Experimental and theoretical study of dicyanocarbene C(CN)2

Abstract

Dicyanocarbene C(CN)2 and its radical cation (m/z 64) were generated by dissociative ionization of tetracyanoethene, dicyanofuroxan and dicyanofurazan and characterized by collisional activation (CA) and neutralization-reionization (NR) mass spectrometries and tandem MS3 experiments performed in a ‘hybrid’ tandem mass spectrometer having a sector-quadrupole-sector configuration. In both neutral and ionized forms, dicyanocarbene is found to be a stable and detectable species. Existence of the peaks at m/z 52, 24 and 12 in the CA/NR spectra suggests however some possible post-collisional rearrangements. The carbene was further studied by ab initio calculations using B3LYP/6-311+G(3df) for geometries and CASPT2(14,12) and MR-SDCI with the cc-pVTZ and aug-cc-pVTZ basis sets for relative energies. The dicyano form NC-C-CN is consistently more stable than its isocyano isomers CN-C-CN and CN-C-NC, irrespective of the electronic state. Each neutral carbene exhibits a triplet ground state lying up to 2.6 eV below the singlets. All the triplet, singlet and ionized states of dicyanocarbene have a linear shape. The other states have either linear or bent shape but small barriers to linearity. Adiabatic ionization energies (IEa) were estimated as follows: NC-C-CN: 11.3 eV with linear 2Πu cation; NC-C-NC: 10.4 eV with linear 2Π cation, and CN-C-NC: 9.9 eV with bent 2A1 cation.