15min:

GALEN SEDO AND KENNETH R. LEOPOLD, Department of Chemistry, University of Minnesota, 207 Pleasant St., SE, Minneapolis, MN 55455.

The microwave spectrum of the partially bound complex (CH₃)₃CCN-SO₃ has been recorded. The nitrogen-sulfur bond length is 2.34 Å, which is almost exactly half way between that in weakly bound N₂-SO₃ and the more strongly bonded (CH₃)₃N-SO₃. A simple Townes and Dailey analysis of the ¹⁴N nuclear quadrupole coupling constant gives a value of about 0.19 e^- transferred away from the (CH₃)₃CCN upon complexation. Ab initio calculations at the MP2/aug-cc-pvtz level yield a binding energy relative to free (CH₃)₃CCN and SO₃ of 11.0 kcal/mol, which is only about a fourth of that of (CH₃)₃N-SO₃. As noted previously for the complex HCCCN-SO₃, comparison of the N-S bond length with those of a series of SO₃ adducts indicates that the proton affinity Lewis base is a good predictor of the properties of the complex. Indeed, spectra of this adduct were readily located on the basis of the proton affinity value for (CH₃)₃CCN.