1 H NMR Interpretation 1. Number of Signals (Resonances) 2. Position of Signals (Chemical Shift) 3. Peak Integrations
4. Splitting Patterns (Multiplicity) 5. Exchangeable Protons Number of Signals Simple Cases Number of Signals Complex Cases
OH 90 MHz spectrum: 11 signals?
6 signals? 400 MHz spectrum Effect of Magnetic Field Strength on Spectrum Quality 60 MHz ppm = Hz/MHz
MHz = Hz/ppm 300 MHz So the instrument frequency tells you how
many Hz there are in 1 ppm Chemical Shift Proton NMR shifts normally range from 0 ppm (TMS) to 10 ppm Inductive effects explain a lot:
CHCl3 in CDCl3 = 7.26 ppm H2O in CDCl3 = 1.55 ppm Chemical Shift
Anisotropic Effects Aromatic Compounds Anisotropic Effects Alkenes and Alkynes Positions of Signals Characteristic Shifts
Peak Integrations 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 intensities 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 has a peak ratio of 1:1. Triplet - For the two Hs causing splitting there are two different ways to align one proton with B0 and one proton against B0 (ab and ab) the middle peak of the triplet is twice as intense as the two outer peaks and the peak ratio is 1:2:1.
Quartet (H signal split by 3 equivalent protons, e.g. an H on a carbon next t o a CH3) Pascals triangle
Rules for predicting splitting patterns If Ha and Hb are not equivalent, splitting is observed when: vicinal germinal
1. The n+1 rule - A set of n protons splits the signal of a nearby proton into n + 1 peaks. 2. Equivalent protons do not split each others signals. 3. Hs bonded to O or N usually do not show splitting Br
Br Br HO
OH HO OH
O The coupling constant, J, is measured in Hz and has to be the same forward and reverse! Exchangeable Protons
From http://web.mnstate.edu/marasing/CHEM429_2008/Documents/1_H_NMR_Interpretation.pdf Solving an NMR unknown With a given formula and NMR spectra, you can determine a molecules structure by a 4-step process
Calculate the degree of unsaturation or hydrogen deficiency index (HDI).
Consider the number of C- 13 NMR signals as an indication of symmetry in the molecule Analyze the proton NMR shifts, integrations and multiplicities draw out the molecular fragments that match. Assemble the fragments into a complete structure like puzzle pieces.
Copyright 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e
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