The rotational spectrum, structure, and molecular properties of the ethylene–HCl dimer

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Abstract
Three isotopic species of a hydrogen bonded dimer formed between ethylene and HCl have been observed through assignment of their rotational spectra using a pulsed, Fourier transformmicrowave spectrometer in which a gas mixture is pulsed into an evacuated Fabry–Perot cavity. The spectra unambiguously reveal a nonplanar, near prolate, asymmetric top (κ = −0.9880) in its ground vibrational state. The equilibrium structure has C 2V symmetry with the HCl on the C 2 symmetry axis of the dimer, perpendicular to the carbon–carbon bonding axis with the hydrogen atom located between the chlorine and the ethylene molecule, pointing midway between the carbon atoms. The chlorine atom is situated, on average, 3.724 Å from the edge of the ethylene molecule. The following spectroscopic constants have been obtained for the ethylene–H35Cl dimer: the rotational constants are A = 25 457(349), B = 2308.143(3), and C = 2167.970(3) in units of MHz; the chlorine nucleus quadrupole coupling constants are χ a a = −54.076(4), χ b b = 27.091(6), and χ c c = 26.985(10) in units of MHz, and the centrifugal distortion constants are D J = 7.2(2) and D J K = 282(5) in units of kHz.