Knowledge Bank

New microwave measurements on the ground state of acetaldehyde have been carried out using a Fourier transform microwave spectrometer in the region of 7 to 26 GHz (typical measurement uncertainty 4 kHz), and a conventional Stark spectrometer in the region from 45 to 116 GHz (uncertainty: 40 kHz). These new ground state measurements and remeasurements have permitted a much better fit, using two theoretical models, of a data set containing far-infrared combination differences from the literature, microwave transitions from the literature and the new microwave transitions measured in this work. The root-mean-square residuals obtained are only slightly larger than the estimated measurement accuracy. The first theoretical model is a high-barrier effective Hamiltonian for one vibrational state only, based on Fourier expansions in terms of the form cos($2\pi /3$) ($\rho K-\sigma$). The second model is based on calculations using an internal-rotation potential function. The present successful fits using either model indicate that earlier fitting problems using the second model were caused by problems in the original microwave data set, rather than by problems inherent to the model, at least for the ground torsional state of acetaldehyde. Similar techniques are now being tried in order to fit simultaneously the ground torsional state ($vt$=0), the first excited torsional state ($vt$=1) and far-infrared transitions from the literature ($vt$=1-0).""