The rotational spectrum and molecular structure of the acetylene–HCl dimer

Abstract

Three isotopic species of a weakly bound T-shaped π complex formed between acetylene and HCl have been observed through the assignment of their rotational spectra by the use of a pulsed, Fourier-transform spectrometer with a high pressure gas being pulsed into an evacuated Fabry–Perot cavity. The spectra clearly show the complex to be a nearly prolate asymmetric top (κ = −0.9898) with the HCl on the a-inertial axis and perpendicular to acetylene and with the hydrogen atom pointing towards the middle of the acetylene triple bond. The chlorine is situated, on average, 3.699 Å from the edge of the acetylene molecule. The following molecular constants have been obtained for the C2H2, H35Cl isotopic species: The rotational constants are A = 36 084(838), B = 2481.065(6), and C = 2308.602(6) MHz; the nuclear quadrupole coupling constants are χaa = −54.342(4), χbb = 26.862(5), and χcc = 27.480(9) MHz, and the rotational centrifugal distortion constants are DJ = 7.9(4) and DJK = 497(8) kHz.