1,721,093 research outputs found
Photochemical processes: potential energy surface topology and rationalization using VB arguments
No full textThe development of quantum chemical methods capable of treating excited and ground states of organic molecules in a balanced way has prompted many applications in the field of mechanistic organic photochemistry. In this paper, we review a few representative computational results which illustrate the currently emerging concept of a photochemical (and photophysical) reaction pathway. In particular, we focus on the shape (topology) of the potential energy surface along the excited state branch of the reaction path as well as on the shape and nature of the photochemical funnel where decay to the ground state occurs. The chemical effect of different topologies and their origin in terms of simple valence bond ideas are discussed. (C) 2001 Elsevier Science B.V. All rights reserved
Modeling photochemical reactivity of organic-systems - a new challenge to quantum computational chemistry
No full textThe applicability of modern methods of quantum computational chemistry to the problem of modelling photochemical mechanisms is discussed. It is demonstrated that one of the central mechanistic features is the surface-crossing region which takes the form of a conical intersection. It is shown that this mechanistic feature can yield a new rationalization of common experimental observations such as the simultaneous production of different photoproducts, the occurrence of rigid stereochemistry, and the occurrence of very low fluorescence quantum yields. In addition, a simple VB model can often lead to accurate predictions about the nature of the surface-crossing region and the nature of the recoupling pathways on the ground-state surface
Simulation of MC-SCF results on covalent organic multi-bond reactions: molecular mechanics with valence bond (MM-VB)
No full textA scheme, MM-VB, for modelling (i.e., simulating MC-SCF results) covalent multi-bond reactivity problems (pericyclic and radical reactions) using a combination of molecular mechanics (MM) and valence bond (VB) theory is presented. The efficiency of the model in locating equilibrium geometries, transition structures, reaction intermediates, excited-state structures, and structures associated with conical intersections of ground and excited states is documented by comparison with ab initio MC-SCF results for structures belonging to four different potential energy surfaces for hydrocarbon reactions. The results have been obtained using the same pool of parameters (71 parameters) for the valence bond parametrization in all computations. The method performs very well for the semiquantitative prediction of structural parameters, and it is always possible to deduce the correct shape (topology) of the global potential energy surface. The application of the method to larger molecules is illustrated with an example where the conformational change in the ring system of ergosterol during electrocyclic ring opening is discussed. From a conceptual point of view, it has been demonstrated that quantum mechanics of the complicated process of the reorganization of the spin recoupling that occurs in a transition state or the driving force for the change in CC bond lengths that occurs in delocalized pi-systems can both be described by the solution of a simple VB problem which allows for the resonance of the possible VB structures involving the active bonds alone. The inactive framework of the molecule can be described by MM. The central parameters in such a VB calculation are the exchange integrals (K(ab)) and Coulomb integrals (Q(ab)). The functional form of these Coulomb and exchange parameters ran be transferred from two active-center model molecules (in the present case ethane and ethylene) to a multi-active-center system via a delocalization algorithm
Potential energy surface crossings in organic photochemistry
No full textModern experiments and quantum chemical computations show that low lying potential energy surface crossings (conical and singlet-triplet intersections) are a general feature of photochemically relevant excited states. This review focuses on the computational and experimental investigation of the efficiency of internal conversion at a surface crossing, the competition with fluorescence when an excited state barrier is present, and the relationship between the molecular structure at the intersection and structure of the photoproducts. It is shown that single or successive low-lying intersections provide the bottlenecks controlling the evolution of a photoexcited molecule from the Franck-Condon region to the photoproduct valleys
Basic Concepts of Electronic Excited States
No full textThis chapter is intended to identify some of the central concepts associated with excited state computation. So it is more like a “tutorial” rather than a review, with an emphasis on work carried out in the authors’ laboratories. The concepts we will discuss include gradients, conical intersections, and non-adiabatic dynamics, and are central to all the topics discussed in the specialist reviews that follow
Substituent Effects in Buta-1,3-diene Photochemistry: A CAS-SCF Study of 2,3-Dimethylbutadiene and 2-Cyanobutadiene Excited-State Reaction Paths
No full textThe low-energy reaction paths on the first excited state of buta-1,3-diene (butadiene), 2,3-dimethylbutadiene (2,3-DMB), and 2-cyanobutadiene (2-CNB) have been studied at the CAS-SCF level with 4-31G and DZ+d basis sets. Intrinsic reaction coordinate calculations indicate that each excited state pathway connects both the s-cis and s-trans excited state equilibrium structures of 2,3-DMB and 2-CNB to a conical intersection of the ground and excited state. The geometry of the lowest energy conical intersection points have been fully optimized. The conical intersections provide ultrafast radiationless decay channels and thus are a central feature in the mechanism of butadiene photolysis. The excited state reaction paths of the model (butadiene) and substituted systems involve a similar reaction coordinate corresponding to the deformation of the butadiene moiety by out-of-plane motion of the carbon framework coupled with asynchronous rotation of the terminal methylenes. The comparison of the excited state pathways for parent and substituted butadienes provides a basis for the rationalization of the dramatic substituent effects observed in the photoproduct quantum yield ratio of 2,3-DMB and 2-CNB. The result supports the idea that substituents mainly affect the nature of the ground-state relaxation occurring just after the decay. However, this effect is intimately related to the particular structure and dynamics that the photoexcited reactant acquires during its excited-state lifetime
Intramolecular Electron Transfer: Independent (Ground State) Adiabatic (Chemical) and Nonadiabatic Reaction Pathways in Bis(hydrazine) Radical Cations
No full textTwo different mechanistic processes in intramolecular electron transfer chemistry have been studied with the complete active space self-consistent field quantum chemical method for a model bis(hydrazine) radical cation. These correspond to (a) a chemical electron transfer, where a reaction intermediate or a transition structure exist with the charge localized on the linker, and (b) a nonadiabatic electron transfer, where the bridge remains neutral. These processes coexist on the same potential energy surface. They are characterized by very different reaction coordinates and are thus distinct elements of the mechanistic spectrum of intramolecular electron transfer in organic radical cations. The energetically favored chemical electron-transfer process involves conventional reaction paths. In contrast, the nonadiabatic electron-transfer process involves an unconventional reaction path, which connects reactant and products via an un-avoided (i.e., real) crossing seam (i.e, an (n-1)-dimensional intersection, where n is the number of vibrational degrees of freedom of the system) between two different adiabatic potential energy surfaces. Our results, computed for a model compound, differ from Nelsen's experimental results, and thus demonstrate the importance of the hydrazine substituents and the aromatic spacer
Origin of the nonstereospecificity in the ring opening of alkyl-substituted cyclobutenes
No full textFalling Down the Singlet Manifold. A CAS-SCF Mechanistic Study of the Far-UV Photochemistry of Hexa-1,5-dienes
No full textThe far-UV photochemistry of hexa-1,5-diene has been characterized using reaction paths obtained from ab-initio CAS-SCF computations. The decay/reaction pathway leading from the S2 potential energy surface to the ground state (S0) [1,2] and [1,3] allyl-shift photoproduct wells has been documented. The S1 and S0 surfaces are entered sequentially through two different (i.e., S2/S1 and S1/S0) radiationless decay channels which have been characterized as conical intersections. This result is consistent with a barrierless concerted mechanism. The structure of the S1 reaction coordinate connecting the two conical intersection points shows that both allyl-shift photoproducts must originate from the same migrating fragment. The proposed mechanism rationalizes the observed far-UV solution photochemistry of a series of variously substituted hexa-1,5-dienes. In particular, it is consistent with the preferential migration of the less substituted allyl fragment regardless of the type of allyl shift considered, the absence of [3,3] and in general [3,2] allyl-shift photoproducts in the reaction mixture, and the retention of the double-bond stereochemistry of the migrating allyl moiety during the reaction. © 1995 American Chemical Society
Potential energy surfaces of pseudoaromatic molecules: An MMVB and CASSCF study of pentalene
No full textThe S0 and S1 potential energy surfaces of pentalene were studied using MMVB - a hybrid force-field/parametrized valence bond (VB) method designed to simulate CASSCF calculations for ground and covalent excited states. The results were calibrated against full CASSCF calculations. Four distinct critical points were optimized: on S0, a C2h minimum (with alternating single and double bonds) and a D2h transition structure; and on S1, a D2h minimum and an adjacent S1/S0 conical intersection. A VB exchange density matrix (which is independent of the choice of the spin-coupled basis) was used to rationalize the S0 and S1 surface topologies. Craig defined pseudoaromatic molecules to be those with nontotally symmetric electronic ground states. For pentalene, this is true for both CASSCF and MMVB calculations: the CASSCF S0 transition structure is an open-shell B1g singlet, and the VB ground state is dominated by a spin-coupling which transforms as B1g. A C2v minimum and a D2h transition structure were located on the CASSCF S2 potential energy surface. This state cannot be represented by MMVB because of the importance of ionic configurations. The characters of the S1 and S2 states of pentalene are shown to be reverse of the S1 and S2 states of benzene. © 1996 John Wiley & Sons, Inc
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