Rotational spectroscopy of mixtures of ethyne and iodine monochloride: isolation and characterisation of the pi-type complex C2H2 center dot center dot center dot ICl

The ground-state rotational spectrum of a complex formed by ethyne and iodi ne monochloride was observed by using pulsed-nozzle, Fourier-transform micr owave spectroscopy. A fast-mixing nozzle was utilised to avoid chemical rea ction of the component gases prior to their supersonic expansion. Rotationa l constants A(0), B-0 and C-0, quartic centrifugal distortion constants Del ta(J), Delta(JK) and delta(J), and halogen nuclear quadrupole coupling cons tants chi(aa)(X) and {chi(bb)(X)-chi(cc)(X)}, where X=Cl or I, were determi ned for the three isotopomers C2H2... (ICl)-Cl-35, C2H2... (ICl)-Cl-37 and C2D2... (ICl)-Cl-35. Detailed interpretation of the rotational constants es tablished that the equilibrium geometry of the complex has a planar, T-shap e of C-2v symmetry in which ethyne acts as the bar of the T. This geometry, with the zero-point distance r(*... I)=3.115(1) Angstrom between the centr e of the pi-bond of ethyne and the nearest halogen atom I, establishes that the interaction in this complex is between ethyne as a pi electron donor a nd I of ICl as the electron acceptor. The halogen nuclear quadrupole coupli ng constants chi(aa)(X) were interpreted on the basis of a simple model to show that, on complex formation, fractions delta(1)=0.026 and delta(2)=0.05 6 of an electronic charge are transferred from the ethyne pi bond to I and from I to Cl, respectively, leading to a net decrease of 0.030e at I. The c omplex is weakly bound, according to the intermolecular stretching force co nstant k(sigma)=12.2(1) Nm(-1) determined for the isotopomers C2H2... (ICl) -Cl-35 and C2H2... (ICl)-Cl-37 from Delta(J) values. The opportunity is tak en to compare the properties of C2H2... ICl established here with those sim ilarly determined for the series C2H2... XY, where XY=Cl-2, ClF, or BrCl.