Knowledge Bank

In this study, the total integrated intensity of the $\pi \ast \leftarrow n$ absorption of some simple aldehydes and ketones were measured over a pressure range of ca, 4-40 torr sample pressure. A plot of these intensities $vs$ sample concentration was of the form of a damped oscillation, leveling off at higher pressures to the reported literature values for the intensities. The gas phase dielectric constants of these compounds were then measured, and plots of the Clausius-Mosotti function vs density showed oscillations at the same pressures that showed extrema in the absorption intensities. Since these measurements were not done under UV excitation, a dependence on ground state properties is clearly indicated. In addition, IR spectra of these compounds were run as a function of pressure, and an additional band appeared slightly lower in energy than the carbonyl stretch at the same ``optimum’’ pressures that showed peaks in the intensity and polarization plots. These findings will be discussed from the point of view of collisional parameters and processes leading to transient molecular aggregation.