THE ARGON-ACETALDEHYDE VAN DER WAALS COMPLEX SPECTRUM, STRUCTURE, DIPOLE MOMENT, AND TUNNELING MOTIONS

Abstract

166 transitions of the argon-acctaldchyde complex have been observed by FTMW spectroscopy. The transitions appear in quartets centered near the rigid rotor frequencies. The overall splitting in the a and b transitions is about 1-3 MHz. The c-type transitions appear as two doublets symmetrically shifted from the rigid rotor frequencies by about $\pm 200$ MHz. Each line is split by methyl rotation in the acetaldehyde, while the additional doubling is attributed to an inversion tunneling motion through a planar configuration. Frequencies were matched by sum rules to experimental accuracy, in order to identify the level structure. They show each rigid rotor level being split into four states, two A and two E. Averaging of the two A-states and subsequent fitting to a rigid rotor Hamiltonian yields the following rotational constants; $A=9306 MHz, B=1806 MHz, C=1570 MHz$. Substitution of the acetaldehyde hydrogen by deuterium and fitting as above yields the additional rotational constants; $A=9035 MHz, B=1767 MHz, C=1550 MHz$. These six parameters determine the position of the Argon as approximately $R_{cm} = 3.59(1)$ {\AA}, $R_{Ar-O} = 3.6(2)$ {\AA}, $R_{Ar-C_{m}} = 3.7(2)$ {\AA}. The complex is nonplanar and the argon is located above the C-C-O triangle. Since the system exhibits multi-dimensional tunneling, the Hougen-Coudert formalism is being employed to fit the spectrum. A program is currently being written in consultation with Dr. J.T. Hougen to implement the formalism for this particular case, and the interpretation is based on the $G_{6}$ group. The correlation of the group to $C_{1}$ suggests that each rigid rotor level should be split into four states, $A_{1} + A_{2} + 2E$, as observed.