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The microwave spectra of four isotopologues of the CHClF(2)-HCCH dimer have been measured and used to determine the structure of the complex. An initial scan over the 7-18 GHz region using the chirped-pulse microwave spectrometer at the University of Virginia provided initial assignments of the (35)Cl and (37)Cl isotopologues, with two additional H(13)C(13)CH species assigned using the resonant cavity Balle-Flygare microwave spectrometer at Eastern Illinois University. For the most abundant isotopologue, the rotational constants and quadrupole coupling constants are: A = 3301.21(4) MHz, B = 1353.4268(19) MHz, C = 1153.7351(18) MHz, χ(aa) = 34.681(12) MHz, χ(bb) = -69.70(3) MHz, χ(cc) = 35.02(2) MHz and χ(ab) = -8.8(3) MHz, in good agreement with ab initio calculations at the MP2/6-311++G(2d,2p) level. The alignment of CHClF(2) with respect to acetylene reveals a C-Hπ interaction, with a secondary C-ClH-C interaction also present between the two monomers. The fitted distance between the CHClF(2) hydrogen atom and the center of the triple bond is 2.730(6) Å, the distance between the chlorine atom and the acetylenic hydrogen is 3.061(38) Å, and the C-Hπ angle is 148.2(6)°. In addition, the centrifugal distortion constants give an estimate of the binding energy for the weak interaction of about 4.9(5) kJ mol(-1), in reasonable agreement with several similar complexes.

The rotational spectra of ( 18 OCS) 3 and (O 13 CS) 3 have been assigned using a pulsed-nozzle Fourier transform microwave spectrometer. The data for these isotopic species have been combined with normal species data from a paper by Connelly et al. [Connelly, J. P., et al. Mol Phys. 1996, 88, 915] to determine the nine structural parameters of the trimer. The carbon atoms form a triangle with the axes of the monomer units roughly parallel to each other (barrel-like structure). The monomers have an antiparallel-like arrangement, with the dipole moment of one monomer opposing those of the other two monomers. The dipole moment of the complex has been measured, giving values of µ a ) 0.537(10) D, µ c ) 0.373(2) D, and µ total ) 0.653(8) D. The b component of the dipole moment was too small to determine from our data but is predicted to be about 0.02 D. The structure of (OCS) 3 has been compared with similar trimers involving linear triatomic molecules and with the OCS dimer. Semiempirical calculations have been performed on the (OCS) 3 system, and they show good qualitative agreement with the experimental structure.

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