Spectroscopy is the study of the interaction of molecules with light. It has long been known that atoms and molecules absorb light. We discussed this in CH1010 in connection with the energy levels of the hydrogen atom. The light is absorbed because it interacts with the various types of motion that a molecule may undergo. These motions occur in different ranges of frequency, so they interact with light in different wavelength (frequency) ranges. For a molecule to absorb a photon of light, one of its motions must occur with the same frequency that the light wave oscillates with. By measuring the frequencies of light absorbed by the molecules of a substance, we obtain information about the molecular motions and often about the molecular structure.
Table 1 shows the molecular motions that interact with light in the various wavelength regions.
| Molecular Motion | Wavelength range of Electromagnetic Radiation | Type of Spectroscopy |
|---|---|---|
| spinning of an atomic nucleus | radio | Nuclear Magnetic Resonance |
| rotation of the molecule in space | microwave | Rotational Spectroscopy |
| vibrations of the molecule (bond stretches and bends) | infrared | Vibrational (Infrared) spectroscopy |
| motions of the electrons from one energy level to another | ultraviolet, visible | UV-Visible Spectroscopy |
NMR spectroscopy examines the spinning motions of certain susceptible nuclei. Among these are the nuclei of commonly occuring atoms like hydrogen and carbon. We will restrict ourselves here to NMR of hydrogen atom nuclei. NMR is very sensitive, in that even the slightest change in the environment of a hydrogen nucleus causes it to absorb radio frequency radiation of slightly different frequency. Thus we obtain one signal in the NMR spectrum for each different type of hydrogen atom in a molecule. the intensity (strength) of the signal is proportional to the number of hydrogen nuclei of a given type. Thus if we have 6 of one type and one of another, we will get 2 signals, one 6 times stronger than the other. Thus NMR spectra allow us to a) tell immediately how many different types of hydrogen atoms there are in a molecule and b) how many H atoms of each type there are. Further, it turns out that the signal due to one type of hydrogen atom may be perturbed by another type of hydrogen atom, provided the 2 types of hydrogen atoms are near each other in a molecule. The result of this "perturbing" is that the signals for both types of hydrogen are split into subsignals. The number of components of the subsignal indicate how many neighboring hydrogen atoms there are, according to the following table:
| Number of neighboring H atoms | Number of subsignals | Intensities of subsignals |
|---|---|---|
| 1 | 2 | 1:1 |
| 2 | 3 | 1:2:1 |
| 3 | 4 | 1:3:3:1 |
| 4 | 5 | 1:4:6:4:1 |
In NMR spectroscopy, we prepare a sample, place it in an NMR spectrometer (an instrument), and obtain the NMR spectrum. The instrument automatically exposes the sample to all of the wavelengths of light in the radio-frequency (RF) region, and measures how much of the light is absorbed by the sample at each frequency. It then produces a plot showing all of the signals produced by the hydrogen atoms of the sample at each wavelength. Look here for the NMR spectrum of 2-butanone, CH3CH2C(=O)CH3. The spectrum shows 3 signals, one for each type of hydrogen, with intensities of 3:2:3. Two of the signals are split to subsignals by neighboring hydrogen atoms. The signal for the 3 H atoms on the left carbon is split to 3 components because there are 2 H atoms on the neighboring carbon. The signal for the 2 H atoms on the second carbon from the left is split to 4 components because there are 3 H atoms on the left carbon.
Sample Preparation. Unfortunately we have only one NMR spectrometer in the Department of Chemistry and Biochemistry, so we will be unable to obtain our own spectra. You may be relieved, because IF YOU WERE TO BREAK THE NMR SPECTROMETER WHILE USING IT, YOU WOULD HAVE TO PAY FOR IT. COST, $300,000.
For more information on this subject, see Chapter 4 of Concepts of Chemistry, or take CH1040!
