MAGNETIC SPIN-TORSION COUPLING IN METHANOL

The hyperfine structure of non-rigid molecules in which hyperfine coupling arises from equivalent nuclei that can be exchanged by large amplitude motions is of great interest and lead to unexpected results. In the non-rigid (C$_2$D$_2$)$_2$ and (D$_2$O)$_2$ dimers, the hyperfine structure arising for nondegenerate tunneling sublevels can be accounted for using an effective quadrupole coupling Hamiltonian with the same coupling constant for all four deuterium atoms.footnote{Bhattacharjee, Muenter, and Coudert, {em J. Chem. Phys.}~{bf 97} (1992) 8850; and Stahl and Coudert, {em J. Mol. Spectrosc.}~{bf 157} (1993) 161.} In the non-rigid species CD$_3$COH and HCOOCH$_3$, the large amplitude torsional motion leads to hyperfine patterns which are qualitatively dependent on the torsional symmetry of the levels.footnote{Coudert and Lopez, {em J. Mol. Spectrosc.}~{bf 239} (2006) 135; and Tudorie, Coudert, Huet, Jegouso, and Sedes, {em J. Chem. Phys.}~{bf 134} (2011) 074314.} The interaction between a large amplitude torsional motion and the hyperfine coupling may also lead to a less known hyperfine effect, the so-called magnetic spin-torsion coupling, which was first studied by Heuvel and Dymanusfootnote{Heuvel and Dymanus, {em J. Mol. Spectrosc.}~{bf 45} (1973) 282 and {em ibid} {bf 47} (1973) 363.} and which has not yet been conclusively evidenced. In this talk, the magnetic hyperfine structure of the non-rigid methanol molecule will be investigated experimentally and theoretically. 13 hyperfine patterns were recorded using two molecular beam microwave spectrometers. These patterns, along with previously recorded ones,$^c$ were analyzed in an attempt to evidence the effects of the magnetic spin-torsion coupling. The theoretical approach setup to analyze the observed data accounts for the spin-torsion coupling, in addition to the familiar magnetic spin-rotation and spin-spin couplings, and relies on symmetry considerations to build a hyperfine coupling Hamiltonian and a spin-rotation-torsion wavefunction compatible with the Pauli exclusion principle. In the talk, the results of the analysis will be presented. The hyperfine coupling parameters retrieved will be discussed and we hope to be able to conclusively evidence the effects of the magnetic spin-torsion