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

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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 ROCH2++ 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.

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