Photoelectron spectroscopy of the halocarbene anions HCF^-, HCCl^-, HCBr^-, HCl^-, CF₂^-, and CCl₂^-

The 488 nm photoelectron spectra are reported for the HCX(X̃ ¹A′) + e^-←HCX^-(X̃ ²A″) and HCX(ã³A″) + e ^-←HCX^-(X̃²A″) transitions in HCF^-, DCF^-, HCCl^-, HCBr^-, and MCI^- and for the CX₂(X̃¹A₁) + e^-←CX₂^- (X̃²B₁) transitions in CF₂^- and CCl₂^-. Adiabatic electron affinities are found to be 0.557 ± 0.005 eV (HCF), 0.552 ± 0.005 eV (DCF), 1.213 ± 0.005 eV (HCCl), 1.556 ± 0.008 eV (HCBr), 1.683 ± 0.012 eV (HCI), 0.179 ± 0.005 eV (CF ₂), and 1.603 ± 0.008 eV (CCl₂). Bounds for the triplet excitation energies are determined for all the halocarbenes for which photoelectron spectra were observed, with the exception of CCl₂. For the HCX halocarbenes, upper bounds for the triplet excitation energies are 14.7 ± 0.2 kcal/mol (HCF, DCF), 11.4 ± 0.3 kcal/mol (HCCl), and 9 ± 2 kcal/mol (HCBr). A more detailed analysis of HCF suggests the actual triplet excitation energy to be 11.4 ± 0.3 kcal/mol, 14.7 ± 0.2 kcal/mol, or 8.1 ± 0.4 kcal/mol, with the first value the most likely. Since the multiplicity of the ground state of HCl is not known, we report the energy of its first excited state to be less than 9 ± 2 kcal/mol. The absence of an observed triplet state in the CF₂^- photoelectron spectrum allows us to assign a lower bound on the triplet excitation energy of CF₂ of 50 ± 2 kcal/mol.