Molecular structure and infrared spectra of dimethyl malonate: A combined quantum chemical and matrix-isolation spectroscopic study

Abstract

The infrared spectra of dimethyl malonate isolated in low-temperature argon and xenon matrices were studied. Theoretical calculations, carried out at the MP4/6-31G**, MP2/6-31++G** and DFT(B3LYP)/6-311++G** levels, predict two different conformers of nearly equal internal energy, both exhibiting the methyl ester moieties in the cis (C–O) configuration. One of the conformers has C₂ symmetry with the two ester groups crossed symmetrically with respect to the C–C–C plane. This structure is doubly degenerated by symmetry. The other form (gauche) belongs to the C₁ point group. Four identical-by-symmetry minima on the PES correspond to this structure. The energy of this form is predicted (at the MP4 level) to be slightly lower than that of the C₂ conformer. The six minima on the PES can be divided into two groups of three (one C₂ isomer and two C₁ forms in a group). Each structure from one group is related to its counterpart from the other group by the operation of reflection in the C–C–C plane. In each group, the conformers are separated by low energy barriers (less than 2 kJ mol⁻¹), while conformational interconversions between the two groups imply a transition state structure with a vis-à-vis orientation of oxygen atoms and thus are associated with considerably higher energy barriers. The infrared spectra of the matrix isolated compound were found to closely match the spectrum predicted for the C₁ conformer. Annealing of the matrices up to 55 K does not lead to significant changes in the spectra, suggesting that the low energy barriers separating the two conformers allow practically all molecules of dimethyl malonate to transform to the more stable gauche conformer, when they are cooled down after landing on the matrix surface. Spectra of the low temperature solid form of the compound (8 K < T < 200 K) also reveal only the presence of the C₁ conformer.