MICROWAVE AND INFRARED-SPECTRA OF C2H4...HCCH - BARRIER TO TWOFOLD INTERNAL-ROTATION OF C2H4

Microwave spectra of C2H4...HCCH, C2H4...DCCH, C2H4...DCCD, D2C = CH2...HCCH, and trans-HDC=CHD...HCCH have been recorded using a pulsed-nozzle Fourier-transform microwave spectrometer. An a-type, DELTA-K(a) = 0 spectrum is observed, with a number of transitions being split into doublets due to tunneling arising from the hindered internal rotation of the ethylene unit about its C=C bond. 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 C2v 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 lo be 2.78 angstrom. Centrifugal-distortion analysis yields a weak-bond stretching force constant of 2.5 N/m (0.025 mdyn/angstrom), corresponding to a stretching frequency of 56 cm-1. Stark effect measurements determine the electric dipole moment of the complex to be 8.852(21) X 10(-31) C m (0.2654(6) D). The observed tunneling-induced splittings yield an internal rotation barrier of 240 cm-1. An infrared spectrum of the asymmetric acetylenic C-H stretch in the complex has also been measured using an optothermal color-center laser spectrometer. The rotational lines are predissociation broadened, preventing the resolution of K structure. The observed band origin, nu-0 = 3271.61 cm-1, is nearly identical to that found for the similar vibration in the acetylene dimer.