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Dr. Harpreet Singh

Head, PG Department of Bioinformatics,

Hans Raj Mahila Maha Vidyalaya,

Jalandhar, Punjab, India

e-module

An Introduction to Nuclear Magnetic Resonance (NMR)

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In this diagram, frequency is specified in units of wavenumbers, defined as 1/λ, which is the number of waves per centimeter.

Wavenumbers are used to specify energy in infrared spectroscopy.

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NMR Schematic

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Many nuclei undergo magnetic resonance.

In general, nuclei composed of an odd number of protons (¹H and its isotopes, ¹⁴N ¹⁹F, and

³¹P) or an odd number of neutrons (¹³C) show magnetic behavior.

If both the proton and neutron counts are even (¹²C or ¹⁶O) the nuclei are non-‐magnetic.

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¹H NMR Spectra

An NMR spectrum is a plot of the intensity of a peak against its chemical shift, measured in parts per million (ppm).

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Structural Information from Features of a

¹H NMR Spectrum

Position of signals: indicates what types of hydrogen the molecule contains.
Number of signals: indicates the number of different types of hydrogen in a molecule.
Intensity of signals: indicates the relative amounts (how many) of each kind of hydrogen in the molecule.
Spin-spin splitting of signals: gives further information of the neighboring environment for the various hydrogens in the molecule.

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Shielding and Chemical Shift

Protons in a given environment absorb in a predictable region in an NMR spectrum.

Two important factors: electronegativity and magnetic anisotropy

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Shielding and Chemical Shift

The degree of shielding of a nucleus depends upon its surrounding electron density.

Adding electrons increases shielding.

Removing electrons causes deshielding.

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Characteristic Chemical Shifts Relative to TMS (0 ppm)

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Electronegativity and Chemical Shift

The chemical shift of a C−H bond increases with increasing alkyl substitution or electronegative atoms attached.

RCH₂-‐H

RCH₂-‐OH

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The absorptions of alkane hydrogens occur at relatively high field.

Hydrogens close to an electron withdrawing group (halogen or oxygen) are shifted to relatively lower field (deshielding).

The more electronegative the atom, the more the deshielded methyl hydrogens are relative to methane.

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Multiple substituents exert a cumulative eﬀect.

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The deshielding inﬂuence of electron withdrawing groups diminishes rapidly with distance.

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Aromatic Deshielding and Anisotropy

In a magnetic field, the six π electrons in benzene circulate around the ring creating a ring current.
The magnetic field induced by these moving electrons reinforces the applied magnetic field in the vicinity of the protons.
The protons thus feel a stronger magnetic field and a higher frequency is needed for resonance meaning they are deshielded and absorb downfield.

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Alkyne Chemical Shifts

In a magnetic field, the π electrons of a carbon-carbon triple bond are induced to circulate, but in this case the induced magnetic field opposes the applied magnetic field (B₀).
The proton thus feels a weaker magnetic field, so a lower frequency is needed for resonance.
The nucleus is shielded and the absorption is upfield.

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Summary of π Electron and Chemical Shift

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Regions in the ¹H NMR Spectrum

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Structural Information from Features of a

¹H NMR Spectrum

Position of signals: indicates what types of hydrogen the molecule contains.
Number of signals: indicates the number of different types of hydrogen in a molecule.
Intensity of signals: indicates the relative amounts (how many) of each kind of hydrogen in the molecule.
Spin-spin splitting of signals: gives further information of the neighboring environment for the various hydrogens in the molecule.

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¹H NMR—Spin-Spin Splitting

The spectra up to this point have been limited to single absorptions called singlets.
Often signals for different protons are split into more than one peak.

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Spin-‐Spin Coupling: The Eﬀect of Non-‐Equivalent Neighboring Hydrogens

When non-‐equivalent hydrogen atoms are not separated by at least one carbon or oxygen atom, an additional phenomenon called “spin-‐spin splitting” or “spin-‐ spin coupling” occurs.

Instead of single peaks (singlets), more complex patterns occur called multiplets (doublets, triplets or quartets).

The number and kind of hydrogen atoms directly adjacent to the absorbing nuclei can be deduced from the multiplicity of the peak.

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Spin-‐spin splitting is usually observed only between hydrogen atoms bound to the same carbon (geminal coupling) or to adjacent carbons (vicinal coupling).

Hydrogen nuclei separated by more than two carbon atoms (1,3 coupling) is usually negligible.

Finally, equivalent nuclei do not exhibit mutual spin-‐spin splitting. Ethane exhibits only a single line at δ = 0.85 ppm.

Splitting is observed only between nuclei with diﬀerent chemical shifts.

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Local-‐field contributions from more than one hydrogen are additive.

Consider the triplet above. It corresponds to the methyl protons being split by the methylene protons.

The methylene proton spins will statistically orient in the external magnetic field as αα, αβ, βα and ββ. Each methyl proton will see an increased field 25% of the time (αα), no change 50% of the time (αβ and βα), and a decreased field 25% of the time (ββ).

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The integrated intensity of the triplet will be 6 since there are a total of 6 equivalent methyl protons.

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In the case of the methylene protons, the methyl proton spins will statistically distribute as ααα, ααβ, αβα, βαα, αββ, βαβ, ββα, and βββ.

This will result in a 1:3:3:1 quartet of peaks.

The integrated intensity of the quartet will be 4, corresponding to the 4 equivalent methylene protons.

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In many cases, spin-‐spin splitting is given by the N+1 rule.

A simple set of rules:

Equivalent nuclei located adjacent to one neighboring hydrogen resonate as a doublet.

Equivalent nuclei located adjacent to two hydrogens of a second set of equivalent nuclei resonate as a triplet.

Equivalent nuclei located adjacent to a set of three equivalent hydrogens resonate as a quartet.

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In many cases, spin-‐spin splitting is given by the N+1 rule.

This table illustrates the N+1 rule: Nuclei having N adjacent equivalent neighbors split into N+1 peaks. The heights of the N+1 peaks follow Pascal’s triangle.

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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.

Two additional examples:

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Magnetic Resonance Imaging (MRI)

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How to determine structures of compounds from their spectra?

Determine the molecular formula and degree of unsaturation

From ¹³C-‐NMR, determine # of signals and chemical shifts

From ¹H-‐NMR, determine # of signals, # of protons in each signal, peak splitting patterns and chemical shifts