A Systematic Model Study of the Mechanisms of Electrophilic Substitutions of Aliphatic Hydrocarbons: CH₄ + E⁺ (E = F⁺, Cl⁺, NO⁺, NO2⁺, HCO⁺, Li⁺, OH⁺, and H₂O-OH⁺)

Peter R. Schreiner, Paul v R. Schleyer, and Henry F. Schaefer III

Institut Für Organische Chemie, Henkestr. 42, D-91054 Erlangen, Germany and The Center for Computational Quantum Chemistry, University of Georgia Athens, GA 30602, USA

The reactions of F⁺, Cl⁺, NO⁺, NO₂⁺, HCO⁺, Li⁺, OH⁺, and H₂O-OH⁺ with methane were investigated ab initio at the MP4SDTQ/6-31G**/MP2/6-31G** + ZPVE level of theory. The more stable electrophiles (NO⁺, NO₂⁺, HCO⁺) attack the methane carbon directly; the first intermediate is a substituted methyl cation-dihydrogen complex (ECH2+--H2). This either loses H2 to give the H2CE⁺ cation or rearranges to the thermodynamically most favored formal insertion product H3CEH⁺. The generally small activation energies for hydrogen loss (0.4 - 2.0 kcal mol-1) are lower than the barriers for other reactions (5 - 50 kcal mol-1). In contrast, less stable, high energy electrophiles (e.g., H⁺, R⁺, F⁺, Cl⁺, OH⁺) react with methane without a barrier, so that direct CH insertion or carbon attack pathways are not differentiated. The most favorable binding arrangements involve the electrophile bound to carbon directly, and not via 3c-2e bonding (CHE⁺). Dihydrogen attachment (ECH₂⁺--H₂) leads to additional stabilization. With the exception of NO⁺ as the electrophile, no transition structures for direct CH attack were located. But even for NO⁺, this CH pathway is less favorable by 14 kcal mol^-1. None of the 3c-2e bonding arrangements involve C, H, and the electrophile E⁺. Even the hydride shift transition structures which lead to the thermodynamic products exhibit 4c-4e instead of 3c-2e binding. The critical activation barriers are related qualitatively to the singlet-triplet separation of the electrophile. As suggested by Bach, the less stable open shell singlet states of electrophiles with triplet ground states (for instance Cl⁺) react most readily with methane. Ground state singlet electrophiles with high lying triplet states are the least reactive. Initial CH₄--E⁺ complexes were only found for the most stable electrophiles (NO⁺, NO₂⁺, HCO⁺, HOOH₂⁺, Li⁺). However, these CH₄---E⁺ complexes need not be involved in the most favorable pathway. The reactions of HOOH₂⁺, a more realistic model, have been compared with those of singlet OH⁺. Since the more charge localized ground states involving HOOH₂⁺ are more stabilized, the overall activation barriers are higher (4 - 7 kcal mol^-1), but the features of the reaction are quite similar. We conclude that no general mechanism describes the reactions of these electrophiles with methane.

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A Systematic Model Study of the Mechanisms of Electrophilic Substitutions of Aliphatic Hydrocarbons: CH4 + E+ (E = F+, Cl+, NO+, NO2+, HCO+, Li+, OH+, and H2O-OH+)

Peter R. Schreiner, Paul v R. Schleyer, and Henry F. Schaefer III

A Systematic Model Study of the Mechanisms of Electrophilic Substitutions of Aliphatic Hydrocarbons: CH₄ + E⁺ (E = F⁺, Cl⁺, NO⁺, NO2⁺, HCO⁺, Li⁺, OH⁺, and H₂O-OH⁺)