Atta-ur-Rahman / Choudhary / Wahab Solving Problems with NMR Spectroscopy

2. Auflage 2015
ISBN: 978-0-12-411613-9
Verlag: William Andrew Publishing
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 534 Seiten, Format (B × H): 152 mm x 229 mm

Solving Problems with NMR Spectroscopy, Second Edition, is a fully updated and revised version of the best-selling book. This new edition still clearly presents the basic principles and applications of NMR spectroscopy with only as much math as is necessary. It shows how to solve chemical structures with NMR by giving many new, clear examples for readers to understand and try, with new solutions provided in the text. It also explains new developments and concepts in NMR spectroscopy, including sensitivity problems (hardware and software solutions) and an extension of the multidimensional coverage to 3D NMR. The book also includes a series of applications showing how NMR is used in real life to solve advanced problems beyond simple small-molecule chemical analysis. This new text enables organic chemistry students to choose the most appropriate NMR techniques to solve specific structures. The problems provided by the authors help readers understand the discussion more clearly and the solution and interpretation of spectra help readers become proficient in the application of important, modern 1D, 2D, and 3D NMR techniques to structural studies. - Explains and presents the most important NMR techniques used for structural determinations - Offers a unique problem-solving approach for readers to understand how to solve structure problems - Uses questions and problems, including discussions of their solutions and interpretations, to help readers understand the fundamentals and applications of NMR - Avoids use of extensive mathematical formulas and clearly explains how to implement NMR structure analysis - Foreword by Nobel Prize winner Richard R. Ernst New to This Edition - Key developments in the field of NMR spectroscopy since the First Edition in 1996 - New chapter on sensitivity enhancement, a key driver of development in NMR spectroscopy - New concepts such as Pulse Field Gradients, shaped pulses, and DOSY (Diffusion Order Spectroscopy) in relevant chapters - More emphasis on practical aspects of NMR spectroscopy, such as the use of Shigemi tubes and various types of cryogenic probes - Over 100 new problems and questions addressing the key concepts in NMR spectroscopy - Improved figures and diagrams - More than 180 example problems to solve, with detailed solutions provided at the end of each chapter

Atta-ur-Rahman, Professor Emeritus, International Center for Chemical and Biological Sciences (H. E. J. Research Institute of Chemistry and Dr. Panjwani Center for Molecular Medicine and Drug Research), University of Karachi, Pakistan, was the Pakistan Federal Minister for Science and Technology (2000-2002), Federal Minister of Education (2002), and Chairman of the Higher Education Commission with the status of a Federal Minister from 2002-2008. He is a Fellow of the Royal Society of London (FRS) and an UNESCO Science Laureate. He is a leading scientist with more than 1283 publications in several fields of organic chemistry.

Zielgruppe

Senior and graduate chemistry students and organic, medicinal, and pharmaceutical chemists.

Autoren/Hrsg.

Atta-ur-Rahman

Choudhary, Muhammad Iqbal

Wahab, Atia-tul-

Fachgebiete

• Naturwissenschaften Chemie Chemie Allgemein Pharmazeutische Chemie, Medizinische Chemie
• Naturwissenschaften Chemie Organische Chemie
• Naturwissenschaften Chemie Analytische Chemie Massenspektrometrie, Spektroskopie, Spektrochemie

Inhaltsverzeichnis

The Basics of Modern NMR Spectroscopy: What is NMR? Instrumentation. Creating NMR Signals.

Spin-Echo and Polarization Transfer: Spin-Echo Formation in Homonuclear and Heteronuclear Systems. Cross-Polarization. Polarization Transfer in Reverse.

The Second Dimension: Data Acquisition in 2D NMR. Data Processing in 2D NMR. Plotting 2D Spectra.

Nuclear Overhauser Effect: nOe and Selective Population Transfer. Relaxation. Mechanism of nOe. Factors Affecting nOe. Some Practical Hints.

Important 2D NMR Experiments: Homo- and Heteronuclear J-Resolved Spectroscopy. Homonuclear and Heteronuclear Shift-Correlation Spectroscopy. Two-Dimensional Nuclear Overhauser Spectroscopy. Two-Dimensional Chemical Exchange Spectroscopy. Homonuclear Hartmann-Hahn Spectroscopy (HOHAHA), or Total Correlation Spectroscopy (TOCSY). Inverse NMR Spectroscopy. Inadequate.

The Third Dimension: Basic Philosophy. Types and Positions of Peaks in 3D Spectra.

Recent Developments in NMR Spectroscopy: Selective Pulses in Modern NMR Spectroscopy. One-Dimensional Experiments Using Soft Pulses. Heteronuclear Selective 1D NMR Experiments. Two-Dimensional Experiments Using Soft Pulses. Soft Excitation in Two Dimensions. Three-Dimensional Experiments Using Soft Pulses. Field Gradients.

Logical Protocol for Solving Complex Structural Problems: 3-Hydroxylupanine. (1). (+)-Buxalongifolamidine. (2). References. Subject Index.

Leseproben

Chapter 1

The Basics of Modern NMR Spectroscopy

Abstract

The basic principles of NMR spectroscopy, including the key components of NMR spectrometers such as magnets and probes, are described. How to make the best use of your NMR spectrometer through optimizing various instrumental parameters is also presented. Thirty penetrating questions and their well described answers help in understanding why NMR is so different from other spectroscopic techniques and how these fundamental differences make this technique a “work horse” in various fields of molecular sciences.

Keywords

radiofrequency

resonance

NMR-active nuclei

sensitivity and resolution

pulse Fourier transform NMR spectroscopy

superconducting magnets

NMR probes

shimming

probe tuning

deuterium lock

NMR tubes

Chapter Outline

1.1 What Is NMR? 1

1.1.1 The Birth of a Signal 3

1.2 Instrumentation 9

1.2.1 The Magnet 10

1.2.2 The Probe 11

1.2.3 Probe Tuning 14

1.2.4 Shimming 17

1.2.5 Deuterium Lock 22

1.2.6 Referencing NMR Spectra 22

1.2.7 NMR Sample Tubes 23

Solutions to Problems 25

References 33

1.1. What is NMR?

Nuclear magnetic resonance (NMR) spectroscopy is the study of molecules by recording the interaction of radiofrequency (Rf) electromagnetic radiations with the nuclei of molecules placed in a strong magnetic field. Zeeman first observed the strange behavior of certain nuclei when subjected to a strong magnetic field at the end of the nineteenth century, but practical use of the so-called “Zeeman effect” was made only in the 1950s when NMR spectrometers became commercially available.

Like all other spectroscopic techniques, NMR spectroscopy involves the interaction of the material being examined with electromagnetic radiation. Why do we use the word “electromagnetic radiation”? This is so because each ray of light (or any other type of electromagnetic radiation) can be considered to be a sine wave that is made up of two mutually perpendicular sine waves that are exactly in phase with each other, i.e., their maxima and minima occur at exactly the same point of line. One of these two sine waves represents an oscillatory electric field, while the second wave (that oscillates in a plane perpendicular to the first wave) represents an oscillating magnetic field – hence the term “electromagnetic” radiation.

Cosmic rays, which have a very high frequency (and a short wavelength), fall at the highest energy end of the known electromagnetic spectrum and involve frequencies greater than 3 × 1020 Hz. Radiofrequency (Rf) radiation, which is the type of radiation that concerns us in NMR spectroscopy, occurs at the other (the lowest energy) end of the electromagnetic spectrum and involves energies of the order of 100 MHz (1 MHz = 106 Hz). Gamma rays, X-rays, ultraviolet rays, visible light, infrared rays, microwaves and radiofrequency waves all fall between these two extremes. The various types of radiations and the corresponding ranges of wavelength, frequency, and energy are presented in Table 1.1.

Table 1.1

The Electromagnetic Spectrum

Radiation

Wavelength (nm) ?

Frequency (Hz) ?

Energy (kJ mol-1)

Cosmic rays

<10-3

>3 × 1020

>1.2 × 108

Gamma rays

10-1 to 10-3

3 × 1018 to 3 × l020

1.2 × l06 to 1.2 × l08

X-rays

10 to l0-1

3 × 1016 to 3 × 1018

1.2 × 104 to 1.2 × 106

Far ultraviolet rays

200 to 10

1.5 × 1015 to 3 × 1016

6 × 102 to 1.2 × l04

Ultraviolet rays

380 to 200

8 × 1014 to 1.5 × 1015

3.2 × 102 to 6 × 102

Visible light

780 to 380

4 × 1014 to 8 × l014

1.6 × l02 to 3.2 × 102

Infrared rays

3 × l04 to 780

1013 to 4 × 1014

4 to 1.6 × 102

Far infrared rays

3 × 105 to 3 × 104

1012 to 1013

0.4 to 4

Microwaves

3 × l07 to 3 × 105

1010 to 1012

4 × 10-3 to 0.4

Radiofrequency (Rf) waves

1011 to 3 × 107

106 to 1010

4 × 10-7 to 4 × 10-3

Electromagnetic radiation also exhibits behavior characteristic of particles, in addition to its wave-like character. Each quantum of radiation is called a photon, and each photon possesses a discrete amount of energy, which is directly proportional to the frequency of the electromagnetic radiation. The strength of a chemical bond is typically around 400 kJ mol-1, so that only radiations above the visible region will be capable of breaking bonds. But infrared, microwaves, and radio-frequency radiations will not be able to do so.

Let us now consider how electromagnetic radiation can interact with a particle of matter. Quantum mechanics (the field of physics dealing with energy at the atomic level) stipulates that in order for a particle to absorb a photon of electromagnetic radiation, the particle must first exhibit a uniform periodic motion with a frequency that exactly matches the frequency of the absorbed radiation. When these two frequencies exactly match, the electromagnetic fields can “constructively” interfere with the oscillations of the particle. The system is then said to be “in resonance” and absorption of Rf energy can take place. Nuclear magnetic resonance involves the immersion of nuclei in a magnetic field, and then matching the frequency at which they are precessing with electromagnetic radiation of exactly the same frequency so that energy absorption can occur.

1.1.1. The Birth of a Signal

Certain nuclei, such as 1H, 2H, 13C, 15N, and 19F, possess a spin angular momentum and hence a corresponding magnetic moment µ, given by

=?h[I(I+1)]1/22p

(1.1)

where h is Planck’s constant and ? is the magnetogyric ratio (also called gyromagnetic ratio). When such nuclei are placed in a magnetic field B0, applied along the z-axis, they can adopt one of 2I + 1 quantized orientations, where I is the spin quantum number of the nucleus (Fig. 1.1). Each of these orientations corresponds to a certain energy level:

=-µzB0=-m1?hB02p

(1.2)

where m1 is the magnetic quantum number of the nucleus and µz is the magnetic moment. In the lowest energy orientation, the magnetic moment of the nucleus is most closely aligned with the external magnetic field (B0), while in the highest energy orientation it is least closely aligned with the external field. Organic chemists are most frequently concerned with 1H and 13C nuclei, both of which have a spin quantum number (I) of 1/2, and only two quantized orientations are therefore allowed, in which the nuclei are either aligned parallel to the applied field (lower energy orientation) or antiparallel to it (higher energy orientation). The nuclei with only two quantized orientations are called dipolar nuclei. Transitions from the lower energy level to the higher energy level can occur by absorption of radiofrequency radiation of the correct frequency. The energy difference ?E between these energy levels is proportional to the external magnetic field (Fig. 1.2), as defined by the equation ?E = ?hB0/2p. In frequency terms, this energy difference corresponds to

0=?B02p

(1.3)

Figure 1.1Representation of the precession of the magnetic moment about the axis of the applied magnetic field, B0. The magnitude µz, of the vector corresponds to the Boltzmann excess in the lower energy (a) state.

Figure 1.2The energy difference between the two energy states ?E increases with increasing value of the applied magnetic field B0, with a...

Über Autor(innen)

Choudhary, Muhammad Iqbal
Muhammad Choudhary, PhD, is a Professor of the International Center for Chemical and Biological Sciences, (H. E. J. Research Institute of Chemistry and Dr. Panjwani Center for Molecular Medicine and Drug Research), University of Karachi, Pakistan. He is a member of the Royal Society of Chemistry, London; American Chemical Society; International Union of Pure and Applied Chemistry (IUPAC); American Society of Pharmacology; New York Academy of Sciences; Federation of Asian Chemical Societies (FACS); and he serves on the executive board of the Asian Network of Research on Anti Diabetic Plants (ANRAP). He is a recipient of the National Book Foundation's Prize for Chemistry and the Economic Cooperation Organization (ECO) Award in Education, 2006, given by the President of Azerbaijan. He has published 24 books, more than 570 papers, and 20 patents.

Atta-ur-Rahman
Atta-ur-Rahman, Professor Emeritus, International Center for Chemical and Biological Sciences (H. E. J. Research Institute of Chemistry and Dr. Panjwani Center for Molecular Medicine and Drug Research), University of Karachi, Pakistan, was the Pakistan Federal Minister for Science and Technology (2000-2002), Federal Minister of Education (2002), and Chairman of the Higher Education Commission with the status of a Federal Minister from 2002-2008. He is a Fellow of the Royal Society of London (FRS) and an UNESCO Science Laureate. He is a leading scientist with more than 930 publications in several fields of organic chemistry.

Wahab, Atia-tul-
Atia-tul-Wahab, PhD, is Assistant Professor at the Dr. Panjwani Center for Molecular Medicine and Drug Research (International Center for Chemical and Biological Sciences), University of Karachi, Karachi, Pakistan. She is trained as a structural and synthetic organic chemist, and received additional training in structural biology in the laboratory of Nobel Laureate Prof. Dr. Kurt Wüthrich at The Scripps Research Institute. She is now establishing the first Structural Biology Laboratory of Pakistan at the PCMD.