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Topochemical transformations of layered materials CaX2 (X=Si, Ge) are the method of choice for the high-yield synthesis of pristine, defect-free two-dimensional systems silicane and germanane, which have advanced electronic properties. Based on solid-state dispersion-corrected calculations, mechanisms for such transformations are elucidated that provide an in-depth understanding of phase transition in these layered materials. While formation of such layered materials is highly favorable for silicane and germanane, a barrier of 1.2 eV in the case of graphane precludes its synthesis from CaC2 topochemically. The energy penalty required for distorting linear acetylene into a trans-bent geometry accounts for this barrier. In contrast it is highly favorable in the heavier analogues, resulting in barrierless topochemical generation of silicane and germanane. Photochemical generation of the trans-bent structure of acetylene in its first excited state (S1) can directly generate graphane through a barrierless condensation. Unlike the buckled structure of silicene, the phase- h of CaSi2 with perfectly planar silicene layers exhibits the Dirac cones at the high symmetry points K and H. Interestingly, topochemical acidification of the cubic phase of calcium carbide is predicted to generate the previously elusive platonic hydrocarbon, tetrahedrane. [ABSTRACT FROM AUTHOR]

Structures and mechanism of pattern formation for theradical fluorinationon selected polyaromatic hydrocarbons (PAH) has been studied usingdensity functional theory (DFT) methods. Our study reveals that theF•radical addition occurs preferentially at theedges of PAHs followed by the hopping of F•to thecenter due to the fluxional nature of C–F bond. F•migrates preferentially over the C–C bonds having a lowerbarrier than that over the aromatic π-cloud in cases of monofluorinatedPAHs. Addition of a second F radical can stabilize the system, cooperatively.When two F•are added to the adjacent C atoms, itforms the minimum energy patterns. However, the addition of two fluorineradicals at the meta position of the same aromatic ring would leadto the stabilization of the triplet state compared to the singletground state. Therefore, depending on the sites of F•addition, these structures exhibit ferromagnetic/antiferromagneticground states. Considering the low barrier heights for the F•hopping, these systems are predicted to be in a dynamic equilibriumwith their less stable ferromagnetic states. Our study also providesan atomistic understanding of the well-known rate determining statefor the fluorine pattern formation in graphene and CNT. [ABSTRACT FROM AUTHOR]

Graphene oxide (GO) materials are widely studied, and yet their atomic-scale structures remain to be fully understood. Here we show that the chemical and configurational space of GO can be rapidly explored by advanced machine-learning methods, combining on-the-fly acceleration for first-principles molecular dynamics with message-passing neural-network potentials. The first step allows for the rapid sampling of chemical structures with very little prior knowledge required; the second step affords state-of-the-art accuracy and predictive power. We apply the method to the thermal reduction of GO, which we describe in a realistic (ten-nanometre scale) structural model. Our simulations are consistent with recent experimental findings, including X-ray photoelectron spectroscopy (XPS), and help to rationalise them in atomistic and mechanistic detail. More generally, our work provides a platform for routine, accurate, and predictive simulations of diverse carbonaceous materials., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Alkali ion intercalation is fundamental to battery technologies for a wide spectrum of potential applications that permeate our modern lifestyle, including portable electronics, electric vehicles, and the electric grid. In spite of its importance, the Nernstian nature of the charge transfer process describing lithiation of carbon has not been described previously. Here we use the ultrathin few-layer graphene (FLG) with micron-sized grains as a powerful platform for exploring intercalation and co-intercalation mechanisms of alkali ions with high versatility. Using voltammetric and chronoamperometric methods and bolstered by density functional theory (DFT) calculations, we show the kinetically facile co-intercalation of Li + and K + within an ultrathin FLG electrode. While changes in the solution concentration of Li + lead to a displacement of the staging voltammetric signature with characteristic slopes ca. 54-58 mV per decade, modification of the K + /Li + ratio in the electrolyte leads to distinct shifts in the voltammetric peaks for (de)intercalation, with a changing slope as low as ca. 30 mV per decade. Bulk ion diffusion coefficients in the carbon host, as measured using the potentiometric intermittent titration technique (PITT) were similarly sensitive to solution composition. DFT results showed that co-intercalation of Li + and K + within the same layer in FLG can form thermodynamically favorable systems. Calculated binding energies for co-intercalation systems increased with respect to the area of Li + -only domains and decreased with respect to the concentration of -K-Li- phases. While previous studies of co-intercalation on a graphitic anode typically focus on co-intercalation of solvents and one particular alkali ion, this is to the best of our knowledge the first study elucidating the intercalation behavior of two monovalent alkali ions. This study establishes ultrathin graphitic electrodes as an enabling electroanalytical platform to uncover thermodynamic and kinetic processes of ion intercalation with high versatility., Competing Interests: The authors declare no competing financial interest., (This journal is © The Royal Society of Chemistry.)

Chalcogenide perovskites constitute an emerging class of promising photovoltaic materials that are stable and less toxic than popular lead-halide perovskites. Transition-metal and chalcogenide doping are the possible strategies for improving the photovoltaic properties of these materials via the band gap engineering. At the same time, doping can facilitate nonradiative charge-carrier recombination in these materials, adversely affecting their photovoltaic properties. We report a systematic study of electronic structure and nonadiabatic dynamics in transition-metal- and chalcogenide-doped barium-zirconium-sulfide-based perovskites. The potential of these doping strategies to modulate the performance of photovoltaic materials is explored. Through the detailed analysis of the factors affecting the dynamics, we illustrate how symmetry (both structural and orbital) and decoherence can be critical to furnishing the most favorable properties. The noted factors of symmetry and decoherence may provide new rational design principles for efficient photovoltaics.

Kinetic and thermodynamic preferences for the reductive elimination of C aryl -CF 3 , C aryl -X, C aryl -P, and CF 3 -X bonds and competitive phosphine dissociation from a series of Au III complexes [(Ph 3 P)Au(Ar)(CF 3 )(X)] (1  X ; Ar=4-Me-C 6 H 4 ; X=F, Cl, Br, I) are studied computationally. Kinetically, the most favorable pathways were found to consist of an initial phosphine dissociation from complex 1  X , which furnished the respective three-coordinate Au III complexes [Au(Ar)(CF 3 )(X)] (2  X ). The computed enthalpy barriers for various reductive elimination reactions from complex 2  X by a direct (or uncatalyzed) mechanism showed that C aryl -CF 3 bond formation was the most favorable fate for any X group. When the direct elimination was compared with an autocatalytic mechanism that proceeded through the formation of a mixed-valent binuclear Au III -Au I intermediate, the preference for the formation of a C aryl -CF 3 bond is dependent on the nature of the bridging halide atom and follows the order F>Cl>Br>I. Concomitantly, the selectivity for the formation of C aryl -X bonds for various X atoms follows the opposite trend. The preference for the direct and autocatalytic processes is controlled entirely by the nature of the halide ligand. The predicted mechanisms and product selectivity trends for various halides show excellent agreement with recent experimental observation. The selectivity of various reductive elimination pathways was rationalized by using molecular orbital theory and distortion-interaction model analyses. Attractive interactions between the Au I complex and complex 2 X were found to reduce the activation barrier for C aryl -X elimination and critically control the selectivity of the product formation., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Topochemical transformations of layered materials CaX2 (X=Si, Ge) are the method of choice for the high-yield synthesis of pristine, defect-free two-dimensional systems silicane and germanane, which have advanced electronic properties. Based on solid-state dispersion-corrected calculations, mechanisms for such transformations are elucidated that provide an in-depth understanding of phase transition in these layered materials. While formation of such layered materials is highly favorable for silicane and germanane, a barrier of 1.2 eV in the case of graphane precludes its synthesis from CaC2 topochemically. The energy penalty required for distorting linear acetylene into a trans-bent geometry accounts for this barrier. In contrast it is highly favorable in the heavier analogues, resulting in barrierless topochemical generation of silicane and germanane. Photochemical generation of the trans-bent structure of acetylene in its first excited state (S1 ) can directly generate graphane through a barrierless condensation. Unlike the buckled structure of silicene, the phase-h of CaSi2 with perfectly planar silicene layers exhibits the Dirac cones at the high symmetry points K and H. Interestingly, topochemical acidification of the cubic phase of calcium carbide is predicted to generate the previously elusive platonic hydrocarbon, tetrahedrane., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Carbon-carbon bond reductive elimination from gold(III) complexes are known to be very slow and require high temperatures. Recently, Toste and co-workers have demonstrated extremely rapid CC reductive elimination from cis-[AuPPh3 (4-F-C6 H4 )2 Cl] even at low temperatures. We have performed DFT calculations to understand the mechanistic pathway for these novel reductive elimination reactions. Direct dynamics calculations inclusive of quantum mechanical tunneling showed significant contribution of heavy-atom tunneling (>25 %) at the experimental reaction temperatures. In the absence of any competing side reactions, such as phosphine exchange/dissociation, the complex cis-[Au(PPh3 )2 (4-F-C6 H4 )2 ](+) was shown to undergo ultrafast reductive elimination. Calculations also revealed very facile, concerted mechanisms for HH, CH, and CC bond reductive elimination from a range of neutral and cationic gold(III) centers, except for the coupling of sp(3) carbon atoms. Metal-carbon bond strengths in the transition states that originate from attractive orbital interactions control the feasibility of a concerted reductive elimination mechanism. Calculations for the formation of methane from complex cis-[AuPPh3 (H)CH3 ](+) predict that at -52 °C, about 82 % of the reaction occurs by hydrogen-atom tunneling. Tunneling leads to subtle effects on the reaction rates, such as large primary kinetic isotope effects (KIE) and a strong violation of the rule of the geometric mean of the primary and secondary KIEs., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Dimethyl-3,6-dichloro-2,5-dihydroxyterephthalate (MCHT) is known to exist in three differently packed crystals having three different colors, namely yellow (Y), light yellow (LY), and white (W). Apart from the difference in their color, the molecules in the crystals also differ in their intramolecular O-H⋅⋅⋅O and O-H⋅⋅⋅Cl hydrogen bonds. Time-dependent DFT calculations reveal the role of the various types of hydrogen bonds in controlling the color of the polymorphs. Mechanistic pathways that lead to such transformations in the crystal are elucidated by solid-state dispersion-corrected DFT studies. Relative stabilities of the various polymorphs rationalize the experimentally observed transformations between them. Calculations reveal that the minimum-energy pathway for the conversion of the Y form to a W form is through stepwise disrotatory motion of the two -OH groups through a hybrid intermediate having one intramolecular OH⋅⋅⋅O and one O-H⋅⋅⋅Cl bond. The LY form is shown to exist on the higher-energy pathway involving a concerted Y→W transformation., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

CH···π and lone-pair···π interactions are estimated for a series of conformationally dynamic bicyclic N-aryliimides. On the basis of their strengths and mutual synergy/competition, the molecules prefer a folded/unfolded conformation. Calculations suggest strategies to selectively isolate the folded form by increasing the strength of the attractive CH···π interaction or removing the lone-pair···π repulsion. While the barrier for the folded ⇄ unfolded transformation is too large to conformationally lock the molecules in either of the conformers, the dynamics for hopping of the alkyl group across rings and tumbling over the rings are found to be facile in the folded conformation.