NMR Multiplet Interpretation

An Infographic Walk-Through

Roman Valiulin

• Editorial: De Gruyter
• Año: 2019
• ISBN:
  9783110608465
• Edición: 1
• Categoría: Naturwissenschaften
• Categoría: Analytische Chemie

A visual guide for the interpretation of complex 1 H-NMR spectra with a concise and illustrative practice problems section. This book is an easy-to-grasp source for (organic) chemists and students that want to understand and practice NMR spectroscopy.

1 Introduction

This workbook provides a guided walk-through of ¹H NMR spectra interpretation, with a carefully sequenced collection of practical exercises designed to reinforce understanding of common concepts. The book focuses on simple and complex first-order ¹H NMR multiplets and is specifically designed for the needs of a developing organic chemist. It can be utilized as a companion to chemistry studies at the undergraduate as well as graduate level. Furthermore, experienced chemists seeking additional practice with the ¹H NMR interpretation can also utilize this book as a visual guide and a workbook.

As discussed before, the book does not present scientific findings and is not intended to be a comprehensive nor exhaustive study of the fundamental theory undergirding NMR. Moreover, those seeking to develop a specialization in this area should consider using this book in conjunction with a series of NMR spectroscopy courses for comprehensive coverage and knowledge.

This workbook embraces the intuitive and elegant algorithm previously presented by Professor Thomas R. Hoye and colleagues [5, 6], which enables a quick and very informative determination of coupling constant values. The literature may contain alternative ways for determining coupling constant values and students may wish to supplement their practice by exploring those publications, as they craft their own personal approach. Uniquely, as a student, teaching assistant, and graduate researcher not too long ago, the author is mindful of knowledge gaps that can present challenges for learning. Thus, the book strategically focuses on the most useful scenarios, provides an intuitive overview, presents a sequential and guided progression in the complexity of the problems, and highlights common trends to anticipate likely challenges that may arise in application. Most uniquely, it aims to provide a highly visual tutorial with diagrammatic and illustrative aids developed by the author. Readers who enjoy this highly visual pedagogical method are invited for further reading on the author’s ChemInfographic Blog⁴ and are invited to follow the author on Twitter™.⁵

The book can be divided into two parts. The first part introduces key concepts, defines main terms, describes a common nomenclature, and sets forth certain assumptions that are used throughout the workbook. Next, it provides detailed, step-by-step walk-throughs with multiple examples: three (3) total core examples, with five (5) steps covered in each example. Each step of every walk-through is supported by a rich illustration and practical tips in the form of mnemonic shortcuts and rules that are either commonly known in the field or suggested by the author. Readers may identify their own patterns and are encouraged to develop their own mnemonic devices and shortcuts to further reinforce learning. Each example concludes with a summary Infographic. Chapter 4 contains mnemonic rule summaries that classify some general trends and that help reinforce a conceptual framework of the entire process.

The second half of the book has a set of carefully sequenced exercises, followed by a set of graphical answers. Four (4) subsets are presented, progressing from simple to more complex: Elementary, Intermediate, Advanced, and Expert. Special attention is given to intermediate and advanced exercises because they represent the most common types of multiplets and are most likely to be encountered in application. The reader is encouraged to look over the entire workbook of problems and to develop a study plan that works best for their goal and level of understanding. Those for whom the entire subject-matter is new are encouraged to go in order and to complete all of the problems. More-experienced organic chemists may wish to skip ahead to the complex problems. All readers are encouraged to carefully follow the first half of the book and to take advantage of the illustrated and detailed answer keys for all of the exercises. Moreover, readers can find a few rich-color infographics at the end of the book (e.g., “Deuterated NMR Solvents”) as useful quick reference guides.

2 Simple First-Order ¹H NMR Multiplets: Pascal’s Triangle

The multiplets observed in ¹H NMR spectra can be grouped into three major categories: First-Order multiplets, Second-Order multiplets, and other Higher-Order multiplets (Figure 2.1).

Figure 2.1: Classification of various multiplets in ¹H NMR spectroscopy.

Second-Order and Higher-Order multiplets are not covered in this workbook. For further reading please refer to more comprehensive NMR spectroscopy literature such as [7–9].⁶

First-Order multiplets are composed of two types: Simple and Complex First-Order multiplets (Figure 2.1). Without going into detailed discussion of the physics and theory of the Nuclear Magnetic Resonance spectroscopy, which is beyond the technical scope of this book, true Simple First-Order multiplets observed in ¹H NMR spectra obey certain rules:

• The number of peaks in the multiplet (multiplicity) can be determined from the n + 1 rule, where n is the number of equivalent neighboring H atoms.
• All the J-coupling constants to the neighboring protons are identical.
• The intensities (≈ heights) of the peaks in the multiplets can be derived from a mnemonic rule: using Pascal’s triangle (Figure 2.2). For example, the intensities (≈ heights) of the peaks in a true triplet should be 1:2:1.
• The distances between each peak in a multiplet are identical and equal to the magnitude of the coupling constants measured in Hertz (Hz).

Additionally, we distinguish a subset of the Simple First-Order multiplets: Fundamental Simple First-Order multiplets (this is not an official term). These represent the simplest and most common examples of Simple First-Order multiplets and have a unique single letter notation: singlet (s), doublet (d), triplet (t), and quartet (q) (top four examples in Figures 2.2 and 2.3).

Figure 2.2: Pascal’s triangle.

Figure 2.3: Simple First-Order multiplets, including the four Fundamental multiplets: s, d, t, q.

Complex First-Order multiplets, on the contrary, do not obey the n+1 rule and Pascal’s triangle cannot be used to derive the peaks’ intensities. A unique and defining feature of complex first-order multiplets is the fact that the J-coupling constants are not identical. In some instances, all the coupling constants to the neighboring protons are completely different: dd, ddd, dddd, ddddd, and so on. In other cases, some constants are identical and some are different: dt, dq, dtd, qt, and so on. For deeper knowledge on when a multiplet would be considered First-Order or Second-Order, refer to a few suggested NMR textbooks [7–9]; there are also several robust electronic resources: a particularly good example is an NMR spectroscopy course and manual by Professor Hans J. Reich [10].

Our tutorial in this book will focus only on the description of Simple and Complex First-Order multiplets. Before we start the visual walk-through, we should be cognizant of a few assumptions that have to be made while discussing the images of the multiplets.

• Only ¹H NMR spectra are discussed in this tutorial.
• Each multiplet represents 1H (single proton), that is, it should integrate as 1 (one) with respect to the other signals in the spectrum. Note, due to the symmetry or degeneracy, integration may be higher: 2H, 3H, …, 6H (e.g., two methyl groups in iso-propyl).
• Only H–H coupling patterns are considered, that is, each proton (represented by the multiplet) is coupled to one or multiple neighboring H atoms. Coupling to other nuclei are not discussed.
• Each multiplet is assumed to be a true First-Order Multiplet, that is, it is governed by first-order coupling rules. In actual examples, the multiplet may only appear to be a first-order multiplet (while being a true second-order multiplet by nature). For simplicity, only their shape and appearance are taken into consideration here.
• It is assumed that the multiplet is symmet...