The first singlet excited state (S1) intramolecular hydrogen bond of malonaldehyde derivatives: a TD-DFT and CIS study

Abstract

In the present study, for the first time, the first singlet excited state (S1) intramolecular hydrogen bond (IMHB) of malonaldehyde derivatives by time-dependent density functional theory (TD-DFT) and configuration interaction singles (CIS) with 6–311++G(d, p) basis set is investigated. In this regard, we initially characterized the equilibrium structures of the first singlet excited states of the benchmark systems and some of their properties such as excitation energies, oscillator strengths and dipole moments were analyzed. Then, the IMHB energies have been calculated by several adopted methods, such as potential energy density (PED) of Espinosa, related rotamers method (RRM) and rotational barrier method (RBM). The RBM and RRM energies appear grossly overestimate the intramolecular hydrogen bond energies as compared to the PED method and are not convenient for estimation the IMHB energy of excited states. Then, the more reliable PED results were used to systematically investigate the halogen substitution effects. It was found that the halogen replacement at R3 position decreases the IMHB energies, both in the ground and the first singlet excited states, while the halogen substitution at R1 and R2 positions of ground and excited states have paradoxical effects on the IMHB energies. Subsequently, the geometrical, topological, molecular orbital and spectroscopic descriptors of IMHB are comprehensively examined and various correlations between the PED energies and different HB descriptors are explored. Our results show that the excellent linear correlations between the PED energies and the majority of HB descriptors exist. Interestingly, the linear character of correlations between the HB descriptors with CIS energies is more than the TD-DFT ones. Consequently, on the basis of these correlations, one can claim that the PED method, E PED ≈ V(r BCP)/2, is a trusty equation for estimation of IMHB energy of RAHB systems, both in the ground and first singlet excited states (S1).