The impact of the solvent dielectric constant on A←NH3 dative bond depends on the nature of the Lewis electron-pair systems

The present work aims to determine to what extent the value of the dielectric constant of the solvent can influence the dative bond in Lewis electron pair bonding systems. For this purpose, two different systems, namely H3B <- NH3 and {Zn <-(NH3)}(2+), were studied in selected solvents with significantly different dielectric constants. Based on the results from state-of-the-art computational methods using DFT, constrained DFT, energy decomposition analyses, solvent accessible surface area, and charge transfer calculations, we found that the stability of the neutral H3B <- NH3 system increases with increasing solvent polarity. In contrast, the opposite trend is observed for the positively charged {Zn <-(NH3)}(2+). The observed changes are attributed to different charge redistributions in neutral and charged complexes, which are reflected by a different response to the solvent and are quantified by changes in solvation energies.Web of Science293

Photodynamics Simulations of Thymine: Relaxation into the First Excited Singlet State

Ab initio nonadiabatic dynamics simulations are reported for thymine with focus on the S2 → S1deactivation using the state-averaged CASSCF method. Supporting calculations have been performed on vertical excitations, S1 and S2 minima, and minima on the crossing seam using the MS-CASPT2, RI-CC2, MR-CIS, and MR-CISD methods. The photodynamical process starts with a fast (\u3c100 fs) planar relaxation from the S2 ππ* state into the πOπ* minimum of the S2 state. The calculations demonstrate that two π-excited states (denoted ππ* and πOπ*) are actually involved in this stage. The time in reaching the S2/S1 intersections, through which thymine can deactivate to S1, is delayed by both the change in character between the states as well as the flatness of the S2 surface. This deactivation occurs in an average time of 2.6 ps at the lowest-energy region of the crossing seam. After that, thymine relaxes to the nπ* minimum of the S1state, where it remains until the transfer to the ground state takes place. The present dynamics simulations show that not only the πOπ* S2 trapping but also the trapping in the nπ* S1 minimum contribute to the elongation of the excited-state lifetime of thymine

The effect of water on gold supported chiral graphene nanoribbons: rupture of conjugation by an alternating hydrogenation pattern

In the last few years we have observed a breakpoint in the development of graphene-derived technologies, such as liquid phase filtering and their application to electronics. In most of these cases, they imply exposure of the material to solvents and ambient moisture, either in the fabrication of the material or the final device. The present study demonstrates the sensitivity of graphene nanoribbon (GNR) zigzag edges to water, even in extremely low concentrations. We have addressed the unique reactivity of (3,1)-chiral GNR with moisture on Au(111). Water shows a reductive behaviour, hydrogenating the central carbon of the zigzag segments. By combining scanning tunnelling microscopy (STM) with simulations, we demonstrate how their reactivity reaches a thermodynamic limit when half of the unit cells are reduced, resulting in an alternating pattern of hydrogenated and pristine unit cells starting from the terminal segments. Once a quasi-perfect alternation is reached, the reaction stops regardless of the water concentration. The hydrogenated segments limit the electronic conjugation of the GNR, but the reduction can be reversed both by tip manipulation and annealing. Selective tip-induced dehydrogenation allowed the stabilization of radical states at the edges of the ribbons, while the annealing of the sample completely recovered the original, pristine GNR.We acknowledge the financial support from MCIN/AEI/10.13039/501100011033 (grant no. PID2019-107338RB-C62 and PID2019-107338RB-C63) and the European Union “NextGenerationEU”/PRTR (TED2021-132388B-C42 and TED2021-132388B-C43), from MCIN/AEI/10.13039/501100011033/ERDF/EU (PID2022-140845OB-C62 and PID2022-140845OB-C64), the Xunta de Galicia (Centro Singular de Investigación de Galicia, 2019-2022, grant no. ED431G2019/03), and European Regional Development Fund (ERDF). P. J. and A. M. acknowledge the support from Praemium Academie of the Academy of Science of the Czech Republic and Czech Science Foundation projects no. 20-13692X. We acknowledge the computational resources provided by the project “e-Infrastruktura CZ” (e-INFRA CZ LM2018140) supported by MEYS CR. A. M. is thankful for the support from the Internal Student Grant Agency of the Palacký University in Olomouc, Czech Republic, IGA_PrF_2023_018 and the Endowment Fund of the Palacký University in Olomouc, Czech Republic. H. L. wants to acknowledge the support by the National Science Foundation [grant number 2107923], Division of Chemistry. B. T. acknowledges the financial support from Czech Science Foundation 23-06781M and CzechNanoLab Research Infrastructure supported by MEYS CR (LM2023051). We acknowledge financial support of the European Union under the REFRESH – Research Excellence For REgion Sustainability and High-tech Industries project number CZ.10.03.01/00/22_003/0000048 via the Operational Programme Just Transition.Peer reviewe

The generality of the GUGA MRCI approach in COLUMBUS for treating complex quantum chemistry

The core part of the program system COLUMBUS allows highly efficient calculations using variational multireference (MR) methods in the framework of configuration interaction with single and double excitations (MR-CISD) and averaged quadratic coupled-cluster calculations (MR-AQCC), based on uncontracted sets of configurations and the graphical unitary group approach (GUGA). The availability of analytic MR-CISD and MR-AQCC energy gradients and analytic nonadiabatic couplings for MR-CISD enables exciting applications including, e.g., investigations of π-conjugated biradicaloid compounds, calculations of multitudes of excited states, development of diabatization procedures, and furnishing the electronic structure information for on-the-fly surface nonadiabatic dynamics. With fully variational uncontracted spin-orbit MRCI, COLUMBUS provides a unique possibility of performing high-level calculations on compounds containing heavy atoms up to lanthanides and actinides. Crucial for carrying out all of these calculations effectively is the availability of an efficient parallel code for the CI step. Configuration spaces of several billion in size now can be treated quite routinely on standard parallel computer clusters. Emerging developments in COLUMBUS, including the all configuration mean energy multiconfiguration self-consistent field method and the graphically contracted function method, promise to allow practically unlimited configuration space dimensions. Spin density based on the GUGA approach, analytic spin-orbit energy gradients, possibilities for local electron correlation MR calculations, development of general interfaces for nonadiabatic dynamics, and MRCI linear vibronic coupling models conclude this overview

Trends in the stability of covalent dative bonds with variable solvent polarity depend on the charge transfer in the Lewis electron-pair system

In general, the stability of neutral complexes with dative bonds increases as the polarity of the solvent increases. This is based on the fact that the dipole moment of the complex increases as the charge transferred from the donor to the acceptor increases. As a result, the solvation energy of the complex becomes greater than that of subsystems, causing an increase in the stabilization energy with increasing solvent polarity. Our research confirms this assumption, but only when the charge transfer is sufficiently large. If it is below a certain threshold, the increase in the complex's dipole moment is insufficient to result in a higher solvation energy than subsystems. Thus, the magnitude of the charge transfer in the Lewis electron-pair system determines the stability trends of dative bonds with varying solvent polarity. We used molecular dynamics (MD) simulations based on an explicit solvent model, which is considered more reliable, to verify the results obtained with a continuous solvent model.Web of Science2538259642596