Gas Phase NMR
Nuclear magnetic resonance (NMR) is a very special kind of molecular spectroscopy. It is extensively used by chemists, primarily for the investigation of organic molecules in liquids and solids. NMR experiments in the gas phase are somewhat less popular, even though the first systematic studies of gases have been started more than 50 years ago. This is presumably connected with the time-consuming procedure of sample preparation, because the NMR observation of gaseous compounds is fairly easy – almost all the new experimental techniques known and applied for liquids can be successfully adopted for the study of gases. Gas phase NMR has certainly its own areas of unique applications in science and for this reason the investigation of gases is emerging as an important tool, involving many new applications in the fields of physics, chemistry, biology, and medicine. It is especially important that gas phase NMR experiments deliver the values of spectral parameters which are free from intermolecular interaction effects and therefore suitable for direct comparison with quantum chemical calculations, usually performed for isolated molecules. The combined use of experimental and theoretical methods in this area gives rise to a new outlook on magnetic properties of molecules and on multinuclear NMR experiments themselves; the gas phase studies are enormously enriched when they are connected with the calculation of spectral parameters.
This book includes eleven chapters, discussing various aspects of NMR spectroscopy; only some of the numerous covered topics are listed below. The starting point is a general overview of problems and challenges encountered in the gas phase. It shows how the dependence of NMR parameters (nuclear magnetic shielding and spin–spin coupling) on gas density and temperature gives insight into the theories of intermolecular effects and intramolecular motion. Microwave spectroscopy and molecular beam resonance methods, described next, provide valuable information about NMR parameters. For instance, the molecular beam resonance methods yield indirect nuclear spin–spin coupling tensors, while the nuclear spin-rotation tensors are especially important for the semi-empirical determination of absolute shielding in molecules.
Having the absolute shielding values in small molecules and the accurate magnetic moment of the proton makes it possible to extend our knowledge to the magnetic moments of other nuclei. The more accurate values of nuclear magnetic moments thus obtained are essential for NMR spectroscopy; moreover they can be applied for the measurement of shielding in routine research work. In addition, such measurements may be used to determine primary isotope effects in shielding. Molecules in the gas phase are of interest as objects of chemical analysis and modern NMR spectrometers are so fast that two-dimensional spectra can be successfully used to monitor the progress of chemical reactions. As an illustration, the gaseous decomposition of di-tert-butyl peroxide is elucidated in detail. As mentioned above, accurate values of the shielding in small molecules are needed for further applications, e.g. for the comparison with calculated shielding parameters or the determination of nuclear magnetic moments. This is important in the reviewed 17O and 33S studies, which are especially difficult because they require advanced NMR techniques for the detection of gaseous compounds when the concentration of molecules is low and in addition the natural abundances of oxygen-17 and sulfur-33 nuclei are low.
The discussion of the theoretical studies of NMR parameters begins with a short explanatory account of the methods used to determine accurate shielding constants. Particular attention is paid to the hierarchy of ab initio methods, because their use permits to improve systematically the results and to estimate the error bars. Next, the development of efficient theoretical methods needed to calculate zero-point vibration and temperature effects and the magnitude of these contributions to NMR parameters are discussed. The nuclear motion effects, related to the rotation and vibration of the molecule, have to be considered when the experimental data are compared with computed shielding or spin–spin coupling constants. Relativistic methods are required to determine reliable values of all the NMR parameters when there is a heavy atom in the molecule. Significant progress made in the last few years in the development and implementation of two- and four-component approaches which yield the relativistic values of NMR parameters, reflected by increasing accuracy of the results, is next reviewed.
Molecules are usually observed exploring the thermal equilibrium of nuclear magnetic moments, but the sensitivity of NMR spectroscopy is enormously increased after gas hyperpolarization. The hyperpolarization can be achieved by parahydrogen induced polarization (PHIP) for molecular hydrogen or by the optical pumping methods for noble gases like 3He or 129Xe. The hyperpolarized gases are utilized in a large variety of NMR experimental studies, with the most spectacular application in magnetic resonance imaging (MRI) of the lung. This non-invasive diagnostic method could be an excellent tool of modern medicine; unfortunately, there is a shortage of helium-3 on the market and for medical purposes the application of alternative gases like xenon-129 and fluorinated compounds must be also examined. The use of fluorinated gases in MRI can be accomplished without the hyperpolarization process.
We are pleased to present the first book which covers so many different aspects of gas phase NMR spectroscopy. We hope that this book will be helpful for everyone familiar with NMR methods, giving a better understanding of spectral parameters and more knowledge about the role and possible applications of gas phase NMR experiments.
Karol Jackowski and Michał Jaszuński