Collective rotational tunneling of methyl groups and quantum solitons in 4-methylpyridine: Neutron scattering studies of single crystals

Abstract

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 I4 1 /a and I4 1 /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(Q a ,Q b ,) has been measured with the inelastic neutron scattering technique at 1.7 K. The energytransfer ranges, បϭϮ(500Ϯ60)eV, 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 ͉Q a ͉ or ͉Q b ͉Ϸ1.55 Å Ϫ1 are distinctive of stationary states for breathers. Weaker peaks at ͉Q a ͉ or ͉Q b ͉Ϸ1.0 Å Ϫ1 reveal collective tunneling. This is a thermally activated process arising from rather heavy pseudoparticles composed of large numbers ͑from 22 to 25͒ 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.

Synchrotron and neutron diffraction study of 4-methylpyridine-N-oxide at low temperature

“…angles, all other distances and angles were kept constant in the refinement processes ( Table 1). The calculated distances agree with those found in the parent compounds (Ü lkü et al, 1971;Chiang, 1974;Ohms et al, 1985;Vorontsov, 2002;Fillaux et al, 2003), with the N-O distance of 1.39 Å typical of a very limited -bond character (Ü lkü et al, 1971). The molecular thermal motions of the rigid body and of the satellite methyl group were refined by means of the appropriate molecular displacement tensors, the TLS matrices (Schomaker & Trueblood, 1968).…”

“…($ 48 J mol À1 ) obtained from inelastic neutron scattering studies (Alefeld et al, 1975). The methyl groups are organized in infinite chains with rotational axes parallel to the crystal c axis, and the collective rotational dynamics in one dimension has been modelled with the quantum sine-Gordon theory Fillaux et al, 2003). In the case of 4MPNO, however, the rotational tunnelling spectrum at 4 K is more complicated.…”

Quantum‐rotor‐induced polarization

“…The energy structure of quantum rotors enables their study by a large range of spectroscopic techniques. Neutron scattering [40][41][42] and infrared spectroscopy [42] can directly probe the energy structure. Magnetic resonance does not directly probe the energy structure, but temperature-jump experiments can be used to study conversion between the A and E states of methyl groups or between the para and ortho states of water.…”

Unconventional density dependence of the stochastic dynamics in an organic liquid

“…Data at hand as discussed by Vorontsov et al [19] identify local configurations where two molecules adopt a T configuration that is having the molecular planes normal to each other, as able to explain such features as well as the presence of crossing (isosbestic) points in the density dependence of the atom-atom functions. In fact, such local configurations are found in the crystal structure of the positional isomer 4-methyl-pyridine [20,21] which shows at low temperatures a tetragonal I4 1 / a with eight molecules per unit cell having the rotational axis of the methyl groups aligned along the crystal c axis.…”