 There are two single quantum transitions, "W"1I, corresponding to αα ➞ αβ and βα ➞ ββ; "W"1S, corresponding to αα ➞ βα and βα ➞ ββ; a zero quantum transition, "W"0, corresponding to βα ➞ αβ, and a double quantum transition corresponding to αα ➞ ββ.While rf irradiation can only induce single-quantum transitions (due to so-called quantum mechanical selection rules) giving rise to observable spectral lines, dipolar relaxation may take place through any of the pathways.The dipolar mechanism is the only common relaxation mechanism that can cause transitions in which more than one spin flips.