Properties of Diazocarbene [CNN] and the Diazomethyl Radical [HCNN] via Ion Chemistry and Spectroscopy

Abstract

We have used negative ion photoelectron spectroscopy to measure the electron affinities of diazocarbene and the diazomethyl radical: EA(Χ 3 Σ - CNN) = 1.771 ± 0.010 eV, EA(Χ 2 A‘‘ HCNN) = 1.685 ± 0.006 eV, and EA(Χ 2 A‘‘ DCNN) = 1.678 ± 0.006 eV. Our experimental findings are accurately reproduced by complete basis set (CBS) ab initio electronic structure calculations: EA(Χ 3 Σ - CNN) = 1.83 ± 0.03 eV, EA(Χ 2 A‘‘ 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 acid H 298 (HCHN 2 ) = 372.2 ± 2.1 kcal mol -1 (CBS calculated value = 373.4 ± 0.7), to find the bond enthalpies of H 2 CNN. We find DH 298 (H−CHN 2 ) equal to 97 ± 2 kcal mol -1, which closely agrees with the CBS-QCI/APNO-calculated value [ DH 298 (H−CHN 2 ) = 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 acid H 298 (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, DH 298 (H−CNN) = 79 ± 4 kcal mol -1 which is in excellent agreement with the CBS-QCI/APNO-calculated value: DH 298 (H−CNN) = 78.7 ± 0.7 kcal mol -1 . If we adopt the CBS-QCI/APNO value for Δ f H 298 (CH 2 N 2 ) [64.1 ± 0.7 kcal mol -1 ] as our reference, we obtain Δ f H 298 (HCN 2 ) = 110 ± 2 kcal mol -1 and Δ f H 298 (CN 2 ) = 136 ± 5 kcal mol -1, which are again in agreement with the CBS-QCI/APNO values: Δ f H 298 (HCN 2 ) = 110.5 ± 0.7 kcal mol -1 and Δ f H 298 (CN 2 ) = 138.4 ± 0.7 kcal mol -1 . We recommend revised experimental values for Δ f H 0 (HCN) = 30.9 ± 0.7 kcal mol -1 and Δ f H 298 (HCN) = 30.8 ± 0.7 kcal mol -1 and find that the reaction CH ( 2 Π) + N 2 → HCN + N ( 4 S) to be slightly endothermic, Δ rxn H 0 = 1.6 ± 0.7 kcal mol -1 .