The photoelectron spectrum of CCl2−: the convergence of theory and experiment after a decade of debate

Abstract

We report new 351 nm negative ion photoelectron spectra of CCl₂⁻, CBr₂⁻, and CI₂⁻. This study was undertaken in an attempt to understand the major discrepancy between dihalocarbene (CX₂, X = Cl, Br, I) singlet–triplet splittings reported by our laboratory (R. L. Schwartz, G. E. Davico, T. M. Ramond, W. C. Lineberger, J. Phys. Chem. A., 1999, 103, 8213) and new theoretical values. Our recent experiments show that a dihalomethyl anion (CHX₂⁻) contaminant in the dihalocarbene anion beam, previously considered insignificant, made a major contribution to the reported CX₂⁻ photoelectron spectra. Thus, the interpretations of the earlier CX₂⁻ spectra and the reported singlet–triplet splittings were incorrect. Replacing O⁻ with OH⁻ in the anion formation process yields a pure dihalomethyl anion, whose highly structured photoelectron spectrum can be subtracted from the contaminated spectrum, yielding a clean CX₂⁻ photoelectron spectrum. The new CCl₂⁻ photoelectron spectrum displays resolved vibronic transitions to the two lowest electronic states of CCl₂: X¹A₁ and a³B₁. The electron affinity of X¹A₁ CCl₂ is 1.593(6) eV. A large change in geometry between the anion and the neutral triplet state precludes the direct observation of the triplet origin. The energy difference between the X¹A₁ and a³B₁ states of CCl₂ is estimated to be ∼0.9(2) eV, consistent with high-level theoretical studies. While we confirm similar dihalomethyl anion contaminants in the earlier photoelectron spectra of CBr₂⁻ and CI₂⁻ and report new photoelectron spectra for these ions, the paucity of resolved features in the spectra provides limited additional thermochemical information.