Nuclear Magnetic Resonance Spectroscopy Organic Chemistry Second Edition David Klein Chapter 15 Nuclear Magnetic Resonance Spectroscopy Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.1 Intro to NMR Spectroscopy Protons and neutrons in a nucleus behave as if they are spinning If the total number of neutrons and protons is an ODD number, the atoms will have net nuclear spin Examples: 1H, 13C, 15N, 19F, 31P The spinning charge in the nucleus creates a magnetic moment Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.1 Intro to NMR Spectroscopy If the normally disordered magnetic moments of atoms are exposed to an external magnetic field, their magnetic moments will align Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.1 Intro to NMR Spectroscopy The aligned magnetic moments can be either with or against the external magnetic field The α and β spin states are not equal in energy. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.1 Intro to NMR Spectroscopy When an atom with an α spin state is exposed to radio waves of just the right energy, it can be promoted to a β spin state The stronger the magnetic field, the greater the energy gap Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.2 Acquiring a 1H NMR Spectrum NMR requires a strong magnetic field and radio wave energy The strength of the magnetic field affects the energy gap Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.2 Acquiring a 1H NMR Spectrum Solvents are used such as chloroform-d. WHY? The magnet is super-cooled, but the sample is generally at room temp Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.3 Characteristics of a 1H NMR Spectrum NMR spectra contain a lot of structural information Number of signals Signal location – shift Signal area – integration Signal shape – splitting pattern Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.4 Number of Signals Protons with different electronic environments will give different signals Protons that are homotopic will have perfectly overlapping signals Protons are homotopic if the molecule has an axis of rotational symmetry that allows one proton to be rotated onto the other without changing the molecule Find the rotational axis of symmetry in each molecule below Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.4 Number of Signals Another test for homotopic protons is to replace the protons one at a time with another atom If the resulting compounds are identical, then the protons that you replaced are homotopic Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.4 Number of Signals There are some shortcuts you can take to identify how many signals you should see in the 1H NMR The 3 protons on any methyl group will always be equivalent to each other Multiple protons are equivalent if they can be interchanged through either a rotation or mirror plane Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.4 Number of Signals Identify all the groups of equivalent H in the molecules below Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

From where does up/downfield come? Early NMRs analyzed samples at a constant energy (ΔE) over a range of magnetic field (Bo) strengths from low field strength = downfield to high field strength = upfield Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.5 Chemical Shifts Alkane protons generally give signals around 1-2 ppm Protons can be shifted downfield when nearby electronegative atoms cause deshielding. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.5 Chemical Shifts To predict chemical shifts, start with the standard ppm for the type of proton (methyl, methylene, or methine) Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.6 Integration Integrations represent numbers of protons, so you must adjust the values to whole numbers (C4H10O) If the integration of the first peak is doubled, the computer will adjust the others according to the ratio 2.96 3.12 2.00 2.10 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity When a signal is observed in the 1H NMR, often it is split into multiple peaks Multiplicity or a splitting patterns results Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity The resulting multiplicity or splitting pattern for Ha is a doublet A doublet generally results when a proton is split by only one other proton on an adjacent carbon Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity Consider an example where there are two protons on the adjacent carbon There are three possible affects the Hb protons have on Ha Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity Ha appears as a triplet WHY? The three peaks in the triplet have an integration ratio of 1:2:1 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity Consider a scenario where Ha has three equivalent Hb atoms splitting it Explain how the magnetic fields cause shielding or deshielding Hb Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity Ha appears as a quartet What should the integration ratios be for the 4 peaks of the quartet? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity The multiplicity trend continues The splitting pattern can tell you the number of equivalent protons on adjacent carbons Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity Remember three key rules Equivalent protons can not split one another Predict the splitting patterns observed for 1,2-dichloroethane To split each other, protons must be within a 2 or 3 bond distance Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity Remember three key rules To split each other, protons must be within a 2 or 3 bond distance Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity A peak with an integration equal to 9 suggests the presence of a tert-butyl group An isolated isopropyl group gives a doublet and a septet Note the integrations Practice with conceptual checkpoint 15.17 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity Splitting is not observed for some protons. Consider ethanol The protons bonded to carbon split each other, but the hydroxyl proton is not split Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e