Ground and Excited State Surfaces for the Photochemical Bond Cleavage in Phenylmethylphenylphosphonium Ions

Abstract

Photolytic bond cleavage is a well-established method to generate carbocations for organic synthesis. Changes in the leaving group have a large influence on the chemical yield. The underlying potential energy surfaces governing the initial process are mostly unknown. We provide potential energy surfaces of ground and excited states on the CASSCF/CASPT2 level of theory for the charged precursor phenylmethylphenylphosphonium ion. We present the electronic and structural changes accompanying the excitation process and the subsequent bond cleavage. Inter-ring charge-transfer processes play a crucial role in the Franck-Condon region. Beyond the Franck-Condon region, competing reaction pathways emerge connected through conical intersections. The phenylmethylphenylphosphonium ion is used as a model system for the commonly used diphenylmethyltriphenylphosphonium ion. The appropriateness of the model is tested by CC2 calculations of the excitation spectrum.

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

“…This results in an increased excited state manifold. The full system ${\text{Ph}}_2\text{CH}-{\text{PPh}}_3^{+}$ exhibits five locally excited states which are energetically quite close—two on the Ph 2 CH moiety and three on the LG 43 . In addition, charge transfer excitations between both moieties are present.…”

“…In addition, charge transfer excitations between both moieties are present. They are energetically well separated from the locally excited states and higher in energy 43 . Like in the case of Ph 2 CH–Cl, the PESs for the QD calculations are evaluated at the ONIOM level of theory.…”

“…The high-level system is calculated at the CASSCF(10,10) level of theory. The active space contains two π - and two π *-orbitals of each phenyl ring as well as the σ - and σ *-orbitals of the C1-P bond 43 . The low-level system is evaluated at the density functional theory (DFT) level of theory (functional M06-2X 44 ).…”

“…To observe bond cleavage experimentally, the system is excited to the locally excited states 4 . The S 1 state opens the preferred reaction path compared to the S 2 as first it has the lower barrier, and second the S 2 population can immediately relax to the S 1 via a S 2 / S 1 CoIn 43 . Therefore, we will focus on the S 1 state from now on.…”

“…But at the same time, an energetically low lying two-state CoIn between S 1 and S 0 can be optimized in the high-level system PhCH 2 –PH 2 Ph + , which is located at a larger C1-P distance 43 . The P atom is located above an allylic C 3 subunit containing C1, C2, and C3 showing a related geometry to the S 1 / S 0 CoIn of Ph 2 CH–Cl calculated at the CASSCF(8,8) level of theory.…”