1H NMR Interpretation Number of Signals (Resonances) Positions of Signals – Chemical Shift Relative Intensities of Signals – Integrals Splitting Patterns – Spin-Spin Coupling Exchangeable Protons

Number of Signals – Simple Cases

Number of Signals and Stereochemistry Watch out when you have rings and/or double bonds! To determine equivalent protons in cycloalkanes and alkenes, always draw all bonds to hydrogen.

Chemical Shift - Local Diamagnetic Shielding

Chemical Shift Induced Anisotropic Shielding - Benzene In an external magnetic field (B0), the six  electrons in a benzene ring circulate around the ring creating a ring current, which has an associated magnetic field.

Experimental Validation H over ring = 0.7 ppm Waugh, Fessenden J. Am. Chem. Soc. 1957, 79, 846. H over ring = -4.0 ppm Pascal, W.; Van Engen J. Am. Chem. Soc. 1989, 111, 3007.

Induced Anisotropic Shielding – Alkenes and Alkynes

Summary of pi electron effects

Positions of Signals – Characteristic Shifts

Intensity of Signals The area under an NMR signal (the integration) is proportional to the number of absorbing protons. The area ratios tell us the ratios of the protons causing the peak. Strategy - find a peak that you can assign unambiguously and set its integration at the appropriate number of Hs.

Splitting Patterns Consider the spectrum below:

Peak ratios in a multiplet. Doublet – The two spin states of the proton causing splitting are nearly equally populated (because the energy difference is so small). Therefore a doublet is has a peak ratio of 1:1. Triplet - Because there are two different ways to align one proton with B0, and one proton against B0—that is, ab and ab—the middle peak of the triplet is twice as intense as the two outer peaks, making the ratio of the areas under the three peaks 1:2:1. Higher – use Pascals triangle

Multiplet names

Rules for predicting splitting patterns Equivalent protons do not split each other’s signals. A set of n nonequivalent protons splits the signal of a nearby proton into n + 1 peaks. Splitting is observed for nonequivalent protons on the same carbon or adjacent carbons. If Ha and Hb are not equivalent, splitting is observed when: