Theoretical Investigation on Halocyanocarbene XCCN (X = F, Cl, Br)

Complete the calculations by means of the active space self-consistent field method (CASSCF) with ANO-S basis sets was performed on the ground and low-lying electronic excited states of halocyanocarbene XCCN and their anions, with X = F, Cl and Br. Our results show that XCCN have triplet X(3)A '' neutral ground states and the singlet-triplet energy gaps Delta ES-T (kJ/mol) follow this order: 7.4 (FCCN) < 13.4 (CICCN) < 6.6 (BrCCN). The calculated most intensive vertical excitation energies of X-3 A '' -> 2(3) A '' transitions for FCCN, CICCN, and BrCCN at 408.3, 385.4 and 345.2 kJ/mol, respectively, are mainly attributed to the pi(alpha') -> n(xy) transitions. The adiabatic electron affinities for XCCN are found to be 235.7, 2330 and 237.2 kJ/mol, respectively. Compared with the results of HCCN, the electron affinities are enhanced.