Interpretation of Organic Spectra

Yong-Cheng Ning

• Publisher: Wiley
• Year: 2011
• ISBN:
  9780470828311
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Spectroscopy & Spectrum Analysis

Although there are a number of books in this field, most of them lack an introduction of comprehensive analysis of MS and IR spectra, and others do not provide up-to-date information like tandem MS. This book fills the gap. The merit of this book is that the author will not only introduce knowledge for analyzing nuclear magnetic resonance spectra including 1 H spectra (Chapter 1), 13 C spectra (Chapter 2) and 2D NMR spectra (Chapter 3), he also arms readers systemically with knowledge of Mass spectra (including EI MS spectra and MS spectra by using soft ionizations) (Chapter 4) and IR spectra (Chapter 5). In each chapter the author presents very practical application skills by providing various challenging examples. The last chapter (Chapter 6) provides the strategy, skills and methods on how to identify an unknown compound through a combination of spectra. Based on nearly 40 years researching and teaching experience, the author also proposes some original and creative ideas, which are very practical for spectral interpretation.

Chapter 1

Interpretation of ¹H NMR Spectra

As described in the preface to this book, the NMR is the most important method to identify the structure of an unknown organic compound, because the information obtained from (one dimensional and two dimensional) NMR spectra is more abundant and interpretable than that obtained by other spectroscopic methods. Since

NMR spectra have higher sensibility than other NMR spectra,

NMR spectra can be acquired more easily in some ways, and we present

NMR spectra in the first chapter of this book.

Because the

NMR spectrum can be interpreted in detail, it is possible to deduce the structure of an unknown compound, whose structure is not complex, only by using its

NMR spectrum,

NMR spectrum and the information about its molecular weight (without two dimensional NMR spectra). When we need to select the most reasonable structure from several possible structures, the

NMR spectrum of that compound can play a very important role.

Even when two dimensional NMR spectra were applied, the information, especially that from the analysis of coupled splittings in the

NMR spectrum, would still be useful to deduce an unknown structure.

The main parameters of

NMR spectra are chemical shifts, coupled constants (and splitting patterns) and peak areas. If we consider a

NMR spectrum from the viewpoint of physics, there is a fourth parameter, that is, relaxation times. However, relaxation times are short for

NMR spectroscopy. Therefore, the variation of relaxation times does not produce variations of peak areas of

NMR spectra. And relaxation times do not affect the interpretation of

NMR spectra.

The abscissa of the

NMR spectrum is the chemical shift δ, which characterizes the position in a

NMR spectrum of the peak of a functional group.

Because of coupling interactions between magnetic nuclei, peaks in the

NMR spectrum will show splittings. The splitting distance between two related split peaks is characterized by the coupling constant, measured in hertz. The magnitude of coupling constants reflects the strength of coupling interaction.

The related knowledge about the chemical shift and the coupling constant will be presented later.

The ordinate of the

NMR spectrum is the intensity of peaks. Because peaks in the

NMR spectrum have some widths, integral values of peak areas are applied as the measurements of intensities of peaks. Integral values, denoted under or beside the corresponding peaks, are proportional to the numbers of hydrogen atoms of related functional groups. The quantitative relationship of the

NMR spectrum is good with errors less than 5%.

The quantitative relationship between the integral values of peak areas and the numbers of hydrogen atoms of corresponding functional groups is important for deducing an unknown structure.

If a measured sample is a mixture, the quantitative ratio of components can be obtained from the quantitative relationship.

By using the quantitative relationship in

NMR spectroscopy, some important results can be obtained. For example, the averaged additional number of ethylene oxides, n, in a non-ionic surfactant, RO(CH₂)_nH, can be measured easily by using

NMR spectroscopy when we analyze this kind of surfactant. And the averaged value of n is more important than individual numbers which participate in average calculation for evaluating the character of this kind of surfactant. Otherwise, if we apply thin layer chromatography to analyze the surfactant, after its development by thin layer chromatography, we will get a series of spots on the thin plate. Each spot corresponds to a particular additional number and all numbers form a normal distribution shape. In this case, an average number is more important than these individual numbers which participate in average calculation.

The

spectrum of Compound C1-1 is shown in Figure 1.1.

Figure 1.1 The

spectrum of compound C1-1

From Figure 1.1 we know that the abscissa of the

spectrum is the chemical shift whose accurate values are denoted under (or above) corresponding peak sets. The ordinate of the

spectrum is the peak intensity. The integral values which show the areas of peak sets are denoted above (or under) corresponding peak sets.

There are split peak sets in the

spectrum. Because split distances are measured in Hz, the higher the frequency of the NMR spectrometer, the shorter the split dist...

Table of contents

• Cover
• Title Page
• Copyright
• Foreword
• Preface
• Chapter 1: Interpretation of 1H NMR spectra
• Chapter 2: Interpretation of 13C NMR spectra
• Chapter 3: Interpretation of 2D NMR spectra
• Chapter 4: Interpretation of mass spectra
• Chapter 5: Interpretation of infrared spectra
• Chapter 6: Identification of unknown compounds or confirmation of structures through comprehensive interpretation of spectra
• List of abbreviations
• Index