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Chapter

Cover NMR Spectroscopy in Inorganic Chemistry

Dynamic NMR spectroscopy  

This chapter focuses on dynamic NMR spectroscopy. Intra- and intermolecular dynamic processes affect the appearance of NMR spectra. When the dynamic process is slow, separate resonances are observed for each site. When the process is fast, a resonance at the weighted average chemical shift is seen. Exchange not only affects the chemical shift but also the couplings seen. If the dynamic process is intermolecular exchange, decoupling is observed due to the breaking of the bonds between the exchanging groups. If the process is intramolecular a weighted average coupling constant is seen to all the neighbouring spins involved. Dynamic processes can be studied by both variable temperature spectroscopy and by EXSY NMR, allowing the intimate mechanism of the dynamic process to be determined.

Chapter

Cover NMR Spectroscopy in Inorganic Chemistry

Experimental methods: pulses, the vector model, and relaxation  

This chapter examines how an NMR spectrum is recorded and how we can tailor the spectroscopic experiment to maximize the information we are interested in. It begins by looking at how the NMR signal is created, and NMR spectra acquired. Traditionally, this was done using the continuous wave method; modern spectrometers use the pulse Fourier transform method. Meanwhile, the vector model provides a simple means to visualize the effect of an excitation pulse on the NMR magnetization and the evolution of the magnetization vector in the rotating frame and can be used to understand simple pulse sequences. Relaxation is an important consideration when using excitation pulses and sample averaging, since we must allow the system to return to equilibrium before starting the excitation sequence again. Relaxation can also provide valuable structural information and dynamic information about a sample that is not accessible through analysis of the chemical shifts and couplings.

Chapter

Cover NMR Spectroscopy in Inorganic Chemistry

Factors influencing the chemical shift and coupling constants  

This chapter surveys the chemical factors that influence the chemical shift and the magnitude of coupling constants. Shielding is influenced by the fields generated by circulation of ground state electrons around the nucleus (diamagnetic term) and by electrons mixed in from excited states (the paramagnetic term). The diamagnetic contribution is usually small and dominates for light elements, resulting in (usually) small chemical shift ranges for these elements. The paramagnetic term dominates for heavier elements and is often large, resulting in large chemical shift ranges for these elements. The chemical shift is influenced by many, often competing factors, including oxidation state, electronegativity, coordination number, and bonding to other atoms. Meanwhile, scalar coupling constants are influenced by similar factors and by geometric factors, particularly interbond and dihedral angles. If all other factors are kept constant, scalar couplings between different isotopes of the same elements scale with gyromagnetic ratio.

Chapter

Cover NMR Spectroscopy in Inorganic Chemistry

Fundamentals  

This chapter provides an overview of nuclear magnetic resonance (NMR) spectroscopy, which is commonly first encountered in organic chemistry, and attention is focused on the NMR spectroscopy of a single element, hydrogen. There are, however, many other elements that have isotopes with nuclear spin; if these elements are taken into account, NMR becomes perhaps the most important spectroscopic technique today for the characterization of inorganic compounds in solution and is of growing importance in the solid state. In solution, spin ½ nuclei give well-resolved, sharp resonances for each NMR active site in the molecule, while in the solid state, cross-polarization results in an increase in sensitivity and magic angle spinning results in an increase in resolution. The rules for interpreting NMR spectra depend only on the nuclear spins present. The chapter then presents some basic theory of NMR and a formalism for the prediction of NMR spectra in solution.

Book

Cover Inorganic Spectroscopic Methods
Inorganic Spectroscopic Methods provides an introduction to common spectroscopic techniques and interpretation of spectra, and their application to inorganic-based systems. The approach taken is aimed at the application of the techniques and interpretation of the spectra obtained. Beginning with an introductory description of electromagnetic radiation and its interaction with matter, each subsequent chapter covers the physical basis of related spectroscopic methods (vibrational, resonance, UV-visible spectroscopy, mass spectrometry) and their applications typical in inorganic compounds. The final chapter offers an integrated approach to the identification of unknown materials—putting together the techniques discussed.

Chapter

Cover Inorganic Spectroscopic Methods

Introduction  

This introductory chapter provides an overview of spectroscopy, which can be defined as the study of the interaction between radiation and matter. Since the spectroscopic techniques described in this book, with the exception of mass spectrometry, involve such an interaction, it begins by considering electromagnetic radiation and some of its properties. To observe a spectrum, the compound must interact either with the electric or magnetic component of the applied radiation. The chapter then explains molecular energy levels, population distributions, selection rules, and time-scales. It also details the basic components of a scanning spectrometer. It is more common these days for an alternative method to be used which essentially allows all the data for the complete spectrum to be recorded very rapidly. Such techniques use Fourier transform methods to mathematically de-convolute individual absorptions at particular energies (or frequencies) from data containing all such energies which are recorded as an interference pattern.

Chapter

Cover Inorganic Spectroscopic Methods

Mass spectrometry  

This chapter focuses on mass spectrometry. Although mass spectrometry is not strictly a form of spectroscopy, in that it does not involve the interaction of radiation with matter, it is nonetheless a very useful method for identifying unknown compounds. One of the reasons it is so useful is that it can provide information about the relative molecular mass of a compound. In fact, mass spectroscopy will usually provide a lot more information than just the mass of compounds; it is frequently possible to determine which ligands or groups of atoms are bonded together from the way the compound breaks up or fragments during the experiment. Mass spectroscopy requires that the molecule of interest is charged; this then provides a basis for separating the different ions due to their mass-to-charge ratio. The chapter then looks at ionization methods and the interpretation of mass spectra.

Book

Cover NMR Spectroscopy in Inorganic Chemistry

Jonathan A. Iggo and Konstantin V. Luzyanin

NMR Spectroscopy in Inorganic Chemistry offers a non-mathematical grounding in the physics of NMR spectroscopy and explores why the spectra look the way they do, providing a useful collection of NMR examples and trends in NMR parameters from inorganic chemistry. The first chapter covers the fundamentals. The second chapter looks at structure determination. The third chapter covers dynamic processes and NMR. The last chapter is about the solid state.

Chapter

Cover NMR Spectroscopy in Inorganic Chemistry

Polarization transfer and 2-D NMR spectroscopy  

This chapter evaluates polarization transfer and 2-D NMR spectroscopy. Polarization transfer, the exchange of excitation between spins, can be used to establish connectivity through scalar coupling interactions, molecular conformation through the nuclear Overhauser effect, and to study exchange. There are many 2-D experiments available, of which the most useful to the inorganic chemist are: correlation spectroscopy (COSY), heteronuclear correlation spectroscopies (HMQC, HSQC, and HMBC), and nuclear Overhauser effect spectroscopy (NOESY and HOESY). Interpretation of 2-D spectra can at first seem challenging, but with a little practice, becomes straightforward in most cases. Meanwhile, hyperpolarization refers to techniques such as DNP, OPNMR, or use of para-hydrogen, in which highly polarized spins—i.e. spins in which the population difference between NMR energy levels has been greatly increased by some physical or chemical process—are introduced into the molecule before the NMR experiment, resulting in greatly enhanced NMR signals.

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Cover Inorganic Spectroscopic Methods

Putting it all together  

This chapter presents some worked problems where a number of spectroscopic techniques are brought to bear on the same problem. It is rarely sensible, and frequently not possible, to rely on a single spectroscopic method to provide an unambiguous answer to the identity of an unknown product. Usually, the more techniques that are used, the more confident one can be in assigning the identity of a compound and its potential structure. One problem concerns the identification of a gaseous product which results from the reaction of (Me2HSi)2S with methanol. The presence of a variety of protons makes nuclear magnetic resonance (NMR) spectroscopy a likely candidate, provided one can find a solvent in which this gas is soluble. For a solid sample, it is possible to try and obtain the molecular composition by elemental analysis. For a volatile sample, mass spectrometry is more straightforward.

Chapter

Cover Inorganic Spectroscopic Methods

Resonance spectroscopy  

This chapter examines resonance spectroscopy. Nuclear magnetic resonance (NMR) spectroscopy is probably the single most widely used and important physical technique available to the modern practical chemist. This is because of its wide applicability, its relative ease of use, and the amount of chemical and structural information that can be obtained from its use. The technique is frequently associated only with proton and carbon nuclei but there are many other elements to which it can potentially be applied. The chapter then looks at nuclear quadrupole resonance (NQR) spectroscopy. In NMR experiments, an external field is applied to cause a splitting of the normally degenerate nuclear spin states, but this is not necessary in NQR experiments because when there is an asymmetric charge distribution within the molecule, a molecular electric field gradient is generated. The chapter also considers electron spin resonance (ESR) spectroscopy.

Chapter

Cover NMR Spectroscopy in Inorganic Chemistry

The solid state  

This chapter studies the solid state. Although NMR spectroscopy in solution continues to be the most widely used magnetic resonance spectroscopic technique, NMR in the solid state is rapidly growing in importance. In addition to the problems of low sensitivity and low natural abundance for many important nuclides encountered in solution, NMR in the solid state faces additional challenges due to interactions such as dipolar coupling, chemical shift anisotropy, and quadrupolar interactions that are averaged out in solution by molecular tumbling but remain in the solid state due to the restricted mobility of the molecules/ions. Cross-polarization can be used to improve the signal to noise in spectra by allowing more scans to be acquired for signal averaging in the same amount of time. Analysis of CP build-up rates also provides information about things such as internuclear distances and molecular motions.

Chapter

Cover NMR Spectroscopy in Inorganic Chemistry

Structure determination  

This chapter discusses the interpretation of NMR spectra to deduce the structure of the compound. When analysing an NMR spectrum, we should consider the groups of spins present, not individual spins. It is important to look for symmetrical patterns to identify the chemical shifts (the number of chemically different groups of spins of the nuclide/element observed) present in the molecule. We should then use the coupling pattern/multiplicity and coupling constants to identify the neighbouring groups and number of spins in each neighbouring group responsible for each coupling pattern. Coupling may be homonuclear—the coupling partner is of the same nuclide, so its resonance appears in the same spectrum —or heteronuclear—the coupling partner is of a different nuclide, so its resonance appears in 'the other' NMR spectrum. The rules for interpreting homo- and heteronuclear couplings are the same.

Chapter

Cover Inorganic Spectroscopic Methods

UV-visible spectroscopy  

This chapter evaluates UV–visible spectroscopy. Within inorganic chemistry, the field of study often associated with UV–visible spectroscopy is that of the coloured transition metal complexes. The energies associated with transitions between different arrangements of valence electrons falls within the ultraviolet (UV) and visible region of the electromagnetic spectrum. Just as the most popular form of vibrational spectroscopy is usually referred to by the region it occurs in—the infrared—so the most popular form of electronic spectroscopy is usually known as UV-visible spectroscopy and is in essence the study of the transitions involved in the rearrangements of valence electrons. The chapter then looks at metal–metal transitions, charge-transfer transitions, and ligand-centred transitions.

Chapter

Cover Inorganic Spectroscopic Methods

Vibrational spectroscopy  

This chapter discusses vibrational spectroscopy, which was one of the first spectroscopic techniques to be widespread in its use, particularly infrared or IR spectroscopy—so called because of the region of the electromagnetic spectrum used. These forms of spectroscopy are tied up with changes in the vibrational state of molecules. The basis of the IR experiment is to pass infrared radiation through a thin sample of compound and measure which energies of the applied infrared radiation are transmitted by the sample. Infrared spectra can be recorded for solids, liquids, solutions, and gases using a variety of different sampling arrangements, but are probably most commonly recorded as liquids or solutions or of a solid which has been ground up with a mulling agent and pressed between two alkali halide plates. The chapter then considers a second form of vibrational spectroscopy—Raman spectroscopy—which provides similar information but has different physical basis.