Larmor frequency
Definition and meaning of Larmor frequency in chemistry.
Larmor frequency is the frequency at which a magnetic moment precesses around an external magnetic field. It is the resonance frequency used in nuclear magnetic resonance and electron spin resonance spectroscopy.
In more detail
When a nucleus with spin is placed in a magnetic field, its magnetic moment does not align statically but instead precesses around the field direction at the Larmor frequency. This frequency depends on two factors: the gyromagnetic ratio of the nucleus (an intrinsic property of each isotope) and the strength of the applied magnetic field. Applying radiofrequency energy at precisely the Larmor frequency causes resonance absorption, allowing the instrument to detect the nuclear spins. This principle underlies all magnetic resonance spectroscopy techniques used in chemistry.
Key facts
| Field | Physical Chemistry |
|---|---|
| Formula | f = (γ/2π) × B0, where γ is gyromagnetic ratio and B0 is magnetic field strength |
| Named after | Joseph Larmor (physicist, 1857-1942) |
| Common applications | NMR and EPR spectroscopy for structure determination |
A proton (hydrogen-1) in a 400 megahertz NMR spectrometer operates at a Larmor frequency of 400 megahertz, corresponding to an applied magnetic field of approximately 9.4 tesla. At this frequency, radiofrequency pulses cause proton spins to absorb energy and transition between spin states.
Frequently asked questions
Why is Larmor frequency important in NMR spectroscopy?
The Larmor frequency is the radiofrequency at which nuclei in a specific magnetic field undergo resonance. NMR instruments are tuned to apply RF energy at this frequency to detect and manipulate nuclear spins, making it essential for all NMR measurements.
Do different nuclei have different Larmor frequencies in the same magnetic field?
Yes. Each nucleus has a unique gyromagnetic ratio, so protons, carbon-13, nitrogen-15, and other nuclei all precess at different frequencies in the same magnetic field. This property allows selective detection of specific isotopes.