R.E.D McClung

A product operator description of AB and ABX spin systems

Abstract A simple product operator description of the strongly coupled AB and ABX spin systems is developed. The utility of this description in the investigation of the effects of strong coupling is illustrated in a number of examples.

Simulation of two-dimensional NMR spectra using product operators in the spherical basis

Abstract The product operator description of magnetic resonance experiments is developed in the spherical-tensor basis, and a computer simulation scheme, employing this description, is reported. The utility of simulations in the design of selective phase cycles and in the study of the effects of spectrometer imperfections is demonstrated.

Selective 2D DEPT heteronuclear shift correlation spectroscopy

Abstract Selective 2D DEPT heteronuclear shift correlation maps for cholesterol in CDCl 3 are reported and demonstrate the selectivity and utility of these experiments. Improvements in the basic 2D DEPT sequence to effect quadrature detection and proton-proton decoupling in the f 1 domain are reported. An implementation in which the 2D data files for proton flip angles τ = 30, 90, and 150° are acquired and used to construct selective CH, CH 2 , and CH 3 maps is shown to be twice as efficient as the previous implementation where separate experiments were performed to construct each selective 2D map.

Computer simulation of multiple-pulse and two-dimensional FT NMR experiments

A simulation program to analyze multiple-pulse and two-dimensional FT NMR experiments using density matrix theory is reported. The program computes analytical expressions for NMR signals arising from an nmSn spin system (where I and S are spin-12 nuclei and 1 ⩽ m, n ⩽ = 3), after the application of a pulse sequence specified by the input data. The simulation program has been designed to be quite general and incorporates a number of features including irradiation from a broadband decoupler, effects of phase cycling the pulses and receiver, and pulses of any given flip angle. The essential features of density matrix description of NMR experiments, implementation of the density matrix computation, and some results from simulation of DEPT, INADEQUATE, and two-dimensional heteronuclear shift correlation experiments are presented.

Rotating-frame relaxation rates of solvent molecules in solutions of paramagnetic ions undergoing solvent exchange

Abstract The theory is developed for the dependence of the rotating-frame relaxation rate (R1ϱ) of bulk solvent nuclei on the magnitude of the spin-locking field H 1 (= − ω 1 γ for a dilute solution of a paramagnetic ion dissolved in a solvent which undergoes exchange between bulk and coordinated sites. When certain conditions are satisfied R1ϱ will vary with ω1, according to R1ϱ = R1ϱ∞ + Sϱ2(R2 − R1ϱ)ω1−2, 2, where R1ϱ∞ = R2s + PmrmR2m(R2m + rm)−1, Sϱs2 = (R1m + rm)(R2m + rm)−1 {(R2m + rm)2 + Δωm2}, R1m, R2m are the relaxation rates in the coordinated site, Δωm is the chemical shift between the bulk and coordinated sites, rm is the solvent exchange rate from a coordinated site, R2s is the sum of the pure solvent and outer-sphere relaxation rates, and Pm = (no. coordinated solvent molecules) × (metal-ion molality/solvent molality). Data for cobalt(II) in CH3OD and nickel(II) in CH3CN are used to show that the R1ϱ measurements are especially useful in the determination of Δωm. The latter can be combined with bulk solvent shifts to determine the number of coordinated solvent molecules.

Improvement of INADEQUATE using compensated delays and pulses

Abstract A J -compensated homonuclear spin-echo sequence is developed from the 90 α o 180 α+2π/3 o composite pulse using a product-operator analogy between homonuclear spin-spin coupling and RF pulses, and a J -compensated INADEQUATE sequence is constructed using this spin-echo sequence. Detailed study of the efficiencies of various composite pulses in minimizing resonance-offset effects in INADEQUATE experiments showed that five-step symmetric phase-altemating composite pulses are most effective. The sequences C-INADEQUATE, which incorporates composite pulses, and JC-INADEQUATE, which includes both offset and J -compensation, are shown to have a sensitivity significantly higher than that of conventional INADEQUATE.

Fourier transform double-resonance NMR on two- and three-spin systems☆

Abstract The flip angle dependence of the line intensities in gated double-resonance FT NMR experiments on the strongly coupled spin systems 2,3-dibromothiophene, 1,2,3-trichlorobenzene, vinyl acetate, and 2,3-dibromopropionic acid is shown to follow the theory of Shaublin, Hohener, and Ernst. A general method for analysis of the flip-angle-dependent intensities in terms of spin level population differences is described, and it is demonstrated that the variation of line intensities with flip angle is sensitive to the relative signs of spin-spin couplings and to the mechanism of spin relaxation.

Experimental and theoretical investigation of 2D-13C DEPT spectra on CDn systems☆

Abstract Coupled 2 D- 13 C DEPT spectra for the CD, CD2, and CD3 systems are reported and a thorough analysis of these experiments using the density matrix theory is presented. DEPT is shown to be useful for selective enhancement of 13 C resonances from CD fragments, but less practical for the selective enhancement of resonances from CD 2 and CD 3 groups.

J-compensated DEPT sequences

J-compensated magnetization manipulation sequences are derived from composite 90° and 180° pulse sequences. Incorporation of J-compensated sequences into each of the three precession periods of DEPT shows that compensation of the first delay period results in a sequence, DEPTC1, which is less sensitive to variations in JIS than DEPT. The compensation of the second and third parts of DEPT does not lead to improved conversion efficiency.

Internal rotation and intramolecular dipolar relaxation: Effect of modulation of internuclear separation☆

Abstract Expressions for the spin-lattice relaxation time for a nucleus experiencing intramolecular dipolar interactions which are modulated by internal motion and by overall molecular reorientation are given for the case when the internal motion modulates the internuclear distance. Rotational diffusion models are used to describe both internal and overall rotational dynamics, and results are given for spherical and symmetric top molecules. It is shown that modulation of the internuclear distance can give rise to substantial effects on the nuclear relaxation rate in certain molecular geometries.