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MICROWAVE INVESTIGATIONS OF C₅H₅N-SO₃ AND HCCCN-SO₃: THE PRINCIPLE OF HARD AND SOFT ACIDS AND BASES APPLIED TO PARTIALLY BONDED SYSTEMS.

S. W. HUNT, D. L. FIACCO, M. CRADDOCK AND K. R. LEOPOLD, Department of Chemistry, University of Minnesota, Minneapolis, MN 55455.

The Lewis acid-base adducts C₅H₅N-SO₃ and HCCCN-SO₃ have been studied by Fourier transform microwave spectroscopy. The spectrum of C₅H₅N-SO₃ indicates a short N-S bond length of 1.91540(66) Å and free rotation of the SO₃ unit. In contrast, HCCCN-SO₃ is more weakly bound with an N-S distance of 2.5676(76) Å, only slightly less than the expected van der Waals interaction distance. The NSO angles are 98.9212(45)^o and 91.89(36)^o for C₅H₅N-SO₃ and HCCCN-SO₃ respectively. A Townes and Dailey analysis of the ¹⁴N quadrupole coupling constant of C₅H₅N-SO₃ indicates a transfer of 0.54 electrons upon formation of the dative bond. This is a 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, noting the correlation of adduct properties, such as electron transfer and bond length, with the energy gap between the donor and acceptor orbital.