Transcript Document

1H
NMR Spectroscopy:
Nuclear Magnetic Resonance
Nuclear Shielding
and
1H Chemical Shifts
What do we mean by "shielding?"
What do we mean by "chemical shift?"
The electrons surrounding a nucleus affect the effective
magnetic field sensed by the nucleus
Shielding
An external magnetic field
affects the motion of the
electrons in a molecule,
inducing a magnetic field
within the molecule.
C
H
H0
Shielding
An external magnetic field
affects the motion of the
electrons in a molecule,
inducing a magnetic field
within the molecule.
C
H
The direction of the induced
magnetic field is opposite to
that of the applied field.
H0
Shielding
The induced field shields the
nuclei (in this case, C and H)
from the applied field.
A stronger external field is
needed in order for energy
difference between spin states
to match energy of rf radiation.
C
H
H0
Chemical Shift
Chemical shift is a measure of
the degree to which a nucleus
in a molecule is shielded.
Protons in different
environments are shielded to
greater or lesser degrees;
they have different chemical
shifts.
C
H
H0
Downfield
Decreased shielding
Upfield
Increased shielding
(CH3)4Si (TMS)
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
Chemical shift (d, ppm)
measured relative to TMS
1.0
0
Cl
d 7.28 ppm
H
C
Cl
Cl
The chemical shift, d ppm, is a
measure of how far the signal is
from the TMS reference signal
which is set to 0 ppm
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
Chemical shift (d, ppm)
2.0
1.0
0
Chemical Shift:
H3C
H3C
Si
CH
CH3 3
H3C
The common scale for chemical shifts = d (ppm)
distance downfield from TMS (Hz)
d=
operating frequency of the spectrometer (MHz)
Effects of Molecular Structure
on
1H Chemical Shifts
protons in different environments experience different
degrees of shielding and have different chemical
shifts
The chemical shift is independent of the operating
frequency of the spectrometer
Chemical Shift
Example: The signal for the proton in chloroform
(HCCl3) appears 1456 Hz downfield from TMS at
a spectrometer frequency of 200 MHz.
d =
d =
position of signal - position of TMS peak
spectrometer frequency
1456 Hz - 0 Hz
200 x 106 Hx
d = 7.28
x 106
x 106
Question
• What is the chemical shift of a proton
that appears at 2065 Hz when recorded
on a 300 MHz NMR spectrometer?
•
A) 6.88 ppm
•
B) 0.14 ppm
•
C) 2.07 ppm
•
D) 1.40 ppm
Electron withdrawal produces NMR signals downfield
at
higher frequency (at larger d values)
Electronegative substituents decrease
the shielding of methyl groups
CH3F
CH3OCH3
CH3N(CH3)2
CH3CH3
CH3Si(CH3)3
d 4.3 ppm
d 3.2 ppm
d 2.2 ppm
d 0.9 ppm
d 0.0 ppm
Electronegative substituents decrease
the shielding of methyl groups
CH3F
CH3OCH3
CH3N(CH3)2
CH3CH3
CH3Si(CH3)3
d 4.3 ppm least shielded H
d 3.2 ppm
d 2.2 ppm
d 0.9 ppm
d 0.0 ppm most shielded H
Effect is cumulative
CHCl3
CH2Cl2
CH3Cl
d 7.3 ppm
d 5.3 ppm
d 3.1 ppm
Question
• Which proton is most shielded?
•
A) CHCl3
•
B) CH2Cl2
•
C) CHBr3
•
D) CBr4
Methyl, Methylene, and Methine
CH3 more shielded than CH2 ;
CH2 more shielded than CH
d 0.9
CH3
H3C
C
CH3
H d 1.6
d 0.9
CH3
H3C
C
CH3
d 1.2
CH2
d 0.8
CH3
Question 3
• Select the most shielded proton in 1,1,2trichlorobutane.
•
A) 1
•
B) 2
•
C) 3
•
D) 4
Protons attached to sp2 hybridized carbon
are less shielded than those attached
to sp3 hybridized carbon
H
H
H
H
H
C
H
H
H
CH3CH3
C
H
H
d 7.3 ppm
d 5.3 ppm
d 0.9 ppm
Diamagnetic Anisotropy
The p electrons are less tightly held by the nuclei than
are s electrons; they are more free to move in response
to a magnetic field
Causes unusual chemical shifts for hydrogen bonded to
carbons that form p bonds
But Protons Attached to sp Hybridized Carbon
are More Shielded than those Attached
to sp2 Hybridized Carbon
d 5.3
H
H
C
H
C
H
d 2.4
H
C
C
CH2OCH3
Protons Attached to Benzylic and Allylic
Carbons are Somewhat Less Shielded than Usual
H3C
CH3
d 0.8
d 1.5
d 0.9
d 1.3
d 0.9
H3C—CH2—CH3
d 1.2
H3C
d 2.6
CH2
Question
• Assign the chemical shifts d 1.6,
•
d 2.2, and d 4.8 to the appropriate
protons of methylene cyclopentane.
•
A) x = 1.6; y = 2.2; z = 4.8
•
B) x = 4.8; y = 1.6; z = 2.2
•
C) x = 1.6; y = 4.8; z = 2.2
•
D) x = 2.2; y = 1.6; z = 4.8
Proton Attached to C=O of Aldehyde
is Most Deshielded C—H
d 2.4
H
H3C
C
O
C
CH3
d 1.1
H d 9.7
Question
• Assign the chemical shifts d1.1,
•
d2.4, and d9.7 to the appropriate
protons of propanal.
•
A) x = 2.4; y = 1.1; z = 9.7
•
B) x = 1.1; y = 9.7; z = 2.4
•
C) x = 9.7; y = 2.4; z = 1.1
•
D) x = 1.1; y = 2.4; z = 9.7
Chemical Shift Table
Type of proton
Chemical shift (d), Type of proton
ppm
H
C
R
0.9-1.8
H
C
C
H
C
C
C 1.6-2.6
H
C
Ar
C
C
C
2.5
2.3-2.8
H
O
H
Chemical shift (d),
ppm
2.1-2.5
C
C
4.5-6.5
Chemical Shift Table
Type of proton
H
Ar
Chemical shift (d), Type of proton
ppm
Chemical shift (d),
ppm
6.5-8.5
H
C
Cl
3.1-4.1
9-10
H
C
Br
2.7-4.1
2.2-2.9
H
C
O
3.3-3.7
O
C
H
H
C
NR
Chemical Shift Table
Type of proton
Chemical shift (d),
ppm
H
NR
1-3
H
OR
0.5-5
H
OAr
6-8
O
HO
C
10-13
Characteristic Values of
Chemical Shifts
1H
NMR spectrum / Integration
1-bromo-2,2-dimethylpropane
Integration
The area under each signal is proportional to the number
of protons that give rise to that signal.
The height of each integration step is proportional to the
area under a specific signal.
The integration tells us the relative number of protons
that give rise to each signal, not absolute number.
1H
NMR spectrum / Integration
1-bromo-2,2-dimethylpropane
4.5
(9)
1 (2)