Nuclear Magnetic Resonance

(1H NMR and 13C NMR)

Magnetic field is created by a spinning charge. The resultant magnetic dipoles of nuclei (I =1/2) are aligned with the external magnetic field Bo as shown

Splitting of energy levels for a nucleus with I = ½, such as hydrogen, in an external magnetic field

A diagram of a continuous wave NMR (CW-NMR) instrument. The sweep coils are used to modulate the strength of the external magnetic field

the NMR tube

20 cm

5 mm diameter

the solution (0.7 ml)

Solvents must not contain protons

CCl4 CDCl3

O

SD3C CD3

dimethyl sulfoxide-d6

(DMSO)

The NMR tube

Si

CH3

CH3

CH3H3C

tetramethylsilane (reference)

(TMS)

The shielding effect

In an applied magnetic field, magnetic nuclei like proton precess at a frequency ν, which is proportional to the strength Bx of the applied field: ν = γBx/2π

precession orbit

magnetic dipole created by proton spin

H0

external magnetic field

δ = (νmol – νTMS)/ν x 106

Proton chemical shift ranges for samples in CDCl3solution. The δ scale is relative to TMS at δ = 0

If electron density is withdrawn from around

the hydrogen nucleus toward a more

electronegative atom, the lower electron

density around this hydrogen atom will

produce a smaller magnetic field (opposite to

the magnetic field of the spectrometer) and, as

a result, this proton will be deshielded and will

resonate at a position farther downfield

(farther to the left in the spectrum). For

example:

CH3-CH3 δ 0.26

CH3-Cl δ 3.06

CH3-OCH3 δ 3.24

Integration of the NMR spectra

The effect of the H – D exchange on the NMR spectra

R-O-H + D2O R-O-D + D-O-H

The hydroxyl proton can resonate over a large range of chemical shifts but hydrogen bonding results in the resonance at a lower magnetic field or higher frequency. Because of their favored hydrogen-bonded dimeric association, the hydroxyl proton of carboxylic acids displays a resonance signal significantly down-field of other functions

Magnetic anisotropy at the benzene ring

The spectra with and without a coupling pattern

Typical coupling patters

If an atom under examination isperturbed or influenced by a nearby magnetic field caused by a nuclear spin (or set of spins), the observed nucleus responds to such influences, and its response is manifested in its resonance signal. This spin-coupling is transmitted through the connecting bonds, and it functions in both directions.

Spin – spin coupling for -CH2-CH3

For a CH2 group adjacent to a methyl group, there will be four peaks, created by the spin

orientations of the methyl protons shown below

1 2 2 2 3 3 3 4

A quartet for –CH2-CH3

Four signals with the relative intensity of 1:3:3:1

= quartet

Energy

1 2 3 4

The “roof effect” for coupled protons

Pascal’s triangle (the intensity ratio)The splitting pattern of a given nucleus (or set of equivalent nuclei) can be predicted by the n+1 rule, where n is the number of neighboring spin-coupled nuclei with the same (or very similar) Js. If there are 2 neighboring spin-coupled nuclei, the observed signal is a triplet (2 + 1 = 3); if there are three spin-coupled neighbors, the signal is a quartet (3 + 1 = 4 ). In all cases the central line(s) of the splitting pattern are stronger than those on the periphery (the “roof effect”).

1

1 1

1 2 1

1 3 3 1

1 4 6 4 1

Typical coupling patterns with a single coupling constant J

Typical coupling patters with different coupling constants Js

Typical values of coupling constants Js (in Hz)

13C NMR spectroscopyWhen significant portions of a molecule lack C-H bonds, little information is forthcoming by 1H NMR.

The following diagram depicts three pairs of isomers (A & B) which display similar proton NMR spectra.

13C NMR spectroscopy

13C isotope has a spin I = ½ (is magnetic)1.1% of natural carbon is the 13C isotope

In 13C NMR spectroscopy, the sample is irradiated with a relatively intense range of frequencies that correspond to precessional frequencies of all protons in the molecule. As a result, these protons become saturated, no further absorption of the irradiation energy is possible, and the protons are no longer coupled to 13C nuclei.

Proton-decoupled 13C NMR and 1H NMR spectra of camphor

13C NMR chemical shifts for various classes of compounds. The δ scale is relative to TMS at δ = 0

The isomeric pairs previously examined as giving very similar proton NMR spectra can be distinguished by carbon NMR spectroscopy.Cyclohexane (A): a single signal at δ 27.1Alkene (B): two signals at δ 20.4 and δ 123.5Fulvene (A): five signalsortho-Xylene (B): four signalsQuinone (A): four signalsQuinone (B): five signals

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Center for Diagnostics and Therapeutics

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