Structural change at the onset of microsolvation: Rotational spectroscopy of HCN⋯HCN-SO₃

The structure and dipole moment of HCN⋯HCN-SO₃ have been determined by rotational spectroscopy. The N-S bond distance is 2.470(20) Å, which is 0.107(21) Å shorter than that in HCN-SO₃. In contrast, the N⋯H distance, 2.213(29) Å, is the same to within experimental uncertainty as that in (HCN)₂. The dipole moment of HC¹⁵N⋯HC¹⁵N-³²SO₃ is 8.640(19) D, representing an enhancement of 1.238(19) D over the sum of the dipole moments of HCN and HCN-SO₃. The results indicate significant changes in the HCN-SO₃ subunit upon interaction with a single HCN "solvent" molecule, with relatively little change in the HCN-HCN interaction. The hypersensitivity of the HCN-SO₃ moiety to the presence of an additional HCN arises because the dative bond is partially formed, and we suggest that partially bound systems may offer sensitive probes of microsolvation.