Knowledge Bank

The existing $theory1,2$ to predict the collision-induced rotational spectrum of non-polar, gas phase, symmetric top molecules has been modified. The quadrupole moment of a symmetric top molecule is axially symmetric and thus only $\Delta K=0$ transitions should be observed for a dipole moment induced in another molecule by a symmetric top. However, 85% of the spectrum observed in our laboratory for allene-allene collisions lies in a frequency range above that predicted using this leading multipole term. The multipole expansion for describing the field of one molecule at another depends on the intermolecular distance, R, being larger than the intramolecular distances, $r1$, of the charge distribution. This condition is not met in collisions between molecules the size of allene and multipole expansions are not appropriate (as is commonly the case in the liquid and solid state). A model which represents the charge distribution in the molecule as discrete charges at the nuclei will be presented and shown to produce the K transitions necessary to explain the experimental spectrum. Perhaps the general unreliability of multipole moments determined from analysis of collision induced spectra is due in part to the inadequacy of the multipole expansion.