Energetics and dynamics of carbene-forming dissociation reactions: The Purdue tandem flowing afterglow-guided ion beam instrument

A method for determining absolute thermochemistry for carbenes is described. The procedure involves the measurement of the threshold energy for collision-induced halide loss from $\alpha$-halocarbanions in a flowing afterglow-triple quadrupole instrument. The threshold energy is combined with gas phase acidities and other thermochemical data according to a simple thermochemical cycle in order to derive the absolute heats of formation for the carbene. This method is applied to the ground state singlet carbenes CHCl, CHF and CClF for which the dissociation from a singlet carbanion is a spin-allowed process. Recommended values for the absolute heats of formation for the five simple chlorine and fluorine containing carbenes, CHCl, CHF, CClF, CCl$\sb2$ and CF$\sb2$. A simple linear correlation between divalent state stabilization energies, derived from these recommended heats of formation, and halocarbene singlet-triplet splittings is presented. The CID method is also applied to three ground state triplet carbenes, CH$\sb2$, CH$\sb2$ = CHCH and C$\sb6$H$\sb5$CH, for which dissociation from a singlet carbanion is spin-forbidden. Comparison of the heats of formation for CH$\sb2$ derived independently from dissociations of CH$\sb2$X$\sp-$ (X = Cl, Br) with the known experimental value indicates that the halomethyl anion are dissociating adiabatically to ground state triplet products. Similar results are found for vinyl and phenyl carbenes. The derived heats of formation are compared to high level ab initio and density functional calculations. In addition, the heats of formation are used to derive $\alpha$-C-H bond energies in allyl and benzyl radicals. The design and construction of a new tandem flowing afterglow-guided ion beam instrument are presented. The instrument was designed in order to optimize the conditions for collision-induced dissociation threshold experiments