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 , receiver coil would see exponentially decaying signal. This decay is due to relaxation.
Fourier TransformThe Fourier transform relates the time-domain f(t) data with the frequency-domain f(w) data.
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 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 : height of the chosen peakNpp : peak to peak noise
ppN
Ans 5.2/
Sensitivity Measurement1H 0.1% ethyl benzene in deuterochloroform13C ASTM, mixture of 60% by volume deuterobenzen
e and dioxan or 10% ethyl benzene in chloroform
31P 1% trimehylphosphite in deuterobenzene15N 90% dimethylformamide in deutero-dimethyl-
sulphoxide19F 0.1% trifluoroethanol in deuteroacetone2H, 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 CouplingThe 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
shielding constant
Chemically non-equivalent nuclei are shielded to different extents and give separate resonance signals in the spectrum
000 1 BBBBeff
0' 1
22BBeff
Chemical Shift – scale or abscissa scale
612
121
12
120
12
20
2
10
1
10parameter shift Chemical
2
12
12
B
B
B
Chemical Shift
Shielding CH3Br < CH2Br2 < CH3Br < TMS
610frequency observing
(ppm) 82.6101090
614CHBr 6
63
90 MHz spectrum
Chemical Shift
is dimensionless expressed as the relative shift in parts per million ( ppm ).
is independent of the magnetic field of proton 0 ~ 13 ppm
of carbon-13 0 ~ 220 ppm
of F-19 0 ~ 800 ppm
of P-31 0 ~ 300 ppm
Chemical Shift
Charge density Neighboring group
AnisotropyRing currentElectric field effectIntermolecular interaction (H-bonding &
solvent)
ieRNlocalpara
localdia
2
//3cos1
43
1
r
N
Chemical Shift – anisotropy of neighboring group
Differential shielding of HA and HB in the dipolar field of a magnetically anisotropic neighboring group
susceptibility
r distance to the dipole’s center
• 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 ele
ctron 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.
Multiplicity RuleMultiplicity M (number of lines in a multiplet)
M = 2n I +1
n equivalent neighbor nuclei
I spin numberFor I= ½
M = n + 1
Order of Spectrum
Zero order spectrum
only singlet
First order spectrum
>> J
Higher order spectrum
~ J
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 anglehybridization of the carbon atomsubstituents
Automated Protein Chemical Shift Predictionhttp://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
Chemical Shift Prediction