Perturbing π‐clouds with Substituents to Study the Effects on Reaction Dynamics of gauche‐1,3‐Butadiene to Bicyclobutane Electrocyclization

Abstract

The conical intersection (CI) governs the ultra-fast relaxation of excited states in a radiationless manner and are observed mainly in photochemical processes. In the current work, we investigated the effects of substituents on the reaction dynamics for the conversion of gauche-1,3-butadiene to bicyclobutane via photochemical electrocyclization. We incorporated both electron withdrawing (À F) and donating (À CH 3 ) groups in the conjugated system. In our study, we optimized the minimum energy conical intersection (MECI) geometries using the multi-configurational state-averaged CASSCF approach, whereas, to study the ground state reaction pathways for the substituted derivatives, dispersion corrected, B3LYP-D3 functional was used. The non-adiabatic surface hopping molecular dynamics simulations were performed to observe the behaviour of electronic states involved throughout the photoconversion process. The results obtained from the multireference second-order perturbation correction of energy at the XMS-CASPT2 level of theory, topography analysis, and nonadiabatic dynamics suggest that the À CH 3 substituted derivatives can undergo faster thermal conversion to the product in the ground state with a smaller activation energy barrier compared to À F substituted derivative. Our study also reveals that the GBUT to BIBUT conversion follows both conrotatory and disrotatory pathways, whereas, on substitution with À F or À CH 3 , the conversion proceeds via the conrotatory pathway.

Deciphering the Internal Conversion Processes Involved in the Photochemical Ring-Opening of 1,3-Cyclohexadiene by Symmetric sp2-Carbon Substitutions

“…A systematic idea about the topography and dynamic behavior of a molecular system near CI n can be accessed by a phenomenological study of the substitution(s) effects. Additionally, a substituent can significantly alter the electronic structure, density of the state, and impact the nonadiabatic dynamics mediated via conical intersection species. − Therefore, in this study, our primary focus is the introduction of symmetric methyl (–CH 3 ) substitutions into the CHD molecule, with the goal of understanding their influence on the reaction dynamics near CI n (s). Our investigation includes two dimethyl derivatives, specifically 2,3-dimethylcyclohexadiene (CHD-Me2-1) and 1,4-dimethylcyclohexadiene (CHD-Me2-2) as well as one tetramethyl derivative, 1,2,3,4-tetramethylcyclohexadiene (CHD-Me4).…”

Minimum-Energy Conical Intersections by Compressed Multistate Pair-Density Functional Theory

“…The CMS-PDFT method gives the correct topology of closely spaced PESs at and near conical intersections. CMS-PDFT may be compared to extended multistate complete active space second-order perturbation theory (XMS-CASPT2), , which has been demonstrated to be a highly accurate quantum chemical method that can describe important dynamic correlation effects at and near the conical intersections that control quenching and other electronically nonadiabatic processes. − Although CMS-PDFT gives results of comparable quality, including near conical intersections, it is advantageous as it is much less expensive.…”