Comparison of the relative stabilities of polycyclic aryl nitrenium ions and arylmethyl cations: ab initio and semiempirical molecular orbital calculations

Syn andanti aryl nitrenium ions

“…A similar phenomenon was observed in Mulliken population analyses of some of the same ions at the HFr3-21G level. 9 However, the impact of those results was tempered by the well-known shortcomings of the Mulliken analysis. In both cases, the reasons can be traced partly to the substantial migration of electron density along the NH bond.…”

“…data for the parent arenes in Table III , are summarized in Table IV The general pattern of varying nitrenium ion stabilities is very similar to that predicted in the earlier semiempirical AM1 calculations. 9 The ab initio stabilization energies are generally larger than those predicted by AM1 and increase in the order Ž . Ž .…”

“…Even in the absence of such obvious electronic asymmetry these energy differences can amount to several kilocalories per mole. At the HFr3-21G level, structure 4 was predicted to be 3.5 kcal mol y1 more stable than 5 9 .…”

“…In the course of our AM1 studies on various carbocyclic aryl nitrenium ions with RϭCOCH , CH , and H, we optimized both forms 3 3 and noticed that their relative stabilities often differed by surprisingly large amounts. 9,13 Qualitative analysis of these differences suggested the presence of both steric and electronic components. Some of the largest energy differences were predicted in an AM1 study of heteroaryl nitrenium ions and attributed to the electrostatic interaction between the polar NH group and an adjacent aza substitutent.…”

“…These variations largely reflect the differing abilities of the aryl moieties to delocalize electron density onto the formally positive NH group. Indeed, a simple semiquantitative model based on AM1 results was formulated 9 on this basis using the ideas of PMO theory.…”