Knowledge Bank

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, $△Ka=0$ 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 $C2v$ 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 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.