Nuclear Polarization, Chemical

the phenomenon of the nonequilibrium population of nuclear Zeeman levels in molecules produced in chemical radical reactions in a constant magnetic field (seeZEEMAN EFFECT).

The phenomenon is detected on the basis of the anomalously large absorption or emission of radio waves by molecules as a result of nuclear transitions between Zeeman levels. Absorption occurs as a result of the excess population of lower levels with nuclei whose spins are oriented in the direction of the magnetic field (positive chemical nuclear polarization) and as a result of the transitions of nuclei from lower to higher levels. Radio-wave emission occurs when upper levels are excessively populated by nuclei with spins oriented in a direction opposite to the field (that is, in negative chemical nuclear polarization) and when the transitions of nuclei occur from upper to lower levels. A nonequilibrium population of nuclear levels is produced in simple reactions upon the recombination of two radicals.

Chemical nuclear polarization is usually observed by the nuclear magnetic resonance method. It is commonly used as a new technique for the study of the mechanisms of chemical reactions and the identification of radicals that participate in chemical reactions.