High Resolution Nmr Spectroscopy Understanding Molecules And Their Electronic Structures

The progress in nuclear magnetic resonance (NMR) spectroscopy that took place during the last several decades is observed in both experimental capabilities and theoretical approaches to study the spectral parameters. The scope of NMR spectroscopy for studying a large series of molecular problems has notably broadened. However, at the same time, it requires specialists to fully use its potentialities. This is a notorious problem and it is reflected in the current literature where this spectroscopy is typically only used in a routine way. Also, it is seldom used in several disciplines in which it could be a powerful tool to study many problems. The main aim of this book is to try to help reverse these trends. This book is divided in three parts dealing with 1) high-resolution NMR parameters; 2) methods for understanding high-resolution NMR parameters; and 3) some experimental aspects of high-resolution NMR parameters for studying molecular structures. Each part is divided into chapters written by different specialists who use different methodologies in their work. In turn, each chapter is divided into sections. Some features of the different sections are highlighted: it is expected that part of the readership will be interested only in the basic aspects of some chapters, while other readers will be interested in deepening their understanding of the subject dealt with in them. Shows how NMR parameters are useful for structure assignment as well as to obtain insight on electronic structures Emphasis on conceptual aspects Contributions by specialists who use the discussed methodologies in their everyday work

Polarization propagators (PP) are powerful theoretical tools that allow carrying out a deep analysis of the electronic mechanisms underlying any molecular response property. The inner projections of the PP and contributions from localized orbitals within the PP approaches described in were developed to fully take advantage of this power of analysis for the study of NMR spectroscopic parameters. They are based on the use of localized molecular orbitals (LMOs) related to chemically intuitive concepts to decompose the mathematical expression of these parameters into coupling pathways or shielding pathways. Each of them may be furthermore decomposed into two new objects: (i) perturbators, which give information on the efficiency of a given magnetic perturbation to produce local excitations and (ii) the principal propagator matrix elements which provide deep understanding on the way perturbations are transmitted within the electronic framework of the molecule under study. Applications are presented in , both within semiempirical and ab initio approaches: the Karplus rule, a general analysis of the signs of J couplings, σ–π decomposition, hyperconjugative effects in transmission of J couplings, general features of 1J couplings, and intermolecular couplings in hydrogen-bonded systems. All applications were especially selected to cover examples in which qualitative physical insight can be gained.

In this introductory chapter are described briefly the aims and scopes of this multiauthor book. One of the main ideas behind them is helping its readership to understand some approaches for extracting information on subtle chemical interactions, defining trends of such parameters from either measured or calculated high-resolution NMR parameters (indirect nuclear spin–spin coupling constants and nuclear magnetic shielding constant) without needing to acquire solid backgrounds on quantum chemistry. However, in all cases, the discussed ideas are based on solid grounds, and adequate references are quoted for readers interested in a better understanding of the basic concepts lying behind such approaches. It is highlighted that such discussions are restricted only to a few methods in order to both avoid overlapping this book with descriptions found in the current literature and to avoid lengthening this book beyond reasonable limits. It was also considered pertinent to include chapters dealing with concepts whose importance are increasing rapidly in modern NMR spectroscopy like for instance, relativistic effects on NMR parameters in compounds containing heavy atoms, and NMR spectroscopy in paramagnetic species, pNMR. Also, it was considered opportune including in this book excellent examples showing how very good excellent sets of experimental values can provide interesting insight in some chemical interactions.

The Ramsey theory of nuclear magnetic resonance spectral parameters has been reformulated in terms of current densities induced in the electron cloud of a molecule by an external magnetic field and intramolecular magnetic dipoles at the nuclei. Conditions for invariance of nuclear magnetic shielding and nuclear spin–spin coupling tensors, in gauge transformations of the vector potentials associated to the magnetic perturbations, have been expressed via quantum mechanical sum rules, also providing constraints for charge conservation. It is shown that the combined use of current density and property density maps provides valuable tools for the rationalization of magnetic response.

This chapter is devoted to the 1–3JCCs and to the factors influencing their magnitude. The experimental and calculated J data presented in the subsequent parts of the chapter are arranged with the thought of showing how hybridization, substituent electronegativity, the complex and hydrogen bond formation, and geometry of the compound bear on the JCC magnitude and which range of changes can be expected for a given type of coupling when all these effects are taken into account. The subsequent sections are devoted to the couplings across single, double, and triple CC bonds and to the couplings in aromatic and heteroaromatic systems and in the compounds of biological importance. It is also shown that at the present level of theory, it is possible in many cases to reproduce the experimental J values very exactly, achieving one-to-one correspondence.

Spin–spin coupling constant J provides decisive data for organic compound characterization. This electron-mediated coupling is usually taught as transmitted between covalently bonded magnetic atoms. However, this physical interaction between nuclear spins is much more complex than that with regard to chemical bonding concept. Independent experimental and theoretical studies related to small organic and organometallic species (molecular mass below 2000gmol−1) have highlighted the existence of J couplings operating via clearly nonbonded interactions and known as “through-space” couplings. Interactions of this type are frequently reported and couplings involving 19F, 13C, 77Se, 15N, 31P, or 1H in hydrogen bonding are now clearly identifiable. This chapter aims to clarify this phenomenon often poorly known by routine users of NMR. Thus, nonbonded spin couplings can provide critical data for studying and determining molecular structures both in solution and in the solid state. This is illustrated herein through selected examples picked in different families of small organic and organometallic compounds.

This chapter describes briefly chemical shifts (or nuclear magnetic shielding constants) and indirect spin–spin coupling constants. They are well known as powerful tools for studying several molecular properties which are very important in different branches of the broad field of molecular sciences. The present description is oriented to an interdisciplinary audience and therefore it is expected that it can be followed for readers without strong backgrounds either in mathematics or physics. After a short revision of basic concepts, a qualitative method devised to extract information on electronic molecular structures is described. This aim is achieved employing this qualitative method for relating such parameters known in different series of compounds with several common chemical interactions. Since both types of NMR parameters present second-rank tensor properties, it is discussed how such property is affected in molecules measured in isotropic phase. Anybody with mathematical and physical background would answer immediately, “in isotropic phase is only observed one-third of the respective tensor trace.” However, in molecules that trace depends on the relative orientation of the Principal Axes System and bonds associated to the atom whose nuclear magnetic shielding is studied, or to the straight line connecting a pair of coupled nuclei. To describe these effects in this chapter is coined the expression “the geometric effect” to identify them. The same expression is also employed in . A list of exercises and appropriate references are included at the end of this chapter.

For heavy atoms, and for molecules with heavy atoms, a theoretical description of the electronic structure needs to consider the finite speed of light and Einstein’s special relativity. This chapter provides a brief introduction to special relativity and to relativistic methods in quantum chemistry. It is shown how these methods can be used to calculate NMR chemical shifts and indirect spin–spin coupling constants (J-coupling). A number of examples are discussed where relativistic effects have a significant influence on these NMR parameters. In some cases for example, for indirect spin–spin coupling involving 199Hg, relativistic effects may be larger than the total value calculated with a nonrelativistic method.