Quantum-Chemical Study of Alkyl Carbenium Ions
in 100% Sulfuric Acid

V. N. Solkan, I. V. Kuz’min, and V. B. Kazanskii

Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia

Received March 14, 2000

Abstract—A quantum-chemical study of neutral and protonated monoalkyl sulfates RHSO₄ and [RH₂SO₄]⁺
(where R = CH₃, C₂H₅, iso-C₃H₇, and tert-C₄H₉) is carried out. Calculations are performed using the Hartree–
Fock method in the 6-31G** and 6-31++G** basis sets taking into account electron correlation according to
the Müller–Plesset perturbation theory MP2/6-31+G*//6-31+G*. Protonated tert-butyl sulfate was also calcu-
lated by the DFT B3LYP/6-31++G** method. It was found that monoalkyl sulfates are covalent compounds,
and the complete abstraction of alkyl carbenium ions from them has substantial energy cost: 196.4, 161.7, 150.8
and 136.0 kcal/mol, respectively. Protonated methyl and ethyl sulfates are also covalent compounds according
to the calculation. They have lower but still high energies of heterolytic dissociation (65.0 and 33.5 kcal/mol,
respectively). The energy of R⁺ abstraction from protonated isopropyl sulfate is much lower: 23.6 kcal/mol. The
main covalent state and the ion–molecular pair, which is a carbenium ion [C(CH₃)₂H]⁺ solvated by the H₂SO₄
molecule, have about the same energy. The ground state of protonated tert-butyl sulfate corresponds to the ion–
molecular complex [C(CH₃H₂SO₄] with still lower energy of carbenium ion [C(CH₃)₃]⁺ abstraction, which
is equal to 10.0 kcal/mol. Calculation according to the DFT B3LYP/6-31++G** method shows the absence of
a minimum for the protonated tert-butyl sulfate with a covalent structure on the potential energy surface.

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