rotational diffusion coefficient

Defined by the equation:

$D θ = t θ ∂ ( f θ Φ ) ∂ θ sin ⁡ θ$

where $f θ Φ sin ⁡ θ d θ d Φ$ is the fraction of particles whose axes make an angle between $θ$ and $θ + d θ$ with the direction $θ = 0$ , and have an azimuth between $Φ$ and $Φ + d Φ$ ; $t θ d Φ$ is the fraction of particles having an azimuth between $Φ$ and $Φ + d Φ$ whose axis passes from values $< θ$ to values $> θ$ in unit time. The axis whose rotational diffusion is considered has to be clearly indicated.

Source:

PAC, 1972, 31, 577 (Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry) on page 617