Investigating the Effects of Electron Correlation and Methylation on the S₁/S₀ Conical Intersection Seam of Fulvene

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 S₁/S₀ 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.