Which is smaller: hydrogen or deuterium? The standard answer is that deuterium (2H) takes up less space than 1H because its greater mass gives it a smaller vibration amplitude in the lowest-energy quantum state. But the veracity of this argument is temperature sensitive, say Jack Dunitz of ETH-Zurich in Switzerland and Richard Ibberson of the Rutherford Appleton Laboratory in Didcot, UK.
By taking careful measurements of the crystal structures of benzene and fully deuterated benzene at temperatures between 5 kelvin and 280 kelvin, they show that the volume of a molecule of the latter exceeds that of the former above about 170 kelvin. At such temperatures, the vibrations of the carbon–deuterium bonds include more of the higher-energy quantum states than those of C–1H bonds, which means that the deuterium atoms effectively occupy more space.