Buildup and Decay of the Optical Absorption in the Ultrafast Photo-Generation and Reaction of Benzhydryl Cations in Solution

Abstract

The identification of the transition state or a short-lived intermediate of a chemical reaction is essential for the understanding of the mechanism. For a direct identification typically transient optical spectroscopy is used, preferentially with high temporal resolution. We combine broad-band femtosecond transient absorption measurements and on-the-fly molecular dynamics calculations to decipher the microscopic evolution of the geometry and solvation of photogenerated benzhydryl cations (Ar(2)CH(+), Ar = phenyl, p-tolyl, m-fluorophenyl, or m,m'-difluorophenyl) in bulk solution. From the high level quantum chemical calculations on the microsolvated cation we can deduce a narrowing and blue shift of the cation absorption that is nearly quantitatively equal to the experimental finding. The roughly 300 fs initial increase in the absorption signal found for all investigated combinations of benzhydryl chlorides or phosphonium salts as benzhydryl cation precursors and solvents is therefore assigned to the planarization and solvation of the nascent fragment of the bond cleavage. The actual cleavage time cannot directly be deduced from the rise of the spectroscopic signal. For alcohols as solvent, the cation combines on the picosecond time scale either with one of the solvent molecules to the ether or to a lesser degree geminately with the leaving group. The study shows that the absorption signal attributable to a species like the benzhydryl cation does not mirror the concentration during the first instances of the process. Rather, the signal is determined by the geometrical relaxation of the photoproduct and the response of the solvent.

Wavepacket Splitting in the First 100 fs Determines the Products from the Bond Cleavage of Diphenylmethylchloride

“…Through micro-solvation calculations performed with on-the-fly molecular dynamics we can identify this signal increase with the planarization of the products from the sp 3 to the sp 2 configuration [4]. This leads to a strong increase of the optical absorption strength in many combinations of substituted diphenylmethylchlorides and solvents EPJ Web of Conferences 05042-p.2 investigated (see Fig.…”

“…The transmission changes of the probe pulses can monitor the appearance and evolution of the radical and cation product, respectively, since they have their strongest absorption bands at these wavelengths. This has been identified by broadband transient absorption measurements [3,4,5]. To avoid any pulse broadening and coherent artifacts due to cell windows, we perform the pump-probe experiment in a 50 µm free-flowing jet.…”

Monitoring conical intersections in the ring opening of furan by attosecond stimulated X-ray Raman spectroscopy

“…Ultrafast photoinduced reactions are widely investigated in photochemistry and photobiology. Prominent examples are the photo damage and repair of the DNA bases, 3–6 the initial event in the process of vision, 7,8 electrocyclic photoreactions, 9–12 photodissociations, 13,14 and energy and charge transfer processes 15 . Ultrafast photodynamics is essential to current and emerging energy conversion and optical storage technologies.…”

Dynamics of Bimolecular Reactions in Solution

“…Instead, it emphasizes the chemical reaction dynamics of neutral species on the ground electronic potential energy surface (PES), mimicking in solution many comprehensive TRIR: time-resolved infrared investigations of the dynamics of gas-phase bimolecular reactions. Although the systems chosen for review involve uncharged species, the same principles might apply to important classes of ionic reactions, such as nucleophilic substitution (S N 1 and S N 2) mechanisms (12)(13)(14)(15)(16). Moreover, polar solvent stabilization of charge-separated states might promote a switch from initially neutral reagents to ionic reaction pathways.…”