The frequency of the nuclear spin precession usually is in the range of several hundreds of MHz, being dependent on the strength of the magnetic field and the type of the nucleus itself. According to the different locations in a given molecule, the electron cloud density around a particular nucleus may be different at various places. O or halides may lead to a decrease of electron density as compared to alkanes. Usually, these splittings may be interpreted by very simple rules and permit even more detailed insight into the structure of a molecule. RF pulse excites all nuclei, irrespective of their chemical shifts and their spin, spin couplings.
Macroscopic magnetization is generated within a sample.
At the end of the pulse, the RF receiver is turned on. The nuclei which precess with their individual frequencies induce oscillating currents in the receiver coil which are amplified and digitized for further analysis. The observation and further exploitation of FIDs plays the central role in NMR spectroscopy. Here is a typical NMR FID and its corresponding Fourier transformed spectrum . If you want to read more about introductory NMR, click here .