Photoelectron Spectroscopy of the NCN⁻ and HNCN⁻ Ions

We have used negative ion photoelectron spectroscopy to measure the electron affinities of the cyanonitrene and the cyanoamino radical: EA(X̃ 3 NCN) = 2.484 ± 0.006 eV, EA(X̃ 2 A‘‘ HNCN) = 2.622 ± 0.005 eV, and EA(X̃ 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(X̃ 3 NCN) = 2.51 ± 0.03 eV and EA(X̃ 2 A‘‘ HNCN) = 2.60 ± 0.03 eV. Our qualitative picture of these species is N C N X̃ 3, [N C N] - X̃ 2 Π u, HNC⋮N X̃ 2 A‘‘, and [HNC⋮N] - X̃ 1 A‘. We make use of the electron affinities of NCN and HNCN, together with the gas phase acidity of cyanamide, Δ acid H 298 (H−NHCN) = 350 ± 3 kcal mol -1, to find the bond enthalpies of H 2 NCN. We find DH 298 (H−NHCN) = 96.9 ± 3.0 kcal mol -1 and Δ f H 298 (HNCN) = 77 ± 4 kcal mol -1, which closely agree with the calculated values: DH 298 (H−NHCN) = 95.5 ± 0.7 kcal mol -1 and Δ f H 298 (H−NCN) = 76.7 ± 0.7 kcal mol -1 . We therefore use the CBS ab initio electronic structure calculations to estimate Δ acid H 298 (H−NCN) ≅ 339 kcal mol -1 . Use of the experimental electron affinity, EA(NCN), leads to the NH bond enthalpy of the cyanoamino radical, DH 298 (H−NCN) = 83 ± 2 kcal mol -1 and Δ f H 298 (NCN) = 108 ± 4 kcal mol -1 .