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

The 364 nm negative ion photoelectron spectra of CF2-, CCl2-, CBr2-, and CI2- exhibit transitions to two different electronic states, the 1A1 and 3B1. The CF2- spectrum exhibits well-resolved transitions to both electronic states. In the cases of CCl2-, CBr2-, and CI2-, 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 CF2-. 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 1A1 state is found to be the lower state for CF2, CCl2, and CBr2 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 CI2, 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 CF2, CCl2, CBr2, and CI2, respectively. In addition, the bending and symmetric stretching vibrational frequencies have been determined for either one or both states. I