Effect of the folding of methylene units in the conformational preferences of small diesters

The conformational preferences of dimethyl succinate and ethylene glycol diacetate have been explored in the gas phase using ab initio quantum mechanical calculations at the HF/6-31G(d) and MP2/6-31G(d)levels. Furthermore, the effects of both aqueous and carbon tetrachloride solvents have been estimated using the self-consistent reaction-field model developed by Miertus, Scrocco, and Tomasi and adapted to semiempirical methods by Orozco and Luque. The results have been interpreted in terms of the so-called ''folding of methylene units'', which is the tendency of the methylene units to adopt a gauche conformations when a short aliphatic segment is placed between two carbonyls. For dimethyl succinate the lowest energy conformation corresponds to the TTGTT as expected from our previous studies on the folding of methylene units. On the other hand, for ethylene glycol diacetate the global minimum is the TGG (S) over bar T conformation, which is stabilized by the simultaneous combination of two stereoelectronic effects: (i) the folding of methylene units and (ii) the gauche oxygen effects.