Understanding NMR Spectra I

Integrals, Multiplicities, and Chemical Shifts in 1H NMR

Introduction A 1H NMR spectrum shows all of the H atoms in a compound as electronic signals on a chart paper (spectrum). Each signal in the spectrum reveals three important pieces of information about H atoms. The integral tells us how many H atoms are responsible for a given signal. The sum of the integrals equals the total number of H atoms in the compound. The word multiplicity means how many times something occurs. A signal composed of three lines has a multiplicity of three. So the number of lines in a given signal is its multiplicity. The multiplicity of a signal tells us how many equivalent H neighbors are affecting our signal. In general, neighbors affect a signal by splitting the signal into multiple lines, depending on how many equivalent H atoms are neighbors. The third piece of information is the chemical shift. The chemical shift is the location of a signal on the chart relative to zero, which is usually marked by TMS (tetramethylsilane). The chemical shift tells us about the chemical environment of the H’s making up our signal. A typical alkane H appears at about 1 ppm to the left of zero, whereas an aromatic H (one bonded to a benzene derivative) usually appears between 7 and 8 ppm. To better understand the fundamental information available from a 1H NMR spectrum, work through the following three tutorials. You may work through the tutorial with SpartanBuild or a model kit. Then go on to the 1H NMR practical.

Understanding NMR Spectra I

Integrals

1.  Build a model of a methyl group. Place a stick in the vacant hole. Hold the methyl group by the stick and spin the model back and forth. Can you distinguish one of the yellow hydrogens from the other two? No, the three hydrogen atoms are equivalent. Therefore, a methyl group will always integrate for three hydrogens. Three equivalent hydrogen atoms will give one signal in a 1H NMR spectrum. They are evident by the integral above the actual spectrum. When the integral is given, a methyl group will be indicated by the number 3 written above the NMR signal. The three means, 3 H atoms!!!

2.  Scan through the 1H NMR problems for this lab. Do any of the spectra have signals with a 3 written above an NMR signal? If so, what kind of group is likely represented by the 3? We represent a methyl group as CH3 squiggly.

3.  Remove one of the yellow hydrogen atoms from your methyl group, leaving its stick in place. The result is a methylene group -CH2-. Now we have a group with two equivalent H atoms. How will the integral for a methylene group be indicated in a 1H NMR spectrum? That’s right, with a 2 written above its 1H NMR signal.

4.  Thus, the integral or number above a signal gives us the number of equivalent H atoms that make up that signal.

5.  What is the maximum number of hydrogen atoms that can be bonded to a single carbon atom in methane?___ What is the maximum number of hydrogen atoms that can be bonded to a single carbon atom in any molecule other than methane?___ If you can’t answer these questions, make models of methane and any other molecule to help you. If you still can’t answer these questions or you are unsure what is being asked, consult the instructor.

6.  What if we have a signal with a 6 written above it? This means that we have six equivalent H atoms. Since we can’t have 6 H atoms bonded to a single C atom, we need two methyl groups to make 6. The two methyl groups are identical, so both methyl carbon atoms are bonded to the same carbon. That is, we need three carbon atoms to satisfy a 6. Make a model of two methyl groups bonded to the same C atom. Bond a yellow H to this same C. You have made a three-carbon group with a free bond (squiggly) at the middle carbon. This group is called an isopropyl group and is very common. Your model shows six equivalent H’s on two carbon atoms and one H atom on an adjacent C atom. Since the single H atom is not the same as the other six, it will have a different 1H NMR signal. What number will be written above the signal for the single H? __ Above the equivalent six? ____ When you see a 6 and a 1 in a spectrum, what group is probably present? ______

7.  Draw a picture of an isopropyl group below, showing the vacant bond with a squiggly.

8.  When we interpret spectra, we make partial structures with squigglies and then join the squigglies together to make a molecule.

Understanding NMR Spectra II

Multiplicity (Multiple Lines

due to Spin-Spin Splitting)

1.  Make a model of ethyl chloride (chloroethane). This model has a methyl (3 yellow H’s) and a methylene (2 yellow H’s) bonded to adjacent black carbon atoms. The integral would be a 3 and a 2 written above the two signals. [Note: only H atoms appear in the 1H NMR spectrum. The C’s and Cl do not show up. We must infer their presence by other data.]

2.  How do the three H’s of the methyl group interact with the two H’s of the methylene group? They interact by the spins of their nuclei, which are protons (1H+). The two groups are neighbors, meaning the methylene group is a neighbor of the methyl group, and the methyl group is a neighbor of the methylene group.

3.  The methylene H’s spin-spin couple with the methyl H’s. This causes the 1H NMR signal of both groups to be altered. A group of H’s without any neighboring H’s gives a single line as a signal. A group with one H neighbor gives two lines, and so on. The number of lines is always one more than the number of neighbors.

4.  How many neighbors (H atoms) does the methyl group have? Answer 2. Therefore, the methyl signal will appear as three lines (one more than 2).

5.  How many neighbors (H atoms) does the methylene group have? Answer 3. How many lines will the methylene group have? _____

6.  Thus, the spectrum shows two signals: one signal of three lines (a methyl group that has two neighbors) and one signal of four lines (a methylene group that has three neighbors). The methylene will have a 2 above its four lines, and the methyl will have a 3 above its three lines. We call three lines a triplet (of lines) and four lines a quartet; the signals are thus indicated by 3H (t) and 2H (q), meaning a three-hydrogen triplet (two neighbors) and a two-hydrogen quartet (three neighbors).

Understanding NMR Spectra III

Chemical Shifts

1.  We have seen that equivalent H atoms give a distinct signal, and the number of lines in the signal is due to the number of equivalent neighboring H’s. Next, we consider where the distinct signals appear in the spectrum.

2.  The location of a signal is found by reference to a standard signal made by a compound that is added to the NMR sample tube. The reference is called tetramethylsilane, TMS. Since TMS has four equivalent H’s with no neighbors, it gives a 12H signal with one line (a singlet). This line is the reference and it is conveniently set at zero, and all other signals are measured in reference to it. Nearly all organic compounds give signals to the left (downfield) of the TMS signal. The measurements are made in parts per million parts (ppm) (relative to the magnetic field strength of the magnet in the instrument making the spectrum). The unit is called delta, a small Greek d, or d.

3.  The signals in delta units (ppm from TMS) are recorded as a whole number and a decimal (e.g., d = 9.1 ppm; d = 2.1 ppm) or several signals as d 12.1, 9.1, 7.4, and 3.4 ppm.

4.  H atoms give 1H NMR signals that range from about 1 ppm to about 13 ppm, depending on their chemical environment. As a general rule, heteroatoms and multiple bonds cause the signals to move downfield or away from TMS. Alkanes have H signals near 1 ppm and Aromatic hydrocarbons (benzene) have H signals between 7 and 8 ppm. Refer to the table on page 5 of the accompanying 1H NMR Aid for the signals of commonly encountered H atoms.

5.  Look at your model of chloroethane. Which H atoms give a signal nearer TMS? Ans. The methyl group—it is further from the heteroatom (Cl). Thus, the spectrum would be: 2H, 4 lines (d = 3.4 to 4.3) and 3H, three lines, (d = about 1).

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