Computation of global reactivity descriptors along the proton transfer co-ordinate of 9-Hydroxy-phenalen-1-one and 6-Hydroxy-benzo[de]anthracen-7-one: a DFT-based comparative study

Potential energy (PE) curves along the proton transfer co-ordinate of ground and excited states of 9-Hydroxy-phenalen-1-one (9HPO) and 6-Hydroxy-benzo[de]anthracen-7-one (6HBAO) have been studied using DFT-CAM-B3LYP/6-311G(d,p) and TD-DFT-CAM-B3LYP/6-31G(d) level of theory, respectively. The choice of these two molecules is based on their symmetric and asymmetric nature for their intramolecular H-bonded six-membered ring systems. Ground and excited states of PE calculations show that due to lower potential energy barrier, 9-Hydroxy-phenalen-1-one (9HPO) and 6-Hydroxy-benzo[de]anthracen-7-one (6HBAO) can be in equilibrium with their enol and keto tautomeric forms. Computed values of global reactivity descriptors, that is, average polarisability (alpha(av)) and chemical hardness (eta) along the proton transfer co-ordinate of both the molecules are very small although their variations conform with their respective optimum principles. Inspired by the significant variation of the first hyperpolarisabilty along the proton transfer co-ordinate of both the compounds, it is concluded that the first hyperpolarisability can serve as a better parameter to identify enol or keto tautomeric forms in enol keto equilibrium. The variation of average first hyper polarisability (beta(av)) along the proton transfer co-ordinate follows the optical gap rule in both the molecules.

Automated summary

Potential energy curves and global reactivity descriptors of 9HPO and 6HBAO in ground and excited states were studied using DFT-CAM-B3LYP/6-311G(d,p) and TD-DFT-CAM-B3LYP/6-31G(d) level of theory. The results showed that both molecules can exist in equilibrium with their enol and keto tautomeric forms due to lower potential energy barrier. The first hyperpolarisability was found to be a better parameter for identifying the tautomeric forms in the enol-keto equilibrium.