Microwave and infrared spectra of C 2H 4…HCCH: barrier to twofold internal rotation of C 2H 4

Abstract

Microwave spectra of C 2H 4…HCCH, C 2H 4…DCCH, C 2H 4…DCCD, D 2CCH 2…HCCH, and trans-HDCCHD…HCCH have been recorded using a pulsed-nozzle Fourier-transform microwave spectrometer. An α-type, Δ K α = 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 and about its CC bond. For the normal 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 π cloud, with the HCCH axis normal to the plane of the ethylene. The hydrogen bond length is found to be 2.78 Å. Centrifugal-distortion analysis yields a weak-bond stretching force constant of 2.5 N/m (0.025 mdyn/Å), 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) × 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, ν 0 = 3271.61 cm −1, is nearly identical to that found for the similar vibration in the acetylene dimer.