Structure and potential energy functions for acetaldehyde: Ab initio calculations of X∼1A' , Ã1A″ , and B∼1A' states
The geometrical structure of the ground state of acetaldehyde obtained by optimization of ab initio SCF energies is compared with a number of experimental structures derived from microwave spectra. The optimum geometries of acetaldehyde in its two lowest singlet excited states, Ã1A″ and B∼1A' , are also determined. A force constant matrix found for each of the X∼1A' and B∼1A' states is used to calculate the vibrational wavenumbers of the four isotopic species, CH 3CHO, CH 3CDO, CD 3CHO, and CD 3CDO, and thus to check assignments of observed wavenumbers from infrared spectra. The potential energy function for internal rotation of the methyl group is also studied in each of the three states and for inversion in the Ã state. Reasonable agreement is obtained between ab initio barrier heights and values derived from microwave, far infrared, and ultraviolet spectra.