4a,4b-Dihydrophenanthrene → cis-stilbene photoconversion: TD-DFT/DFT study

Area of Science:

• Computational chemistry
• Photochemistry
• Organic reaction mechanisms

Background:

• First-time analysis of DHP → CS photoconversion focusing on electron density redistribution.
• Investigates the non-recovery of the C4a-C4b bond after CS relaxation.
• Examines structural changes in the linker connecting benzene rings during CS relaxation.

Purpose of the Study:

• To elucidate the mechanism behind the non-recovery of the C4a-C4b bond in DHP → CS photoconversion.
• To analyze electron density redistribution and its role in bond dynamics.
• To understand the structural and electronic factors influencing the C4a-C4b bond stability.

Main Methods:

• Utilized Gaussian16 software with B3LYP/6-311+G(d,p)/IEFPCM level of theory.
• Employed Natural Population Analysis (NPA) for quantitative electron density analysis.
• Used cyclohexane as an implicit solvent and Gaussview6 for visualization.

Main Results:

• Coulomb repulsion energy between C4a-C4b atoms increases significantly during CS relaxation.
• Bonding by a single electron at the Lowest Unoccupied Molecular Orbital (LUMO) is insufficient to restore the C4a-C4b bond.
• The C4a-C4b bond distance increases from 3.00 Å to 3.28 Å upon CS relaxation.
• Calculated IR spectra show very low intensity for C4a-C4b vibrations, indicating minimal contribution from thermal motion.

Conclusions:

• The C4a-C4b bond cleavage in DHP is primarily driven by electronic factors during CS relaxation, not thermal effects.
• Increased Coulombic repulsion and inadequate electron density at the LUMO prevent bond reformation.
• Significant structural rearrangement of the linker moiety accompanies CS relaxation.