Rotational tunneling of bound H₂ in a tungsten complex

The rotational tunnel splitting of the librational ground state of activated molecular hydrogen bound in a tungsten complex has been determined to be 0.95 (1) cm^-1. With the assumption of a twofold cosine potential with one degree of freedom for the rotation, this result yields a barrier height of 760 cm^-1 in good agreement with the analysis of the librational data. These quantum mechanical rotations are also found to persist up to temperatures above 150 K. The relationship of these measurements to the binding of molecular hydrogen to the metal center are discussed.