Exchange Rules for Diradical π-Conjugated Hydrocarbons

A variety of planar π-conjugated hydrocarbons such as heptauthrene, Clar’s goblet and, recently synthesized, triangulene have two electrons occupying two degenerate molecular orbitals. The resulting spin of the interacting ground state is often correctly anticipated as S = 1, extending the application of Hund’s rules to these systems, but this is not correct in some instances. Here we provide a set of rules to correctly predict the existence of zero mode states as well as the spin multiplicity of both the ground state and the low-lying excited states, together with their open- or closed-shell nature. This is accomplished using a combination of analytical arguments and configuration interaction calculations with a Hubbard model, both backed by quantum chemistry methods with a larger Gaussian basis set. Our results go beyond the well established Lieb’s theorem and Ovchinnikov’s rule, as we address the multiplicity and the open-/closed-shell nature of both ground and excited states.J.F.-R. and R.O. acknowledge financial support from MINECO-Spain (grant no. MAT2016-78625-C2) and from the Portuguese “Fundação para a Ciência e a Tecnologia” (FCT) for the project P2020-PTDC/FIS-NAN/4662/2014. J.F.-R., M.M.-F. and N.G.-M. acknowledge support from the P2020-PTDC/FISNAN/3668/2014. J.F.-R. acknowledges support from UTAPEXPL/NTec/0046/2017 projects as well as Generalitat Valenciana funding (Prometeo2017/139). R.O. and J.C.S.-G. acknowledge ACIF/2018/175 (Generalitat Valenciana and Fondo Social Europeo). M.M.-F. and R.B. acknowledge the Portuguese “Fundação para a Ciência e a Tecnologia” (FCT) for the project IF/00894/2015 and FCT ref. UID/CTM/50011/2019 for CICECO - Aveiro Institute of Materials. This project received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 664878

Optomechanical effects in nanocavity-enhanced resonant Raman scattering of a single molecule

In this paper, we address the optomechanical effects in surface-enhanced resonant Raman scattering (SERRS) from a single molecule in a nanoparticle on mirror (NPoM) nanocavity by developing a quantum master-equation theory, which combines macroscopic quantum electrodynamics and electron-vibration interaction within the framework of open quantum system theory. We supplement the theory with electromagnetic simulations and time-dependent density functional theory calculations in order to study the SERRS of a methylene blue molecule in a realistic NPoM nanocavity. The simulations allow us not only to identify the conditions to achieve conventional optomechanical effects, such as vibrational pumping, nonlinear scaling of Stokes and anti-Stokes scattering, but also to discovery distinct behaviors, such as the saturation of exciton population, the emergence of Mollow triplet side bands, and higher-order Raman scattering. All in all, our study might guide further investigations of optomechanical effects in resonant Raman scattering.This work is supported by the Projects No. 12004344 and No. 21902148 from the National Nature Science Foundation of China, joint Project No. 21961132023 from the NSFC-DPG, Project No. PID2019-107432GB-I00 from the Spanish Ministry of Science and Innovation, and Grant No. IT1526-22 for consolidated groups of the Basque University, through the Department of Education, Research, and Universities of the Basque Government.Peer reviewe

The role of dispersion forces in metal-supported self-assembled monolayers

International audienceThe self-assembly of organic molecules (octylamine) is studied by means of periodic DFT calculations including Grimme D2 dispersion scheme. It has been found that in the gas-phase dispersive forces are crucial when packing is considered, modifying considerably the potential energy surface compared to the case of a molecular dimer. In interaction with a metallic surface (platinum), the electronic interaction between adsorbate and substrate is predominant, dispersion forces mainly increasing the adsorption strength. A detailed study on the structural parameters of the molecule–metal system is reported to gain understanding on the factors affecting their stability. The intermolecular bonding in these systems is analyzed by means of the non-covalent interactions NCI index. The decomposition of the 2D packing along each direction (x, y) highlights the importance of the diagonal contributions in the stabilization of the square pattern. Moreover, the stabilizing nature of H–H interactions between neighboring molecules is supported in this model

Steric clash in real space: biphenyl revisited

A textbook case of twisted structure due to hydrogen–hydrogen steric clash, the biphenyl molecule, has been studied in real space from a new perspective. Long-term discrepancies regarding the origin of the steric repulsion are now reconciled under the NCI (Non Covalent Interaction) method, which reflects in 3D the balance between attractive and repulsive interactions taking place in the region between the phenyl rings. The NCI method confirms that the steric repulsion does not merely come from the H–H interaction itself, but from the many-atom interactions arising from the Cortho–H region, therefore providing rigorous physical grounds for the steric clash. This method allows a continuous scan of all the subtle changes on the electron density on going from the planar to the perpendicular biphenyl structure. The NCI results agree with other topological approaches (IQA, ELF) and are in line with previous findings in the literature regarding controversial H–H interactions in steric clash situations: H–H interactions are attractive, but repulsion appears between (Cortho–H)⋯(Cortho–H), raising the intraatomic energy of the ortho H. ELF is also used to support these conclusions. Indeed, deformations are observed in compressed basins that allow to visualize the intraatomic effect of steric repulsion. These results can be easily extrapolated to systems with similar topological features in which steric clash is claimed to be the reason for instability.R. Laplaza gratefully thanks École Doctoral ED388 for his PhD grant. R. A. Boto and J. Contreras-García express their gratefulness to LabEx CALSIMLAB (public grant ANR-11-LABX-0037-01 constituting a part of the “Investissements d’Avenir” program – reference: ANR-11-IDEX-0004-02). M. M. Montero-Campillo is grateful to the Ministerio de Ciencia, Innovación y Universidades from Spain for her José Castillejo International Mobility grant (CAS19/00070) at the Laboratoire of Chimie Théorique (Sorbonne Universitè) in Paris. Financial support from the Spanish PGC2018-094644-B-C21 project and computational time from the Centro de Computación Científica (CCC) at the Universidad Autónoma de Madrid are also acknowledged.Peer reviewe

Sulfites detection by surface-enhanced Raman spectroscopy: A feasibility study

The exhaustive control required for the correct wine production needs of many chemical analysis throughout the process. The most extended investigations for wine production control are focused on the quantification of total and free SO2. Most methods described in the literature have an adequate detection limit, but they usually lack reproducibility and require a previous sample treatment for the extraction of the SO2 from the wine-matrix. In this context, Surface-Enhanced Raman Spectroscopy (SERS) can be a promising technique for free SO2 determination without the need for any sample pre-processing. This work describes a proof of concept of a new methodology based on SERS and supported by Density Functional Theory (DFT) calculations to identify the active vibrational modes of the key molecules that contribute to the concentration of free SO2 in solution. Theoretical predictions and experimental outcomes are brought together to chemometrics to get a simple and real-time free SO2 monitoring. This general procedure could pave the way towards an implementation of a portable SERS detection module for in-field measurements.This work has been funded in part by the Fondo Europeo de Desarrollo Regional (FEDER), in part by the Ministerio de Ciencia, Innovación y Universidades under projects PID2021-122505OB-C31, PID2019-107432GB-I00, in part by the GobiernoVasco/Eusko Jaurlaritza IT1452-22, IT1526-22, ELKARTEK KK-2021/00092 and ELKARTEK KK-2021/00082.Peer reviewe

Chapter 5 - Chemical Bonding Under Pressure

International audienc

NCIPLOT and the analysis of noncovalent interactions using the reduced density gradient

This article is categorized under: Software > Molecular Modeling Quantum Computing > Theory Development Structure and Mechanism > Computational Biochemistry and BiophysicsNoncovalent interactions are of utmost importance. However, their accurate treatment is still difficult. This is partially induced by the coexistence of many types of interactions and physical phenomena, which hampers generality in simple treatments. The NCI index has been successfully used for nearly over 10 years in order to identify, analyze, and understand noncovalent interactions in a wide variety of systems, ranging from proteins to molecular crystals. In this work, the development and implications of the method will be reviewed, and modern implementations will be presented. Afterward, some sophisticated examples will be given that showcase the current advances toward the fast, robust, and intuitive identification of noncovalent interactions in real space.Agence Nationale de la Recherche, Grant/Award Numbers: ANR-11-IDEX-0004-02, ANR-11-LABX-0037-01; PICS-CNRS; Ecole doctorale 388; H2020 European Research Council, Grant/Award Number: 810367; Institut Universitaire de France; PHC PROCOPE 2017.Peer reviewe

NCIPLOT

This article is categorized under: Software > Molecular Modeling Quantum Computing > Theory Development Structure and Mechanism > Computational Biochemistry and BiophysicsNoncovalent interactions are of utmost importance. However, their accurate treatment is still difficult. This is partially induced by the coexistence of many types of interactions and physical phenomena, which hampers generality in simple treatments. The NCI index has been successfully used for nearly over 10 years in order to identify, analyze, and understand noncovalent interactions in a wide variety of systems, ranging from proteins to molecular crystals. In this work, the development and implications of the method will be reviewed, and modern implementations will be presented. Afterward, some sophisticated examples will be given that showcase the current advances toward the fast, robust, and intuitive identification of noncovalent interactions in real space.Agence Nationale de la Recherche, Grant/Award Numbers: ANR-11-IDEX-0004-02, ANR-11-LABX-0037-01; PICS-CNRS; Ecole doctorale 388; H2020 European Research Council, Grant/Award Number: 810367; Institut Universitaire de France; PHC PROCOPE 2017.Peer reviewe

The Houk–List transition states for organocatalytic mechanisms revisited

International audienceThe ten year old Houk–List model for rationalising the origin of stereoselectivity in the organocatalysed intermolecular aldol addition is revisited, using a variety of computational techniques that have been introduced or improved since the original study. Even for such a relatively small system, the role of dispersion interactions is shown to be crucial, along with the use of basis sets where the superposition errors are low. An NCI (non-covalent interactions) analysis of the transition states is able to identify the noncovalent interactions that influence the selectivity of the reaction, confirming the role of the electrostatic NCH d+ /O dÀ interactions. Simple visual inspection of the NCI surfaces is shown to be a useful tool for the design of alternative reactants. Alternative mechanisms, such as proton-relays involving a water molecule or the Hajos–Parrish alternative, are shown to be higher in energy and for which computed kinetic isotope effects are incompatible with experiment. The Amsterdam manifesto, which espouses the principle that scientific data should be citable, is followed here by using interactive data tables assembled via calls to the data DOI (digital-object-identifiers) for calculations held on a digital data repository and themselves assigned a DOI