Do enantiomers of benzenesulfonic acid exist? Electron diffraction and quantum chemical study of molecular structure of benzenesulfonic acid

Abstract

Molecular structure of benzenesulfonic acid was studied by gas-phase electron diffraction and quantum chemical (B3LYP/cc-pVTZ, МР2/cc-pVDZ, МР2/cc-pVTZ) methods. On the base of mass spectrometric analysis it was found that saturated vapor at Т=396(9)K is represented by only molecular species, monomeric benzenesulfonic acid. Theoretical calculations showed that the molecule has two mirror conformers of C1 symmetry which can invert to each other via transition state of Cs symmetry by rotation of OH-group around SO(H) bond. Both computational methods, B3LYP and MP2, resulted in the same structure of enantiomers; the MP2/cc-pVDZ calculations denoted a over-barrier transition between enantiomers at the temperature of electron diffraction experiment, while B3LYP and MP2 calculations with cc-pVTZ basis set estimated the barrier height to be comparable with the thermal energy value. Two geometric models of C1 and Cs symmetry were examined in gas electron diffraction structural analysis. It was established that the structure of C1 symmetry (Rf=3.3%) demonstrated the best fit with GED data in comparison with Cs structure (Rf=3.8%).In conformer of C1 symmetry an ordinary bond SO(Н) is located almost orthogonal to benzene ring plane, and an OH bond practically eclipses one of SO bonds of SO3H fragment. The following internuclear distances (Å) in benzenesulfonic acid were determined: rh1(CH)av=1.116(6), rh1(CC)ср=1.402(4), rh1(CS)=1.770(5), rh1(SO)av=1.438(4), rh1(SO)=1.623(4), rh1(OH)=0.870(17).Calculations of internal rotation potential functions and NBO-analysis of electron density distribution in a conformer and transition states between enantiomers were performed to establish the reasons of stability of the found asymmetric structure of the studied molecule. The structure of free molecule of benzenesulfonic acid was compared with that of molecular form in crystal.