Structure and Stability of the Hypervalent Na₂CN Molecule:  An Experimental and <i>ab Initio</i> Study

Abstract

The Na 2 CN molecule has been detected in the vapor over a mixture of sodium metal and sodium cyanide by means of Knudsen effusion mass spectrometry. The ionization potential, IP(Na 2 CN + ) = 4.9 ± 0.2 eV, and the energy of dissociation into NaCN and Na, D 0 °(NaCN−Na) = 24.8 ± 1.6 kcal/mol, have been determined. The measured value of the dissociation energy indicates moderately strong bonding of sodium to the NaCN fragment, although theoretical calculations indicate a somewhat lower value of 17.4 kcal/mol. The experimental ionization potential agrees well with the theoretical value of 4.66 eV calculated for the vertical ionization. The lowest energy structures are planar with C s symmetry. The two linear isomers are ca . 12 kcal/mol less stable and may be described as “electronomers” Na + (CN) - Na • and Na • (CN) - Na + . The molecule can be described as a salt of the Na 2 + radical cation with the CN - anion, like the previously reported Li 2 CN molecule. The presence of the Na 2 + unit justifies the term hypervalent for this system.