The stabilities of α-oxy and α-thio carbenium ions: the importance of the ground-state energies of the neutral precursors

Abstract

The abilities of oxy and thio substituents to stabilize an adjacent carbenium ion centre have been evaluated by ab initio methods, up to the MP3/6-31G* level of theory. The relative stabilities of RXCH₂⁺(X = O or S; R = H or CH₃) have been calculated by using the hydride-transfer equation ROCH_2⁺+ CH₃SR → CH₃OR + RSCH₂⁺. HOCH₂⁺ is calculated to be more stable than HSCH₂⁺ by 2.3 kcal mol^–1, and CH₃SCH₂⁺ is more stable than CH₃OCH₂⁺ by 0.7 kcal mol^–1. On the other hand, when chlorides are used as the precursors, the equation ROCH₂⁺+ RSCH₂Cl → RSCH₂⁺+ ROCH₂Cl is exothermic, e.g. by 2.9 kcal mol^–1 for R = H (MP3/6-31G*) and ca. 2–3 kcal mol^–1(estimated for MP3/6-31G*) for R = CH₃. The latter value is in excellent agreement with recent ion cyclotron resonance experiments. The apparent contrast between the conclusions from the foregoing equations regarding the relative stabilities of RSCH₂⁺ and ROCH₂⁺ results from significant ground-state stabilization of ROCH₂Cl relatively to RSCH₂Cl. The energies of the two isodesmic equations show a strong dependence on the basis set, and reliable results are obtained only when both d-functions and correlation energy are included in the calculations. The possible correlation of the π-donation abilities and other charge-related criteria of the RS and RO substituents with the stabilities of RSCH₂⁺ and ROCH₂⁺ is analysed and critically discussed. It is concluded that such correlations should be treated with great caution, in particular when first-and second-row substituents are compared.