Chemistry 350 Organic Chemistry I

Study Guide :: Unit 13

Structure Determination: Nuclear Magnetic Resonance Spectroscopy

Unit Preview

In Unit 12, you learned how an organic chemist could use two spectroscopic techniques, mass spectroscopy and infrared spectroscopy, to assist in determining the structure of an unknown compound. This unit introduces a third technique, nuclear magnetic resonance (NMR). The two most common forms of NMR spectroscopy, ¹H NMR and ¹³C NMR, will be discussed in this unit, the former in much more detail than the latter. Nuclear magnetic resonance spectroscopy is a very powerful tool, particularly when used in combination with other spectroscopic techniques. You will encounter frequent references to NMR as you proceed through the rest of this course.

Unit Objectives

After you have completed Unit 13, you should be able to

1. fulfil all of the detailed objectives listed under each individual section.
2. solve road-map problems which may require the interpretation of ¹H NMR spectra in addition to other spectral data.
3. define, and use in context, the key terms introduced.

13.1  Nuclear Magnetic Resonance Spectroscopy

Objectives

After completing this section, you should be able to

1. discuss the principles of NMR spectroscopy.
2. identify the two magnetic nuclei that are most important to an organic chemist.

Learning Activities

Read 13.1 Nuclear Magnetic Resonance Spectroscopy and do any associated exercises.

13.2  The Nature of NMR Absorptions

Objectives

After completing this section, you should be able to

1. explain, in general terms, the origin of shielding effects in NMR spectroscopy.
2. explain the number of peaks occurring in the ¹H or ¹³C NMR spectrum of a simple compound, such as methyl acetate.
3. describe, and sketch a diagram of, a simple NMR spectrometer.
4. explain the difference in time scales of NMR and infrared spectroscopy.
5. predict the number of peaks expected in the ¹H or ¹³C NMR spectrum of a given compound.

Learning Activities

Read 13.2 The Nature of NMR Absorption and do any associated exercises.

Note: In this course you will not be expected to develop a deep understanding of ¹³C NMR. However, as this section indicates, the principles involved in ¹³C NMR are very similar to those in ¹H NMR.

13.3  Chemical Shifts

Objectives

After completing this section, you should be able to

1. describe the delta scale used in NMR spectroscopy.
2. perform calculations based on the relationship between the delta value (in ppm), the observed chemical shift (in Hz), and the operating frequency of an NMR spectrometer (in Hz).

Learning Activities

Read 13.3 Chemical Shift and do any associated exercises.

In Section 13.9 we discuss ¹H NMR chemical shifts in more detail. Although you will eventually be expected to associate the approximate region of a ¹H NMR spectrum with a particular type of proton, you will always be supplied with a general table of ¹H NMR chemical shifts for your examinations and you will find one on the course homepage.

While you are still responsible for the ¹³C NMR components of Objectives 2 and 5 in Section 3.2, we are not concerned with the details of ¹³C NMR. Hence, you may omit Sections 13.4 through 13.7. Interested students may wish to read them for enrichment purposes.

13.4  ¹³C NMR Spectroscopy: Signal Averaging and FT-NMR

13.5  Characteristics of ¹³C NMR Spectroscopy

13.6  DEPT ¹³C NMR Spectroscopy

13.7  Uses of ¹³C NMR Spectroscopy

13.8  ¹H NMR Spectroscopy and Proton Equivalence

Objectives

After completing this section, you should be able to

1. identify those protons which are equivalent in a given chemical structure.
2. use the ¹H NMR spectrum of a simple organic compound to determine the number of equivalent sets of protons present.

Learning Activities

Read 13.8 ¹H NMR Spectroscopy and Proton Equivalence and do any associated exercises.

13.9  Chemical Shifts in ¹H NMR Spectroscopy

Objectives

After completing this section, you should be able to

1. state the approximate chemical shift (δ) for the following types of protons:
   1. aromatic.
   2. vinylic.
   3. those bonded to carbon atoms which are in turn bonded to a highly electronegative element.
   4. those bonded to carbons which are next to unsaturated centres.
   5. those bonded to carbons which are part of a saturated system.
2. predict the approximate chemical shifts of each of the protons in an organic compound, given its structure and a table of chemical shift correlations.

Learning Activities

Read 13.9 Chemical Shifts in ¹H NMR Spectroscopy and do any associated exercises.

13.10  Integration of ¹H NMR Absorptions: Proton Counting

Objectives

After completing this section, you should be able to

1. explain what information can be obtained from an integrated ¹H NMR spectrum, and use this information in the interpretation of such a spectrum.
2. use an integrated ¹H NMR spectrum to determine the ratio of the different types of protons present in an organic compound.

Learning Activities

Read 13.10 Integration of ¹H NMR Absorptions: Proton Counting and do any associated exercises.

13.11  Spin-spin Splitting in ¹H NMR Spectra

Objectives

After completing this section, you should be able to

1. explain the spin-spin splitting pattern observed in the ¹H NMR spectrum of a simple organic compound, such as chloroethane or 2-bromopropane.
2. interpret the splitting pattern of a given ¹H NMR spectrum.
3. determine the structure of a relatively simple organic compound, given its ¹H NMR spectrum and other relevant information.
4. use coupling constants to determine which groups of protons are coupling with one another in a ¹H NMR spectrum.
5. predict the splitting pattern which should be observed in the ¹H NMR spectrum of a given organic compound.

Learning Activities

Read 13.11 Spin-spin Splitting in ¹H NMR Spectra and do any associated exercises.

13.12  More Complex Spin-spin Splitting Patterns

Objectives

After completing this section, you should be able to

1. explain how multiple coupling can give rise to complex-looking ¹H NMR spectra.
2. predict the splitting pattern expected in the ¹H NMR spectrum of an organic compound in which multiple coupling is possible.
3. interpret ¹H NMR spectra in which multiple coupling is evident.

Learning Activities

Read 13.12 More Complex Spin-spin Splitting Patterns and do any associated exercises.

13.13  Uses of ¹H NMR Spectroscopy

Objective

After completing this section, you should be able to use data from ¹H NMR spectra to distinguish between two (or more) possible structures for an unknown organic compound.

Learning Activities

Read 13.13 Uses of ¹H NMR Spectroscopy and do any associated exercises.

Go to the ¹H NMR website and review the theory section and explore the virtual lab and quizzes for additional practice in interpreting proton NMR spectra of unknown organic compounds. This website was developed at the University of Alberta by J. James Bailey, Robert Carmichael, Dr. Albin Otter and Dr. Lois M. Browne of the Chemistry Dept.

Go to the Organic Structure Elucidation website at Notre Dame University developed by Dr. Bradley Smith, et al. The practice problems require a combination of MS, IR, and NMR spectra interpretation to identify unknown organic compounds. Try a few problems now and use the site to help prepare for the final examination. Answer (Problems 1-32 and Problems 33-64), as well as a few selected fully detailed answers (Problems 1-8).

Summary

In this unit, you studied the principles of nuclear magnetic resonance spectroscopy. You have seen that NMR spectra can be obtained for both ¹H and ¹³C nuclei, although the unit concentrated on the former.

We suggest that you try a number of the online spectroscopy exercises in earlier Learning Activities before you proceed to the next unit, and do the remainder when you are reviewing in preparation for an examination.