Properties of diazocarbene [CNN] and the diazomethyl radical [HCNN] via ion chemistry and spectroscopy

We have used negative ion photoelectron spectroscopy to measure the electron affinities of diazocarbene and the diazomethyl radical: EA(X̃ ³∑^- CNN) = 1.771 ± 0.010 eV, EA(X̃ ²A″ HCNN) = 1.685 ± 0.006 eV, and EA(X̃ ²A″ DCNN) = 1.678 ± 0.006 eV. Our experimental findings are accurately reproduced by complete basis set (CBS) ab initio electronic structure calculations: EA(X̃ ³∑^- CNN) = 1.83 ± 0.03 eV, EA(X̃ ²A″ HCNN) = 1.69 ± 0.03 eV. We make use of the electron affinities of CNN and HCNN, together with the gas phase acidity of diazomethane, Δ_acidH₂₉₈(HCHN₂) = 372.2 ± 2.1 kcal mol^-1 (CBS calculated value = 373.4 ± 0.7), to find the bond enthalpies of H₂CNN. We find DH₂₉₈(H-CHN₂) equal to 97 ± 2 kcal mol^-1, which closely agrees with the CBS-QCI/APNO-calculated value [DH₂₉₈(H-CHN₂) = 98.5 ± 0.7 kcal mol^-1]. From proton transfer experiments in a Fourier transform mass spectrometer and a tandem flowing afterglow-selected ion flow tube (FA-SIFT), we find Δ_acidH₂₉₈(HCNN) = 352 ± 4 kcal mol^-1 in agreement with the CBS-QCI/ APNO-calculated value of 351.8 ± 0.7 kcal mol^-1. Use of the experimental electron affinity, EA(CNN), leads to the CH bond enthalpy of the cyanoamino radical, DH₂₉₈(H-CNN) = 79 ± 4 kcal mol^-1 which is in excellent agreement with the CBS-QCI/APNO-calculated value: DH₂₉₈(H-CNN) = 78.7 ± 0.7 kcal mol^-1. If we adopt the CBS-QCI/APNO value for Δ_fH₂₉₈(CH₂N₂) [64.1 ± 0.7 kcal mol^-1] as our reference, we obtain Δ_fH₂₉₈(HCN₂) = 110 ± 2 kcal mol^-1 and Δ_fH₂₉₈(CN₂) = 136 ± 5 kcal mol^-1, which are again in agreement with the CBS-QCI/APNO values: Δ_fH₂₉₈(HCN₂) = 110.5 ± 0.7 kcal mol^-1 and Δ_fH₂₉₈(CN₂) = 138.4 ± 0.7 kcal mol^-1. We recommend revised experimental values for Δ_fH₀(HCN) = 30.9 ± 0.7 kcal mol^-1 and Δ_fH₂₉₈(HCN) = 30.8 ± 0.7 kcal mol^-1 and find that the reaction CH (²Π) + N₂ → HCN + N (⁴S) to be slightly endothermic, Δ_rxnH₀ = 1.6 ± 0.7 kcal mol^-1.