Knowledge Bank

The Lewis acid-base adducts $C5H5N-SO3$ and $HCCCN-SO3$ have been studied by Fourier transform microwave spectroscopy. The spectrum of $C5H-5N-SO3$ indicates a short N-S bond length of $1.91540(66)$ \AA and free rotation of the $SO3$ unit. In contrast, $HCCCN-SO3$ is more weakly bound with an N-S distance of $2.5676(76)$ \AA, only slightly less than the expected van der Waals interaction distance. The NSO angles are $98.9212(45)\circ$ and $91.89(36)\circ$ for $C5H5N-SO3$ and $HCCCN-SO3$ respectively. A Townes and Dailey analysis of the $\mathrm{<mrow\; data-mjx-texclass="ORD">14</mrow>}N$ quadrupole coupling constant of $C5H5N-SO3$ indicates a transfer of 0.54 electrons upon formation of the dative bond. This is physical measurement of the ``soft'' portion of the chemical interaction and comparisons are made with other adducts of $SO3$. Bonding is considered in light of Pearson's concept of Hard and Soft Acids and $Bases,a$ nothing the correlation of adduct properties, such as electron transfer and bond length, with the energy gap between the donor and acceptor orbital.