C--H...$\pi$ and C--F...H--C INTERACTIONS IN THE ACETYLENE--FLUOROFORM DIMER

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Rotational spectra for four isotopomers of the HCCH--HCF$_3$ complex have been identified by Fourier-transform microwave spectroscopy. The spectra exhibit considerable fine splittings, presumably arising from the internal motion of one or both subunits; both first- and second-order Stark effects have been observed. A fit of selected $K$=0 and 1 components for the normal isotopomer (assigned using second-order Stark effect data) gave an rms of around 4 kHz and provided rotational constants $A = 5450.236(4)$ MHz, $B = 1381.3361(9)$ MHz, $C = 1375.1333(12)$ MHz. These rotational constants and the planar moments are consistent with a structure of $C_s$ symmetry exhibiting C--H...$\pi$ and C--F...H--C interactions and are in excellent agreement with a structure obtained from optimizations at the MP2/6-311++G(2d,2p) level.

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Author Institution: Department of Chemistry, Eastern Illinois University, 600 Lincoln Ave.,; Charleston, IL 61920 USA

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