Structure of the disordered phase of t-butyl cyanide determined using neutron scattering methods

The structure of the high-temperature phase of t-butyl cyanide has been determined in detail by neutron scattering experiments on single crystals of a fully deuteriated specimen. The structure is tetragonal (P4/n, a = 6.85 Å, c = 6.77 Å) with two molecules per unit cell. Analysis of the intensities of 82 independent reflections show the structure to be disordered, each molecule having one-quarter occupancy of four positions related by π/2 rotations about the four-fold axes. The diffuse scattering intensity shows clearly that local correlations exist which are closely similar in structure to that of the low-temperature monoclinic phase, with a correlation length of ca. 7 Å. New incoherent quasi-elastic scattering measurements provide the basis for a slightly modified analysis of previous results, which are then in complete accord with the above model. The correlation time for rotation of the molecules about their three-fold axes is 10^-12s and for the cooperative relaxations among the four local structures the correlation time is ≲10^-11 s.