MICROWAVE AND INFRARED SPECTRA OF $C_{2}H_{4}--HCCH$

Microwave spectra of $C_{2}H_{4}--HCCH, C_{2}H_{4}--DCCH, C_{2}H_{4}--DCCD, D_{2}C=CH_{2}--HCCH$, and trans-HDC=CHD--HCCH have been recorded using a pulsed-nozzle Fourier-transform microwave spectrometer. An a-type, $\\bigtriangleup K_{a} = 0$ spectrum is observed, with a number of transitions being split into doublets due to tunneling. For the normal isotopic species we find A = 25981 (33) MHz, B+C = 3478.2560(13) MHz, and B-C = 89.45(18) MHz. The complex is shown to have a $C_{2v}$ structure in which the HCCH unit hydrogen bonds to the ethylene $\\pi$ cloud, with the HCCH axis normal to the plane of the ethylene. The hydrogen-bond length is found to be 2.78 {\\AA}. Centrifugal-distortion analysis yields a weak-bond stretching force constant of 0.025 mdyn/{\\AA}, corresponding to a stretching frequency of 56 $cm^{-1}$. Stark effect measurements determine the electric dipole moment of the complex to be 0.2654(6) D. An infrared spectrum of the asymmetric acetylenic C-H stretch in the complex has also been measured using an color-center laser optothermal spectrometer. The rotational lines are predissociation broadened, preventing the resolution of K structure. The Q-branch for the band overlaps the Q-branch for the equivalent band in the acetylene dimer.