NMR and theoretical study on conformation of methacrolein dimer, 2,5-dimethyl-3,4-dihydro-2H-pyran-2-carboxyaldehyde

Detailed structures of methacrolein dimer (MAD) were analyzed by nuclear magnetic resonance (NMR) and theoretical calculations. Two dimensional H-1 and C-13 NMR peaks were assigned. Relative stability of conformations and interconversion barrier height were studied by semi-empirical MNDO and nb initio DFT calculations. Barrier height between pseudoaxial and pseudoequatorial half-chair conformers was 5.8 kcal mol(-1). Separations of chemical shifts of two protons bonded to the same methylene carbons became larger as measurement temperature decreased, due to reduced mobility of the ring framework.