CHEM 347 Laboratory Methods in Organic Chemistry Introduction to NMR Spectroscopy Splitting of magnetic energy levels in presence of external field -1/2 state E 300 MHz 400 MHz 600 MHz +1/2 state 7.02 tesla 9.46 tesla B0 14.1 tesla General condition for nuclear magnetic resonance νo = γBo/2π where: νo = applied radiofrequency pulse γ = gyromagnetic ratio (a nucleus-dependent constant) Bo = external magnetic field Basic NMR 1-pulse sequence FID magnitude time What structural information does NMR spectroscopy provide? 1) Chemical 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-Coupling provides H,H and C-13,H connectivity information (how nuclei are connected to each other). 4) The nuclear Overhauser effect (NOE) provides 3-D H,H spatial information. 5) Polarization transfer experiments (DEPT) provide C-13,H connectivity information. Chemical Shift Consider the general equation for resonance, νo = γBo/2π For a “bare” proton, the “effective” external field (Beff) is equal to the “actual” applied field (Bo). + ν (Hz) Bo However, for protons surrounded by an electron cloud, Beff = Bo - Bind or Bo (1 - σ), leading to a smaller νo for the resonance condition, Binduced νo = γBeff/2π e ν (Hz) + Bo The proton is thus said to be “shielded” and its chemical shift is defined as δ  (ppm) = resonance frequency (Hz) – reference frequency (Hz) ---------------------------------------------------------------------operating frequency of instrument (MHz) relative to a reference (CH3)4Si, where δ is defined as 0.00 ppm. 1H NMR chemical shift ranges for important functional groups: Deshielding of aromatic protons due to magnetic anisotrpy How 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). Chemically equivalent nuclei have the same chemical shift. 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. H H c2 Cl Cl H H Cl Cl 2. Interchange by reflection Protons are enantiotopic and thus chemical shift equivalent in achiral solvents. H H F Cl σ H H F Cl 3. No interchange by symmetry Methylene protons are diastereotopic and thus chemical shift nonequivalent except by coincidental signal overlap. H H H 3C H CO2H ≠ HO H HO H 4. Interchange by rapid interconversion Protons are chemical shift equivalent when the kinetics are fast on the NMR time-scale. H HO C H H H H H HO C H HO H CO2H CH2CO2H H H H HO k ~ 10 6 s-1 H H3C CH2CO2H CH2CO2H H3C H C H H H CO2H Predict the theoretical number of signals that could be observed in the 1H and 13C NMR spectra of the following organic structures. CH3 CH2CH 2Br H3C CH CH3 OH CH3 CH2OCH2 CH3 H CH 3 H H H H H H H CH 3 H H H H H C C CH3 H CH3 H C C Cl H CH 3 CH 3 CH 3 H CH 3 Cl 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 1H NMR spectrum of ethyl chloride, CH3CH2Cl, would be comprised of two signals with an integrated area ratio of 3/2. What will each NMR signal’s splitting pattern be? Spin-Spin or J-Coupling Consider 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. J-Coupling of an AX System with Spin of +1/2 (1H, 13C, 19F, 31P) AX (J = 0) (J > 0) ββ νX E νA νX + J/2 νA + J/2 αβ νX νA νX - J/2 βα νX JXA νA νA - J/2 αα where: α = "aligned" spin state β = "opposed" spin state νA and νX = transition (precession) frequencies in Hz J = coupling constant in Hz J AX νX + J/2 νA + J/2 νX - J/2 νX ν A - J/2 νA NMR Splitting Tree Diagrams level 1: set Ha no spin-spin coupling N+1 = 1 singlet (s) level 2: set Ha coupled to Hb N+1 = 2 1:1 doublet (d) Jab level 3: set Ha coupled to 2 x H b N+1 = 3 1:2:1 triplet (t) level 4: set Ha coupled to 3 x H b N+1 = 4 1:3:3:1 quartet (q) Jab Jab Second Order Effects (Δν/J < 10) Spin-Spin Coupling Constants Connection Coupling-Type Ha J-value geminal (alkane) 12-15 Hz vicinal (alkane) 2-9 Hz* geminal (alkene) 0.5-3 Hz 7-12 Hz C C vicinal (cis-alkene) vicinal (trans-alkene) 13-18 Hz C C C Hb Ha Hb C C Ha C C Hb Ha Hb Ha Hb Ha C C C Hb vicinal (allylic) 4-10 Hz Spin-Spin Coupling Constants (Continued) Connection Ha Hb Coupling-Type J-value ortho-aromatic 6-9 Hz meta-aromatic 1-3 Hz para-aromatic 0-1 Hz Ha Hb Ha Hb Karplus Curve for Estimating Vicinal Coupling Constants H φ C H C 10 J (Hz) 5 0 0° 90° φ 180° In-Class Problem: Predict the 1H NMR spectrum of ethyl acetoacetate. O O H3C C CH2 C OCH2CH3 1H O NMR spectrum of ethyl acetoacetate: O H3C C CH2 C OCH2CH3 Solvent: CDCl3 Broad band proton decoupled 13C NMR pulse sequence 13C chemical shift ranges as a function of molecular environment In-Class Problem: Predict the 13C NMR spectrum of ethyl acetoacetate. O O H3C C CH2 C OCH2CH3 Broad band decoupled 13C NMR spectrum of ethyl acetoacetate: Solvent: CDCl3 How can 1H NMR be used to ascertain if the following reaction worked? Ac2O, pyridine 1H NMR (400 MHz, DMSO-D6) δ 2.75 (1 H, t, J = 8.49), 3.11 (1 H, m), 3.20 (1 H, t, J = 7.65), 3.29 (1 H, d, J = 1.34), 3.38 (1 H, t, J = 7.67), 3.44 (1 H, dd, 6.39, 11.99), 3.69 (1 H, dd, J = 5.44, 11.66), 3.76 (3 H, s), 4.50 (1 H, t, J = 5.78), 4.66 (1 H, t, J = 7.26), 4.76 (1 H, d, J = 5.24), 4.87 (1 H, d, J = 5.56), 6.48 (1 H, d, J = 6.63), 6.95 (2 H, d, J = 8.21), 7.65 (2 H, d, J = 8.18), 8.08 (1 H, s). 1H NMR (400 MHz, DMSO-D6) δ 2.75 (1 H, t, J = 8.49), 3.11 (1 H, m), 3.20 (1 H, t, J = 7.65), 3.29 (1 H, d, J = 1.34), 3.38 (1 H, t, J = 7.67), 3.44 (1 H, dd, 6.39, 11.99), 3.69 (1 H, dd, J = 5.44, 11.66), 3.76 (3 H, s), 4.50 (1 H, t, J = 5.78), 4.66 (1 H, t, J = 7.26), 4.76 (1 H, d, J = 5.24), 4.87 (1 H, d, J = 5.56), 6.48 (1 H, d, J = 6.63), 6.95 (2 H, d, J = 8.21), 7.65 (2 H, d, J = 8.18), 8.08 (1 H, s). 1H NMR (600 MHz, DMSO-D6 with D2O) δ 2.77 (1 H, t, J = 8.61), 3.11 (1 H, t, J = 9.24), 3.21 (1 H, dd, J = 5.41, 10.02), 3.39 (1 H, t, J =7.6), 3.44 (1 H, m), 3.68 (1 H, dd, J = 2.34, 11.65), 3.76 (3 H, s), 4.66 (1 H, d, J = 7.66), 6.96 (2 H, d, J = 8.25), 7.65 (2 H, d, J = 8.21), 8.08 (1 H, s). 1H NMR (400 MHz, CDCl3) δ 1.86 (3 H, s), 2.00 (3 H, s), 2.02 (3 H, s), 2.08 (3 H, s), 3.43 (1 H, t, J = 9.01), 3.82 (3 H, s), 3.95 (1 H, dd, J = 2.74, 10.12), 4.11 (1 H, dd, J = 2.30, 12.32), 4.36 (1 H, dd, J = 4.53, 12.41), 5.12 (1 H, t, J = 9.77), 5.41 (1 H, t, J = 9.60), 5.92 (1 H, d, J = 8.27), 6.89 (2 H, d, J = 8.41), 7.63 (2 H, d, J = 8.32), 8.14 (1 H, s).