Lithium-7 qNMR as a method to quantify lithium content in brines using benchtop NMR.

A novel 7Li quantitative NMR (qNMR) method to analyze lithium was developed to determine the lithium content in real brine samples using benchtop NMR instruments. The method was validated, and limits of detection and quantification of 40 and 100 ppm, respectively, were determined. Linearity, precision, and bias were also experimentally determined, and the results are presented herein. The results were compared to those obtained using atomic absorption (AA) spectroscopy, currently one of the few validated methods for the quantification of lithium. The method provides both accurate and precise results, as well as excellent correlation with AA. The absence of matrix effects, combined with no need for sample preparation or deuterated solvents, shows potential applicability in the mining industry.

The amide rotational barrier in isonicotinamide: Dynamic NMR and ab initio studies.

We report use of dynamic nuclear magnetic resonance (NMR) to measure the amide rotational barrier in isonicotinamide. A significant challenge to obtaining good transition rates from dynamic NMR data is suppression of errors due to inherent line widths associated with transverse relaxation. We address this challenge with a fitting procedure that incorporates transverse relaxation over the temperature range of interest simply and reliably. The fitting model is nonlinear in only one of the fit parameters, namely, the activation enthalpy. This reduces parameter estimation to solution of a single transcendental equation, which avoids both a fine search over a multidimensional parameter space and extrapolation of a "limiting line width" solely from slow-exchange data. The activation enthalpy Delta H++ measured for isonicotinamide, +14.1 +/- 0.2 kcal/mol, falls between those of its regioisomers picolinamide and nicotinamide, which were reported in an earlier study. In that study, ab initio calculations of the rotational barriers helped to discern the relative importance of steric, electronic, and hydrogen-bonding effects in this biochemically significant combination of pyridine-ring and carboxamide moieties. A direct comparison between isonicotinamide and nicotinamide, where steric and hydrogen-bonding effects differ only slightly, permits a closer study of electronic considerations.

Uniform-sign cross-peak double-quantum-filtered correlation spectroscopy.

We detail the uniform-sign cross-peak double-quantum-filtered correlation spectroscopy (UC2QF COSY) experiment, a new through-bond correlation method for disordered solids. This experiment is a refocused version of the popular double-quantum-filtered correlation spectroscopy experiment in liquids. Its key feature is that it provides in-phase and doubly absorptive line shapes, which renders it robust for chemical shift correlation in solids. Both theory and experiment point to distinct advantages of this protocol, which are illustrated by several experiments under challenging conditions, including fast magic-angle spinning (30kHz), anisotropic molecular motion, and (13)C correlation spectroscopy at the natural abundance isotope level.