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A Practical Introduction to Nuclear Magnetic Resonance Spectroscopy

Basic Theory

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Key Learning

• Why NMR Spectroscopy• Introduction to NMR• Chemical Shift• Shielding Demystified• Chemical Shift and PPM Scale• Integration• Multiplet Analysis• Summary

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Why NMR Spectroscopy?

picoSpin45

ethyl acetate spectrum

•Arguably the most powerful analytical method in organic chemistry

•Provides clues to neighboring functional groups and their location

•Yields quantitative concentration ratios (+/- 1%)

•Reveals the structural arrangement of a molecule for a given nuclei

•Fast, non-destructive analysis

•An excellent complimentary technique to Infra-red and Mass spec.

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What NMR Signals Tell Us

• The number of signals shows how many different kinds of protons (Hydrogen) are present.

• The location (chemical shift) of the signals shows how shielded or deshielded the proton is.

• The intensity of the signal shows the number of protons of that type.

• Signal splitting shows the number of protons on adjacent atoms.

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Chemical Shift

Chemical Shift is measured as a shift along the x axis relative to a standard (in most cases Tetramethylsilane or TMS). TMS always has a chemical shift of 0.0 Hz

TMS

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This shift is caused by the magnetic environment experienced by the nuclei (H for picoSpin) which changes based on the electron density surrounding the nuclei being measured.

Blue indicates areas of low electron density. Red indicates areas of high electron density.

EthanolCH3CH2OH

Image From: Dr. Sheila Woodgate, University of Auckland, 2002, http://www.bestchoice.net.nz/chemistry/913/p14418.htm

Chemical Shift

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Blue indicates areas of low Be strength. Red indicates areas of high Be strength.

EthanolCH3CH2OH

Chemical Shift

Because electrons are moving and have charge, they induce a magnetic field Be. This field opposes the applied field B0 (the magnetic field from the instrument’s magnet). This is called “shielding” because the electrons are shielding the nucleus from the effect of B0.

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Chemical Shift

e-

e- e-

e-

e-e-

e-

e-

nucleus

e- cloud(shields the nucleus from B0)

B0

Because the chemical shift is dependent upon the total magnetic field strength experienced by the nuclei, different electron densities surrounding a given nuclei will cause it’s signal to appear at different positions along the x axis.

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Chemical Shift

Highly electronegative atoms (like oxygen, halogens, etc.) pull electron density away from the nucleus “deshielding” it from the magnetic field of the instrument.

H

e-

e- e-

e-

e-

e-

e-

e-

smaller e- cloud(deshielded by the oxygen atom)

B0

The electron density surrounding the nucleus is dependent upon the electronegativity and atoms bonded to it, therefore…

Chemical shift tells us about the functional groups surrounding a given NMR signal.

O

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Shielding Demystifiede- cloud

(shields the nucleus from B0)

nucleus

Ma

gn

eti

c F

ield

fro

m I

ns

tru

me

nt

+ =

Down-FieldDeshielded

Up-FieldShielded

Magnetic field from the instrument

Opposing field from e- cloud

Opposing field from e- cloud

Total magnetic field experienced by the

nucleus

smaller e- cloud(less shielding)

Nuclei with less electron density surrounding

them are said to be de-shielded, and appear

further “downfield” or to the left of the spectrum.

B0

BeBe

+ =

Magnetic field from the instrument

Opposing field from e- cloud

Total magnetic field experienced by the

nucleus

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Examples:

2 ppm = 90 Hz chemical shift / 45 MHz magnet

1 ppm = 400 Hz chemical shift / 400 MHz magnet

This example eludes to the advantage of stronger magnet systems: chemical shift dispersion

Chemical Shift and the ppm Scale

As we’ve seen, chemical shift is dependent on the field strength. Instrument’s with different strength magnets (45 MHz magnet vs. a 400 MHz magnet) produce different chemical shifts for the same signal.

To make the chemical shifts of NMR spectra magnet strength independent, we use the ppm, or parts-per-million scale.

chemical shift in ppm = chemical shift in Hz / Frequency of the magnet in MHz

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1H Chemical Shifts of Protons Accompanying Common Functional Groups.

http://academic.reed.edu/chemistry/alan/201_202/lab_manual/appendices/nmr_ir.html

Note: Anything that can change the magnetic environment around a given nuclei (solvent, concentration, temperature, dielectric strength, etc.) can influence chemical shift. Protons that are susceptible to exchange and/or hydrogen bonding (OH groups for example) tend to show these shifts the most.

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Integration

NMR is inherently quantitative. Two protons will produce a signal with twice the area of a signal produced by a single proton.

If the sample contains only a single component, the area of the peaks can be used in conjunction with the chemical shift and multiplet information to determine or verify the chemical structure of the sample as shown in the next slide.

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Ethyl Alcohol

Integration

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If the sample contains more than one component, and the chemical structures of those components are known, the integration values can be compared to determine the relative concentrations of the components in the sample.

Integration

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Both of these signal groups represents two protons. Since the relative integration of these two signals is one to one, it indicates that the two components of the sample (ethanol and ethyl acetate) are present in a one to one mole ratio.

Integration

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Both of these signal groups represents two protons. Since the relative integration of these two signals is two to one, it indicates that there are twice as many moles of ethanol in this sample as there are moles of ethyl acetate.

Note: The entire spectrum doesn’t need to be resolved to accomplish the quantitation.

Integration

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Multiplet Analysis

Multiplet splitting is a splitting of a NMR signal into multiple peaks due to interactions between the quantum spins of neighboring nuclei.

or

Signal of Interest

Neighboring nuclear spin

1 neighbor

2 neighbors

couples to

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number of peaks in the multiplet

number of neighboring protons within three bonds of

the proton responsible for the

signal

= + 1

The n+1 rule

Count all protons within 3 bonds to

determine the number of neighbors

Multiplet Analysis

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Ethyl Alcohol

These protons have two neighboring protons (from the CH2 group) within

three bonds.

These protons have three neighboring protons from the

terminal CH3 group. Note: splitting is not normally observed

through hetero-atoms such as oxygen.

There are no neighboring protons on the same side of the

oxygen atom.

Multiplet Analysis

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Configuration Peak Ratios

A 1 Singlet

AB 1:1 Doublet

AB2 1:2:1 Triplet

AB3 1:3:3:1 Quartet

AB4 1:4:6:4:1 Pentet or Quintet

AB5 1:5:10:10:5:1 Sextet

AB6 1:6:15:20:15:6:1 Septet

number of peaks in the multiplet

number of neighboring protons within three bonds of the proton responsible for the

signal

= + 1

The n+1 rule

Multiplet Analysis

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Summary

NMR is an incredibly important analytical technique that provides both quantitative and qualitative information quickly and non destructively.

NMR Spectra are interpreted using three main properties

• Signals – the number of signals tells us about the number of different proton types

• Chemical Shift – displacement along the x axis, indicates electron withdrawing potential of neighboring functional groups.

• Integration – quantifies relative amounts by measuring area under peaks

• Multiplet Splitting – signal splitting : Doublets, triplets, quartets, etc.reveals neighboring nuclei and structural conformation

Follow the “n+1 rule”