Transcript NMR – Effect of Magnetic Field

Chapter 13 NMR Spectroscopy
NMR - Nuclear Magnetic Resonance
NMR is a form of spectroscopy that uses an
instrument with a powerful magnet to
analyze organic compounds.
Invented by physicists (1950’s), then used
by chemists (1960’s).
MRI – Magnetic Resonance Imaging (1980’s)
A special form of NMR used in medicine.
What is NMR?
NMR – a tool to determine the structure of an
organic compound.
Why is it called NMR?
Nuclear Magnetic Resonance
Nuclear – because it looks at the nucleus of
an atom, most commonly a hydrogen atom.
The nucleus of a hydrogen atom consists of
one proton. It has a +1 charge and “spin” of
½, and acts like a tiny bar magnet.
NMR – Effect of Magnetic Field
hydrogen
nucleus
aligned with field
(lower energy)
aligned against field
(higher energy)
No external magnetic
field applied to sample
Sample placed in an external
magnetic field
Random orientation of
nuclear spins
Spins align with or against
field (most align with field)
NMR: Absorption of Energy
radio waves
nucleus
absorbs
energy
Initial State – nucleus
at low energy level
Scan with RF field – nucleus
absorbs energy, giving a
signal in the NMR spectrum
NMR: Information Obtained
from a Spectrum
An NMR Spectrum will generally provide three types
of information:
Chemical Shift – indicates the electronic environment of
the nucleus (shielded or deshielded)
Integration – gives the relative number of nuclei producing
a given signal
Spin-Spin Coupling – describes the connectivity
1H
NMR Spectrum – H2O
signal from
protons in H20
scanning
A sample of water is placed in an NMR instrument, and a
proton spectrum is recorded (scanned from left to right).
An NMR signal appears. This proves that
water contains hydrogen atoms!
When does nucleus absorb energy?
Not all
protons are
the same!
Magnetic Fields:
1. from magnet
2. from protons
3. from electrons
External Field (Ho)
from magnet
Absorption depends on shielding by electron cloud around the nucleus.
More electron density = more shielding = signal shifted to the right.
NMR: Scanning for All Nuclei
13C
area is
much wider
1H
area
is small
To see both proton and C-13
nuclei, a very wide region
would have to be scanned.
An instrument can only
examine one area at a time.
NMR: Simple 1H NMR Spectrum
Showing Chemical Shift
Chemical Shift:
location of the signal
on the spectrum.
Left Side:
low electron
density
Right Side:
high electron
density
Two types of protons (a CH2 and a CH3 group) give two
separate signals at two different chemical shifts.
NMR: Chemical Shift Practice
Group
EN
-O-CH3
3.5
-Si-CH3
1.8
-C-CH3
2.5
Cl3C-H
3.0
Cl3C-H
-OCH3
CH3CH3
Left Side:
low electron
density
(high EN)
Assign the four groups shown to the four NMR
singals, based on each element’s electronegativity.
-SiCH3
NMR: Chemical Shift Reference
Chemical shift zero is set to
TMS (tetramethylsilane).
CH3
TMS =
H3C Si CH3
CH3
(silicon – low electroneg.)
Chemical shift measured in ppm. For 1H: roughly 0 to 10 ppm.
NMR: Chemical Shift Regions
-O-CH3
Double Bond
Region 5-10 ppm
Alkane region (high electron density) is from about .8 – 2.5 ppm.
Heteroatom region (low electron density) is from about 2.5 to 5.
Double bond region is on the left, from about 5 – 10 ppm.
NMR: Chemical Equivalence and
Number of Signals
How many signals will the following compounds show in their
1H NMR Spectrum?
(Hint: check for symmetry)
O
OMe
Br
2
4
H
Cl
H
NH2
5
Cl
Cl
H
H
H
Cl
H
2
4
7
NMR: Chemical Equivalence and
Number of Signals
How many signals should appear in the proton NMR
spectrum for these compounds?
O
octane
In theory:
9
4
Signals actually
resolved:
3-4
2
NMR: Overlapping Proton Signals
The -CH2- groups all
appear in the same spot
(not resolved)
Protons b, c, and d are in roughly the same environment,
and their chemical shifts are also about the same.