Nmr  (Page 44/6)

Experiment 1 NMR (Nuclear Magnetic Resonance Spectroscopy

Objective

To introduce or re-acquaint you to the fundamentals of Nuclear Magnetic Resonance spectroscopy (NMR spectroscopy) and to show you how the information obtained from this technique can be used to determine molecular composition and structure. Mass (MS), infrared (IR), and nuclear magnetic resonance (NMR) spectrums ( ${}^{1}H$ and ${}^{\text{13}}C$ ) are useful tools for analyzing unknown organic compounds.

Grading

• The correctness and thoroughness of your observations.
• The ability to deduce the chemical structure from the NMR spectra.
• Completion of Laboratory Revision Questions and Report Questions.

Background Information

Nuclear Magnetic Resonance (NMR) spectroscopy is an analytical technique based upon the nuclear properties of some types of atoms. Many atoms have isotopes which possess a nuclear magnetic moment, just as the electron does, having a spin of 1/2. Atomic nuclei may have no spin, spin of 1/2, or other spins which are increments of 1/2 (1, 2, etc.). For organic chemists, the most useful nuclei for observation are usually those with spin 1/2. Nuclei with other spins may be studied, but their signals are sometimes observed only under special circumstances and will not be discussed here. A number of nuclei have spin 1/2 and are very useful for study by NMR spectroscopy. Table 1 lists a number of these elements and their natural abundances. ${}^{1}H$ , ${}^{\text{13}}C$ , ${}^{\text{31}}P$ , ${}^{\text{19}}F$ , and ${}^{\text{15}}N$ are of particular interest to organic chemists.

Table 1. Common Nuclei with Spin 1/2

If we place the nuclei in a strong external magnetic field, these energy levels are no longer equal.

Most commonly a sample solution is prepared and placed in a glass tube that is very uniform and has thin walls. The sample is then placed inside a high field magnet. In older instruments, a variable Rf frequency was applied to the sample to sweep out a range of radiofrequencies, thereby generating a spectrum of the radiofrequency energy absorbed. In newer instruments, a short pulse of radiofrequency energy is used that excites nuclei over a range of frequencies, and the response of all of these nuclei is measured all at once. The spectrum is then obtained by a mathematical transformation of the total signal using the Fourier Transform technique. This is known as FT-NMR. NMR spectra may also be obtained on solid samples, but the technical difficulties are much greater and will not be covered here.