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.