Photoelectron imaging of NCCCN-: The triplet ground state and the singlet-triplet splitting of dicyanocarbene

The photoelectron spectra of NCCCN- have been measured at 355 and 266 nm by means of photoelectron imaging. The spectra show two distinct features, corresponding to the ground and first excited states of dycianocarbene. With support from theoretical calculations using the spin-flip coupled-cluster methods, the ground electronic state of HCCCN is assigned as a triplet state, while the first excited state is a closed-shell singlet. The photoelectron band corresponding to the triplet is broad and congested, indicating a large geometry change between the anion and neutral. A single sharp feature of the singlet band suggests that the geometry of the excited neutral is similar to that of the anion. In agreement with these observations, theoretical calculations show that the neutral triplet state is either linear or quasilinear ((X) over tilde B-3(1) or (3)Sigma(-)(g)), while the closed-shell singlet ((a) over tilde (1)A(1)) geometry is strongly bent, similar to the anion structure. The adiabatic electron binding energy of the closed-shell singlet is measured to be 3.72 +/- 0.02 eV. The best estimate of the origin of the triplet band gives an experimental upper bound of the adiabatic electron affinity of NCCCN, EA <= 3.25 +/- 0.05 eV, while the Franck-Condon modeling yields an estimate of EA(NCCCN) =3.20 +/- 0.05 eV. From these results, the singlet-triplet splitting is estimated to be Delta E-ST((X) over tilde B-3(1)/(3)Sigma(-)(g)-(a) over tilde (1)A(1))=0.52 +/- 0.05 eV (12.0 +/- 1.2 kcal/mol). (C) 2010 American Institute of Physics. [doi:10.1063/1.3436717]