Conformational analysis: Part 42. A modelling and LIS/NMR investigation of the conformations of some aromatic SO compounds

Abstract

AbstractA refined Lanthanide‐Induced‐Shift Analysis (LISA), in which both the paramagnetic and the diamagnetic lanthanide induced chemical shifts are normalised separately, is used with molecular mechanics (MMFF94) and ab initio (RHF/6‐31G, RHF/6‐311G** and B3LYP/6‐311G**) calculations to investigate the conformations of methyl para‐tolyl sulphoxide 1, sulphone 2 and sulphonate 3 and the conformational equilibria of methyl 2,4‐dimethylphenyl sulphoxide 4 and sulphone 5. In 1 acceptable agreement factors (AFs) in the LISA analysis were only obtained when the SO/phenyl dihedral angle was increased from that (10–15°) of the computed geometries to 30–40°. The calculated geometries for 2 had the methyl group orthogonal to the phenyl ring and gave good AF in the LISA analysis. In both compounds, the AF were in the order MMFF94 ≤ B3LYP < RHF with the larger basis sets giving better agreement than the limiting 6‐31G* function. In 3 all the geometries again had the methoxy group orthogonal to the phenyl ring but, while the ab initio geometries still gave good AFs, the MMFF94 structure gave an AF which was only just acceptable. The unsymmetrical compounds 4 and 5 display conformational equilibria. In 4 two energy minima were found, both with the SMe group orthogonal to the phenyl ring, with the SO oxygen cis or trans to the 2‐methyl group. The energy difference (cis–trans) was calculated as 1.6 kcal/mol (MMFF94), 2.7 (6‐31G*), 1.9 (6‐311G**) but −0.11 (DFT). The LISA analyses on the separate conformers gave poor agreement (ca. 3%) for the trans conformer but increasing the SO/Ph dihedral angle again gave an acceptable value (ca. 1.4%). Inclusion of even small amounts of the cis conformer in the LIS analysis gave poorer results. Thus, the LISA analysis confirms the MM and RHF prediction that the trans conformer is more stable and suggests that the conformer energy difference in solution is equal to or greater than the largest calculated value of 2.7 kcal/mol. In 5 all the calculations gave two mirror image conformations with the SMe orthogonal to the phenyl ring. The LISA analysis in this case is complicated by the fact that in the stable conformer the complexing oxygen atoms are not equivalent. This was resolved by considering both coordination sites in one Z‐matrix and varying the % coordination on each oxygen atom. This gave an acceptable AF of ca. 1.0% for all the geometries with ca. 2:1 coordination on the oxygen atoms trans and cis to the C2‐methyl. The difference in the geometries and conformer energies found by LISA for these molecules in solution compared to the theoretical values may be due to interactions with the chloroform solvent for these polar compounds. Copyright © 2010 John Wiley & Sons, Ltd.