Nature of Dynamic Processes Associated with the S<sub>N</sub>1 Reaction Mechanism

Peters, Kevin S.

doi:10.1021/cr068021k

Cited by 62 publications

(58 citation statements)

References 115 publications

(277 reference statements)

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“…The reaction starts with a π-π * excitation to the S 1 state (red). The excitation energy is 4.88 eV (254 nm), which is in good agreement with the experimental value of 4.66 eV (266 nm) 20 . At 2.12 Å, the lone pair states (green) cross the S 1 forming a three-state CoIn and are more and more stabilized with increasing C1-Cl distance, while their character changes from nπ * to nσ *.…”

Section: Bond Cleavage Of Ph2ch–clsupporting

confidence: 88%

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

Thallmair¹,

Roos

Vivie-Riedle

2016

Structural Dynamics

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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.

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Section: Bond Cleavage Of Ph2ch–clsupporting

confidence: 88%

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

Thallmair¹,

Roos

Vivie-Riedle

2016

Structural Dynamics

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“…A similar broad distribution was reported for photo-detached electrons from I - [29]. The underlying assumption of all previous interpretations on the reaction dynamics of photo-excited benzhydryl halides was that homolysis would exclusively lead to geminate radical pairs (GRPs) [6][7][8][9][10]. We see no good arguments for this assumption.…”

Section: Conversion Of Radical Pairs To Ion Pairs By Electron Transfersupporting

confidence: 48%

“…At first sight, the early reaction steps leading to the chemically relevant fragment populations might not seem very important, as the large majority of bimolecular reactions proceed at best in the diffusional limit. Yet, the early events have been investigated in great detail by Peters and co-workers [6][7][8][9][10]. Their aim was not only the elucidation of the bond cleavage and the subsequent reaction steps, but the hope to obtain information on the rates and barriers for the reverse processes leading to bond formation [10].…”

Section: Introductionmentioning

confidence: 99%

Photogeneration and reactions of benzhydryl cations and radicals: A complex sequence of mechanisms from femtoseconds to microseconds

Sailer

Riedle

2013

Pure and Applied Chemistry

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Benzhydryl radicals and cations are reactive intermediates central to the understanding of organic reactivity. They can be generated from benzhydryl halides by UV irradiation. We performed transient absorption (TA) measurements over the range from femtoseconds to microseconds to unravel the complete reaction scheme. The 290-720-nm probe range allows the unambiguous monitoring of all fragments. The appearance of the radical is delayed to the optical excitation, the onset of the cation signal is found even later. Ab initio calculations show that this non-rate behavior in the 100 fs range is due to wavepacket motion from the Franck-Condon region to two distinct conical intersections. The rise of the optical signal with a quasi-exponential time of 300 fs is assigned to the planarization and solvation of the photoproducts. The bond cleavage predominantly generates radical pairs. A subsequent electron transfer (ET) transforms radical pairs into ion pairs. Due to the broad interradical distance distribution and the distance dependence, the ET is strongly non-exponential. Part of the ion pairs recombine geminately. The ET and the recombination are terminated by the depletion of close pairs and diffusional separation. The remaining free radicals and cations undergo further reactions in the nanosecond to microsecond regime.Benzhydryl cation and radical photogeneration 1489 Fig. 1 (a) Transient spectrum at a pump-probe delay of 40 ps and (b) false color representation of the 2D TA data matrix of tol(Ph)CHCl in CH 3 CN after 270 nm UV excitation. The delay time axis is linear between -1 and 1 ps and logarithmic beyond 1 ps. (c) Scheme of the ultrafast bond cleavage.

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“…To examine the applicability of the Q-Chem–Amber interface, we simulated the S N 1 fragmentation of tert -butyl chloride in water (Hartsough & Merz, 1995; Peters, 2007), t-Bu-Cl → t-Bu + + Cl − , in which water plays an indispensable role. For such a charge separation reaction, the proper treatment of long range electrostatic interactions and boundary condition is expected to be very important and spherical boundary condition would not sufficient in this case.…”

Section: Examplesmentioning

confidence: 99%

Born–Oppenheimer Ab Initio QM/MM Molecular Dynamics Simulations of Enzyme Reactions

Zhou

Wang

et al. 2016

Methods in Enzymology

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There are two key requirements for reliably simulating enzyme reactions: one is a reasonably accurate potential energy surface to describe the bond forming/breaking process as well as to adequately model the heterogeneous enzyme environment; the other is to perform extensive sampling since an enzyme system consists of at least thousands of atoms and its energy landscape is very complex. One attractive approach to meet both daunting tasks is Born-Oppenheimer ab initio QM/MM molecular dynamics simulation (aiQM/MM-MD) with umbrella sampling. In this chapter, we describe our recently developed pseudobond Q-Chem–Amber interface, which employs a combined electrostatic-mechanical embedding scheme with periodic boundary condition and the particle mesh Ewald method for long-range electrostatics interactions. In our implementation, Q-Chem and the sander module of Amber are combined at the source code level without using system calls, and all necessary data communications between QM and MM calculations are achieved via computer memory. We demonstrate the applicability of this pseudobond Q-Chem–Amber interface by presenting two examples, one reaction in aqueous solution and one enzyme reaction. Finally, we describe our established aiQM/MM-MD enzyme simulation protocol, which has been successfully applied to study more than a dozen enzymes.

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Nature of Dynamic Processes Associated with the S_N1 Reaction Mechanism

Abstract: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Cited by 62 publications

References 115 publications

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

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

Photogeneration and reactions of benzhydryl cations and radicals: A complex sequence of mechanisms from femtoseconds to microseconds

Born–Oppenheimer Ab Initio QM/MM Molecular Dynamics Simulations of Enzyme Reactions

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Nature of Dynamic Processes Associated with the SN1 Reaction Mechanism

Abstract: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Cited by 62 publications

References 115 publications

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

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

Photogeneration and reactions of benzhydryl cations and radicals: A complex sequence of mechanisms from femtoseconds to microseconds

Born–Oppenheimer Ab Initio QM/MM Molecular Dynamics Simulations of Enzyme Reactions

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Nature of Dynamic Processes Associated with the S_N1 Reaction Mechanism