1,721,081 research outputs found
Predicting forbidden and allowed cycloaddition reactions: potential surface topology and its rationalization
No full textAdiabatic and diabatic surfaces in the treatment of chemical reactivity. II. An illustrative application to the Diels-Alder reaction
No full textThe 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
Unlocking the Double Bond in Protonated Schiff Bases by Coherent Superposition of S1and S2
Full text linkThe primary event occurring during the E-to-Z photoisomerization reaction of retinal protonated Schiff base (rPSB) is single-to-double bond inversion. In this work we examine the nuclear dynamics that occurs when the initial excited state is a superposition of the S1 and S2 electronic excited states that might be created in a laser experiment. The nuclear dynamics is dominated by double bond inversion that is parallel to the derivative coupling vector of S1 and S2. Thus, the molecule behaves as if it were at a conical intersection even if the states are nondegenerate
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
Quantum and quantum-classical studies of the photoisomerization of a retinal chromophore model
No full textWe report an in-depth analysis of the photo-induced isomerization of the 2-cis-penta-2,4-dieniminium cation: a minimal model of the 11-cis retinal protonated Schiff base chromophore of the dim-light photoreceptor rhodopsin. Based on recently developed three-dimensional potentials parametrized on ab initio multi-state multi-configurational second-order perturbation theory data, we perform quantum-dynamical studies. In addition, simulations based on various quantum-classical methods, among which Tully surface hopping and the coupled-trajectory approach derived from the exact factorization, allow us to validate their performance against vibronic wavepacket propagation and, therefore, a purely quantum treatment. Quantum-dynamics results uncover qualitative differences with respect to the two-dimensional Hahn-Stock potentials, widely used as model potentials for the isomerization of the same chromophore, due to the increased dimensionality and three-mode correlation. Quantum-classical simulations show, instead, that three-dimensional model potentials are capable of capturing a number of features revealed by atomistic simulations and experimental observations. In particular, a recently reported vibrational phase relationship between double-bond torsion and hydrogen-out-of-plane modes critical for rhodopsin isomerization efficiency is correctly reproduced
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
Validation of recombinant chicken liver bile acid binding protein as a tool for cholic acid hosting
Full text linkBile acids (BAs) are hydroxylated steroids derived from cholesterol that act at the intestinal level to facilitate the absorption of several nutrients and also play a role as signaling molecules. In the liver of various vertebrates, the trafficking of BAs is mediated by bile acid‐binding proteins (L‐BABPs). The ability to host hydrophobic or amphipathic molecules makes BABPs suitable for the distribution of a variety of physiological and exogenous substances. Thus, BABPs have been proposed as drug carriers, and more recently, they have also been employed to develop innovative nanotechnology and biotechnology systems. Here, we report an efficient protocol for the production, purification, and crystallization of chicken liver BABP (cL‐BABP). By means of target expression as His6‐tag cL‐BABP, we obtained a large amount of pure and homogeneous proteins through a simple purification procedure relying on affinity chromatography. The recombinant cL‐BABP showed a raised propensity to crystallize, allowing us to obtain its structure at high resolution and, in turn, assess the structural conservation of the recombinant cL‐BABP with respect to the liverextracted protein. The results support the use of recombinant cL‐BABP for the development of drug carriers, nanotechnologies, and innovative synthetic photoswitch systems
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
The 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
- …
