Structure and gas-phase acidity of oxalic acid and its disila derivative. A theoretical study by means of the DFT quantum theoretical method

Abstract

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.