Conformations of methylbenzenesulfonate and its substituted derivatives: gas-phase electron diffraction versus vibrational spectroscopy

Abstract

The conformational composition of the vapor at T = 376(5) K and the conformer structures of para-nitro-methylbenzenesulfonate (NO2C6H4SO2OCH3, 4-NMBS) molecule have been studied by a combined gas-phase electron diffraction and mass spectrometry (GED/MS) method complemented by quantum chemical calculations (DFT/B3LYP/cc-pVTZ and MP2/cc-pVTZ). The vapor contains the only species of 4-NMBS existing as two conformers with different mutual positions of C–S and O–C bonds: synclinal (sc), C1 symmetry, and antiperiplanar (ap), Cs symmetry. The geometrical parameters (A and deg.) of conformers (sc/ap) obtained from the experiment (uncertainties are in parentheses) are: r h1(C–H) = 1.062(6)/1.062(6), r h1(C–C)av. = 1.395(4)/1.395(4), r h1(C–S) = 1.786(5)/1.779(5), r h1(S–O)av. = 1.435(3)/1.439(3), r h1(O–C) = 1.445(6)/1.450(6), ∠C–CS–C = 121.8(6)/122.1(6), ∠S–O–C = 119.2(21)/116.4(21), ϕC–O–S–CS = 74(8)/180 and ϕO–S–CS–C = 73(12)/90. The calculated barriers to internal rotation of the SO2OCH3, OCH3, CH3 and NO2 groups exceed the thermal energy RT corresponding to the temperature of the GED experiment. The conformational properties of the SO2OCH3 group are the same in the unsubstituted methyl ester (MBS) and its para-methyl and para-nitro substituted derivatives. The conformational sensitivity of vibrational spectrum was studied. Natural bond orbital analysis was used to explain the structural peculiarities of the molecule 4-NMBS.