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

The conformational composition of the vapor at T = 376(5) K and the conformer structures of para -nitro-methylbenzenesulfonate (NO 2 C 6 H 4 SO 2 OCH 3 , 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), C 1 symmetry, and antiperiplanar (ap), C s symmetry. The geometrical parameters (Å 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–C S –C = 121.8(6)/122.1(6), ∠S–O–C = 119.2(21)/116.4(21), ϕ C–O–S–C S = 74(8)/180 and ϕ O–S–C S –C = 73(12)/90. The calculated barriers to internal rotation of the SO 2 OCH 3 , OCH 3 , CH 3 and NO 2 groups exceed the thermal energy RT corresponding to the temperature of the GED experiment. The conformational properties of the SO 2 OCH 3 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. Graphical Abstract