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

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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 χ a a = −54.342(4), χ b b = 26.862(5), and χ c c = 27.480(9) MHz, and the rotational centrifugal distortion constants are D J = 7.9(4) and D J K = 497(8) kHz.