1,721,104 research outputs found
Conical intersections as a mechanistic feature of organic photochemistry
No full textThe photochemical decay channel is the central point of a photochemical mechanism. The traditional view of the mechanism of a photochemical reaction assumes that the excited state reaction path is similar to the ground state path and the decay channel corresponds to an avoided crossing minimum (van der Lugt and Oosterhoff model). In contrast, we find that there is no evidence for a decay path via such an avoided crossing minimum and that excited state reaction paths are very different from those on the ground state. Rather, the photochemical decay channel corresponds to a conical intersection point and plays the central role in mechanistic photochemistry.
The nature of the conical intersection point and its mechanistic implications will be discussed for 3 illustrative types of photochemical reactions (2+2 cycloadditions, polyene electrocyclization/isomerisation, and the di-pi-methane rearrangement)
Predicting forbidden and allowed cycloaddition reactions: potential surface topology and its rationalization
No full textFollowing reaction paths in organic photochemistry: The special role of surface crossings
No full textIn this paper we discuss the mechanistic importance of surface crossings in various types of organic photochemical reactions which include the photocycloaddition of alkenes and the photochemistry of butadiene and acrolein. We discuss both the role of conical intersections which occur between states of the same multiplicity (most commonly singlet-singlet) and of intersystem crossings that occur between states of different multiplicity (typically singlet-triplet)
Adiabatic and diabatic surfaces in the treatment of chemical reactivity. II. An illustrative application to the Diels-Alder reaction
No full textThe short-chain acroleiniminium and pentadieniminium cations: towards a model for retinal photoisomerization. A CASSCF/PT2 study
No full textIn this communication we report the results obtained in a computational study of the Minimum Energy Paths (MEP) found in the first excited state S1 and in the ground state S0 of two short-chain protonated Schiff bases (PSB): the s-cis 1-iminium-2-propene cation H2C=CH-CH=NH2 + and the tZt 1-iminium- 2,4-pentadiene cation H2C=CH-CH=CH-CH=NH2 +. This computational study has been performed at an high ab-initio level where the geometries of the relevant points have been optimized at the CAS-SCF level and the energetics have been refined via single-point computations at the CAS-PT2 level. This communication provides the important information that the photochemistry of the two studied PSB is driven by the spectroscopic 1B ionic state which remains the lowest excited state along all the optimized MEP. Both PSB show a S1/S0 Conical Intersection which is reached through a low barrier (barrierless) relaxation path for the shorter (longer) system and has an almost 90°twisted double bond (the CH2=CH-double bond for the shorter and the central double bond for the longer PSB) which provides a route for fully efficient non-adiabatic cis- > trans isomerization. In both PSB the crossing involves also a charge transfer between the two twisted fragments and the isomerization reaction coordinates on S1 are dominated by a stretching planar mode in the initial part of the MEP.
Acroleiniminium; CASSCF/PT2; Pentadieniminium; Photoisomerization; Retina
Diabatic surfaces for two-bond addition reactions. Role of resonance interaction
No full textDiabatic surfaces for two-bond cycloaddition reactions are examined in terms of a diabatic surface analysis which includes the computation of the resonance interaction between the reactant-like and product-like diabatic surfaces. A qualitative analysis and rigorous numerical computations are presented for a concerted synchronous mechanism (a two-bond process), a concerted asynchronous mechanism (a concerted one-bond process), and the first step of a two-step mechanism (a nonconcerted one-bond process) for both “allowed” and “forbidden” processes. The results illustrate that the resonance interaction is the dominant factor which controls the mechanistic preference between two-bond and one-bond processes. For a Woodward-Hoffmann forbidden process, the magnitude of the resonance interaction is found to be much smaller for the (forbidden) synchronous process than for the one-bond process; this leads to the expected preference for the one-bond process. For a Woodward-Hoffmann allowed process in the comparison of a concerted two-bond mechanism and the first step of a two-step mechanism, it is found that magnitude of the resonance interaction at the transition structure geometry can lead to a preference for the concerted process. © 1987, American Chemical Society. All rights reserved
Potential Energy Surface Crossings and the Mechanistic Spectrum for Intramolecular Electron Transfer in Organic Radical Cations
No full textThe structure of the potential energy surface for the intramolecular electron transfer (IET) of four different model radical cations has been determined by using reaction path mapping and conical intersection optimization at the ab initio CASSCF level of theory. We show that, remarkably, the calculated paths reside in regions of the ground-state energy surface whose structure can be understood in terms of the position and properties of a surface crossing between the ground and the first excited state of the reactant. Thus, in the norbornadiene radical cation and in an analogue compound formed by two cyclopentene units linked by a norbornyl bridge, IET proceeds along direct-overlap and super-exchange concerted paths, respectively, that are located far from a sloped conical intersection point and in a region where the excited-state and groundstate surfaces are well separated. A second potential energy surface structure has been documented for 1,2-diamino ethane radical cation and features two parallel concerted (direct) and stepwise (chemical) paths. In this case a peaked conical intersection is located between the two paths. Finally, a third type of energy surface is documented for the bismethyleneadamantane radical cation and occurs when there is, effectively, a seam of intersection points (not a conical intersection) which separates the reactant and product regions. Since the reaction path cannot avoid the intersection, IET can only occur nonadiabatically. These IET paths indicate that quite different IET mechanisms may operate in radical cations, revealing an unexpectedly enriched and flexible mechanistic spectrum. We show that the origin of each path can be analyzed and understood in terms of the one-dimensional Marcus-Hush model
Factors controlling the synchronous versus asynchronous mechanism of the Cope rearrangement
No full textMC-SCF potential surfaces for the Cope rearrangement of 1,5-hexadiene have been modeled by using a valence bond (VB) scheme parametrized with effective Hamiltonian methods. It is demonstrated that the mechanistic preference for a synchronous mechanism with an aromatic transition state versus an asynchronous mechanism with a biradicaloid intermediate is controlled by two factors: (i) the stability of the long bond in the Dewar VB structure and (ii) the softness of the Coulomb interactions between the terminal methylenes of the allylic fragments. Thus, the mechanism may be strongly affected by substituents. © 1990, American Chemical Society. All rights reserved
Can a photochemical reaction be concerted? A theoretical study of the photochemical sigmatropic rearrangement of but-1-ene
No full textMC-SCF computations at the 4-31G level using a complete active space (CAS) of four orbitals demonstrate the existence of a concerted photochemical pathway for [1,2] and [1,3] alkyl sigmatropic shifts. The central feature of this concerted path is a conical intersection (e.g., a genuine crossing) between ground and excited state from which a fully efficient return to the ground state is possible. Thus the excited-state surface has no minimum with zero gradient (i.e., a critical point) but only a singularity which corresponds to the lowest energy point of a conical intersection between ground and excited states. Thus there is no bottleneck corresponding to a short-lived intermediate that would correspond to the minimum on the excited-state surface at an avoided crossing. Intrinsic reaction coordinate computations have been performed on the excited-state surface that demonstrate the existence of two "channels" on the excited-state surface that simply continue on the ground-state surface. One of these channels leads to a [1,2] sigmatropic shift, the other to a [ 1,3] sigmatropic shift. The proposed mechanism is consistent with experimental observations where both [1,2]- and [1,3]-shift products are observed, and where the migrating group moves according to a supra process with retention of configuration of the migrating group
The mechanism of ground-state-forbidden photochemical pericyclic reactions: evidence for real conical intersections
No full textThe presence of a conical intersection between the S0 and S1, surfaces for ground-state-forbidden photochemical pericyclic reactions is demonstrated by using results from an effective (valence bond) Hamiltonian and MC-SCF computations. The existence of such topological features is an important feature in the mechanism since it permits a fully efficient return to S0 from the S, excited state. An example is presented for the 2 + 2 cycloaddition reaction of two ethylene molecules and the electrocyclic reaction of cir-butadiene. © 1990, American Chemical Society. All rights reserved
- …
