Pulsed nozzle Fourier transform microwave spectroscopic and ab initio investigations on the weakly bound $Ar-(H_2S)_2$ trimer

A pulsed nozzle Fourier transform microwave spectrometer has been used to obtain the rotational spectra of $Ar-(H_2S)_2$ and $Ar-(D_2S)_2$ trimers. Two states have been observed for each trimer and these are likely to arise due to tunneling/internal rotation of the two H2S monomers within the trimer. The rotational constants for the lower state of $Ar-(H_2S)_2$ are: A = 1810.410(6) MHz; B = 1596.199(9) MHz and C = 848.814(2) MHz; and those for $Ar-(D_2S)_2$ are A = 1725.49(1) MHz; B = 1566.26(2) MHz; and C = 826.818(3) MHz. From these rotational constants, the $Ar-H_2S$ and $H_2S-H_2S$ distances have been estimated to be 4.04 and 4.08 \AA, respectively. The splitting observed between the two states is of the order of a few MHz for $Ar-(H_2S)_2$ and a few kHz for $Ar-(D_2S)_2$. Ab initio alculations at MP2 level with large basis sets up to aug-cc-PVQZ level lead to three different minima, including a pseudo-linear one. Experimental rotational constants are consistent with the other two T-shaped structures in which Ar approaches the $(H_2S)_2$ along either its B or C axis. Single-point calculations at CCSD(T)/aug-cc-pVTZ level identifies the global minimum as having Ar along the B/C axis of $(H_2S)_2$ and it has a stabilization energy of $1.0\hspace{2mm} kcal\hspace{2mm} mol^{-1}$, after zero-point and counter-poise corrections