Singlet−Triplet Splittings in CX2 (X = F, Cl, Br, I) Dihalocarbenes via Negative Ion Photoelectron Spectroscopy

The 364 nm negative ion photoelectron spectra of CF 2 -, CCl 2 -, CBr 2 -, and CI 2 - exhibit transitions to two different electronic states, the 1 A 1 and 3 B 1 . The CF 2 - spectrum exhibits well-resolved transitions to both electronic states. In the cases of CCl 2 -, CBr 2 -, and CI 2 -, the spectra exhibit extended, partially resolved vibrational progressions and the two states are overlapped, making a direct determination of the origin transition energy not possible. The overlapped spectra show that the singlet−triplet splittings in the heavier halocarbenes are much smaller than for CF 2 - . The results of ab initio calculations have been used to generate Franck−Condon simulations of the spectra, which aid in the determination of the band origins. The 1 A 1 state is found to be the lower state for CF 2, CCl 2, and CBr 2 and the electron affinities have been determined to be 0.180 ± 0.020, 1.59 ± 0.07, and 1.88 ± 0.07 eV, respectively. For CI 2, the triplet state is apparently the lower lying state with an electron affinity of 2.09 ± 0.07 eV. The singlet−triplet splitting energy has been determined to be 54 ± 3, 3 ± 3, 2 ± 3, −1 ± 3 kcal/mol for CF 2, CCl 2, CBr 2, and CI 2, respectively. In addition, the bending and symmetric stretching vibrational frequencies have been determined for either one or both states.