Theoretical Study On The Tautomerisms Of Monochalcogenocarboxylic Acids And Monochalcogenosilanoic Acids

In this paper, all of the tautomers, complexes and transition states of monochalcogenocarboxylic acids and monochalcogenosilanoic acids have fully optimized at the HF/6-311+G(d,p) level first, then re-optimized by MP2/6-311+G(d,p). The further single point calculation of all the optimized species has also been carried at the MP2/6-311++G(2df,2pd)//MP2/6-311+G(d,p)level. Systematically investigations on the tautomerisms of monochalcogenocarboxylic acids and monochalcogenosilanoic acids in the gas phase and in the polar and aprotic solution THF lead to the following conclusions:For the tautomerism of monochalcogenocarboxylic acids by direct intramolecular proton transfer, the keto forms [CH3COXH] are more stable than the enol forms [CH3CXOH (X = S, Se and Te)] both in the gas-phase and in the polar solution THF; the tautomeric barriers and the reaction enthalpy changes increase in the order S < Se < Te with a decrease of electronegativity of X, similar to previous theoretical results. The forming of supermolecule by binding of CH3OCH3 with monochalcogenocarboxylic acid with a scale of 1:1 will increase the stabilities of the enol froms [CH3CXOH…O(CH3)2(X = S, Se and Te)] due to the stronger OH…O interactions, and make all of the enol forms exist predominantly in the polar solution THF. Meanwhile the tautomeric barriers from the keto to the enol forms are also lower than those in the cases of bare keto forms. The results supply some reasonable explanations for the previous experimental results.While for the tautomerism of monochalcogenosilanoic acids by direct intermolecular proton transfer, the enol forms [CH₃SiXOH] are more stable than the keto forms [CH₃SiOXH (X = S, Se and Te)] both in the gas-phase and in the polar solution THF. That is different from the case in the monochalcogenocarboxylic acids because the more stable enol forms of monochalcogenosilanoic acids are not from the strength of Si=X double bond, but the Si-O and O-H bonds. The forming of supermolecule by binding of CH3OCH3 with monochalcogenosilanoic acid with a scale of 1:1 also increase the stabilities of the enol forms [CH₃SiXOH…O(CH₃)₂(X = S, Se and Te)] due to the stronger OH…O interactions. While the electrostatic interactions will weaken the relative stabilities of the enol forms of monochalcogenosilanoic acids if the continuum solvent effects are taken into account due to the differences of their dipole moments, which is contrasted with the case in the monochalcogenocarboxylic acids.In a word, in views of thermodynamics and kinetics, enol forms of the monochalcogenocarboxylic acids are predominantly present in polar solvents at lower temperature, while the keto forms are preferred in the gas phase; for the monochalcogenosilanoic acids, the enol forms are always the preference either in the gas-phase or in the polar solvent .