Hund's cases
The Hund's coupling case a)
The Hund's coupling case a) occurs in less interaction between the molecular rotation and the electron motion. The orbital and spin angular momentum vectors of the electrons and to be tied to the molecular axis, about which they rotate in rapid precession. The sum of their projections onto the molecular axis ( and ) is called the total electron angular momentum. The total angular momentum, however, is constant in time, both the magnitude and direction of the forth.