Rotational constants of malonaldehyde and isotopic species derived from ab initio results

A quantum mechanical treatment based on ab initio results for the 21-dimensional potential energy surface of malonaldehyde is extended to yield the rotational constants of the lowest two states of the hydrogen transfer motion. The approximate separation of rotation from internal motion by an Eckart type transformation allows one to use a restricted basis set involving not more than one quantum of excitation for the 20 small vibrations. The results agree with experiment on the apparent structural effects of tunnelling excitation and their dependence on isotopic substitution. In the case of asymmetrically substituted species, the extent of localization confirms qualitative estimates from observation. The second moments derived from the rotational constants indicate that the molecular extension is underestimated in the O···O direction and overestimated perpendicular to this direction within the molecular plane. The agreement with experiment is shown to improve significantly by correcting the equilibrium geom...