Etude théorique de la réaction C+CH

The reaction of interstellar interest C(3Pg) + CH(X2Π) → C2 + H presents a very complicated dynamics reflecting the complexity of the topology of the potential energy surfaces (PES) involved in the reaction process. The ab initio study (CASSCF level) of several surfaces shows that the C2 molecule can be produced in different electronic states even at very low temperature. Ab initio of higher accuracy (MRCI level) provided the required information for an analytical representation of the different PES involved. The fitting of these surfaces has been limited to the two lowest electronic states, namely C2H(X2Σ+) and C2H(A2Π). These two electronic states give rise to two 2A’ states and one 2A’’ state when the C2H system is bent. The two 2A’ electronic states display an avoided crossing arising from the crossing of the states C2H(X2Σ+) and C2H(A2Π). We have chosen the “ Double Many Body Expansion ” (DMBE) method to represent these three PES analytically. Two different models have been used to describe the two 2A’states. The first one is based on an adiabatic representation and is not able to represent the conical intersection between the X2Σ+ and A2Π states. The second one uses a diabatic representation that enables a rigorous description of these PES. The classical trajectories simulation on the PES issued from the first model provided an estimation of state selected rate constants. A more realistic treatment of the dynamics taking into account the non adiabatic transitions has been achieved using the diabatic model for the 2A’ electronic states. The study of the energetic and angular distributions on the products revealed some statistical behaviour in the dynamics.La réaction d’intérêt interstellaire C(3Pg) + CH(X2Π) → C2 + H présente une dynamique extrêmement compliquée reflétant la topologie complexe des surfaces d’énergie potentielle (SEP) mise en jeu dans le mécanisme réactionnel. L’étude ab initio (niveau CASSCF) de ces différentes surfaces a permis de montrer que la molécule C2 peut être formée dans différents états électroniques même à très basse température. Des calculs ab initio de meilleure qualité (niveau MRCI) nous ont permis d’obtenir les informations nécessaires en vue d’une représentation analytique des différentes SEP mises en jeu. Nous avons restreint la modélisation de ces surfaces aux deux états électroniques les plus bas, à savoir C2H(X2Σ+) et C2H(A2Π). Ces deux états électroniques donnent naissance à deux états 2A’ et un état 2A’’ lorsque le système C2H est coudé. Les deux états électroniques 2A’ présentent un croisement évité résultant du croisement des états C2H(X2Σ+) et C2H(A2Π). Nous avons choisi la méthode “ Double Many Body Expansion ” (DMBE) pour représenter analytiquement ces trois SEP. Deux modèles différents ont été utilisés pour décrire les deux états 2A’. Le premier fait appel à une représentation adiabatique et ne permet pas de représenter l’intersection conique entre les états X2Σ+ et A2Π. Le deuxième utilise une représentation diabatique permettant une description rigoureuse de ces SEP. La simulation de trajectoires classiques sur les SEP issues du premier modèle nous a permis de donner une estimation des constantes de vitesse thermiques. Un traitement plus réaliste de la dynamique, à travers la prise en compte des transitions non-adiabatiques, a été réalisé en utilisant le modèle diabatique pour les états électroniques 2A’. L’étude des distributions énergétiques et angulaires sur les produits a permis de révéler des caractères statistiques et d’autres non statistiques dans le comportement de la dynamique réactionnelle

Assessing Excited State Energy Gaps with Time-Dependent Density Functional Theory on Ru(II) Complexes

A set of density functionals coming from different rungs on Jacob's ladder are employed to evaluate the electronic excited states of three Ru(II) complexes. While most studies on the performance of density functionals compare the vertical excitation energies, in this work we focus on the energy gaps between the electronic excited states, of the same and different multiplicity. Excited state energy gaps are important for example to determine radiationless transition probabilities. Besides energies, a functional should deliver the correct state character and state ordering. Therefore, wavefunction overlaps are introduced to systematically evaluate the effect of different functionals on the character of the excited states. As a reference, the energies and state characters from multi-state second-order perturbation theory complete active space (MS-CASPT2) are used. In comparison to MS-CASPT2, it is found that while hybrid functionals provide better vertical excitation energies, pure functionals typically give more accurate excited state energy gaps. Pure functionals are also found to reproduce the state character and ordering in closer agreement to MS-CASPT2 than the hybrid functionals

Computational mechanistic photochemistry: The central role of conical intersections

In this thesis, I review my own contributions in the field of computational photochemistry. This manuscript is written as an introduction to this field of research. It is not intended to be a textbook, as more emphasis has been made on illustrations rather than on methodologies and technical guidelines. In this way, I hope that it will be accessible to a large audience, from undergraduate students to more experienced scientists who would be interested in learning about this fascinating and relatively young field of research

Assessing the Performances of CASPT2 and NEVPT2 for Vertical Excitation Energies

Methods able to simultaneously account for both static and dynamic electron correlations have often been employed, not only to model photochemical events, but also to provide reference values for vertical transition energies, hence allowing to benchmark lower-order models. In this category, both CASPT2 and NEVPT2 are certainly popular, the latter presenting the advantage of not requiring the application of the empirical ionization-potential-electron-affinity (IPEA) and level shifts. However, the actual accuracy of these multiconfigurational approaches is not settled yet. In this context, to assess the performances of these approaches the present work relies on highly-accurate ($\pm 0.03$ eV) \emph{aug}-cc-pVTZ vertical transition energies for 284 excited states of diverse character (174 singlet, 110 triplet, 206 valence, 78 Rydberg, 78 $n \to \pi^*$, 119 $\pi \to \pi^*$, and 9 double excitations) determined in 35 small- to medium-sized organic molecules containing from three to six non-hydrogen atoms. The CASPT2 calculations are performed with and without IPEA shift and compared to the partially-contracted (PC) and strongly-contracted (SC) variants of NEVPT2. We find that both CASPT2 with IPEA shift and PC-NEVPT2 provide fairly reliable vertical transition energy estimates, with slight overestimations and mean absolute errors of $0.11$ and $0.13$ eV, respectively. These values are found to be rather uniform for the various subgroups of transitions. The present work completes our previous benchmarks focussed on single-reference wave function methods (\textit{J.~Chem. Theory Comput.} \textbf{14}, 4360 (2018); \emph{ibid.}, \textbf{16}, 1711 (2020)), hence allowing for a fair comparison between various families of electronic structure methods. In particular, we show that ADC(2), CCSD, and CASPT2 deliver similar accuracies for excited states with a dominant single-excitation character.Comment: 21 pages, 3 figure (supporting information available

Arginine52 controls the photoisomerization process in photoactive yellow protein

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Reference Vertical Excitation Energies for Transition Metal Compounds

To enrich and enhance the diversity of the \textsc{quest} database of highly-accurate excitation energies [\href{https://doi.org/10.1002/wcms.1517}{V\'eril \textit{et al.}, \textit{WIREs Comput.~Mol.~Sci.}~\textbf{11}, e1517 (2021)}], we report vertical transition energies in transition metal compounds. Eleven diatomic molecules with singlet or doublet ground state containing a fourth-row transition metal (\ce{CuCl}, \ce{CuF}, \ce{CuH}, \ce{ScF}, \ce{ScH}, \ce{ScO}, \ce{ScS}, \ce{TiN}, \ce{ZnH}, \ce{ZnO}, and \ce{ZnS}) are considered and the corresponding excitation energies are computed using high-level coupled-cluster (CC) methods, namely CC3, CCSDT, CC4, and CCSDTQ, as well as multiconfigurational methods such as CASPT2 and NEVPT2. In some cases, to provide more comprehensive benchmark data, we also provide full configuration interaction estimates computed with the \textit{"Configuration Interaction using a Perturbative Selection made Iteratively"} (CIPSI) method. Based on these calculations, theoretical best estimates of the transition energies are established in both the aug-cc-pVDZ and aug-cc-pVTZ basis sets. This allows us to accurately assess the performance of CC and multiconfigurational methods for this specific set of challenging transitions. Furthermore, comparisons with experimental data and previous theoretical results are also reported.Comment: 17 pages, 3 figure

Chromophore Protonation State Controls Photoswitching of the Fluoroprotein asFP595

Fluorescent proteins have been widely used as genetically encodable fusion tags for biological imaging. Recently, a new class of fluorescent proteins was discovered that can be reversibly light-switched between a fluorescent and a non-fluorescent state. Such proteins can not only provide nanoscale resolution in far-field fluorescence optical microscopy much below the diffraction limit, but also hold promise for other nanotechnological applications, such as optical data storage. To systematically exploit the potential of such photoswitchable proteins and to enable rational improvements to their properties requires a detailed understanding of the molecular switching mechanism, which is currently unknown. Here, we have studied the photoswitching mechanism of the reversibly switchable fluoroprotein asFP595 at the atomic level by multiconfigurational ab initio (CASSCF) calculations and QM/MM excited state molecular dynamics simulations with explicit surface hopping. Our simulations explain measured quantum yields and excited state lifetimes, and also predict the structures of the hitherto unknown intermediates and of the irreversibly fluorescent state. Further, we find that the proton distribution in the active site of the asFP595 controls the photochemical conversion pathways of the chromophore in the protein matrix. Accordingly, changes in the protonation state of the chromophore and some proximal amino acids lead to different photochemical states, which all turn out to be essential for the photoswitching mechanism. These photochemical states are (i) a neutral chromophore, which can trans-cis photoisomerize, (ii) an anionic chromophore, which rapidly undergoes radiationless decay after excitation, and (iii) a putative fluorescent zwitterionic chromophore. The overall stability of the different protonation states is controlled by the isomeric state of the chromophore. We finally propose that radiation-induced decarboxylation of the glutamic acid Glu215 blocks the proton transfer pathways that enable the deactivation of the zwitterionic chromophore and thus leads to irreversible fluorescence. We have identified the tight coupling of trans-cis isomerization and proton transfers in photoswitchable proteins to be essential for their function and propose a detailed underlying mechanism, which provides a comprehensive picture that explains the available experimental data. The structural similarity between asFP595 and other fluoroproteins of interest for imaging suggests that this coupling is a quite general mechanism for photoswitchable proteins. These insights can guide the rational design and optimization of photoswitchable proteins