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

We report new 351 nm negative ion photoelectron spectra of CCl(2)(-), CBr(2)(-), and CI(2)(-). This study was undertaken in an attempt to understand the major discrepancy between dihalocarbene (CX(2), 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 (CHX(2)(-)) contaminant in the dihalocarbene anion beam, previously considered insignificant, made a major contribution to the reported CX(2)(-) photoelectron spectra. Thus, the interpretations of the earlier CX(2)(-) 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 CX(2)(-) photoelectron spectrum. The new CCl(2)(-) photoelectron spectrum displays resolved vibronic transitions to the two lowest electronic states of CCl(2): X(1)A(1) and a(3)B(1). The electron affinity of X(1)A(1) CCl(2) 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 X(1)A(1) and a(3)B(1) states of CCl(2) is estimated to be approximately 0.9(2) eV, consistent with high-level theoretical studies. While we confirm similar dihalomethyl anion contaminants in the earlier photoelectron spectra of CBr(2)(-) and CI(2)(-) and report new photoelectron spectra for these ions, the paucity of resolved features in the spectra provides limited additional thermochemical information.