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 (CWNMR) 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
20 cm
5 mm diameter
CDCl3
O
D3C S CD3
dimethyl sulfoxide-d6
(DMSO)
the solution (0.7 ml)
CH3
H3C Si CH3
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 field
δ = (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If electron density is withdrawn from around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
CH3-Cl
CH3-OCH3
δ 0.26
δ 3.06
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 downfield 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
4
A quartet for –CH2-CH3
Four signals with the relative intensity of 1:3:3:1
= quartet
1
2
3
Energy
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 spincoupled 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
3
3
1
4
1
2
1
6
1
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 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
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 27.1
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
Graduate Studies in Chemistry
•
Competitive stipends and fellowships;
waived tuition; and assisted health
insurance (PhD’s supported: 82)
•
Ranked top 10 of 178 by National
Research Council in “Student support and
Outcomes” and “Faculty Diversity”
MS and PhD Programs offered in:
•
•
•
•
Analytical
Biological / Biochemical
Biophysical / Computational
Organic / Medicinal
http://www.nap.edu/rdp/
•
For more information:
www.chemistry.gsu.edu
chegsc@langate.gsu.edu
Masters program ranked number 9 in the
United States (number one in the
Southeast) by the American Chemical
Society for MS degrees conferred in 20082009
http://pubs.acs.org/cen/email/html/8834acsnews1.html
30
Center for Diagnostics and
Therapeutics
31
32
33