Quantum-chemical study of alkyl carbenium ions in 100% sulfuric acid

A quantum-chemical study of neutral and protonated monoalkyl sulfates RHSO4 and [RH2SO4](+) (where R = CH3, C2H5, iSO-C3H7, and terr-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 accordi ng to the Muller-Plesset perturbation theory MP2/6-31+G*//6-31+G*. Protonat ed tert-butyl sulfate was also calculated by the DFT B3LYP/6-31++G** method . It was found that monoalkyl sulfates are covalent compounds, and the comp lete abstraction of alkyl carbenium ions from them has substantial energy c ost: 196.4, 161.7, 150.8 and 136.0 kcal/mol, respectively. Protonated methy l and ethyl sulfates are also covalent compounds according to the calculati on. They have lower but still high energies of heterolytic dissociation (65 .0 and 33.5 kcal/mol, respectively). The energy of R+ abstraction from prot onated isopropyl sulfate is much lower: 23.6 kcal/mol. The main covalent st ate and the ion-molecular pair, which is a carbenium ion [C(CH3)(2)H]+ solv ated by the H2SO4 molecule, have about the same energy. The ground state of protonated tert-butyl sulfate corresponds to the ion-molecular complex [C( CH3)(3)(+) H2SO4] With Still lower energy of carbenium ion [C(CH3)(3)](+) a bstraction, which is equal to 10.0 kcal/mol. Calculation according to the D FT 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 surfa ce.