Computational Investigations on Stability and Equilibrium Composition of (Z)-indol-3-ylidenemethanol: An Enol-tautomer of 1H-indole-3-carbaldehyde via DFT Approach

Background: The stability and molecular properties of 1H-indole-3- carbaldehyde (A) and its tautomer, (Z)-indol-3-ylidenemethanol (B) are examined using B3LYP/6-311++G(d,p), M06/6-311++G(d,p), wB97xD/6-311++G(d,p) and MP2/6- 311++G(d,p) methods. Method: The molecular reactivity indices calculated and discussed are the EHOMO, ELUMO, Δ(EHOMO-LUMO), dipole moment, softness (σ), chemical hardness (η), chemical potential (μ), ionization potential (IP), electron affinity (EA), nucleophilicity index (ω) and thermodynamics parameters. Observations: The band gap reveals that compound A is hard, more thermodynamically stable and less labile than B. This is in agreement with the total energy, chemical hardness and chemical softness calculated for the compounds. Result: The equilibrium constant calculation shows that the mixture of the tautomers in aqueous medium is 99.98 % and 0.12 % for compounds A and B respectively. This reveals that compound B is unstable in aqueous medium and is fast converted to compound A, this might explain the reason for compound B not to have been synthesized or reported in any literature. Conclusion: Critical analysis of the charge population distributions shows that charge distribution responded more to the basis sets rather than the computational methods used for the calculations.