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Nuclear Magnetic Resonance

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Nuclear Magnetic Resonance Nuclear Magnetic Resonance Yale Chemistry 800 MHz Supercooled Magnet Bo Atomic nuclei in Atomic nuclei in the presence The absence of of a magnetic field a magnetic field α spin - with the field β spin - opposed to the field The Precessing Nucleus resonance α spin β spin Ho resonance no resonance no resonance Rf The Precessing Nucleus Again The Continuous Wave Spectrometer The Fourier Transform Spectrometer Observable Nuclei • Odd At. Wt.; s = ±1/2 1 13 15 19 31 Nuclei H1 C6 N7 F9 P15 …. Abundance (%) 99.98 1.1 0.385 100 100 • Odd At. No.; s = ±1 2 14 Nuclei H1 N7 … • Unobserved Nuclei 12 16 32 C6 O8 S16… •Δ E = hν h = Planck’s constant:1.58x10-37kcal- sec •Δ E = γh/2π(Bo) γ = Gyromagnetic ratio: sensitivity of nucleus to the magnetic field. 1H = 2.67x104 rad sec-1 gauss-1 • Thus: ν = γ/2π(Bo) • For a proton, if Bo = 14,092 gauss (1.41 tesla, 1.41 T), ν = 60x106 cycles/sec = 60 MHz and •Δ EN = Nhν = 0.006 cal/mole Rf Field vs. Magnetic Field for a Proton E 60 MHz 500 MHz ΔE = γh/2π(Bo) 100 MHz 1.41 T 2.35 T 11.74 T Bo Rf Field /Magnetic Field for Some Nuclei o Nuclei Rf (MHz) B (T) γ/2π (MHz/T) 1H 500.00 11.74 42.58 13C 125.74 11.74 10.71 2H 76.78 11.74 6.54 19F 470.54 11.74 40.08 31P 202.51 11.74 17.25 Fortunately, all protons are not created equal! at 1.41 T 60 MHz 60,000,600 Hz 60,000,000 Hz upfield downfield Me Si 60 Hz shielded 4 deshielded 240 Hz } 10 9 8 7 6 5 4 3 2 1 0 δ scale (ppm) } at 500 MHz 500 Hz chemical δ = (νobs - νTMS)/νinst(MHz) = (240.00 - 0)/60 = 4.00 shift Where Nuclei Resonate at 11.74T 12 ppm 2 H RCO2H 12 ppm 1 H (TMS) ~380 ppm 31 P (H3PO4) ~220 ppm CH4 76.78 220 ppm 19 F (CFCl3) 13C 500.00 PX3 CPH2 -SO2F -CF2-CF2- 470.54 202.51 125.74 R2C=O RCH3 100 200 300 400 500 MHz Chemical Shifts of Protons The Effect of Electronegativity on Proton Chemical Shifts TMS CHCl3 CH2Cl2 CH3Cl CH4 δ=7.26 δ=5.30 δ=3.05 δ=-0.20 10 9 8 7 6 5 4 3 2 1 0 δ TMS CHBr3 CH2Br2 CH3Br δ=6.83 δ=4.95 δ=2.68 10 9 8 7 6 5 4 3 2 1 0 δ Chemical Shifts and Integrals δ 2.2 TMS area = 3 δ 4.0 area = 2 10 9 8 7 6 5 4 3 2 1 0 δ homotopic enantiotopic protons: protons: chemically chemically and and magnetically Cl magnetically equivalent equivalent H3C C CH2Cl Cl Spin-Spin Splitting δ = 1.2 “anticipated” spectrum area = 9 TMS δ = 6.4 δ = 4.4 area = 1 area = 1 10 9 8 7 6 5 4 3 2 1 0 δ Br Br CH3 Br C C C CH3 H H CH3 Spin-Spin Splitting δ = 1.2 observed spectrum area = 9 TMS δ = 6.4 δ = 4.4 area = 1 area = 1 Hβ Hα Hα Hβ 10 9 8 7 6 5 4 3 2 1 0 δ Br Br CH3 Br C C C CH J is a constant 3 and independent Jcoupling constant = 1.5 Hz H H CH3 of field Spin-Spin Splitting δ = 6.4 δ = 4.4 J = 1.5 Hz J = 1.5 Hz Hβ Hα Hα Hβ νlocal νlocal νobserved νobserved 8 7 6 5 4 3 δ νapplied νapplied Br Br CH3 This spectrum is not recorded at ~6 MHz! Br C C C CH3 H H CH3 δ and J are not to scale. Multiplicity of Spin-Spin Splitting for s = ±1/2 multiplicity (m) = 2Σs + 1 # equiv. pattern spin (1/2) multiplicity symbol neighbors (a + b)n 0 0 1 1 singlet (s) 1 1/2 2 1:1 doublet (d) 2 2/2 = 1 3 1:2:1 triplet (t) 3 3/2 4 1:3:3:1 quartet (q) 4 4/2 = 1 5 1:4:6:4:1 quintet (qt) 1 H NMR of Ethyl Bromide (90 MHz) CH3CH2Br δ = 1.68 area = 3 for H spins ααβ αββ αβα βαβ δ = 3.43 ααα βαα ββα βββ area = 2 1 3 3 1 90 Hz/46 pixels = 3J/12 pixels for H spins : J= 7.83 Hz αβ αα βα ββ 1 2 1 1 H NMR of Isopropanol (90 MHz) (CH3)2CHOH 1H septuplet; no coupling to OH 6H, d, J = 7.5 Hz (4.25-3.80)x90/6 = 7.5 Hz 1H, s δ4.25 δ3.80 Proton Exchange of Isopropanol (90 MHz) (CH3)2CHOH + D2O (CH3)2CHOD Diastereotopic Protons: 2-Bromobutane homotopic sets Br of protons H pro-R H H H H H H H R Diastereotopic protons H pro-S Diastereotopic Protons: 2-Bromobutane at 90MHz Br H H H H H H H H H δ1.03 (3H, t) δ1.70 (3H, d) δ1.82 (1H, m) δ1.84 (1H, m) δ4.09 (1H, m (sextet?)) 1H NMR of Propionaldehyde: 300 MHz H H H O δ1.13 (3H, t, J = 7.3 Hz) H H Is this pattern at 2.46 H δ a pentuplet given that there are two different δ9.79 (1H, t, J = 1.4 Hz) values for J? No! 1H NMR of Propionaldehyde: 300 MHz δ2.46 H H 7.3 Hz 7.3 Hz 7.3 Hz H O 1.4 Hz H H 7.3 Hz H quartet of doublets 7.3 Hz δ9.79 (1H, t, J = 1.4 Hz) δ1.13 (3H, t, J = 7.3 Hz) 1H NMR of Ethyl Vinyl Ether: 300 MHz H 3H, t, J = 7.0 Hz J = 14.4 Hz H J = 1.9 Hz O H H H J = 6.9 Hz H H H δ3.96 (1H, dd, J = 6.9, 1.9 Hz) 2H, q, J = 7.0 Hz δ4.17 (1H, dd, J = 14.4, 1.9 Hz) δ6.46 (1H, dd, J = 14.4, 6.9 Hz) δ6.46 ABX Coupling in Ethyl Vinyl Ether 14.4 H 6.9 H O H H δ4.17 H H H H 1.9 14.4 δ3.96 (1H, dd, J = 6.9, 1.9 Hz) δ4.17 (1H, dd, J = 14.4, 1.9 Hz) δ3.96 6.9 δ6.46 (1H, dd, J = 14.4, 6.9 Hz) 1.9 Another example Dependence of J on the Dihedral Angle The Karplus Equation 1H NMR (400 MHz): cis-4-t-Butylcyclohexanol OH He He He Ha triplet of triplets Ha He 3.0 3.0 He Hδe 4.03, δ4.03, J( HJ(aH) a=) =3.0 3.0 Hz Hz J( HJ(H) =) =2.7 2.7 Hz Hz 2.7 2.7 e e 11.4 Hz 400 Hz 1H NMR (400 MHz): trans-4-t-Butylcyclohexanol Ha He OH H e triplet of triplets Ha Ha Ha Ha δ3.51, J(Ha) = 11.1 Hz 11.1 11.1 J(He) = 4.3 Hz 4.3 4.3 30.8 Hz X Y W Z Peak Shape as a Function of Δδ vs. J H H J Δδ >> J Δδ 10 9 8 7 6 5 4 3 2 1 0 δ Δδ ~= J 10 9 8 7 6 5 4 3 2 1 0 δ Δδ = 0 10 9 8 7 6 5 4 3 2 1 0 δ Magnetic Anisotropy H H H H H H Downfield shift for aromatic protons (δ 6.0-8.5) Bo Magnetic Anisotropy R C C H Upfield shift for alkyne protons (δ 1.7-3.1) Bo 13C Nuclear Magnetic Resonance 13C Chemical Shifts Where’s Waldo? One carbon in 3 molecules of squalene is 13C What are the odds that two 13C are bonded to one another? ~10,000 to 1 13C NMR Spectrum of Ethyl Bromide at 62.8 MHz JCH = 151 Hz H H J = 3 Hz J = 126 Hz CH CH C1 H Br C2 H H J = 5 Hz TMS CH J = 118 Hz H CH H H H H H Si H H H H H H 30 20 10 0 26.6 18.3 ppm (δ) 13C NMR Spectrum of Ethyl Bromide at 62.8 MHz JCH = 151 Hz H H Off resonance decoupling J = 3 Hz of the 1H region removes J = 126 Hz CH CH C1 small C-H coupling. H Br Broadband decoupling C2 removes all C-H coupling. H H J = 5 Hz TMS CH J = 118 Hz H CH H H H H H Si H H H H H H 30 20 10 0 26.6 18.3 ppm (δ) 13C Spectrum of Methyl Salicylate (Broadband Decoupled) 13C Spectrum of Methyl p-Hydroxybenzoate (Broadband Decoupled) The 13C Spectrum of Camphor H H H H H H H H H H H H H H O H H The End F. E. Ziegler, 2004 .
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