Hyperfine Coupling Constant (HFCC)
Definition and meaning of Hyperfine Coupling Constant (HFCC) in chemistry.
HFCC (hyperfine coupling constant) measures the strength of the interaction between an unpaired electron's spin and the nuclear spin of a nearby magnetic nucleus, observed as the splitting of lines in an electron paramagnetic resonance (EPR) spectrum.
In more detail
The coupling arises from two contributions: the isotropic Fermi contact interaction, which requires nonzero electron spin density directly at the nucleus, and the anisotropic dipolar interaction between the two magnetic moments through space. Because the Fermi contact term depends on s-orbital electron density, HFCC values reveal how unpaired spin is distributed across a radical's atoms. The McConnell equation, a_H = Q·ρ, relates the proton HFCC (a_H) to the pi-electron spin density (ρ) on an adjacent carbon in conjugated radicals, making HFCCs a key tool for mapping molecular structure and reactivity of radicals.
Key facts
| Field | Physical Chemistry |
|---|---|
| Typical units | gauss (G) or megahertz (MHz) |
| Common symbol | a or A |
| Key relation | McConnell equation: a_H = Q·ρ |
The methyl radical (CH3•) shows a proton HFCC of about 23 gauss, splitting its EPR signal into a 1:3:3:1 quartet from coupling to the three equivalent hydrogen nuclei.
Frequently asked questions
What causes hyperfine splitting in an EPR spectrum?
It arises from the coupling between the magnetic moment of an unpaired electron and the magnetic moments of nearby nuclei with nonzero spin (such as 1H, 14N, or 13C), via the Fermi contact and dipolar interactions.
How is an HFCC actually measured?
It is read directly as the spacing between adjacent lines in the EPR spectrum of a radical or paramagnetic species, typically reported in gauss and convertible to megahertz.