Assessment Of Density Functional Theory For Describing The Correlation Effects On The Ground And Excited State Potential Energy Surfaces Of A Retinal Chromophore Model

In the quest for a cost-effective level of theory able to describe a large portion of the ground and excited potential energy surfaces of large chromophores, promising approaches are rooted in various approximations to the exact density functional theory (DFT). In the present work, we investigate how generalized Kohn-Sham DFT (GKS-DFT), time-dependent DFT (TDDFT), and spin-restricted ensemble-DFT (REKS) methods perform along three important paths characterizing a model retinal chromophore (the penta-2,4-dieniminium cation) in a region of near-degeneracy (close to a conical intersection) with respect to reference high-level multiconfigurational wave function methods. If GKS-DFT correctly describes the closed-shell charge transfer state, only TDDFT and REKS approaches give access to the open-shell diradical, one which sometimes corresponds to the electronic ground state. It is demonstrated that the main drawback of the usual DFT-based methods lies in the absence of interactions between the charge transfer and the diradicaloid configurations. Hence, we test a new computational scheme based on the State-averaged REKS (SA-REKS) approach, which explicitly includes these interactions into account. The State-Interaction SA-REKS (SI-SA-REKS) method significantly improves on the REKS and the SA-REKS results for the target system. The similarities and differences between DFT and wave function-based approaches are analyzed according to (1) the active space dimensions of the wave function-based methods and (2) the relative electronegativities of the allyl and protonated Schiff base moieties

Geometrical Embedding Governs a Dramatic Variation of Electron Paramagnetic Resonance Hyperfine Coupling Constants of Disulfide Radical Anions

International audienceAmong the large variety of experimental techniques amenable to probe disulfide radical anions, electron paramagnetic resonance (EPR) spectroscopy provides the most definitive assignment of these versatile transient intermediates in biochemistry [Stubbe et al. Proc. Natl. Acad. Sci. U.S.A.1999, 96, 8979–84; J. Am. Chem. Soc.2009, 131, 200–211]. EPR parameters along both a series of 12 aliphatic 1,2-dithia-cycloalkane radical anions and a representative set of 18 short-loop peptides are investigated by means of density functional theory. While the g-tensor remains quasi-isotropic (with diagonal terms very close to 2.0, as expected for a σ* singly occupied orbital), we evidence a dramatic conformational dependence of isotropic sulfur hyperfine coupling constants (hcc). Potential energy surface exploration of the prototypical dimethyldisulfide rationalizes their 3–4-fold amplitude, with values ranging between 10 and 29 G for aliphatic moieties. Sulfur hcc’s are readily decomposed into three geometrical components: intersulfur distance, dihedral, and valence angles, with the latter being predominant. Increasing (respectively decreasing) contribution of sulfur atomic s orbital to the σ* molecular orbital, with a concomitant higher (respectively weaker) density around the sulfur nuclei, can be monitored on Walsh diagrams along each degree of motion. In peptidic disulfide radical anionic systems, sulfur hcc’s are dissymmetrized and span an even larger range of values, from 14 to 40 G. Again, dependence is governed by the mechanical embedding of the −CH2–S∴S–CH2– motif, this time with a noticeable contribution from the hemibond lenghtening and some punctual short-range additive electrostatic contributions. This analysis comes within the scope of a unified picture of both spectroscopy and reactivity of the mechanochemistry of disulfide hemibonds

Theoretical Study of the Photochemical Initiation in Nitroxide-Mediated Photopolymerization

Nitroxide-mediated photopolymerization (NMP²) is a promising novel route to initiate radical polymerization. In NMP², alkoxyamines bounded to a monomer are attached to a chromophore. Upon light absorption, the excitation energy is transferred from the chromophore to the alkoxyamine moiety, inducing the cleavage of the oxygen–carbon bond and thus initiating the polymerization. The NMP² mechanism depends strongly on several factors like the type of chromophore, the monomer, the connectivity pattern, etc. This complexity makes it difficult to design new NMP² initiators with increased polymerization efficiency and selectivity. In the present article, we characterize by means of quantum mechanical calculations the main steps of the NMP² initiation for alkoxyamines attached to aromatic ketones. We show how the excitation energy can be transferred from the chromophore to the alkoxyamine moiety, and present two easily computed parameters which can account for the selectivity of the O–C bond photocleaveage. Finally, using results obtained for a series of isomers, we give some rules that may help the design of more efficient NMP² initiators

What Are the Physical Contents of Hubbard and Heisenberg Hamiltonian Interactions Extracted from Broken Symmetry DFT Calculations in Magnetic Compounds?

Analytical expressions of the interactions present in the Heisenberg–Dirac van Vleck and Hubbard Hamiltonians have been derived as functions of both the energy of several broken symmetry DFT solutions and their expectation value of the <i>S</i>² spin operator. Then, following a strategy of decomposition of the magnetic exchange coupling into its main contributions (direct exchange, kinetic exchange, and spin polarization) and using a recently proposed method of spin decontamination, values of these interactions have been extracted. As already observed, they weakly depend on the correlation functional but strongly depend on the exchange one. In order to distinguish between the effect of the delocalization of the magnetic orbitals and that of the amount of Hartree–Fock exchange (HFX) when hybrid exchange-correlation functionals are used, we have disentangled these two contributions by either freezing the magnetic orbitals and varying the amount of HFX or varying the magnetic orbitals while keeping the same amount of HFX. As expected, increasing the amount of HFX induces a slight relocalization of the magnetic orbitals on the magnetic center which results in a weak increase of the repulsion energy <i>U</i> parameter and a weak decrease of both the direct exchange <i>K</i>_<i>ab</i> and hopping |<i>t</i>| parameters. Conversely, the amount of HFX has a huge effect on all the parameters, even when some of the parameters should be exchange-independent, like <i>U</i>. Indeed, it is analytically demonstrated that the physical content of the <i>U</i> parameter extracted from several broken-symmetry solutions depends on the amount of HFX and that this pathological behavior has the same origin as the self-interaction error. This result is interesting not only to theoretical chemists working in the field of magnetic systems but also to DFT methodologists interested in using this theory for studying either excited states or strongly correlated systems. Finally, the performance of the range-separated ωB97XD functional for both ferromagnetic and antiferromagnetic transition-metal compounds and organic systems must be noted

Additive Decomposition of the Physical Components of the Magnetic Coupling from Broken Symmetry Density Functional Theory Calculations

The procedure to extract and identify from broken-symmetry density functional theory (BS-DFT) calculations the various components of the magnetic couplings in diradicals [<i>J. Chem. Phys.</i> <b>2012</b>, <i>137</i>, 114106] is re-examined. It is shown that this previous decomposition scheme fails for systems exhibiting large core polarization effects and hence becomes not additive in such cases. At variance, the new scheme which differs from the previous one in the assessment of the polarization effects is perfectly additive. As done previously, the direct exchange is calculated from the <i>M</i>_S = 1 and <i>M</i>_S = 0 restricted solutions. We show that allowing first the delocalization of the magnetic orbitals in the field of the closed shell frozen core furnishes a good evaluation of the kinetic exchange contribution to the magnetic exchange coupling, i.e. the intersite delocalization of the magnetic electrons in the low-spin state. In a second step, allowing the polarization of the core to take place in the field of the so-revised magnetic orbitals practically leads to the same total value of the magnetic coupling obtained by the brute-force BS-DFT calculation. The success of this decomposition is illustrated on a representative series of inorganic and organic diradicals. The obtained quasi-additivity of the effects is rationalized thanks to a careful theoretical analysis of the broken-symmetry solutions

Hybrid QM/MM Simulations of the Obelin Bioluminescence and Fluorescence Reveal an Unexpected Light Emitter

Obelia longissima, a tiny hydrozoan living in temperate and cold seas, features the Obelin photoprotein, which emits blue light. The Obelin bioluminescence and the Ca²⁺-discharged Obelin fluorescence spectra show multimodal characteristics that are currently interpreted by the concomitant participation of several light emitters. Up to now, the coelenteramide luminophore is thought to exist in different protonation states, one of them engaged in an ion-pair with the nearby residue, His22. Using hybrid quantum mechanics/molecular mechanics (QM/MM) calculations, we demonstrate that such an ion-pair cannot exist as a stable light emitter. However, when His22 electric neutrality is maintained by means of another proton transfer, the phenolate state of coelenteramide exhibits emission properties in agreement with experiment. Finally, an alternative nonradiative decay pathway, involving the formation of a diradical excited state, is postulated for the first time

Can the Closed-Shell DFT Methods Describe the Thermolysis of 1,2-Dioxetanone?

The chemiluminescent decomposition of 1,2-dioxetanone has in the past been studied by state-of-the-art multireference quantum chemical calculations, and a stepwise biradical mechanism was established. Recently, this decomposition has been reinvestigated, and a concerted mechanism has been proposed based on calculations performed at the closed-shell density functional theory (DFT) level of theory. In order to solve this apparent mechanistic contradiction, the present paper presents restricted and unrestricted DFT results obtained using functionals including different amounts of Hartree–Fock (HF) exchange, repeating and complementing the above-mentioned DFT calculations. The calculated results clearly indicate that the closed-shell DFT methods cannot correctly describe the thermolysis of 1,2-dioxetanone. It is found that unrestricted Kohn–Sham reaction energies and barriers are always lower than the ones obtained using a restricted formalism. Hence, from energy principles, the biradical mechanism is found to be prevailing in the understanding of the 1,2-dioxetanone thermolysis

Sub-picosecond C=C bond photo-isomerization: Evidence for the role of excited state mixing

Sub-picosecond photo-isomerization is the major primary process of energy conversion in retinal proteins and has as such been in the focus of extensive theoretical and experimental work over the past decades. In this review article, we revisit the long-standing question as to how the protein tunes the isomerization speed and quantum yield. We focus on our recent contributions to this field, which underscore the concept of a delicate mixing of reactive and non-reactive excited states, as a result of steric properties and electrostatic interactions with the protein environment. Further avenues and new approaches are outlined which hold promise for advancing our understanding of these intimately coupled chromophore-protein systems

Assessment of Density Functional Theory for Describing the Correlation Effects on the Ground and Excited State Potential Energy Surfaces of a Retinal Chromophore Model

In the quest for a cost-effective level of theory able to describe a large portion of the ground and excited potential energy surfaces of large chromophores, promising approaches are rooted in various approximations to the exact density functional theory (DFT). In the present work, we investigate how generalized Kohn–Sham DFT (GKS-DFT), time-dependent DFT (TDDFT), and spin-restricted ensemble-DFT (REKS) methods perform along three important paths characterizing a model retinal chromophore (the penta-2,4-dieniminium cation) in a region of near-degeneracy (close to a conical intersection) with respect to reference high-level multiconfigurational wave function methods. If GKS-DFT correctly describes the closed-shell charge transfer state, only TDDFT and REKS approaches give access to the open-shell diradical, one which sometimes corresponds to the electronic ground state. It is demonstrated that the main drawback of the usual DFT-based methods lies in the absence of interactions between the charge transfer and the diradicaloid configurations. Hence, we test a new computational scheme based on the State-averaged REKS (SA-REKS) approach, which explicitly includes these interactions into account. The State-Interaction SA-REKS (SI-SA-REKS) method significantly improves on the REKS and the SA-REKS results for the target system. The similarities and differences between DFT and wave function-based approaches are analyzed according to (1) the active space dimensions of the wave function-based methods and (2) the relative electronegativities of the allyl and protonated Schiff base moieties