1 lab period; work in groups. Complete the Preparation page before laboratory.
Goals
Background
The nuclei of some atoms, but not all, behave as if they were rotating, or spinning, about an axis passed through them, much as a top spins about its central axis. Examples of atoms whose nuclei spin are hydrogen, fluorine, the 13 isotope of carbon, and the 15 isotope of nitrogen. There are many others not listed here. Under certain conditions, this spinning motion, which is of course an oscillatory motion, can interact with and absorb energy from electromagnetic radiation in the radio frequency region of the spectrum. The nuclear spin is a motion of relatively low energy, so low energy electromagnetic radiation is required to interact with it. A typical radio frequency photon has a frequency of 108 s-1, a wavelength of 300 meters (about the length of three football fields), and an energy of 6.6 x 10-26 Joules. By measuring the energies of photons absorbed by the spinning nuclei, we obtain the nuclear magnetic resonance (NMR) spectrum of the molecule. This spectrum provides information about the chemical environment of the spinning nucleus, and can be used to deduce the atomic bonding patterns in the molecule. Sophisticated NMR techniques are used to determine the 3-dimensional structures of huge protein molecules in solution. Recently, nuclear magnetic resonance spectroscopy has found medical applications, in which context it is known as magnetic resonance imaging (MRI). MRI uses the spinning motions of atomic nuclei to provide a map of the internal structure of human body tissue.
The nuclear magnetic resonance spectrum resulting from spin of the hydrogen atoms in the molecule acetaldehyde is shown. The structure of acetaldehyde is also shown. Several aspects of this spectrum are important. First, the single sharp signal at exactly 0 ppm is not due to acetaldehyde, but is instead due to a reference compound named tetramethylsilane, or TMS. The structure of TMS is also shown in Figure 1. By general agreement, the positions of NMR signals are always reported relative to the position of the signal for TMS. Second, the unit used to measure resonance position, ppm (parts per million), is not a unit of energy. It has no units at all. The ppm unit is a measure of the shift in position between the TMS resonance and a particular resonance in the compound of interest. It is usually called the chemical shift. The chemical shift gives an indication of the chemical environment in which the spinning nucleus is found. Third, there are two groups of signals, or resonances, due to acetaldehyde. These are centered at 2.2 ppm and 10 ppm in Figure 1. Note that one group consists of two sharp lines of equal size (i.e., intensity). The total intensity of this pair of sharp lines is 24 (obtained from the integration, which measures the area under the signal). Taken together, the pair of lines is called a doublet. The 10-ppm signal consists of four equally spaced lines which, taken together, are called a quartet, with total intensity of 8. Study this spectrum and try to make some sense of it in terms of the acetaldehyde structure. Discussion of this system in lab will enable us to discover some general rules about NMR spectra.
In this experiment, you will be given three compounds for which the formulas are known but the arrangement of atoms is not. Your goal will be to answer the following questions for each compound:
Focus Questions
Equipment and Materials
Safety
Safety glasses must be worn at all times in the laboratory. Deuterochloroform is hazardous. If you break an NMR tube, tell your instructor immediately so that the solution may be cleaned up.
Experimental
Record all data in your lab notebook. Sign out a molecular model kit. Obtain three NMR tubes from the instructor. Each contains a deuterochloroform solution of a compound of known formula but unspecified structure (arrangement of atoms). First, for each compound, draw as many structures as you can consistent with the formula. Work together. Remember that carbon atoms form 4 bonds, nitrogen atoms 3, oxygen atoms 2, halogen atoms 1, and hydrogen atoms 1. When satisfied that you have drawn all possible structures, predict the appearance of the NMR spectrum for each structure drawn. Predictions should address the number of signals (i.e., the number of distinct types of hydrogen atoms in the structure) and the splitting of each signal due to interaction with spins of hydrogens on neighboring atoms. Predict the relative chemical shifts of the various types of protons, based on electronegativity arguments.
Second, obtain the NMR spectra of the samples using the NMR spectrometer. When you are finished obtaining spectra, return all sample tubes to the instructor.
Disposal Methods
You are not required to do any disposal. Samples will be disposed of by course personnel at the completion of the experiment.
Preparation
Dynamics: NMR
