Observation and Properties of the van der Waals Dimer Ar···HCCCN Produced in Electrical Discharge

The weakly bound van der Waals dimer between a molecule of cyanoacetylene and an argon atom was observed by rotational spectroscopy of supersonic expansion. Cyanoacetylene was generated in situ from vinyl cyanide precursor by means of a high voltage discharge just downstream of the expansion nozzle. Both $\text{}^{b}$R- and $\text{}^{b}$Q-type rotational transitions were measured and rotational, centrifugal distortion, and nuclear quadrupole splitting constants were determined. Analysis of the data shows that ground state geometry of the dimer is characterised by R$\text{}_{cm}$=3.65Å andθ=94.9º, with the Ar atom shifted from the position above the cyanoacetylene centre of mass in the direction of the CN group.