Knowledge Bank

Theoretical expressions for tunneling splittings in the methanol dimer (a complex which may be involved in the methanol-to-gasoline conversion process) are being derived using a formalism analogous to that developed some time ago for the water dimer. The permutation-inversion group for the methanol dimer is $G36$. This group is isomorphic with the permutation-inversion groups for dimethylacetylene, ethane, acetone, and propane. Unlike these latter molecules, however, the point group of the equilibrium configuration of the methanol dimer exhibits no symmetry operations, and tunneling motions thus connect a set of 36 equivalent frameworks. Such tunnelings could lead to a maximum splitting of ``one asymmetric-rotor line” into 36 components, but the actual splitting of one line is into only 16 components, because of the presence of two-fold and four-fold degenerate representations in $G36$. The precise form of the tunneling splitting pattern depends on which of the 26 symmetrically inequivalent tunneling paths are assumed to be dominant. About 100 microwave lines belonging to the methanol dimer have been recorded in the region from 9 to 23 GHz on the NIST pulsed-beam Fourier-tansform microwave spectrometer, involving transitions with $K=0$ and 1 and $J=2\leftarrow 1,3\leftarrow 2,4\leftarrow 3$, and $5\leftarrow 4$. The J and K assignments were confirmed in many cases by Stark-effect measurements. We shall report on initial attempts to fit the observed spectrum using the formalism described above.