Pulsed-nozzle, Fourier-transform microwave spectroscopy of the methyl cyanide-acetylene dimer

The ground-state rotational spectra of five isotopic species of a weakly bound dimer formed between methyl cyanide and acetylene have been investigated by pulsed-nozzle, Fourier-transform microwave spectroscopy. The rotational constant B0, the centrifugal distortion constants DJ and DJK, and the 14N–nuclear quadrupole coupling constant χ(14N) have been determined for the symmetric top species CH3C14N⋅⋅⋅HCCH, CH3C14N⋅⋅⋅DCCH, CH3C14N⋅⋅⋅HCCD, CH3C14N⋅⋅⋅DCCD, and CH3C15N⋅⋅⋅HCCH. The values for the parent isotopic species are B0=977.4659(1) MHz, DJ=0.718(1) kHz, DJK=139.36(7) kHz, and χ(14N)=−3.95(6) MHz. The nature of the observed spectra and the magnitudes of the B0 values and DJ values are interpreted in terms of a hydrogen-bonded geometry having C3v symmetry, with the nuclei in the order H3CCN⋅⋅⋅HCCH, with r(N⋅⋅⋅C)=3.425(1) Å and with the hydrogen bond stretching force constant kσ=4.7(1) Nm−1.