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# NMR Spectroscopy - PowerPoint

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```									NMR Spectroscopy

Part II. Signals of NMR
Free Induction Decay (FID)
• FID represents the time-domain
response of the spin system following
• 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
2
AW
y
W  4x0  x 
2                2

A      amplitude
W      half-line width
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     A:    height of the chosen peak
s / n  2.5        Npp : peak to peak noise
N pp
Sensitivity
 Measurement
1H        0.1% ethyl benzene in deuterochloroform
13C       ASTM, mixture of 60% by volume deuterobenzene
and dioxan or 10% ethyl benzene in chloroform
31P         1% trimehylphosphite in deuterobenzene
15N         90% dimethylformamide in deutero-dimethyl-
sulphoxide
19F         0.1% trifluoroethanol in deuteroacetone
2H, 17O     tap water
Sensitivity

   Factors affect sensitivity
Probe: tuning, matching, size
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 spin-
spin coupling constant J.
   Intensity of the signal
Chemical Shift
   Origin of chemical shift

Beff  B0  sB0  1  s B0
s       shielding constant

         
  '
Beff     1  s B0
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        1  s 1 
2
B0
 2         1  s 2 
2
B0
 2  1         s 2  s 1 
2
 2  1
 s 2 s1
1
Chemical shift parameterd  s 2  s 1  106
Chemical Shift

d                     106
observing frequency

Shielding s
CH3Br < CH2Br2 < CH3Br < TMS

d CHBr3  
614
106  6.82 (ppm)
90 106

90 MHz spectrum
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    s dia
local
s   local
para    s N s R s e si
   Charge density
   Neighboring group
Anisotropy
Ring current
Electric field effect
Intermolecular interaction (H-bonding & solvent)
Chemical Shift –
anisotropy of neighboring group

sN 
1
 //    1  cos2 
3r 3 4
 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 system
I=1; n=3
AX4
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
dihedral angles

   Karplus curves

3      1 3

J  2 J g 
3
  
1 3
3
Jt 
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

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