THE ACETYLENE-SULFUR DIOXIDE VAN DER WAALS COMPLEX

Thirty-three a- and c-dipole transitions of the acetylene $SO_{2}$ van der Waals complex have been observed by Fourier transform microwave spectroscopy. They are fit with a Watson S-reduced Hamiltonian to rotational constants $A = 7176.804(2) MHz, B = 2234.962(1) MHz, C = 1796.160(1) MHz$. The structure of the complex has the $C_{2}H_{2}$ and $SO{2}$ straddling an ac symmetry plane (i.e. only the S atom lies in the planel. The two subunits are separated by a distance $R_{cm} = 3.431 (1) {\\AA}$ and the $C_{2}$ axis of the $SO_{2}$ is tilted $14.1(1)^{\\circ}$ from the perpendicular to the Rcm vector with the S atom closer to the acetylene. The dipole moment of the complex is 1.683(5) D. The deuterium nuclear quadrupole hyperfine splitting was resolved and assigned for both the $C_{2}HDSO_{2}$ and $C_{2}D_{2}SO_{2}$ species. Although the $C_{2}H_{4}-SO_{2}- C_{2}H_{4}O_{3}$ and $C_{2}H_{2}-O_{3}$ complexes all exhibit tunneling doublets from an internal rotation of the hydrocarbon subunit, no doubling was observed for the $C_{2}H_{2}-SO_{2}$ complex.