NMR Measurement of Biomolecule Diffusion

The thermodynamic property of a molecule describes its energy state, often envisioned in terms of enthalpy, entropy, and free energy. Such a formulation accounts for energy flow and yields insight into the strength of molecular interaction as well as the direction of a chemical reaction. Despite its macroscopic overview, the energy states of thermodynamics have roots in the probability of quantum states. In a triumph of twentieth-century physics, Boltzmann linked entropy with the probability of quantum states and created a bridge from statistical to classical thermodynamics.

In the kinetic theory of gases, Boltzmann's ideas take form. Molecules diffuse randomly and exhibit velocities that follow a probability distribution and depend upon temperature. But each degree of motion contributes ½kT to the average energy of the system, where k is Boltzmann's constant and T is temperature. Because of the low density of molecules in gas, intermolecular collision doesn't significantly alter the average energy of the system. In liquid, these same laws still regulate molecular motion. However, they cannot overlook the concerns of intermolecular interaction as well as molecular size and shape. Molecules no longer exist in low density. Moreover, large molecules move slower than small molecules, and spherical shape molecules will move faster than a cylindrical one.