CHEM 322 Laboratory Methods in Organic Chemistry. Introduction to NMR Spectroscopy

Transcription

1 EM 322 Laboratory Methods in Organic hemistry Introduction to NMR Spectroscopy

2 What structural information does NMR spectroscopy provide? 1) hemical shift (δ) data reveals the molecular (functional group) environment of the observed nucleus. 2) Peak integration (area) is proportional to the relative number of nuclei giving rise to a signal. 3) J-oupling provides, and -13, connectivity information (how nuclei are connected to each other). 4) The nuclear Overhauser effect (NOE) provides 3-D, spatial information. 5) Polarization transfer experiments (DEPT) provide -13, connectivity information.

3 hemical Shift onsider the general equation for resonance, ν o = γb o /2π For a bare proton, the effective external field (B eff ) is equal to the actual applied field (B o ). + ν (z) B o

4 owever, for protons surrounded by an electron cloud, B eff = B o - B ind or B o (1 - σ), leading to a smaller ν o for the resonance condition, ν o = γb eff /2π B induced e + ν (z) B o The proton is thus said to be shielded and its chemical shift is defined as δ = [(ν sample - ν reference )/ν reference ] x 10 6 (in ppm) relative to a standard such as ( 3 ) 4 Si (TMS) where δ is defined as 0.00 ppm.

5 1 NMR chemical shift ranges for important functional groups:

6 ow many NMR signals are to be expected for a given structure? Determine sets of chemical equivalent nuclei. Nuclei are said to be chemically equivalent if they can be interchanged by (1) a symmetry operation (reflection through a plane, rotation about an axis) or (2) a fast (k > 1000 sec -1 ) intramolecular dynamic process (bond rotation, tautomerization). hemically equivalent nuclei have the same chemical shift.

7 NMR-active nuclei are chemical shift equivalent if they are interchangeable through any symmetry operation (rotation, reflection, inversion) or by a rapid process. Examples: 1. Interchange by rotation Protons are homotopic and thus chemical shift equivalent in achiral and chiral solvents. l l c 2 l l 2. Interchange by reflection Protons are enantiotopic and thus chemical shift equivalent in achiral solvents. F l σ F l

8 3. No interchange by symmetry Methylene protons are diastereotopic and thus chemical shift nonequivalent except by coincidental signal overlap. 3 O O 2 3 O O 2 4. Interchange by rapid interconversion Protons are chemical shift equivalent when the kinetics are fast on the NMR time-scale. 3 O O 2 O 2 O 2 O 2 O 2 O 2 O 2 k ~ 10 6 s -1

9 Predict the theoretical number of signals that could be observed in the 1 and 13 NMR spectra of the following compounds Br O 2 3 O 3 3 l l 3 3

10 Integration of NMR Signals Since the number of absorption/relaxation events is proportional to the number of nuclei in a chemically equivalent set, the area under a signal is proportional to the relative number of those nuclei in the molecule. This is usually obtained by electronic integration. For example, the 1 NMR spectrum of ethyl chloride, 3 2 l, would be comprised of two signals with an integrated area ratio of 3/2.

11 What will each NMR signal s splitting pattern be? Spin-Spin or J-oupling onsider two sets of chemically equivalent nuclei A and X each having a nuclear spin of +1/2. The NMR signal due to each will be mutually split by the coupled nucleus. In general, we will restrict ourselves to nuclei that are not separated by more than 3 bonds. A X The splitting pattern can be predicted by applying the N + 1 Rule, where N is the number of equivalent nuclei coupled to the observed nucleus.

12 J-oupling of an AX System with Spin of +1/2 ( 1, 13, 19 F, 31 P) AX (J = 0) (J > 0) E ββ αβ ν X ν A ν X + J/2 ν A + J/2 ν X ν A βα αα ν X ν A ν X - J/2 ν A - J/2 ν X + J/2 J XA ν X - J/2 ν A + J/2 J AX ν A - J/2 where: α = "aligned" spin state β = "opposed" spin state ν A and ν X = transition (precession) frequencies in z J = coupling constant in z ν X ν A

13 NMR Splitting Tree Diagrams level 1: set a no spin-spin coupling N+1 = 1 singlet (s) level 2: set a coupled to b N+1 = 2 1:1 doublet (d) J ab level 3: set a coupled to 2 x b N+1 = 3 1:2:1 triplet (t) J ab level 4: set a coupled to 3 x b N+1 = 4 1:3:3:1 quartet (q) J ab

14 Second Order Effects (Δν/J < 10)

15 Spin-Spin oupling onstants onnection oupling-type J-value a b geminal (alkane) z a b vicinal (alkane) 2-9 z* a b geminal (alkene) z a b vicinal (cis-alkene) 7-12 z a vicinal (trans-alkene) z b a b vicinal (allylic) 4-10 z

16 Spin-Spin oupling onstants (ontinued) onnection oupling-type J-value a b ortho-aromatic 6-9 z a meta-aromatic 1-3 z b a para-aromatic 0-1 z b

17 Karplus urve for Estimating Vicinal oupling onstants 10 φ J (z) φ

18 In-lass Problem: Predict the 1 NMR spectrum of ethyl acetoacetate. O O 3 2 O 2 3

19 300 Mz 1 NMR spectrum of starting material: O O 3 2 O 2 3 Solvent: Dl 3

20 300 Mz 1 NMR spectrum of reaction aliquot after extractive workup: O O 3 2 O O O O Solvent: Dl 3