Polyatomic ions, orientation rotation

Molecular rotation In a normal crystal every atom occupies a precise mean position, about which it vibrates to a degree depending on the temperature molecules or polyatomic ions have precisely defined orientations as well as precise mean positions. When such a crystal is heated, the amplitude of the thermal vibrations of the atoms increases with the temperature until a point is reached at which the regular structure breaks down, that is, the crystal melts. But in a few types of crystal it appears that notation of molecules or polyatomic... 

FIGURE 1.24 The device for evaluation of orientationally averaged projections of polyatomic ions through exposure of wax models rotating around three perpendicular axes. (From Mack, Jr., E., J. Am. Chem. Soc., 47, 2468, 1925.)... 

Though at high E/N the ion transport depends in a complex way on collision integrals of various orders (2.2.1), all those quantities in classic transport theory are orientationally averaged. This implies free rotation of ions regardless of E, which is true only for atomic ions or at sufficiently low E. Polyatomic ions in gas can be oriented with respect to E by two unrelated mechanisms—collisional alignment considered here and dipole alignment (2.7). 

TABLE 5.10 Rotational orientation Correlation Times x/ps of Several Polyatomic Ions in Various Solvents... 

---