ROTATIONAL SPECTRA AND STRUCTURES OF SMALL CLUSTERS: $Ar_{2}-HCl$ and $Ar_{3}-HCl^{\ast}$

$^{\ast}$ Work supported by NSF and PRF. $^{1}$ W. H. Kirchoff, J. Mol., Spctros. 41, 333 (1972). Address: Noyes Chemical Laboratory, University of Illinois, Ubrana, IL 61801.Microwave rotational spectra have been observed for the small clusters $Ar_{2}-H_{35}Cl/^{37} Cl$ and $Ar_{3}-H^{35}Cl/^{37}Cl$. The hyperfine structure of the transitions has been analyzed and the chlorine quadrupole coupling and the line centersdetermined. The line centers were used to obtain the ground state rotational costants. The trimeric cluster, $Ar_{2}-HCl$, has an equilibrium, T-shaped structure with the H pointed at the $Ar_{2}m$ dpoint. The values of A, B. and C determined for the $^{35}cl$ species with the kirchoff $program^{1}$ are 1 733,857, 1 667,932, and 844.492 Mhz: $\tau_{aaaa}$, $\tau_{bbbb}$, $\tau_{aabb}'$ and $\tau_{abab}$ are -120, -80, 41, and -65 KHz. The quadrupole coupling constants $X_{aa}$ and $X_{bb}-X_{cc}$ are -28.11 and -3. 18 MHz. A value of 3.828 A is found for the Ar-Ar interatomic distance. The $Ar_{3}-HCl$ is a symmetric top with the H of the HCl pointing towards the $Ar_{3}$. The spectroscopic constants for the $^{35} cl$ version are $B_{o} = 843. 897$ MHz. $D_{j} = 2.33 KHz D_{jk} = 1.82 KHz$, and $X_{aa} = 30.006$ MHz. By combining $B_{o}$ for the $H^{37}cl$ complex (825.981 MHz) with that for $H^{35}Cl$, We find the Ar-Ar distance in $Ar_{3}$ to be 3.852 A. Several aspects of the results will be considered; The longer Ar-Ar distance in $Ar_{3}$ than in $Ar_{2}$. A comparison of results for the HF and HCl containing clusters. The dependence of the average torsional angle for the HX upon size of the $Ar_{n} (n = 1,2,3)$, as determined from the hyperfine interactions. Use of the centrifugal distortion to obtain some insight concerning the weak van der waals interactions and vibrational modes