Page 365 - Instant notes

Magnetic resonance spectroscopy 351

where B is the applied field strength, g is the g-factor for a free electron and µ B=eћ/2m eis
the Bohr magneton (m e is the mass of an electron). The value of the Bohr magneton
−1
(9.3×10 −24 J T ) is 1837 times greater than the nuclear magneton because of the
difference in mass between an electron and proton. Consequently, the energy separation
of the two electron spin orientations is greater for a given applied field than for nuclear
spin orientations and ESR resonance absorptions occur in the microwave rather than the
radiofrequency region of the spectrum.
As with NMR spectroscopy, ESR signals can show hyperfine structure caused by the
coupling of the electron with magnetic nuclei. If an electron interacts with just one
nucleus of spin ½, then its ESR signal consists of two peaks of equal intensity (separated
in frequency by the appropriate coupling constant), corresponding to the two possible
orientations of the nucleus. The ESR signal for the CH 3 radical consists of four peaks in a
1:3:3:1 ratio because of the combinations in which an electron spin can couple to the
spins of three equivalent protons.
ESR provides information on the electronic structure of radicals. The value of the
coupling constant between the electron and the nucleus in a free hydrogen atom is 1420
MHz. If the value of the coupling constant of an electron to a hydrogen atom in a radical
is A MHz, then the spin population on that hydrogen atom is very approximately A/1420.
13
Similarly, the coupling constant between a 2s electron and a C nucleus is 3330 MHz, so
3
the coupling constant of an electron in an sp hybridized orbital is approximately one-
quarter of this. Different values of coupling constants indicate different degrees of
hybridization.