PHOTOELECTRON-SPECTROSCOPY OF THE NCN- AND HNCN- IONS

We have used negative ion photoelectron spectroscopy to measure the el ectron affinities of the cyanonitrene and the cyanoamino radical: EA(( X) over tilde (3) Sigma(g)(-) NCN) = 2.484 +/- 0.006 eV, EA((X) over t ilde: (2)A '' HNCN) = 2.622 +/- 0.005 eV, and EA((X) over tilde (2)A ' ' DNCN) = 2.622 +/- 0.005 eV. Our experimental findings are accurately reproduced by complete basis set (CBS) ab initio electronic structure calculations: EA((X) over tilde (3) Sigma(g)(-) NCN) = 2.51 +/- 0.03 eV and EA((X) over tilde (2)A '' HNCN) = 2.60 +/- 0.03 eV. Our qualita tive picture of these species is N=C=N (X) over tilde (3) Sigma(g)(-), [N=C=N](-) (X) over tilde (2) Pi(u), HNC=N (X) over tilde (2)A '', an d [HNC=N](-) (X) over tilde (1)A'. We make use of the electron affinit ies of NCN and HNCN, together with the gas phase acidity of cyanamide, Delta(acid)H(298)(H-NHCN) = 350 +/- 3 kcal mol(-1), to find the bond enthalpies of H2NCN. We find DH298(H-NHCN) = 96.9 +/- 3.0 kcal mol(-1) and Delta(f)H(298)-(HNCN) = 77 +/- 4 kcal mol(-1), which closely agre e with the calculated values: DH298(H-NHCN) = 95.5 +/- 0.7 kcal mol(-1 ) and Delta(f)H(298)(H-NCN) = 76.7 +/- 0.7 kcal mol(-1). We therefore use the CBS ab initio electronic structure calculations to estimate De lta(acid)H(298)(H-NCN) congruent to 339 kcal mol(-1). Use of the exper imental electron affinity, EA(NCN), leads to the NH bond enthalpy of t he cyanoamino radical, DH298(H-NCN) = 83 +/- 2 kcal mol(-1) and Delta( f)H(298)(NCN) = 108 +/- 4 kcal mol(-1).