C−H···O Hydrogen Bond between N-Methyl Maleimide and Dimethyl Sulfoxide: A Combined NMR and Ab Initio Study

The proton chemical shifts in N-methyl maleimide (NMM) in dry dimethyl sulfoxide-d6 (DMSO) and in cyclohexane-d12 solutions have been measured between 30 and 70 °C. The proton shieldings have also been calculated using the gauge-including atomic orbital method at the Hartree−Fock and hybrid Becke−Lee−Yang−Parr (B3LYP) density functional levels, using the 6-311++G and 6-311++G** basis sets. There is a 0.4 ppm discrepancy between the observed chemical shift of the ring protons in DMSO solution and the value derived from the calculations, whereas only 0.1 ppm or less is expected (see Wang, B.; Hinton, J. F.; Pulay, P. J. Comput. Chem. 2002, 23, 492). The discrepancy almost certainly arises from a C−H···O hydrogen bond between the acidic ring proton and the oxygen of DMSO. This is supported by its strong temperature coefficient and by model calculations on NMM−DMSO, NMM−acetone, and NMM−water complexes at the B3LYP and second-order Møller−Plesset levels with large basis sets (up to 6-311++G(3df,3pd)). The latter predict bonding strengths of 3−4 kcal/mol, large for a C−H···O hydrogen bond. The sign and approximate magnitude of the effect of hydrogen bonding on the ring proton chemical shift agree with the calculations.