Chapter 10

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Unformatted text preview: Chapter 10: Nuclear Magnetic Resonance (NMR) Spectroscopy Resonance What i s spectr oscopy? What M ol ecul ar r esponse to r adi ati ve sti mul us i s quanti zed (“gear ed”). (M ol ecul e = nucl ei + el ectr ons). quanti (“gear ). ΔE = h ν ν = c/ λ c/ E xci tati on: 1. El ectr oni c (UV-vi si bl e spectr a) 1. 2. Vi br ati onal (I R) 2. 3. Rotati on (mi cr owave) 4. Nucl ear spi n or i entati on i n magnet (NM R) 4. Spectr ometer scans ν t o fi nd ΔE : The spectr um T he E xci tati on causes an “ absor pti on ”, a an peak i n a gr aph A General Spectrometer General kcal mol -1 k cal What i s ∆E i n NM R? Nucl ei What behave as ti ny m agnets (r andom or i entati on). I n a magneti c fi el d H , t hey or gani ze m agneti they Protons as Tiny Magnets Line Up With and Against an External Magnetic Filed and Rati o α:β ~ 1:1 Rati Absorption of Light, Spin Flip, and Resonance ⇒ Spectral Line Spectral ν i s pr opor ti onal t o H . pr (ν 0 F or 1H : at H 0 = 21,150 gauss, ν0 = 90 M H z. A t H 0 = 70500 gauss, ν0 = 300 M H z. At H 0 (ear th): 0.7 gauss! ∆E (300 M H z) ~ 10-5 k cal mol -1. (300 N α/ N β = 1.000004 12 A Hypothetical NMR Spectrum: Active Elements Absorb at Differing Frequencies Frequencies The NMR Spectrometer The Sol vents: CDCl , CD Cl , TH F-d , etc. 3 2 2 8 High Resolution NMR! High Why ar e ther e two peaks? The Chemi cal Shi ft Why E l ectr ons i n Vi ci ni ty of Nucl eus Affect El ν Causes shi el di ng, i .e., Causes shi Peak moves to the r i ght Consi der H : N o e, no shi el di ng, peak Consi No n o peak H HCH H El ectr ons cause shi el di ng: shi Peak moves upfi el d (to r i ght ). u pfi When we add an e-wi thdr awi ng gr oup: e-wi e.g., CH 3 Cll , causes deshi el di ng (to l eft ). e.g., CH C d eshi Chemi cal shi ft pr ovi des a fi nel y tuned pi ctur e of ell ectr oni c...
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