Knowledge Bank

A year ago at this meeting, Hougen et $al.1$ reported a preliminary theoretical interpretation of the a-type rotational spectrum of ($CH3OH)2$ observed at NIST. While the theory remains at the qualitative stage for interpreting the original spectra, it pointed to the need for further isotopic studies in order to unravel the complexities of the tunneling motions. In essence the theoretical model indicates that sixteen tunneling states should occur for each rotational level. The internal motions which give rise to these tunneling states are: methyl internal rotation of two inequivalent tops, hydrogen bond interchange between donor and acceptor sub-units, and interchange of the lone-pair orbital of the acceptor oxygen atom. Last year only 13 of the expected 16 states were clearly assigned for the $Ka=0$ manifold by means of qualitative Stark effect. More detailed examination of the earlier measurements plus new measurements in the $J=1-0$ region with the Kiel pulsed-beam Fabry-Perot cavity Fourier-transform spectrometer show that 15 states are resolved and one appears to be a degenerate pair. We have also augmented the initial data with observations of new isotopic forms, namely: $CH3OD-CH3OH,(CH3OD)2,(13CH3OH)2,\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}CH3OH-CH3OH,CH3OH-13CH3OH,CD3OD-CD3OH$, and $(CD3OD)2$. The variation in the tunneling state splittings allow us to interpret the spectra in terms of the postulated internal motions. In particular, we conclude that the largest splittings arise from the internal rotation of the methyl groups and the smallest splitting (constant with increasing rotational state), arises from the hydrogen-bond interconversion motion. Further details on the observed spectra and analysis will be presented.