A comparative study of intramolecular hydrogen bond on N-formylformamide derivatives in ground and first singlet excited state: a DFT and TD-DFT study

Abstract

Intramolecular hydrogen bond (IHB) of N-formylformamide derivatives in the both ground (GS) and first singlet excited state (S1) have been investigated by DFT and TD-DFT methods with the standard basis set 6-311++G (d,p), respectively. Also the potential energy density method was used to calculate of the IHB energies in the both GS and S1. Our calculations show that IHB energies in the GS are much higher than the S1 one. In order to explain this energy difference, molecular orbital analysis (MOS) has been employed. According to the MOS, it was found that electron density on antibonding orbital (δ O–H * ) increases after excitation, which can be a reason for the weakened IHB in the S1. In this work, we evaluated and compared the substitution effects on the IHB strength in the both GS and S1 by some of the IHB descriptors such as geometrical, topological and orbital charge transfer parameters. The results proved that the substitution effects on the both GS and S1 are same. As with replacing of the halogen atoms at R2 position the IHB energies increased while for the R2 and R1,2 decreased. Moreover, for the first time, various geometry—based resonance indices such as Gilli (Q and λ) parameters, original harmonic oscillator model of aromaticity, harmonic oscillator model of electron delocalization and harmonic oscillator model for heterocyclic with π-electrons delocalization were calculated and compared in the both GS and S1. Interestingly, it is found from the resonance indices that electron delocalization in the S1 increased that this result is against the RAHB theory in the studied molecules. Finally, the linear equations of the IHB energies versus the IHB descriptors parameters and resonance indices in the both GS and S1 indicate that the potential energy density is an inappropriate method for calculation of the IHB energies of the heterocyclic RAHB systems in the S1.