NMR-3

Introduction

The primary use of NMR in chemistry is the determination of the structure of molecules. By examining the 1H and 13C NMR spectra of a compound it is possible (often in conjunction with other complementary techniques, such as infra red spectroscopy and mass spectrometry, to determine the atom-connectivity within a molecule, and so build a molecular model of the compound.

In this activity, we will examine the 1H and 13C NMR spectra of some simple organic compounds. Working in small groups, you will use these spectra, together with the molecular mass and chemical formulae of the compounds to build molecular models of the compounds consistent with the data. Molecular masses of compounds are often obtained through mass spectrometry, gas law calculations, or osmotic pressure experiments. Chemical formulae (empirical or molecular) may be obtained by combustion analyses.

There are many methods to determine the structure of a molecule from NMR spectra. It is best to treat the data given be the spectra as clues to solve a puzzle. By fitting all these clues together, you will be able to determine one (or possibly more) molecular structure(s) that fit all of the data. The following procedure usually works well, but you and your group may wish to follow a different order when determining the structures of different molecules.

Steps for Molecular Structure Determination:

Look at the 13C NMR spectrum. The number of signals in the spectrum will tell you the number of chemically distinct carbon atoms in the molecule. Furthermore, the position of each signal in the spectrum (its chemical shift δ (ppm)) will tell you about the chemical environment of the corresponding carbon atom in the molecule:

Use the following chart to help you

List the different chemically distinct carbon atoms you can see from the spectrum. By each atom note any information you have about the atom from its signal’s chemical shift. Label the spectrum to indicate which signal corresponds to each carbon atom in your list.

Compare the number of carbon atoms in the molecule you have determined with the number of carbon atoms given in the molecular formula. If these are the same, you are well on the way to solving the structure. If the number of chemically distinct carbon atoms you can see in the spectrum is less than the number of carbon atoms in the molecular formula, the molecule must contain symmetry elements, such as mirror planes or axes of rotation, that make some carbon atoms within the molecule chemically equivalent. Your final model must account for this.

Now look at the 1H NMR spectrum. Again, the number of signals will indicate the number of chemically distinct hydrogen molecules within the molecule. Remember that in 1H NMR, individual signals may be split into doublets, triplets, quartets, and multiplets by non-chemically equivalent hydrogens attached to neighboring carbon atoms according to the N+1 Rule. The position of each signal in the spectrum (its chemical shift δ (ppm)) will tell you about the chemical environment of the corresponding hydrogen atom in the molecule:

Use the following chart to help you

Hydrogen atoms bonded to oxygen or nitrogen atoms usually give rise to broad signals, the position of which may vary according to the type of solvent the sample is dissolved in and the temperature at which the NMR experiment is conducted.

The area under each signal tells us the relative number of chemically equivalent hydrogen atoms giving rise to that particular signal. * Comparing these so-called “integration values” will tell you the relative number of hydrogen atoms in each distinct chemical environment.

List the different chemically distinct hydrogen atoms you can see from the spectrum. By each atom note any information you have about the atom from its signal’s chemical shift. Label the spectrum to indicate which signal corresponds to each carbon atom in your list.

Now look at the splitting patterns (if any) in the 1H NMR spectrum. Remember that the signal of a particular hydrogen atom will be split by hydrogen atoms on neighboring carbon atoms according to the “N+1 Rule”. For a signal originating from a hydrogen atom with n hydrogen atoms attached to neighboring carbons:

Remember, hydrogen atoms bonded to oxygen or nitrogen do not split the signals of hydrogens bonded to adjacent carbon atoms. Neither are their signals split by other hydrogen atoms.

Characteristic splitting patterns for common groups are:

Use the signal-splitting data and the N+1 Rule to find signals originating from chemically distinct hydrogens attached to adjacent carbon atoms.

Now link all the information you have from both 13C and 1H spectra with the given molecular formula. Does all the information fit your model? If so, congratulations! If not, think about rearranging your model to better fit the data.

Finally, can you think of any other arrangements of atoms that might also fit the data? If so, make models of these structures also. There are other data within NMR spectra (and other analytical techniques) to help determine which structure is correct. We will meet these methods in later activities…

Exercises

A compound has the molecular formula, C7H16, and the following NMR spectra:

A compound has the molecular formula, C7H16, and the following NMR spectra:

A compound has the molecular formula, C2H4Br2, and the following NMR spectra:

A compound has the molecular formula, C8H18, and the following NMR spectra:

A compound has the molecular formula, C2H4Cl2, and the following NMR spectra: 1H NMR:

A compound has the molecular formula, C4H8O2, and the following NMR spectra: 1H NMR:

Make models of the following molecules, in each case sketch the expected 1H and 13C spectra indicating the approximate chemical shifts for their signals: