Correlation of dative bond length and donor proton affinity in adducts of SO₃: A good predictor for HCCCN-SO₃

The rotational spectrum of HCCCN-SO₃ has been observed using Fourier transform microwave spectroscopy. The vibrationally averaged structure is that of a symmetric top, with the HCCCN axis along the C₃ axis of the SO₃, and the nitrogen end near the sulfur. The N-S bond length is 2.567(13) Å, which is slightly shorter than the sum of the van der Waals radii. The NSO angle is 91.7(4)°, indicating a small but distinct distortion of the SO₃ from planarity, and the N-S interaction can be described as a chemical bond in an early stage of its formation. The N-S bond lengths in a series of SO₃ adducts with amines, nitriles, and pyridine are shown to correlate well with the proton affinities of the bases. In addition, for the bases considered here, the proton affinities vary in a regular manner with the ionization energies corresponding to removal of a lone pair electron. Thus, the trend in proton affinities follows the variation in energy gap between donor and acceptor orbitals in these complexes, accounting for the utility of the proton affinity in correlating aspects of structure and bonding across the series.