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.