WARNING NOTICE: The experiments described in these materials are potentially hazardous and require a high level of safety training, special facilities and equipment, and supervision by appropriate individuals. You bear the sole responsibility, liability, and risk for the implementation of such safety procedures and measures. MIT shall have no responsibility, liability, or risk for the content or implementation of any of the material presented. Legal Notice Experiment #2: Nuclear Magnetic ResonanceMASSACHUSETTS INSTITUTE OF TECHNOLOGY Department of Chemistry 5.311 Introductory Chemical Experimentation Experiment #2 NUCLEAR MAGNETIC RESONANCE I. Purpose This experiment is designed to introduce the basic concepts of nuclear magnetic resonance (NMR) spectroscopy – spin, energy levels, absorption of radiation, and several NMR spectral parameters, and to provide experience in identification of unknowns via 1H (proton) NMR spectra. A series of known samples will be used to introduce methods of sample preparation, operation of the NMR spectrometers, and 1H-NMR spectra from which students will measure chemical shifts, J-couplings and spectral intensities. Subsequently, students will record spectra of three unknowns and will use these spectra to determine the structure and identity of the compounds. II. Safety II.A. Chemicals: A number of different chemicals are involved in this experiment and should be handled with care to avoid harm to yourself or your colleagues. Because the presence of solvent protons would obscure the NMR signals of the desired compound, NMR measurements are commonly carried out in deuterated solvents such as acetone-d6 or chloroform-d3. II.B. Glass: In preparing NMR samples, you will use glass pipettes and NMR sample tubes. In placing a rubber bulb on the pipette or plastic cap on the sample tube, the experimenter should hold the tube immediately below the point of attachment to avoid breakage of the glass. II.C. Magnetic Fields: The field generated by an NMR magnet can have deleterious effects on watches (battery-powered watches with liquid crystal displays are an exception), magnetic credit cards (VISA, Mastercharge, American Express, etc.), and cardiac pacemakers. Thus, when working in the vicinity of an NMR spectrometer, leave these items in an alternative location. III. Reading 1. Hore, P. J. Nuclear Magnetic Resonance, Oxford University Press, Oxford, 1998. A delightful introduction to NMR Spectroscopy. 2. Mohrig, J.R.; Hammond, C.N.; Schatz, P.F.; Morrill, T.C. Techniques In Organic Chemistry W.H. Freeman: New York, 2003. Chapter 19. 3. Pavia, D. l.; Lampman, G. M.; Kriz, G. S. Introduction to Spectroscopy: A Guide for Exp. #2-1Experiment #2: Nuclear Magnetic Resonance Students of Organic Chemistry, Saunders: Fort Worth, 1996. Chapters 3-5. 4. Canet, D. Nuclear Magnetic Resonance: Concepts and Methods, Wiley: New York, 1996. Enables students to understand the physical and mathematical background which underlines liquid state NMR. 5. Derome, A. E. Modern NMR Techniques for Chemistry Research, Pergamon Press: 1987. IV. Theory IV.A. Energy Levels of Nuclei in a Magnetic Field Many isotopes of elements in the periodic table possess nuclear spin angular momenta, which results from coupling the spin and orbital angular momenta of the protons and neutrons in the nucleus. In 5.11, you learned that the orbital angular momentum, l, of an electron can be oriented in 2l + 1 discrete states (e.g. a p electron (l = 1) can have ml = +1, 0, or –1 orientation states). Nuclear spin angular momenta behave the same way. Nuclei with spin I can assume 2 I + 1 values of the spin orientation, characterized by quantum numbers, mI, which are [I, I-1, ..., -(I-1), -I]. For I = 1/2 the allowed mI are +1/2 and -1/2, and for I = 1 the allowed mI are-1, 0, and +1. If I is non-zero, the nucleus has a magnetic moment, and when placed in a magnetic field, the component of the moment along the field direction (conventionally taken as the z-direction) is mz = gh mI, where g, which is a constant specific for each isotope, is termed the gyromagnetic ratio; h is the Planck constant divided by 2p (h=1.055x10-34 J s), and mI is the quantum number. Some g values for chemically important nuclei are given in Table 1. Note that nuclei such as 12C and 16O are not included in the table, since they have I=0. Table 1. Spin, gyromagnetic ratios, NMR frequencies (in a 1 T (10 kG) field) and the natural abundance of selected nuclides)* Nucleus Spin Gyromagnetic Ratio, g (107T-1s-1) n (MHz) Natural Abundance (%)1H 1/2 26.75 42.576 99.985 2H 1 4.11 6.536 0.015 13C 1/2 6.73 10.705 1.108 19F 1/2 25.18 40.054 100.00 14N 1 1.93 3.076 99.63 15N 1/2 -2.71** 4.31 0.37 31P 1/2 10.84 17.238 100.0 * T stands for Tesla (1 Tesla = 104 Gauss. The earth’s magnetic field is around 0.5 Gauss) ** What is the physical meaning of a negative gyromagnetic constant? Exp. #2-2Experiment #2: Nuclear Magnetic ResonanceIn a laboratory field B0, the nuclei can assume 2I+1 orientations corresponding to the values of mI. Each value of mI corresponds to an energy given by (Figure 1) EmI =-mB0 = -mz B0=-h g B0mI (1) which can be rewritten as EmI = - mI h w0 (2) where the Larmor frequency is w0 = g B0. z B0 m mz Figure 1. The relationship between the laboratory magnetic field B0 and, the magnetic moment of the nucleus, m, and its mz component (m component along the z axis) As shown in Figure 2, the energy separation between the two levels a and b of an I=1/2 system is then DE = Eb-Ea = (1/2) h gB0 – (-1/2) h gB0 = h g B0 (3) (h/2p) b a mI = - 1/2 gB0 mI = + 1/2 Zero field Magnetic field on Figure 2. The nuclear spin energy levels of a spin-1/2 nucleus (e.g. 1H or 13C) in a magnetic field. Resonance occurs when the energy separation of the levels, as determined Exp. #2-32.01 x 10-5Experiment #2: Nuclear Magnetic Resonanceby the magnetic field strength, matches the energy of the photons in the electromagnetic field. Thus, when the sample resides in a magnetic field and is bathed with radiation of frequency w0/2p = n0 (energy hw0) matching the difference between the levels resonance occurs -- i.e. energy is absorbed, as illustrated schematically in Figure 2. Equation (3) is the resonance condition. Using the values for g in Table 1, we can calculate that for 1H in a 4.7 T field, the resonance frequency is 200 MHz, and for an 11.7 T field n0 = 500 MHz. The amount of energy absorbed (and therefore