Hydrogen bond strengthening between o-nitroaniline and formaldehyde in electronic excited states: A theoretical study

To study the hydrogen bonds upon photoexcited, the time dependent density function method (TD DFT) was performed to investigate the excited state hydrogen bond properties of between o-nitroaniline (ONA) and formaldehyde (CH₂O). The optimized structures of the complex and the monomers both in the ground state and the electronically excited states are calculated using DFT and TD DFT method respectively. Quantum chemical calculations of the electronic and vibrational absorption spectra are also carried out by TD DFT method at the different level. The complex ONA⋯CH₂O forms the intramolecular hydrogen bond and intermolecular hydrogen bonds. Since the strength of hydrogen bonds can be measured by studying the vibrational absorption spectra of the characteristic groups on the hydrogen bonding acceptor and donor, it evidently confirms that the hydrogen bonds is strengthened in the S₁/S₂/T₁ excited states upon photoexcitation. As a result, the hydrogen bonds cause that the CH stretch frequency of the proton donor CH₂O has a blue shift, and the electron excitations leads to a frequency red shift of NO and NH stretch modes in the o-nitroaniline(ONA) and a small frequency blue shift of CH stretch mode in the formaldehyde(CH₂O) in the S₁ and S₂ excited states. The excited states S₁, S₂ and T₁ are locally excited states where only the ONA moiety is excited, but the CH₂O moiety remains in its ground state.