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 Σg NCN) ) 2.484 ( 0.006 eV, EA(X̃ 2A′′ HNCN) ) 2.622 ( 0.005 eV, and EA(X̃ 2A′′ DNCN) ) 2.622 ( 0.005 eV. Our experimental findings are accurately reproduced by complete basis set (CBS) ab initio electronic structure calculations: EA(X̃ Σg NCN) ) 2.51 ( 0.03 eV and EA(X̃ 2A′′ HNCN) ) 2.60 ( 0.03 eV. Our qualitative picture of these species is NdCdN X̃ Σg , [NdCdN]X̃ Πu, HNCtN X̃ 2A′′, and [HNCtN]X̃ 1A′. We make use of the electron affinities of NCN and HNCN, together with the gas phase acidity of cyanamide, ∆acidH298(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 ∆fH298(HNCN) ) 77 ( 4 kcal mol-1, which closely agree with the calculated values: DH298(H-NHCN) ) 95.5 ( 0.7 kcal mol-1 and ∆fH298(H-NCN) ) 76.7 ( 0.7 kcal mol-1. We therefore use the CBS ab initio electronic structure calculations to estimate ∆acidH298(H-NCN) = 339 kcal mol-1. Use of the experimental electron affinity, EA(NCN), leads to the NH bond enthalpy of the cyanoamino radical, DH298(H-NCN) ) 83 ( 2 kcal mol-1 and ∆fH298(NCN) ) 108 ( 4 kcal mol-1.