Edwin D. Becker

Further conventions for NMR shielding and chemical shifts (IUPAC Recommendations 2008)

IUPAC has published a number of recommendations regarding the reporting of nuclear magnetic resonance (NMR) data, especially chemical shifts. The most recent publication [Pure Appl. Chem.73, 1795 (2001)] recommended that tetramethylsilane (TMS) serve as a universal reference for reporting the shifts of all nuclides, but it deferred recommendations for several aspects of this subject. This document first examines the extent to which the 1H shielding in TMS itself is subject to change by variation in temperature, concentration, and solvent. On the basis of recently published results, it has been established that the shielding of TMS in solution [along with that of sodium-3-(trimethylsilyl)propanesulfonate, DSS, often used as a reference for aqueous solutions] varies only slightly with temperature but is subject to solvent perturbations of a few tenths of a parts per million (ppm). Recommendations are given for reporting chemical shifts under most routine experimental conditions and for quantifying effects of temperature and solvent variation, including the use of magnetic susceptibility corrections and of magic-angle spinning (MAS). This document provides the first IUPAC recommendations for referencing and reporting chemical shifts in solids, based on high-resolution MAS studies. Procedures are given for relating 13C NMR chemical shifts in solids to the scales used for high-resolution studies in the liquid phase. The notation and terminology used for describing chemical shift and shielding tensors in solids is reviewed in some detail, and recommendations are given for best practice.

Infra-red spectroscopic studies of hydrogen bonding in methanol, ethanol, and t-butanol

Abstract The infra-red spectra of methanol, ethanol and t -butanol have been investigated in the 3 μ, region (fundamental frequency of the OH stretching vibration). The alcohols were studied in CCl 4 solution over a concentration range 0.005–1.0 M and a temperature range − 15° to +60°C. Quantitative measurements of the intensity of the sharp OH band near 3630 cm −1 yielded values for the equilibrium constants and the following values for the enthalpies of dimer formation: methanol, 9.2 ± 2.5; ethanol, 7.2 ± 1.6; t -butanol, 4.8 ± 1.1 kcal/mole of dimer. Integrated intensities are given for the dimer band near 3500 cm −1 and the polymer band near 3350 cm −1 . The high values of ΔH, together with the frequency shift and intensity data, suggest that the alcohol dimer is cyclic with two non-linear hydrogen bonds.

Rapid scan Fourier transform NMR spectroscopy

Abstract The theoretical basis for the use of cross correlation to extract an undistorted spectrum from a rapid scan response (as first suggested by Dadok) is examined in detail. It is shown that the desired information can be obtained either by cross correlating the rapid scan responses of the unknown and a reference consisting of only a single sharp NMR line or alternatively by applying to the Fourier transformed response an appropriate analytical function. Practical considerations in the use of rapid scan FT-NMR are explored, and illustrative examples are given. The method compares favorably with pulse FT methods in sensitivity and, in some instances, has distinct advantages over the pulse technique.

Infrared studies of hydrogen bonding in alcohol-base systems

Infrared spectroscopic studies in the 3000–4000 cm−1 region are reported for the eighteen hydrogen-bonding systems involving the proton donors methanol, ethanol and t-butanol and the proton acceptors acetone, ethyl acetate, dioxane, benzophenone, dimethylformamide and pyridine, in solution in CCl4. Equilibrium constants for the formation of 1:1 hydrogen-bonded complexes have been determined. Enthalpies of complex formation (calculated from the temperature dependence of the spectra over the range −10° to +60°C) are in the range 2.2–4.0 kcalmole. Both the frequency differences (Δν) between bands due to uncomplexed and complexed alcohol molecules and the integrated intensities (B°) of the bands due to the complexes were found to be temperature dependent, but the half-widths of the bands due to H-bonded complexes were virtually independent of temperature. Data for the fifteen O-HṫO complexes lie fairly close to the Badger-Bauer line correlating ΔH and Δν, but the results for the three pyridine complexes are in marked disagreement with this relation. Data for all eighteen systems studied, as well as other data available in the literature, show a correlation between ΔH and B°.

Nuclear magnetic resonance studies of hydrogen bonding in ethanol

Abstract The proton magnetic resonance of ethanol in CCl 4 solution has been studied as a function of concentration over the range pure ethanol (17 M) to 0.03 M. The shift of the OH proton frequency with concentration is interpreted in terms of hydrogen bonding between alcohol molecules, and an equilibrium constant for dimer formation is reported. The OH proton resonance of the ethanol monomer occurs slightly to the high field side of the CH 3 resonance. The value of the characteristic OH resonance frequency of the dimer (deduced from the NMR results and pertinent infrared data) suggests that the dimer is structurally different from higher polymers.

The choice of optimal parameters for measurement of spin-lattice relaxation times. II. Comparison of saturation recovery, inversion recovery, and fast inversion recovery experiments

Abstract Using the theory developed in Part I of this series (1) we determine the optimum pulse spacings using saturation recovery, inversion recovery, fast inversion recovery, and Freeman-Hill techniques for the measurement of spin-lattice relaxation times, T1, for conditions of ideal 90° and 180° pulses. We also compare the performance of these techniques, using as the criterion the total experimental time required to reach a specified precision in the estimate of T1. The important assumptions made are (1) that an interval of uncertainty is known for the value of T1, i.e., that the experimenter has a priori knowledge of an interval (TA, TB) in which T1 lies, and (2) that both the equilibrium magnetization M(∞) and T1 are unknown. For the conditions that we consider we find that fast inversion recovery is the most efficient technique in obtaining a fixed precision in the estimate of T1. For any interval of uncertainty the waiting time W = 2 TB proves to be optimal or near optimal. For the ideal 90° and 180° pulse situation, we find that over a wide range of uncertainty intervals the inversion recovery technique is always quicker than saturation recovery in obtaining a fixed precision in the estimate of T1. Analyses that make preliminary estimates of the equilibrium magnetization are found to be slower than those that determine T1 and M(∞) simultaneously from a set of data.

A modified fast inversion-recovery technique for spin-lattice relaxation measurements☆

Abstract A modified version of the fast inversion-recovery (FIR) method for measuring spin-lattice relaxation times is proposed. Whereas the FIR method employs a fixed time W between successive 180°, τ, 90°, sequences, the modified FIR method uses a fixed value of δ = W + τ , so that W decreases as τ increases. Both FIR and modified FIR methods are shown to be capable of nearly equal precision under conditions of ideal 180 and 90° pulses. With imperfect pulses, however, the modified FIR method circumvents certain systematic errors inherent in the FIR method. Conditions under which such errors might become significant are discussed.

The choice of optimal parameters for measurement of spin-lattice relaxation times. I. Mathematical formulation

Abstract This paper, the first in a series on optimization of T 1 measurements, describes the mathematical procedures for the analysis. Detailed results are presented in papers to follow. We assume here that a least-squares analysis is used to process magnetization data and that a first-order analysis is sufficient for practical values of the signal-to-noise ratio. In contrast to earlier analyses, we assume an initial uncertainty in the a priori knowledge of T 1 , specifically that T 1 is known to be between T A and T B . We then calculate σ( T 2 ) T 1 , i.e., the dimensionless standard deviation of the estimate of T 1 . The optimum set of measurement times τ 1 , τ 2 , …, τ 3 ., is then defined to be that which minimizes the maximum of σ( T 1 ) T 1 for T 1 in the interval ( T A , T B ). The overall objective is that of obtaining maximum precision in the shortest time. We show how to calculate the optimum number of experiments required to achieve a specified precision and how to calculate the associated total spectrometer time. Finally, we calculate σ(T 1 ) T 1 , for experiments in which separate measurements are made of the equilibrium magnetization, M (∞).

Further conventions for NMR shielding and chemical shifts (IUPAC Recommendations 2008).

IUPAC has published a number of recommendations regarding the reporting of nuclear magnetic resonance (NMR) data, especially chemical shifts. The most recent publication [Pure Appl. Chem. 73, 1795 (2001)] recommended that tetramethylsilane (TMS) serve as a universal reference for reporting the shifts of all nuclides, but it deferred recommendations for several aspects of this subject. This document first examines the extent to which the 1H shielding in TMS itself is subject to change by variation in temperature, concentration, and solvent. On the basis of recently published results, it has been established that the shielding of TMS in solution [along with that of sodium‐3‐(trimethylsilyl)propanesulfonate, DSS, often used as a reference for aqueous solutions] varies only slightly with temperature but is subject to solvent perturbations of a few tenths of a part per million (ppm). Recommendations are given for reporting chemical shifts under most routine experimental conditions and for quantifying effects of temperature and solvent variation, including the use of magnetic susceptibility corrections and of magic‐angle spinning (MAS).

The driven equilibrium fourier transform NMR technique: An experimental study☆

Abstract The Driven Equilibrium Fourier Transform (DEFT) technique for signal enhancement in pulsed 13C magnetic resonance spectroscopy has been investigated for several small 60%-enriched molecules. The experimental results demonstrate that enhancements in signal/noise over the conventional repetitive single-pulse method are indeed obtainable but are much lower than originally predicted. The principal reason is the fact that T2