Knowledge Bank

We have carried out comparative single-state and global fits to the millimeter wave spectra of the ground and excited torsional states of acctaldehyde measured by ourselves and other $workers(1,2,3,4)$, using an IAM technique not calling for large matrix $diagonalisation(5)$. The Hamiltonian used also utilises a planar rotation which transforms the quartic and sextic centrifugal distortion terms to an internal axes $system(1,2)$. The structure of the molecule derived from the single-state fits shows changes in both methyl and aldehyde group structures corresponding to increasing proton repulsions as the barrier is surmounted, but the use of global fit parameters leads to quite unreasonable molecular geometries.