Collective rotational tunneling of methyl groups and quantum solitons in 4-methylpyridine: Neutron scattering studies of single crystals
The structure of the 4-methylpyridine crystal has been determined at 10 and 260 K with the single-crystal neutron-diffraction technique. The space-group symmetries are I41/a and I41/amd, respectively. In both cases, there are eight molecular entities in the unit cell. The rotational axes of the methyl groups are aligned along the c axis. The shortest intermolecular distances occur between face-to-face methyl groups. The next shortest distances correspond to infinite chains of rotors parallel to the orthogonal axes a and b. The angular probability densities of the methyl groups are clear evidence of orientational disorder. The incoherent scattering function S(Qa,Qb,ω) has been measured with the inelastic neutron scattering technique at 1.7 K. The energy-transfer ranges, ħω=±(500±60)μeV, encompass transitions due to rotational tunneling. The anisotropy in Q is distinctive of dynamics in one dimension. These are represented with the quantum sine-Gordon equation that is an approximation to the Hamiltonian for an infinite chain of coupled rotors. Spots of intensity observed for neutron-energy loss at |Qa| or |Qb|≈1.55 Å-1 are distinctive of stationary states for breathers. Weaker peaks at |Qa| or |Qb|≈1.0 Å-1 reveal collective tunneling. This is a thermally activated process arising from rather heavy pseudoparticles composed of large numbers (from 22 to 25) of kinks or antikinks. Quantization of the kinetic momentum arises from the chain discreteness and from conservation of the angular momentum. Traveling states are stationary when the kinetic energy is within the tunneling energy band. In the ground state, the chain dynamics are represented with a single collective angular coordinate. The incoherent scattering function for neutron-energy-gain reveals bound excited states with lifetimes of several days.