nmr-spectroscopy kjernemagnetisk resonans history physical requirements principles theory...
TRANSCRIPT
NMR-spectroscopyKjernemagnetisk resonans
• History
• Physical requirements
• Principles
• Theory
• Interpretation of spectra
History
- 1946: NMR discovered- 1949: Chemical shift- 1953: First commercial high resolution instrument- 1960: Structure determination of organic molecules- 1970: FT-NMR- 1971 – 75 : 2D-NMR- 1973: MR-tomography
An example
Schematic diagram of NMR
Physical requirements
1.Nuclear spin(12C and 16O
are NMR silent)
Number of spin states in the presence of a magnetic field = 2I +1
Magnetic moment
1H and 13C # spin states = 2 2H # spin states = 3
2. Magnetic field
ΔE = hν ν = γB0/2π
ν = frequency, B0 = applied magnetic field, γ = magnetogyric ratio
N1 = number of nuclei in the α stateN2 = number of nuclei in the β state
At 60 MHz: excess in α state is 9 nuclei in a population of 2 million.The N1/N2 ratio increases with increasing B0.
N1/N2 is given by Boltzmann’s distribution:
Net magnetisation
Net magnetisation vector
FT-NMR
Magnetic field data
Properties
Sample preparation
Chemical shift
Electron density
δ-scale
Tetramethylsilane, Si(Me)4 is used as reference in both 1H and 13C
TMS: - The nuclei are strongly shielded and appear to the right - 12 1H and 4 13C nuclei - One singel peak - Chemically inert - Non toxic
Carbon 13 and proton
Integration
Integration
CW(1H): The area is proportional to the number of nuclei in the peak
FT(1H): Same as CW given that the relaxtion time is long enough (2-3 s)
FT(13C): Not commonly used due to slow relaxtion and large differencein relaxtion rates for different 13C nuclei.