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

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 S52.003 60.014 eV and EA~DCCN X̃ S)52.00960.020 eV. Photodetachment of HCCN to HCCN X̃ S shows a 0.4 eV long vibrational progression in n5 , the H–CCN bending mode; the HCCN photoelectron spectra reveal excitations up to 10 quanta in n5 . The term energies for the excited singlet state are found to be T0(HCCN ã A8)50.51560.016 eV and T0(DCCN ã A8) 50.51860.027 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 A8)51.883 60.013 eV and EA(X̃ A8 DCNC)51.87760.010 eV. The term energy for the excited triplet state is T0(HCNC ã A9)50.05060.028 eV and T0(DCNC ã A9)50.06360.030 eV. Proton transfer kinetics in a flowing afterglow apparatus were used to re-measure the enthalpy of deprotonation of CH3NC to be DacidH298(CH3NC)5383.660.6 kcal mol . The acidity/EA thermodynamic cycle was used to deduce D0(H–CHCN)510462 kcal mol 21 @D fH0(HCCN)511064 kcal mol # and D0(H–CHNC)510664 kcal mol 21 @D fH0(HCNC)513365 kcal mol # . © 2002 American Institute of Physics. @DOI: 10.1063/1.1496473#