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-
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
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
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.