Nuclear Magnetic Resonance - (1H NMR and 13C NMR)
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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
the NMR tube
Solvents must not contain protons
CCl4 CDCl3
O
20 cm
D3C S CD3
5 mm diameter dimethyl sulfoxide-d6
(DMSO)
CH3
H3C Si CH3
the solution (0.7 ml)
CH3
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 fieldδ = (νmol – νTMS)/ν x 106
Proton chemical shift ranges for samples in CDCl3 solution. 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If electron density is withdrawn from around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 3.24Integration of the NMR spectra
The effect of the H – D exchange on
the NMR spectra
R-O-H + D2O R-O-D + D-O-HThe 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 is perturbed 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 4A quartet for –CH2-CH3
Four signals with the relative intensity of 1:3:3:1
= quartet
1 2 3 4
EnergyThe “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 1Typical coupling patterns with a single coupling
constant JTypical coupling patters with different coupling
constants JsTypical values of coupling constants Js (in Hz)
13C NMR spectroscopy When 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 13Cisotope 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 camphor13C
NMR chemical shifts for various classes of
compounds. The δ scale is relative to TMS at δ = 0The 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 27.1 Alkene (B): two signals at δ 20.4 and δ 123.5 Fulvene (A): five signals ortho-Xylene (B): four signals Quinone (A): four signals Quinone (B): five signals
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