Features of an NMR spectrum: SHAPE Spin Coupling: Neighboring nuclei “split” NMR signals Usually n neighbors splits the signal into n+1 peaks Multiplicity = n+1 Which Isomer of C5H10O gives rise to this spectrum? Triplet => 2 neighbors Quartet means the nuclei responsible for this signal have 3 H’s “next door” 1 Origin of Spin Coupling: Nearby nuclei effect the magnetic field E B0 Hb Magnetic field of Hb subtracts from the applied field; Hb signal appears at a higher applied field Hb Magnetic field of Hb adds to the applied field; Ha signal appears at a lower applied field Ha 2 When a neighbor adds to H0 the E is larger, and so is the chemical shift 3 Pascals Triangle predicts pattern 4 Multiplets have predictable shapes Doublets: 1:1 Triplets: 1:2:1 Quartets: 1:3:3:1 Quintet: 1:4:6:4:1 5 The extent to which neighboring nuclei effect the E changes depends on the geometry of the molecule. It does not depend on the magnet strength. Coupling is MUTUAL, so extent of splitting is equal for both signals. The distance between the individual peaks making up the doublet is called the “coupling constant” (J). Here J is 7 Hz. 6 Coupling constants are dependant primarily on conformation/ orientation between coupled signals They are independent of magnet strength 7 Generally coupling is only observed when neighbors are ≥ 3 bonds away from eachother 8 Putting the features together allows for structure elucidation. The following spectrum is for C2H4Cl2 The Area of signal a is 3 times larger than b (CH3?) It is a doublet (ie it has only one neighbor) Therefore a is a methyl next to a methine: CH3-CH 9 10 It is important to note that non-equivalent nuclei split each other. A split in one requires a split in the other. In addition, the coupling constants will be the same for each type of nuclei. 11 Draw the structure of C3H7I consistent with this 1H NMR spectrum 12 Complications in 1H NMR Hb is split into more than 5 peaks. Coupling to “non-equivalent” neighbors may modify the simple n+1 rule: Max splitting is (n+1)(m+1) where n and m are the numbers of neighbors on each side 13 With 3 neighbors on left and 1 neighbor on right max multiplicity = (3+1)(1+1)= 8 14 Despite this complication, the splitting may still simplify to n+1 even with non-equivalent neighbors Multiplicity = 6 (predicted as (3+1)(2+1) = 12 15 Complications in 1H NMR: Signal distortion in non-first-order spectra Signals from coupled protons that are close together (in Hz) can show distorted patterns. When ν >> J, the spectra is said to be first-order. Non-first-order spectra assume more complex shapes than Pascal’s triangle predicts and can only be analyzed with the help of computers. J >>J >J ≈J <<J 16