NMR Spectroscopy
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Transcript NMR Spectroscopy
NMR Spectroscopy
Part II. Signals of NMR
Free Induction Decay (FID)
• FID represents the time-domain
response of the spin system following
application of an radio-frequency pulse.
• With one magnetization at w0, receiver
coil would see exponentially decaying
signal. This decay is due to relaxation.
Fourier Transform
The Fourier transform relates the
time-domain f(t) data with the
frequency-domain f(w) data.
Fourier Transform
Fourier Transform
NMR line shape
Lorentzian line
y
AW
2
W 4x0 x
2
A
amplitude
W
half-line width
2
Resolution
Definition
For signals in frequency domain it is the deviation of the
peak line-shape from standard Lorentzian peak. For time
domain signal, it is the deviation of FID from exponential
decay. Resolution of NMR peaks is represented by the
half-height width in Hz.
Resolution
Resolution-digital resolution
Resolution
Measurement
half-height width:
10~15% solution of 0-dichlorobenzene
(ODCB) in acetone
Line-shape:
Chloroform in acetone
Resolution
Factors affect resolution
Relaxation process of the observed nucleus
Stability of B0 (shimming and deuterium locking)
Probe (sample coil should be very close to the sample)
Sample properties and its conditions
Sensitivity
Definition
signal to noise-ratio
A
s / n 2 .5
N pp
A:
height of the chosen peak
Npp : peak to peak noise
Sensitivity
Measurement
1H
0.1% ethyl benzene in deuterochloroform
13C
ASTM, mixture of 60% by volume deuterobenzene
31P
15N
and dioxan or 10% ethyl benzene in chloroform
1% trimehylphosphite in deuterobenzene
19F
90% dimethylformamide in deutero-dimethylsulphoxide
0.1% trifluoroethanol in deuteroacetone
2H, 17O
tap water
Sensitivity
Factors affect sensitivity
Probe: tuning, matching, size
Dynamic range and ADC resolution
Solubility of the sample in the chosen solvent
Spectral Parameters
Chemical Shift
Caused by the magnetic shielding of the nuclei by their
surroundings. d-values give the position of the signal relative to
a reference compound signal.
Spin-spin Coupling
The interaction between neighboring nuclear dipoles leads to a
fine structure. The strength of this interaction is defined as spinspin coupling constant J.
Intensity of the signal
Chemical Shift
Origin of chemical shift
Beff B0 sB0 1 s B0
s
shielding constant
1 s B0
Beff
2
2
'
Chemically non-equivalent nuclei are shielded to different
extents and give separate resonance signals in the spectrum
Chemical Shift
Chemical Shift
d – scale or abscissa scale
B0
1 s 1
1
2
B0
1 s 2
2
2
B0
s 2 s 1
2 1
2
2 1
s 2 s1
1
Chemical shift parameter d s 2 s 1 106
Chemical Shift
d
106
observingfrequency
Shielding s
CH3Br < CH2Br2 < CH3Br < TMS
d CHBr3
90 MHz spectrum
614
90106
106 6.82 (ppm)
Abscissa Scale
Chemical Shift
d is dimensionless expressed as the relative
shift in parts per million ( ppm ).
d is independent of the magnetic field
d of proton
0 ~ 13 ppm
d of carbon-13
0 ~ 220 ppm
d of F-19
0 ~ 800 ppm
d of P-31
0 ~ 300 ppm
Chemical Shift
s
local
s dia
local
s para
s N s R s e s i
Charge density
Neighboring group
Anisotropy
Ring current
Electric field effect
Intermolecular interaction (H-bonding & solvent)
Chemical Shift –
anisotropy of neighboring group
sN
1
3r 3 4
// 1 cos2
susceptibility
r distance to the dipole’s center
Differential shielding of HA and HB in
the dipolar field of a magnetically
anisotropic neighboring group
Chemical Shift –
anisotropy of neighboring group
d~2.88
d~9-10
• Electronegative groups are "deshielding" and tend to move NMR signals from
neighboring protons further "downfield" (to higher ppm values).
• Protons on oxygen or nitrogen have highly variable chemical shifts which are
sensitive to concentration, solvent, temperature, etc.
• The -system of alkenes, aromatic compounds and carbonyls strongly deshield
attached protons and move them "downfield" to higher ppm values.
•Electronegative groups are "deshielding" and tend to move NMR signals
from attached carbons further "downfield" (to higher ppm values).
•The -system of alkenes, aromatic compounds and carbonyls strongly
deshield C nuclei and move them "downfield" to higher ppm values.
•Carbonyl carbons are strongly deshielded and occur at very high ppm
values. Within this group, carboxylic acids and esters tend to have the
smaller values, while ketones and aldehydes have values 200.
Ring Current
The ring current is induced form the delocalized
electron in a magnetic field and generates an additional
magnetic field. In the center of the arene ring this
induced field in in the opposite direction t the external
magnetic field.
Ring Current -- example
Spin-spin coupling
Spin-spin coupling
AX system
AX2 system
Spin-spin coupling
AX3 system
Multiplicity Rule
Multiplicity M (number of lines in a multiplet)
M = 2n I +1
n equivalent neighbor nuclei
I spin number
For I= ½
M=n+1
Example
AX4
AX4 system
I=1; n=3
Order of Spectrum
Zero order spectrum
only singlet
First order spectrum
>> J
Higher order spectrum
~ J
AMX system
Spin-spin coupling
Hybridization of the atoms
Bond angles and torsional angles
Bond lengths
Neighboring -bond
Effects of neighboring electron lone-pairs
Substituent effect
JH-H and Chemical Structure
Geminal couplings 2J
(usually <0)
H-C-H bond angle
hybridization of the carbon atom
substituents
Geminal couplings J
2
bond angle
Geminal couplings J
2
Substituent Effects
Effect of Neighboring
-electrons
Vicinal couplings JH-H
3
Torsional or dihedral angles
Substituents
HC-CH distance
H-C-C bond angle
Vicinal couplings JH-H
3
3
Karplus curves
1 3
1 3
J 2 J g
Jt
3
3
dihedral angles
Chemical
Shift of
amino acid
http://bouman.chem.georgeto
wn.edu/nmr/interaction/chems
hf.htm
Chemical Shift Prediction
Automated Protein Chemical Shift Prediction
http://www.bmrb.wisc.edu:8999/shifty.html
BMRB NMR-STAR Atom Table Generator for
Amino Acid Chemical Shift Assignments
http://www.bmrb.wisc.edu/elec_dep/gen_aa.html
http://bouman.chem.georgetown.edu/nmr/interaction/chemshf.htm
Example 1