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

ion of H2 + by O occurs with 55% yield to produce the desired CCl2 ; however, CHCl2 produced by proton abstraction, with m/z only 1 amu greater than the desired product, is a significant minor ionic reaction product. Since theWien filter has a mass resolution of at bestm/DmE 40, we can only partially separate ions within 1 amu of each other at m/z E 80, specifically CCl2 and CHCl2 . It was initially anticipated that the electron affinity of CHCl2 was sufficiently low that it would not be an important contaminant in the CCl2 photoelectron spectrum. However, the significant difference between the experimental and calculated photoelectron spectra of CCl2 , as well as of the heavier dihalocarbenes, has led us to investigate the dihalomethyl anionic product of this reaction as a possible cause of the discrepancy. In order to evaluate the CHCl2 contribution to the CCl2 spectrum, we replace O with OH in the above reaction. Only the dihalomethyl anionic product is now formed, OH + CH2Cl2 CHCl2 + H2O Hydroxide ion is formed by reacting O with methane in the flow tube. Sufficient methane is added to ensure complete removal of O before CH2Cl2 is introduced and guarantee that CHCl2 is the only species present near m/z E 83. The photoelectron spectrum of CHCl2 is remarkably broad and exhibits extensive vibrational structure. Comparison to the earlier data shows conclusively that dihalomethyl anion contamination was present in the previously reported photoelectron spectrum of CCl2 and is responsible for the bulk of the progression previously attributed to the triplet state of CCl2. Spectra of CHBr2 and CHI2 , similarly broad and highly structured, overlap with the spectra of the corresponding dihalocarbene anion, indicating that dihalomethyl anion contamination was also present in the photoelectron spectra of CBr2 and CI2 . We take several steps to obtain dihalocarbene anion photoelectron spectra with minimal contamination. We first minimize CHX2 contribution to the contaminated spectrum by tuning the Wien mass filter to 50% of the maximum on the low-mass side of the appropriate unresolved mass peak. In the synthesis of CCl2 , we took the additional step of using deuterated dichloromethane precursor (CD2Cl2) to increase the mass difference between CCl2 and CDCl2 , improving our ability to separate the two ions with theWien filter. As this approach is less effective for the heavier dihalocarbenes, CH2Br2 and CH2I2 precursors were used to make CBr2 and CI2 , respectively. Next, the appropriately scaled authentic CHX2 photoelectron spectrum is subtracted from the two-component spectrum. The multiple peaks that are attributable solely to dihalomethyl anion are used to determine the scaling factor. The final constraint is that the scaling factor must leave the subtracted spectrum everywhere as non-negative. As discussed below, this procedure was applied to each of the dihalocarbenes, but the amount of new information gained diminished for the heavier dihalocarbenes.