupled spin systems dissolved in a nematic phase—the AX and A2 spin systems

Relaxation in coupled AX(13CHCl3) and A2(12CH2Cl2) spin systems has been studied in a nematic solvent. The process is modeled as intramolecular dipole–dipole interactions plus other mechanisms that are treated collectively as external random magnetic fields. The AX system provides evidence that extreme narrowing arguments are valid for small molecules dissolved in liquid crystals. Also, values of spectral densities show that relaxation by the intramolecular dipole–dipole mechanism and by the combined external random field mechanisms are of equal importance for proton longitudinal relaxation whereas the former mechanism dominates the carbon‐13 relaxation. Important details of the A2 system are discussed for the first time with the two relaxation observables that are obtained when superposition of the transitions is eliminated by the anisotropic solvent. Extreme narrowing is assumed, and the relative importance of the various mechanisms is discussed. The random field type mechanisms, which include intermole...

Computerized analysis of 2D INADEQUATE spectra

Abstract The extraction of carbon-carbon bond information from two-dimensional INADEQUATE spectra is both time consuming and complex due to the low sensitivity of the method, the incomplete suppression of single-quantum signals, and the large size of the data sets. A computerized analysis technique is introduced which detects bonds through a nonlinear regression analysis of carefully chosen subsets of the spectral data. A quantitative one-dimensional carbon spectrum is used to establish initial values for the regression and to determine the data subsets to be used, Using statistical analysis techniques, bonds are detected with reliability and sensitivity comparable to those of careful manual interpretation.

A new high pressure sapphire nuclear magnetic resonance cell

A new version of a single‐crystal sapphire high pressure nuclear magnetic resonance (NMR) cell is described that is capable of controlling the sample pressure independent of the temperature. A movable piston inside the cell adjusts and controls the sample pressure from ambient conditions to 200 atm within ±0.3 atm. The linewidth at half‐height for a 13C spectrum of carbon dioxide at 15 °C and 57.8 atm is found to be 0.5 Hz. The carbon dioxide gas/liquid phase transition is clearly observed by measuring 13C chemical shifts as the sample pressure approaches equilibrium. The time required for this NMR cell to reach equilibrium with its surroundings is relatively short, usually 15–30 min. The cell body has the same outer dimensions of a standard spinning turbine and fits into a standard 10 mm commercial probehead capable of controlling the sample temperature using the spectrometer’s variable temperature unit. The flexibility of the device and the increased speed in making the measurement is demonstrated. Such...

Cylindrical spinner and speed controller for magic angle spinning nuclear magnetic resonance

A new design for a cylindrical spinner suitable for magic angle spinning nuclear magnetic resonance (NMR) is described. The rotor has a large sample volume (0.66 cm3) and is capable of spinning speeds from 500 Hz to 5 kHz at drive gas pressures below 50 lb/in.2. The rotor is completely supported by gas bearings. Axial positioning is provided by the rotor’s T shape, leaving the bottom surface available for a photoelectric tachometer. The coil is wrapped on a form machined into the outer surface of the stator, producing a high filling factor. Also described is an instrument which controls the spinning speed of such a rotor to ±0.6 Hz around a set speed in the range of 3–4 kHz. The rotor’s speed is determined by means of a photoelectric tachometer. The instrument then compares this measured speed to the desired value and adjusts the gas flow to the rotor to correct any difference.

Two-dimensional J spectra in solids

Various two-dimensional NMR techniques exploiting the scalar coupling between nuclear spins have been demonstrated in liquids (I). These methods have proved to be valuable tools in the application of NMR to various problems, such as structure determination, kinetics, molecular dynamics, etc. Such techniques, applied in conjunction with those that are rapidly developing for obtaining high resolution spectra of solids (2, 3), offer many new avenues for investigation of complex solids. Work recently published from this laboratory (4) and simultaneously from another (5) demonstrated that high resolution 13C spectra of certain solid materials can be obtained which exhibit the proton-carbon scalar coupling. An obvious extension of this work leads to 2D spectra similar to those referred to as heteronuclear .I spectra (1) in liquids. Chemical shifts are displayed along one frequency axis, and scalar couplings are displayed along a perpendicular axis. Such a spectrum of polycrystalline camphor is shown in Fig. 1. This spectrum was obtained using the pulse sequence shown in Fig. 2. The sample is spun at the magic angle. Carbon transverse magnetization is generated using the usual cross polarization technique (2). This is followed by multipulse decoupling using the MREV-8 pulse sequence (3) for a period t, /2 so that the carbon magnetization evolves with scalar coupling but dipolar interactions are suppressed. The carbon magnetization is then inverted by a ?r pulse and allowed to evolve for an additional time t,/2 under continuous proton decoupling. The isotropic part of the chemical shift refocuses during this period so that the amplitude of the echo is modulated only by scalar coupling. The second half of this echo is then digitized still under proton decoupling. A data matrix indexed by 2, and t2, the time variable of the FID, is then Fourier transformed with respect to t, and f2 to yield the spectrum shown. The combination of MREV-8 irradiation and magic-angle spinning removes the effects of dipolar interactions, both proton-proton and proton-carbon, as well as chemical shift anisotropy during the first half of the evolution period tl . During the second half of the evolution period switching to continuous proton irradiation removes the effect of scalar coupling as well. The overall effect is then very similar to carbonproton .I spectroscopy using the gated decoupler method (1). However, in contrast to the liquid experiment where the scalar coupling is reduced only by a factor of onehalf, the couplings are scaled by an additional factor of about 0.488 due to the MRFV8 pulse sequence (3). This scaling by almost a factor of one-fourth limits the usefulness

Relaxation in an A2 coupled spin system dissolved in a perdeuterated liquid crystal: The intermolecular dipolar random field cross relaxation term

A perdeuterated nematic liquid crystal N‐ (p‐ethoxybenzylidene) ‐p‐n‐butylaniline (EBBA‐d23, 98% deuterium) was synthesized to study relaxation of small molecules dissolved in anisotropic solvents. Proton spin–lattice relaxation in the coupled A2 spin system of methylene chloride (CH2Cl2) was studied using both perhydro and perdeutero EBBA as the solvent. The relaxation was modeled as intramolecular dipole–dipole interactions plus other mechanisms that were treated collectively as fluctuating external random magnetic fields. Comparison of relaxation in EBBA and EBBA‐d23 allowed the first experimental measurement of the correlation coefficient between external random fields at the two protons due to dipolar interactions of the methylene protons with the protons or deuterons of the solvent (78% correlated). The selective nuclear Overhauser effect was also studied.

Visualization of enantiomers in cholesteric solvents through deuterium NMR

Abstract Proton decoupled deuterium NMR of mixtures of enantiomers in homogeneously oriented cholesteric solvents produces simple spectra with linewidths of 10 to 50 Hz in cases where the proton spectra would give second order patterns so complicated as to defy analysis. The chiral solvent orders each of a pair of enantiomers differently which results in a difference in the residual quadrupolar coupling constant yielding well resolved spectra for each enantiomer. That the technique constitutes a new tool for measurement of enantiomeric ratios is illustrated using several chiral benzylic alcohols.