OC307-8-11-Ch13-1 - Organic Chemistry Nuclear Magnetic...

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Organic Chemistry Nuclear Magnetic Resonance H. D. Roth 1 Chemistry 307 Chapter 13 – Nuclear Magnetic Resonance Nuclear magnetic resonance (NMR) spectroscopy is one of three spectroscopic techniques that are useful tools for determining the structures of organic compounds. You know about infrared (IR) spectroscopy (chapter 13) and will learn about ultraviolet/visible (UVVis) spectroscopy in chapter 15.] Spectroscopic techniques probe the energy differences between two “states” in a molecule by irradiating it with electromagnetic radiation of known frequency. We can observe “transitions”, i.e., signals, when the incident radiation has the exact frequency, ν (that is a Greek nu) for which the energy of the photon, h ν , matches the energy difference, Δ E, between the two states, Δ E = h ν (p. 562). Spectroscopic techniques are non-destructive; the excited molecules decay back to the ground state without decomposition. Nuclear magnetic transitions are probed with radio waves. Compared to other spectroscopic techniques NMR has an additional complication: the energy differences between nuclear states and the “resonance frequency” are not constant, but depend on the magnetic field, H 0 , at which the spectrometer operates i.e., Δ E, ν α B 0 . At a magnetic field, B 0 = 14, 47, or 70 kGauss, 1 H nuclei resonate at 60, 200, or 300 MHz, respectively.
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Organic Chemistry Nuclear Magnetic Resonance H. D. Roth 2 1 H Magnetic Field Strength, B [kGauss] 0 60 20 40 Δ E ν α B 0 1 H 1 H Therefore, it is not sufficient to give the frequency at which an NMR transition occurs; in addition to the photon frequency we have to specify the magnetic field strength to describe our results unambiguously. Instead of denoting these two parameters for each NMR transition, we define chemical shift as the ratio, δ , of the response frequency relative to that of a standard (TMS) divided by the resonance frequency: δ = shift from TMS (in Hz) spectrometer frequency (in MHz) Since the resonance frequency is proportional to the magnetic field strength, this is equivalent to denoting the frequency of the signal and the magnetic field strength. This ratio is given in ppm (parts per million, 10 –6 ); it has no dimension. Different nuclei have different ranges of chemical shifts, e.g., Nucleus Range Standard 1 H (10 ppm) Si(CH 3 ) 4 (TMS) 13 C (200 ppm) Si(CH 3 ) 4 19 F (400 ppm) CF 3 COOH 31 P (700 ppm) 85 % H 3 PO 4
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Organic Chemistry Nuclear Magnetic Resonance H. D. Roth 3 Because of the very small energy difference between the two nuclear spin levels transitions between them are very fast: both levels are in equilibrium (see Chapter 2). Δ = –RTlnK or lnK = Δ G°/RT The energy difference, Δ E, between 1 H nuclear levels at 70 kGauss is only ~3x10 –5 kcal mol –1 . Because of this very low energy difference the equilibrium populations of the nuclear spin levels are almost identical; typically they differ by much less than 1%. We will focus our discussion on the magnetic resonance of
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This note was uploaded on 11/17/2011 for the course ORGANIC CH 307 taught by Professor Boikes during the Fall '09 term at Rutgers.

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OC307-8-11-Ch13-1 - Organic Chemistry Nuclear Magnetic...

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