Molecular features in complex environment: Cooperative team players during excited state bond cleavage

Abstract

Photoinduced bond cleavage is often employed for the generation of highly reactive carbocations in solution and to study their reactivity. Diphenylmethyl derivatives are prominent precursors in polar and moderately polar solvents like acetonitrile or dichloromethane. Depending on the leaving group, the photoinduced bond cleavage occurs on a femtosecond to picosecond time scale and typically leads to two distinguishable products, the desired diphenylmethyl cations (Ph2CH+) and as competing by-product the diphenylmethyl radicals (Ph2CH•). Conical intersections are the chief suspects for such ultrafast branching processes. We show for two typical examples, the neutral diphenylmethylchloride (Ph2CH–Cl) and the charged diphenylmethyltriphenylphosphonium ions (Ph2CH−PPh3+) that the role of the conical intersections depends not only on the molecular features but also on the interplay with the environment. It turns out to differ significantly for both precursors. Our analysis is based on quantum chemical and quantum dynamical calculations. For comparison, we use ultrafast transient absorption measurements. In case of Ph2CH–Cl, we can directly connect the observed signals to two early three-state and two-state conical intersections, both close to the Franck-Condon region. In case of the Ph2CH−PPh3+, dynamic solvent effects are needed to activate a two-state conical intersection at larger distances along the reaction coordinate.

Simulating Coherent Multidimensional Spectroscopy of Nonadiabatic Molecular Processes: From the Infrared to the X-ray Regime

“…CoIns between more than two electronic states, like three-state CoIns, have long been proposed , in highly symmetric molecules as an additional photophysical relaxation mechanism. More recent theoretical investigations have predicted them in a variety of molecules (e.g., methyl cation, ethyl radical, allyl radical, pyrazolyl, cytosine, , uracil and andenine, malonic acid, and diphenyl methyl radical) and demonstrate that three-state CoIns are more common than initially believed. Four-state CoIns have been reported as well .…”

Optimization of Three State Conical Intersections by Adaptive Penalty Function Algorithm in Connection with the Mixed-Reference Spin-Flip Time-Dependent Density Functional Theory Method (MRSF-TDDFT)

“…Although the three-state CIs are important for the excited-state dynamics, − the algorithms for their search and optimization are less developed than for the two-state CIs. The most accurate method of finding ME3CI geometries is, perhaps, the Lagrange–Newton method of Matsika and Yarkony, which requires the nonadiabatic couplings.…”

Two New Methods To Generate Internal Coordinates for Molecular Wave Packet Dynamics in Reduced Dimensions

“…One common way to reduce the number of degrees of freedom is the transformation from Cartesian coordinates to a set of internal coordinates describing bond distances, bond angles, and dihedral angles. − Ultrafast chemical reactions can often be approximated by a small number of internal coordinates providing an efficient description is found, exemplary shown by the reactive coordinates derived for the dissociation of diphenylmethyl systems by Thallmair et al On the other hand there exist many reactions with complex deformation of the molecular structure. A possible scenario is the relaxation of a conjugated π system due to a change in the bond order of a reactive coordinate.…”