Lec 3 NMR Spectroscopy PDF

5. Nuclear magnetic resonance (NMR) 5.1 Introduction NMR provides information about the carbon and hydrogen framework of molecules. Certain nuclei have the property of nuclear spin and act like tiny bar magnets (eg 1H and 13C)..…. applied field no applied field (random orientation) aligned nuclei E...

5. Nuclear magnetic resonance (NMR) 5.1 Introduction NMR provides information about the carbon and hydrogen framework of molecules. Certain nuclei have the property of nuclear spin and act like tiny bar magnets (eg 1H and 13C)..…. applied field no applied field (random orientation) aligned nuclei E excitation from lower to higher energy spin state when E = h NMR MRI The absorption frequency is different for each unique kind of 1H or 13C in any molecule. The applied magnetic field experienced by the nucleus, and hence the E for nuclear spin flip, will depend on the shielding of the nucleus. In an NMR spectrum positions of absorptions are known as chemical shifts and measured in  (delta) units (ppm) from the peak of a reference compound tetramethylsilane (TMS), (CH3)4Si. TMS is defined as 0 .  increasing frequency 5.2 13C NMR All 13C NMR spectra considered in this course will be proton decoupled spectra. Proton decoupling means one does not observe splitting of signals which would otherwise be seen because of 13C−1H interaction. As stated above: each unique carbon will give rise to a different signal. Consider the proton decoupled 13C NMR spectrum of butan-2-ol: 1 2 3 4 CH3CHCH2 CH3 OH 'four unique carbons' 32.1→ 10.8  69.3→ 22.9 TMS 13C chemical shift range is generally  0 –220. sp3 carbons show signals between  0 – 90 sp2 carbons are between  100 – 220. 32.1→ 10.8  1 2 3 4 CH3CHCH2 CH3 OH 69.3→ 22.9 TMS C’s bonded to electronegative substituents (O, N, halogen) will occur at higher  values. The electronegative atom pulls electrons towards itself and away from the attached carbon, whose nucleus is thus deshielded. [C2 is the signal at  69.3 in butan-2-ol.] 5.2.1 13C NMR problems 1. How many signals would be expected in the proton decoupled 13C NMR spectra of : 2 A O CH3CCH3 C 4 O CH3CCH2CH3 O 3 CH3COCH3 B   ◼  6 OCH2CH3 D 2. What structural information can you work out for a compound which has the proton decoupled 13C NMR spectrum? Three unique environments for C. One environ sp2 carbon. Two environs sp3 carbons. Given molecular formula is C3H6O2 could be: CH3CH2CO2H (1) or HCO2CH2CH3 (2) or CH3CO2CH3 (3). [ Actually 3 ]. Practice Problem Which structure is consistent with the 13C NMR spectrum? OH A O O A = 3 x sp3, B = 2 x sp3 + 1 x sp2, C = 3 x sp2 B C 5.3 1H NMR In all 1H NMR spectra only hydrogens give rise to absorption signals. To interpret 1H NMR spectra, we consider: 1. Number of absorption signals. 2. Position of absorption signals. 3. Relative areas under absorption signals. 4. Splitting pattern for absorption signals. 5.3.1 The number of absorption signals H’s in identical molecular environments give only one absorption signal. O Examples: CH3CCH3 ClCH2CH2Cl Hs in ‘X’ different molecular environments give ‘X’ absorption signals. No. of signals CH3CH2Cl 2 CH3CHCl2 2 CH3CH2OH 3 No. of signals CH3CH2OCH2CH3 2 3 sometimes 1 usually 5.3.2 Position of the absorption signal Chemical shift range for 1H generally  0 – 15. Most signals observed in the  0 – 10 range. For hydrogen bonded to sp3 carbon, considering the signal for: C E H as E increases in electronegativity (en) signal is observed at a higher  value. C Si H en Si 1.9  value (H) 0 C C H C 2.5 1 C N C O (or halogen) H H N 3.0 O 3.5 ~3 ~3.5 Note: Substituents containing double bonds deshield. For hydrogen bonded to sp2 carbon signals are at higher  values. TABLE 2 1H NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY (1H NMR) Approximate Chemical Shifts () of Hydrogens in Organic Compounds - supplied in test and examination. 1H NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY (1H NMR) Approximate Chemical Shifts () of Hydrogens in Organic Compounds Hydrogens (H) in various chemical environments Hydrogens bonded to sp3 carbon Hydrogens bonded to sp2 carbon Hydrogen bonded to oxygen Notes:  (ppm downfield from TMS) (CH3)4Si (TMS) 0 R2CHCR 1.0 (approx.) R2CHCZ* 1.0 - 2.0 R2CHC=C 1.6 - 1.9 R2CHC=O 2.0 - 2.5 R2CHAr 2.5 - 3.5 R2CHX** 2.0 - 3.7 R2CHO− 3.3 - 4.5 HC=C 4.5 - 7.0 H−Ar 6.5 - 8.0 HCR O 9.5 - 10.0 HOR 3.0 - 6.0 (variable) HOCR O 10.5 - 12.0 **X = Cl, Br, I or the N of an amine *Z = O, C=O, or X (as defined above) R = H or alkyl group Problem: For each environment for hydrogen give the approximate chemical shift value for the hydrogen(s). CH3CH2OCH 3 CH3CH 2Cl 1.0-2.0 2.0-3.7 1.0-2.0 3.3-4.5 3.3-4.5 CH 3CH 2OCH2CH 3 1.0-2.0 CH3CH 2OH 3.3-4.5 There are 2 signals in 3.3-4.5 region. O CH3CH2COCH 3 1.0-2.0 3.0-6.0 1.0-2.0 3.3-4.5 3.3-4.5 2.0-2.5