# Investigating the Effects of Electron Correlation and Methylation on the S 1/S 0 Conical Intersection Seam of Fulvene

## TL;DR

This paper studies how electron correlation and methylation affect the conical intersection seam of fulvene, a molecule important for understanding radiationless decay processes.

## Contribution

The study introduces a detailed comparison of electron correlation effects and methylation impacts on fulvene's conical intersection seam using advanced computational methods.

## Key findings

- Methylation does not significantly alter the energetics of the conical intersection seam in fulvene.
- Single methylation (6MFulv) allows easier access to the planar region of the seam compared to parent fulvene or DMF.
- DMF is suggested as a suitable experimental surrogate system for testing predictions on fulvene's excited state reactivity.

## Abstract

The benzene isomer fulvene has established itself as
a computational
benchmark for characterizing conical intersections of potential energy
surfaces and modeling radiationless decay. However, in contrast to
other benchmark systems such as benzene itself or the DNA/RNA nucleobases,
there is as yet no time-resolved experimental data for fulvene to
compare computational predictions with. This article has three main
aims. The first is to thoroughly characterize the S
1/S
0 conical intersection
seam of fulvene mediating ultrafast decay along its main reaction
coordinates. The second is to understand how radiationless decay is
shaped by the electron correlation included in the model and is carried
out by comparing XMS-CASPT2 with CASSCF results and extracting how
dynamic correlation effects impact differently the critical points
encountered along the seam. The third and final aim is to compare
fulvene against 6-methyl-fulvene (6MFulv) and 6,6-dimethyl-fulvene
(DMF), potential surrogate systems available experimentally. We find
methylation does not alter the energetics along the majority of the
conical intersection seam itself, while single methylation (6MFulv)
enables easier access to the planar region of the seam through the
C1–C6 stretching coordinate compared
to the parent fulvene or DMF. We thus suggest DMF as an appropriate
surrogate system accessible by current experimental setups and where
predictions on fulvene excited state reactivity can be tested.