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Azacyclohexatriene-2-ylidene (1), the 2-isomer of pyridine (2), has been generated by one-electron reduction of the corresponding radical cation in neutralization−reionization mass spectrometric experiments. The experimental finding that this molecule is stable on the microsecond time scale is in agreement with results of quantum chemical calculations that indicate both 1 and its radical cation, 1•+ , correspond to minima on the C5H5N and C5H5N•+ potential energy surfaces. The calculations predict that 1 is less stable than pyridine, 2, by 50 and 49 kcal/mol (MP2/6-31G** and CASSCF-MP2/6-31G**, respectively) or 47 kcal/mol (B3LYP/6-31G**), whereas the radical cations 1•+ and 2•+ are much closer in energy. The ylid ion 1•+ is predicted to be 6 and 7 kcal/mol lower in energy than 2•+ at the MP2 and CASSCF-MP2/6-31G** levels, respectively, and 1 kcal/mol higher according to the hybrid density functional theory. Calculations also suggest that facile isomerization of the ions is prohibited by an energy barrier, amounting to 62 and 57 kcal/mol at MP2/6-31G** and B3LYP/6-31G**, respectively, relative to 1•+ , which is even larger than the 38 kcal/mol obtained at both levels of theory required for the neutral transformation. Despite the substantial impediments, isomerization of excited species is possible since the lowest dissociation channels lie even higher in energy but the experimental observations confirm that neither the ions or neutrals undergo particularly facile isomerization. Using known thermochemical data a value for ΔH f (1•+ ) = 237 ± 5 kcal/mol was obtained from the measured appearance energy, 10.14 eV, of the C5H5N•+ ion generated from methyl picolinate, which is completely consistent with the theoretical predictions of 237−242 kcal/mol derived from the calculated energy differences between the various species and the known heat of formation of 2.

In order to gain a deeper understanding of the specific reactivity of arene ± Cr(CO) 3 complexes, the structures and energies of the reactive intermediates formally generated by abstraction of a proton (H ), a hydride (H À ), or a hydrogen atom (H . ) from the methyl group of toluene ± Cr(CO) 3 (2) were computationally investigated by using density functional theory based quantum chemical techniques. The solid-state structure of the parent complex (2) was determined by low-temperature X-ray crystallography and this confirmed the high accuracy of the computational methods. Besides calculating the geometry of the lowest energy conformation, particular emphasis was laid on the rotational barrier of the Cr(CO) 3 group as well as on that of the exocyclic carbon ± carbon bond which exhibited a significant amount of double-bond character in all of the reactive intermediates investigated. The results are put into broader perspective by discussing their relevance for the rationalization and prediction of the selectivity of synthetically relevant reactions of arene chromium tricarbonyl complexes.

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Theoretical investigation of the energies and geometries of photoexcited uranyl(VI) ion: A comparison between wave-function theory and density functional theoryThe quadrupole moment of the 3 ∕ 2 + nuclear ground state of Au 197 from electric field gradient relativistic coupled cluster and density-functional theory of small molecules and the solid state