Photoelectron spectroscopy of HCCN− and HCNC− reveals the quasilinear triplet carbenes, HCCN and HCNC

Abstract

Negative ion photoelectron spectroscopy has been used to study the HCCN− and HCNC− ions. The electron affinities (EA) of cyanocarbene have been measured to be EA(HCCN X̃ 3Σ−=2.003±0.014 eV and EA(DCCN X̃ 3Σ−)=2.009±0.020 eV. Photodetachment of HCCN− to HCCN X̃ 3Σ− shows a 0.4 eV long vibrational progression in ν5, the H–CCN bending mode; the HCCN− photoelectron spectra reveal excitations up to 10 quanta in ν5. The term energies for the excited singlet state are found to be T0(HCCN ã 1A′)=0.515±0.016 eV and T0(DCCN ã 1A′)=0.518±0.027 eV. For the isocyanocarbene, the two lowest states switch and HCNC has a singlet ground state and an excited triplet state. The electron affinities are EA(HCNC X̃ 1A′)=1.883±0.013 eV and EA(X̃ 1A′ DCNC)=1.877±0.010 eV. The term energy for the excited triplet state is T0(HCNC ã 3A″)=0.050±0.028 eV and T0(DCNC ã 3A″)=0.063±0.030 eV. Proton transfer kinetics in a flowing afterglow apparatus were used to re-measure the enthalpy of deprotonation of CH3NC to be ΔacidH298(CH3NC)=383.6±0.6 kcal mol−1. The acidity/EA thermodynamic cycle was used to deduce D0(H–CHCN)=104±2 kcal mol−1 [ΔfH0(HCCN)=110±4 kcal mol−1] and D0(H–CHNC)=106±4 kcal mol−1 [ΔfH0(HCNC)=133±5 kcal mol−1].