Electron spin resonance - ESR

 

Basis :            Absorption of radiation by molecules with unpaired electrons

Unpaired electrons :

• transition metals

• free radicals

 

Spin probes = stable org. radicals

 

Impulse of the e^- :                    J = L + S

                                                L = Bahn impulse

                                                S = Eigen impulse

 

Magnetic moment :               m_J = m_L + m_S

 

For org. radicals L may be neglected, for transition metals L is quite significant.

 

For Radicals

Magnetic quantum number: m_S = S, S-1, S-2,... -S

Allowed values for S : ½, 3/2, 5/2,...

Adjustment within the magnetic field:     2S + 1

 

-->  DE = h . n = g . m_B . B_o  . m_s

 

B_o = magnetic flow density (often not correct as H)

m_B= Bohrs Magneton

 

g = Landes factor   g = 1 + (J(J+1) + S(S+1) + L(L+1)) / 2J(J+1)

 

Technical Setup

• Magnetic field in coil ( 0.1 -1 Tesla)

• micro wave radiation (cm)

 

Comparison with light absorption

 

• light absorption is based on the electric field vector,

• ESR is based on the magnetic properties of the molecule.