Rotational relaxation

Rotational relaxation refers to the decay of certain autocorrelation magnitudes related to the orientation of molecules. If a molecule has an orientation along a unit vector ${\mathbf n}$ , its autocorrelation will be given by

c_1(t)=\langle \mathbf{n}(0)\cdot\mathbf{n}(t) \rangle.

From the time decay, or relaxation, of this function, one may extract a characteristic relaxation time (either from the long-time exponential decay, or from its total integral, see autocorrelation). This magnitude, which is readily computed in a simulation is not directly accessible experimentally, however. Rather, relaxation times of the second spherical harmonic are obtained:

c_2(t)=\langle P_2( \mathbf{n}(0)\cdot\mathbf{n}(t) ) \rangle,

where $P_2(x)$ is the second Legendre polynomial.

According to simple rotational diffusion theory, the relaxation time for $c_1(t)$ would be given by $\tau_1 = \frac{1}{2D_\mathrm{rot}}$ , and the relaxation time for $c_2(t)$ would be $\tau_2 = \frac{1}{6D_\mathrm{rot}}$ . Therefore, $\tau_1= 3 \tau_2$ . This ratio is actually lower in simulations, and closer to $2$ ; the departure from a value of 3 signals rotation processes "rougher" than what is assumed in simple rotational diffusion (Ref 1).

Water[edit]

Main article Rotational relaxation of water

Often, molecules are more complex geometrically and can not be described by a single orientation. In this case, several vectors should be considered, each with its own autocorrelation. E.g., typical choices for water molecules would be:

symbol	explanation	experimental value, and method
HH	H-H axis	$\tau_2=2.0$ ps (H-H dipolar relaxation NMR)
OH	O-H axis	$\tau_2=1.95$ ps (¹⁷O-H dipolar relaxation NMR)
$\mu$	dipolar axis	not measurable, but related to bulk dielectric relaxation
$\perp$	normal to the molecule plane	not measurable