| 1. |
- Coto, PB, et al.
(author)
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Characterization of the conical intersection of the visual pigment rhodopsin at the CASPT2//CASSCF/AMBER level of theory
- 2006
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In: Molecular Physics. - : Informa UK Limited. - 1362-3028 .- 0026-8976. ; 104:5-7, s. 983-991
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Journal article (peer-reviewed)abstract
- The branching plane associated with the conical intersection controlling the photochemical Z -> E isomerization reaction of rhodopsin has been mapped using a CASPT2//CASSCF/ AMBER quantum mechanics-molecular mechanics method. The nature of the derivative coupling and gradient difference vectors spanning the branching plane has been investigated, showing that the conical intersection is not only associated with the isomerization process but also to a charge transfer along the retinal backbone. Using a simple Landau-Zener model, the paper discusses the possible effects of the documented conical intersection topologies on the efficiency of the reactive process. It is argued that the peculiar shape of the conical intersection favours decay at structures that are geometrically displaced towards the photoproduct bathorhodopsin.
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| 2. |
- De Vico, Luca, et al.
(author)
-
Chemiluminescence of 1,2-dioxetane. Reaction mechanism uncovered
- 2007
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In: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 111:32, s. 8013-8019
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Journal article (peer-reviewed)abstract
- The thermal decomposition of 1,2-dioxetane and the associated production of chemiluminescent products, model for a wide range of chemiluminescent reactions, has been studied at the multistate multiconfigurational second-order perturbation level of theory. This study is in qualitative and quantitative agreement with experimental observations with respect to the activation energy and the observed increase of triplet and singlet excited products as substituents are added to the parent molecule. The, previously incomplete, reaction mechanism of the chemiluminescence of 1,2-dioxetane is now rationalized and described as mainly due to a particular form of entropic trapping.
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| 3. |
- De Vico, Luca, et al.
(author)
-
Location of Two Seams in the Proximity of the C-2v pi pi Minimum Energy Path of Formaldehyde
- 2009
-
In: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 5:1, s. 186-191
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Journal article (peer-reviewed)abstract
- Photochemical reactions rationalization is a key aspect for the understanding and setup of novel experiment and novel photoinitiated pathways. In this respect, the relationship between minimum energy paths over an excited-state and the intersection to lower potential energy surfaces is fundamental. In order to help the understanding of this relationship, in this study we present a novel kind of constraint for geometry optimizations, namely, an “orthogonality” constraint. Its possible applications are described. A complete example on how to retrieve the direct relationship between a minimum energy path over an excited-state potential energy surface and a conical intersection seam is given for C-2v symmetry constrained formaldehyde. The advantages of using the novel constraint when rationalizing a (photo)chemical reaction are presented.
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| 4. |
- De Vico, Luca, et al.
(author)
-
New general tools for constrained geometry optimizations
- 2005
-
In: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 1:5, s. 1029-1037
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Journal article (peer-reviewed)abstract
- A modification of the constrained geometry optimization method by Anglada and Bofill (Anglada, J. M.; Bofill, J. M. J. Comput. Chem. 1997, 18, 992-1003) is designed and implemented. The changes include the choice of projection, quasi-line-search, and the use of a Rational Function optimization approach rather than a reduced-restricted-quasi-Newton-Raphson method in the optimization step. Furthermore, we show how geometrical constrains can be implemented in an approach based on nonreclunclant curvilinear coordinates avoiding the inclusion of the constraints in the set of redundant coordinates used to define the internal coordinates. The behavior of the new implementation is demonstrated in geometry optimizations featuring single or multiple geometrical constraints (bond lengths, angles, etc.), optimizations on hyperspherical cross sections (as in the computation of steepest descent paths), and location of energy minima on the intersection subspace of two potential energy surfaces (i.e. minimum energy crossing points). In addition, a novel scheme to determine the crossing point geometrically nearest to a given molecular structure is proposed.
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| 5. |
- De Vico, Luca, et al.
(author)
-
The ozone ring closure as a test for multi-state multi-configurational second order perturbation theory (MS-CASPT2)
- 2008
-
In: Chemical Physics Letters. - : Elsevier BV. - 0009-2614. ; 461:1-3, s. 136-141
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Journal article (peer-reviewed)abstract
- The open and closed forms of ozone and the path connecting them through a transition state, in C-2v symmetry, are explored using the multi-state, multi-configurational second order perturbative method, MS-CASPT2. It is demonstrated that, by using an ANO-L triple-zeta basis set, it is possible to set up an active space able to describe the otherwise troublesome transition state region. Both a conical intersection and a near degeneracy region between the 1(1)A(1) and 2(1)A(1) states are located in the vicinity of the transition state. The relative position of the intersection and the transition state are discussed. (c) 2008 Elsevier B. V. All rights reserved.
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| 6. |
- Liu, Fengyi, et al.
(author)
-
A CASSCF/CASPT2 approach to the decomposition of thiazole-substituted dioxetanone : Substitution effects and charge-transfer induced electron excitation
- 2009
-
In: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 484:1-3, s. 69-75
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Journal article (peer-reviewed)abstract
- The gas-phase decompositions of a thiazole-substituted dioxetanone, in both the natural and anionic forms, were investigated theoretically in a CASSCF/CASPT2 study. The neutral conjugated thiazole (with or without a hydroxyl group) substitution on the dioxetanone has no evident effect on the dissociation; however, a subsequent deprotonation - invoking charge-transfer excitations from the thiazole to the dioxetanone moiety - will dramatically change the reaction mechanism from stepwise to concerted, and reduce the activation barrier by similar to 8 kcal mol(-1). These findings are helpful for the better understanding of dioxetanone chemiluminescence and the charge-transfer induced electron excitation in chemi- and bioluminescence processes.
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| 7. |
- Liu, Fengyi, et al.
(author)
-
Theoretical Study of the Chemiluminescent Decomposition of Dioxetanone
- 2009
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 131:17, s. 6181-6188
-
Journal article (peer-reviewed)abstract
- The unimolecular chemiluminescent decomposition of unsubstituted dioxetanone was studied at the complete active space self-consistent field level of theory combined with the multistate second-order multiconfigurational perturbation theory energy correction. The calculations revealed interesting features. Two transition states, two conical intersections, and one intermediate stable biradical structure along the lowest energy reaction path were identified. It was noted that the conical intersections are found at or in very close proximity to the transition states. The first and second transition states correspond to O-O and C-C cleavages, respectively. In particular, a planar structure is supported by the(1)(sigma,sigma*) state during the O-O dissociation up to the first transition state and conical intersection. At this point the (1)(sigma,sigma*) state dissociation path bifurcates, corresponding to a torsion of the O-C-C-O angle. Simultaneously, the (1)(n,sigma*) state becomes lower in energy while still favoring a planar structure. As the lowest-energy reaction path proceeds toward the second transition state and conical intersection, the (1)(n,sigma*), (3)(n,sigma*), and (1)(sigma,sigma*)states are close in energy. This work suggests that the vibrational distribution at the first conical intersection and the interactions among the states as the reaction proceeds between the two transition states are the origin of the population of the chemiluminescent (n,sigma*) states.
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| 8. |
- Liu, Ya-Jun, et al.
(author)
-
Ab initio investigation on the chemical origin of the firefly bioluminescence
- 2008
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In: Journal of Photochemistry and Photobiology, A: Chemistry. - : Elsevier BV. - 1873-2666 .- 1010-6030. ; 194, s. 261-267
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Journal article (peer-reviewed)abstract
- The chemical origin of the firefly bioluminescence has been investigated by means of density functional and multireference theoretical methods. Different hypotheses on the mechanism of multicolour emission have been investigated: twisting around the central carbon–carbon bond, polarizability of the oxyluciferin microenvironment and presence of resonance structures. The calculated results indicated that the higher the polarizability of the microenvironment is, the larger the red shift of the bioluminescence is. Moreover, a quite flat potential energy surface should allow the easy shifting of the anion minimum between different pseudo-resonance structures. The possible effects of a tight or loose protein pocket has also been considered.
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| 9. |
- Liu, Ya-Jun, et al.
(author)
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Spin-orbit ab initio investigation of the ultraviolet photolysis of diiodomethane
- 2007
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In: ChemPhysChem. - : Wiley. - 1439-4235 .- 1439-7641. ; 8:6, s. 890-898
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Journal article (peer-reviewed)abstract
- The UV photodissociation (<5 eV) of diiodomethane (CH(2)l(2)) is investigated by spin-orbit ab inito calculations. The experimentally observed photodissociation channels in the gas and condensed phases are clearly assigned by multi-state second-order multiconfigurational perturbation theory in conjunction with spin-orbit interaction through complete active space-state interaction potential energy curves. The calculated results indicate that the fast dissociations of the first two singlet states of CH(2)l(2) and CH(2)l-l lead to geminate-radical products, CH(2)l(+) +l(P-2(3/2)) or CH(2)l(+) + l*(P-2(1/2)). The recombination process from CH(2)l-l to CH(2)l(2) is explained by an isomerazation process and a secondary photodissociation reaction of CH(2)l-l. Finally, the study reveals that spin-orbits effects are sifnificant in the quantitative analysis of the electronic spectrum of the CH(2)l-l species.
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| 10. |
- Rocha-Rinza, Tomas, et al.
(author)
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A theoretical study of singlet low-energy excited states of the benzene dimer
- 2006
-
In: Chemical Physics Letters. - : Elsevier BV. - 0009-2614. ; 426:4-6, s. 268-272
-
Journal article (peer-reviewed)abstract
- The singlet ground and low-energy excited states of the benzene dimer in D-6h geometry are characterized using second order multireference perturbation theory (CASPT2). The counterpoise-corrected spectroscopic parameters of the benzene excimer are in quantitative agreement with experiment. The same method was used to compute a submanifold of the potential energy hypersurface of the first excited state of the benzene dimer, exhibiting two local minima in addition to a saddle point between them. This study demonstrates the suitability of CASPT2 to describe the benzene excimer and suggests that the method can be used to describe weak intermolecular interactions involving excited states. (c) 2006 Elsevier B.V. All rights reserved.
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