High Resolution NMR Spectroscopy: Understanding Molecules and their Electronic Structures

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• Publisher: Elsevier
• Year: 2013
• ISBN:
  9780444594143
• Topic: Scienze fisiche
• Subtopic: Chimica fisica e teoretica

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

Chapter 1

Introduction

Rubén H. Contreras, Physics Department, Faculty of Exact and Natural Sciences, University of Buenos Aires and IFIBA, Argentina

Abstract

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.

Keywords

Scope and aims; Qualitative model; Exchange interactions; FC term; PSO term; Chemical shifts

During the past decades, there has been a very important progress in nuclear magnetic resonance spectroscopy (NMR). This progress took place both in experimental instrumentation and in theoretical approaches that help extracting invaluable molecular information from the spectral parameters commonly known as “high-resolution NMR parameters” (HR-NMR parameters). In this way, the scope of this spectroscopy for studying a large series of molecular problems has notably broadened. At first sight, this scenario could lead many people to wonder why a new book dealing with HR-NMR parameters is worthwhile when there are plenty of books dealing with related subjects. However, looking at that scenario from inside, it is noted that NMR spectroscopy has become a quite specialized field that requires of specialists if its potential is to be used at its best, which is the aim of this book. This is a notorious problem, and it is reflected in many scientific papers published in the current literature where this spectroscopy is only used on a routinely base. In several disciplines, where it could be used as a powerful tool to study many molecular problems, it is seldom used, narrowing notably its potential scope.

Taking into account comments made above, the main aim of this multiauthor book is intending to help reversing, at least in part, this trend, broadening notably the present scope for employing HR-NMR parameters to study a variety of molecular problems. For undertaking this task, it would be adequate to analyze first what are the main problems defining the mentioned situation. Perhaps the main reason is not difficult to be sorted out. To extract at its best possibilities electronic molecular information from HR-NMR parameters, it is required to have an interdisciplinary team of specialists to be able to (a) acquire the spectra using modern and sophisticated techniques. (b) After that comes the often not easy task of extracting and assigning HR-NMR parameters to atoms belonging to the molecular system under study. (c) From HR-NMR parameters thus found it is necessary to employ adequate approaches to extract, as much as possible, information on the electronic structure of the molecular system being studied. Without forgetting points (a) and (b), in this book are described few useful approaches to study aspects connected with point (c). The ability to extract in-depth electronic structure information from HR-NMR parameters requires of mathematical and physical backgrounds that are seldom acquired by undergraduate students in several disciplines. In fact, according to this editor’s own experience, it is observed that for graduates in several disciplines to face either a rather involved equation or to deal with a tensor property is something similar to be confronted to a monster. For this reason, in this book, emphasis is placed on points like this: even complicated equations are rather easy to be “read physically” after a short and adequate training is acquired, which can easily be achieved even without having a sound background either in Mathematics or in Physics. Since it is expected that this book be adequate as a textbook for either advanced undergraduate or graduate student courses in disciplines like Biology, different branches of Chemistry, Physics, or Material Sciences, all authors were asked to explain ideas requiring complex concepts in the easiest possible way keeping at the same time, a sound background.

When organizing this book the following important point was also taken into account. In many laboratories where NMR spectroscopy is not an end in itself but just a powerful tool to investigate a large series of molecular problems, it is important to obtain insight on trends of HR-NMR parameters in terms of chemical interactions affecting them. Generally, in this type of laboratories that task is usually more important than just obtaining accurately calculated values that agree well with their experimental counterparts. That is the reason why in several parts of this book HR-NMR parameter trends are described employing either qualitative or approximate approaches. In other parts of this book, some other important concepts like that of the relationship between the transmission of the FC interaction and the Fermi hole (see Ref. [7] in Chapter 8) is used to rationalize several spin–spin coupling constant (SSCC) trends. On this line of thought, it is very easy to realize that any hyperconjugative interaction becomes a kind of “carrier” for the spin information associated with the FC term. In turn, this conclusion helps to identify, by just observing the general configuration of a molecular system, if the FC interaction of a given SSCC is transmitted through more than one pathway between two given isotopes. This physical understanding of the Fermi hole can also lead to an easy conceptual analysis on the sign of several types of couplings mainly transmitted by exchange interactions. One conspicuous case is that of geminal couplings where, for the same types of isotopes, it is experimentally known that in some cases, it is positive and in some other cases, negative. This task is notably achieved if discussions presented in Chapter 5 are followed in detail. From that chapter, it is easy to understand that in a given molecule the overlap between two adjacent bonds (or one bond and one nonbonding electron pair) is negative at the position of their common nucleus. This indicates that interactions favoring the corresponding positive electronic cloud algebraically increase the FC term of the geminal coupling under consideration and vice versa.

Several other FC contributions that can easily be rationalized on similar grounds are briefly cited here, like the phenomenon reported many years ago, about the FC transmission by an intermediate fragment; the long-range transmission of the FC term by sequences of concatenated hyperconjugative interactions and the different signs that can be observed in J_FH SSCCs mainly transmitted through space. A point that should be stressed when performing a qualitative analysis of this type is this one: in all cases when explaining concepts in easy terms, much care is taken in keeping all explanations on sound grounds.

In each chapter is included a “References” section, which includes some seminal works, some basic references and others that could satisfy highly motivated readers. However, an important point that should be taken into account when reading this book is this: quoted references by no means cover exhaustively the current literature corresponding to each chapter. The reason is that each chapter does not constitute a kind of a review article; in all cases, all authors intended to cover didactic aspects of each chapter contents instead of citing exhaustively the corresponding current literature. It is stressed that a huge amount of articles were not quoted not because they were considered “second rate,” but simply because those which were quoted referred adequately to the subject under consideration. Besides, all in all, special care was taken for the length of each chapter, not to exceed reasonable limits.

Each chapter includes an exercise section that could help understanding different concepts. Additionally, such section will be particularly useful if this book is adopted as a textbook in a course either for advanced undergraduate or for graduate students. It is important to note that the levels of those exercises are not uniform. It is expected that, when using this book as a textbook, the advice of each tutor or professor will be of particular relevance when addressing such exercises and in using them in the most appropriate way.

It is remarked that no chapter of this book deals with NMR parameters for compounds measured in anisotropic phase. This does not mean that this field is not adequately appreciated as a very valuable one whose importance is expected to continue increasing notably during the next few years. Similar advice can be made about NMR relaxation times. Also, it is important to remark that no chapter is devoted neither to describe nor to compare theoretical approaches for calculating HR-NMR parameters. Such apparent omission originates in these three important reasons: (a) the goal of avoiding significant overlap of the contents of this book with any other book dealing with HR-NMR parameters; (b) the fact that in the current literature there are excellent books and review articles dealing with this subject suggested that their adequate citations in several chapters of this book would be the best way to proceed; (c) the constraint on the length of the whole book to be kept within reasonable limits.

Chapters are written by different well-known specialists that use different methodologies and approaches in their everyday scientific works. Each chapter is divided into sections helping in this way the part of the readership that could be interested only on some basic aspects of a given subject. In such case, they could skip some sections and concentrate in those parts that the reader considers more appealing for her/his studies. On the other hand, interest can arise in going further on several other parts of the subject being read. In that case, interested readers could resort to the basic literature quoted at the end of the respective chapter. Finally, a few cross-references between chapters are marked alongside the book.

Besides comments made above, in this introductory chapter, succinct descriptions of aims and contents for different chapters are given. However, such comments do not necessarily follow the chapters’ ordering. Hence, this outlook is expected to highlight notably the coherence and the unity of the whole book, while remarking some generalized interesting conclusions. Whenever considered convenient, some relevant comments are included describing in simple terms how chemical shifts and coupling constants can be powerful indicators of some chemical interactions being present in the molecular system under consideration.

As mentioned above, all authors took care of presenting each subject taking into account didactic aspects. In this editor’s opinion, the goal has been successfully achieved. As a complementary point, most authors agreed with the editor about the usefulness of opening adequate channels for asking questions related to this book. The suggested e-mail contact is [email protected].

First, Chapter 2 starts describing some basic aspects of electron–nucleus magnetic interactions originating the HR-NMR parameters known as nuclear magnetic shielding constants, σ(A), and indirect spin–spin coupling constants, J_AB. Only aspects of these parameters corresponding to “light atoms” are considered in that chapter. Besides, it is assumed that the molecular system under study corresponds to a “closed shell” one. Furthermore, only the case where these parameters are measured in an isotropic phase is taken into account.

The expression “light atoms” is used here to denote atoms whose parameters are described correctly within the framework of nonrelativistic quantum chemistry. A very didactic introduction to relativistic quantum chemistry is presented in Chapter 4. This also includes its application to HR-NMR parameters. It is highly recommended for students dealing with organometallic compounds to study Chapter 4 carefully.

Although nowadays most HR-NMR spectroscopy applications correspond to closed-shell molecular systems, it is expected that in the near future this spectroscopy will also be applied to study, in an extensive way, paramagnetic compounds. For this reason, when planning the organization of this book, it was considered important to include Chapter 3, entitled “Chemical Shift in Paramagnetic Systems,” which is dubbed as pNMR. This is a very clearly written chapter, where an interesting set of references is given. It is noted that this subject is seldom described in the current literature, and for this reason, it was considered convenient to include a reasonable exhaustive references set in Chapter 3.

The first HR-NMR parameter mentioned above, σ(A), is closely related to chemical shift usually denoted by δ(A) for the same atomic nucleus A. This quantity is contributed by two different terms, that is, its diamagnetic, σ^D(A), and its paramagnetic, σ^P(A) parts. They are, respectively, first- and second-order terms, indicating with this terminology that, within the Raleigh–Schrödinger perturbation theory, the order that can be employed to calculate each of them. The second type of HR-NMR parameters mentioned above is observed between a pair of magnetic nuclei where the magnetic interaction between them is transmitted through the molecular electronic system. Within the nonrelativistic theory they are contributed, when measured in isotropic phase, by four terms, J(A,B) = J^FC(A,B) + J^PSO(A,B) + J^SD(A,B) + J^DSO(A,B), where only the first one, FC, is a scalar quantity (isotropic), while the only first-order quantity is J^DSO(A,B), which is an anisotropic contribution, that is, a second rank tensor quantity. In some chapters, J(A,B) is referenced as J coupling constants and in other chapters is employed the common acronym SSCCs to designate them. In Chapter 7, this acronym is changed to NSSC.

Both types of parameters correspond to second rank tensors. The only exception to this last assertion is the FC term, while being a second-order quantity, it is isotropic. As already mentioned, Chapter 2 is limited to discussing HR-NMR parameters measured in an isotropic phase. A particular question seems relevant at this point, about physical properties having second rank tensor character: what is it observed when measurements of that property are carried out in an isotropic phase? Everybody having acquired basic training in Mathematics and/or Physics is expected to answer “in isotropic phase only 1/3 of its trace is observed.” Yes, this answer is correct; however, it is important to take into account that HR-NMR parameters are molecular parameters and most molecules are anisotropic. This fact originates a subtle effect. For instance, the trace of a given σ(A) tensor depends on the angle formed between its “principal axes system,” PAS, and one of the bonds containing the atom whose nuclear magnetic shielding constant is sought. This property is not frequ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributors
• Chapter 1. Introduction
• Chapter 2. Brief Account of Nonrelativistic Theory of NMR Parameters
• Chapter 3. Chemical Shift in Paramagnetic Systems
• Chapter 4. Relativistic Effects on NMR Parameters
• Chapter 5. The Polarization Propagator Approach as a Tool to Study Electronic Molecular Structures from High-Resolution NMR Parameters
• Chapter 6. Analysis of Contributions to Spin–Spin Coupling Constants by the Natural J-Coupling Method
• Chapter 7. Electronic Current Densities Induced by Magnetic Fields and Nuclear Magnetic Dipoles: Theory and Computation of NMR Spectral Parameters
• Chapter 8. Transmission Mechanisms of the Fermi-Contact Term of Spin–Spin Couplings
• Chapter 9. Nonbonded Indirect Nuclear Spin–Spin Couplings (J Couplings “Through-Space”) for Structural Determination in Small Organic and Organometallic Species
• Chapter 10. Chemical Shift Trends in Light Atoms
• Chapter 11. Application of 13C–13C Spin–Spin Couplings in Structural Studies on Organic Compounds
• Index