Structure and gas-phase acidity of oxalic acid and its disila derivative. A theoretical study by means of the DFT quantum theoretical method
Density functional theory at the B3LYP/6-311 + G* level has been used to study the rotational conformers, gas-phase acidities and vibrational spectra of oxalic and disilaoxalic acid. For oxalic acid the geometry with two five-membered intramolecular hydrogen bonds is calculated to have the lowest energy. In the absence of the stabilizing effect of intramolecular H bonds in disilaoxalic acid (due to the elongation of the M–M distance to 2.35 Å) the most stable conformer corresponds to a rectangular structure making the angle between the two SiOOH groups equal to 90°. The energies calculated for stable conformers lie within 4 kcal mol–1. Experimental and B3LYP computed geometries of oxalic acid agree well. Disilaoxalic acid is, according to calculated ionizations of the first and second hydrogen, about 10–25 kcal mol–1 more acidic than its carboxylic acid analogue. Oxalic and disilaoxalic acid are about 15 and 25 kcal mol–1 stronger acids than formic and silanoic acid, respectively. Computed vibrational frequencies for oxalic, formic and silanoic acids were on average 1–3% higher than the observed values.