Abstract
We have investigated the conformational dependences of the torsional potential and dipole moment of double‐rotor molecules related to acetone, using semiempirical and ab initio calculations and expressing the results in terms of limited Fourier‐series expansions. The use of the isodynamic operations of nonrigid molecules to obtain symmetry‐adapted quasianalytic forms for the various properties helps compute the representative surfaces with a minimum number of points. Potential surfaces have been calculated for planar ground‐state acetone (CNDO/2, STO/3__G__, and STO/4‐31__G__) and both pyramidal excited‐triplet acetone and ground‐state dimethylamine (CNDO/2). For groundstate acetone STO/4‐31__G__ brings the results obtained with STO/3__G__ closer to those from CNDO/2 and from experiment. The potential surface of excited‐triplet acetone appears intermediate between those of ground‐state acetone and dimethylamine. For dipole moments the convergence of the harmonic expansions of the vector components is slower than that of the torsional potential whereas that of the vector magnitude is faster.