Collision-induced absorption in cyclopropane (C3H6)

Abstract

We have measured the far-infrared absorption spectrum of cylopropane (C3H6) at frequencies up to 400 cm−1, at densities up to 5 amagat at 295 K and up to 16 amagat at 360 K, using a combination of Fourier-transform and laser techniques. (An amagat is defined as the density of a gas at N.T.P.). By studying the density-dependence of absorption, we have isolated the collisional component of the spectrum. This arises from pair dipole moments induced by the permanent quadrupole, octopole, and hexadecapole moments of the C3H6 molecule. To the extent that the intermolecular potential is isotropic, the induced spectrum can be expressed as the convolution of symmetric top free rotation spectra with translational spectra representing the relative motion of an interacting molecular pair. The former are calculated using expressions recently derived. For the latter we use information theory to provide a "least-biased" estimate based only on available information concerning the zeroth and second moments of the translational frequency spectrum, which can be calculated without approximation within the framework of equilibrium statistical mechanics. The result of these computations is a simple analytic expression for the absorption coefficient [Formula: see text], which is entirely free of adjustable parameters, and yet yields a good representation of the experimental results over most of the frequency range. Our results lend strong support to the theoretical values of the multipole moments calculated by other workers.