TD-DFT analysis of the photo-ring-opening reaction of the flav-3-en-2-ol in acetonitrile solution: Functional choice and effect of solvated hydrogen bond

Abstract

Of the forty hybrid functionals supported by Gaussian 16, only fourteen functionals with small contributions from the exact Hartree-Fock exchange demonstrated the breaking of the C2-O1 bond of flav-3-en-2-ol: TPSSh, O3LYP, τHCTHhyb, B3LYP, B3P86, B3PW91, B971, B972, X3LYP, B98, APF, APFD, mPW1LYP, and mPW3PBE. TPSSh and O3LYP gave rise to a cis-2-hydroxychalcone transition accompanied by ground-state intramolecular proton transfer (GSIPT), and the remaining twelve functionals selected showed a return to the flav-3-en-2-ol with the recovery of the C2-O1 bond. The strong H-bond of acetonitrile with flav-3-en-2-ol has a striking effect on the consequences of the excitation in comparison with a single flav-3-en-2-ol molecule: optimization of the S1 excited state of the solvated complex leads to the breaking of the C2-O1 bond by O3LYP functional only. However, upon relaxation of the excited enol form, this bond was recovered, and a return to the flav-3-en-2-ol occurred. Nevertheless, the O3LYP functional gives a theoretical model consistent with the experiment: the H-bond of flav-3-en-2-ol with the acetonitrile molecule is a switch in the reaction path. If this H-bond is present at the excitation moment, then the flav-3-en-2-ol molecule undergoes a C2-O1 bond cleavage but recovers upon relaxation. If this H-bond is absent, then the flav-3-en-2-ol molecule transforms into the enol form which goes further into cis-2-hydroxychalcone. The excitation of the solvated complex causes a pronounced strengthening of this intermolecular H-bond. GSIPT is caused by a change in the polarization of the enol form during its electronic relaxation, as a result of which the proton begins to experience a stronger attraction to the opposite part of the molecule.