Searching for stable copper borozene complexes in CuB7- and CuB8.

Copper has been shown to be an important substrate for the growth of borophenes. Copper-boron binary clusters are ideal platforms to study the interactions between copper and boron, which may provide insight about the underlying growth mechanisms of borophene on copper substrates. Here we report a joint photoelectron spectroscopy and theoretical study on two copper-doped boron clusters, CuB7- and CuB8-. Well resolved photoelectron spectra are obtained for the two clusters at different wavelengths and are used to understand the structures and bonding properties of the two CuBn- clusters. We find that CuB8- is a highly stable borozene complex, which possesses a half-sandwich structure with a Cu+ species interacting with the doubly aromatic η8-B82- borozene. The CuB7- cluster is found to consist of a terminal copper atom bonded to a double-chain B7 motif, but it has a low-lying isomer composed of a half-sandwich structure with a Cu+ species interacting with an open-shell η7-B72- borozene. Both ionic and covalent interactions are found to be possible in the binary Cu-B clusters, resulting in different structures.

Design and Implementation of a Hardware and Software System for Visual Assessment of Bituminous Coating Quality.

Typically, the quality of the bitumen adhesion in asphalt mixtures is assessed manually by a group of experts who assign subjective ratings to the thickness of the residual bitumen coating on the gravel samples. To automate this process, we propose a hardware and software system for visual assessment of bituminous coating quality, which provides the results both in the form of a discrete estimate compatible with the expert one, and in a more general percentage for a set of samples. The developed methodology ensures static conditions of image capturing, insensitive to external circumstances. This is achieved by using a hardware construction designed to provide capturing the samples at eight different illumination angles. As a result, a generalized image is obtained, in which the effect of highlights and shadows is eliminated. After preprocessing, each gravel sample independently undergoes surface semantic segmentation procedure. Two most relevant approaches of semantic image segmentation were considered: gradient boosting and U-Net architecture. These approaches were compared by both stone surface segmentation accuracy, where they showed the same 77% result and the effectiveness in determining a discrete estimate. Gradient boosting showed an accuracy 2% higher than the U-Net for it and was thereby chosen as the main model when developing the prototype. According to the test results, the evaluation of the algorithm in 75% of cases completely coincided with the expert one, and it had a slight deviation from it in another 22% of cases. The developed solution allows for standardizing the data obtained and contributes to the creation of an interlaboratory digital research database.

Neural network atomistic potentials for global energy minima search in carbon clusters.

The global energy optimization problem is an acute and important problem in chemistry. It is crucial to know the geometry of the lowest energy isomer (global minimum, GM) of a given compound for the evaluation of its chemical and physical properties. This problem is especially relevant for atomic clusters. Due to the exponential growth of the number of local minima geometries with the increase of the number of atoms in the cluster, it is important to find a computationally efficient and reliable method to navigate the energy landscape and locate a true global minima structure. Newly developed neural network (NN) atomistic potentials offer a numerically efficient and relatively accurate approach for molecular structure optimization. An important question that needs to be answered is "Can NN potentials, trained on a given set, represent the potential energy surface (PES) of a neighboring domain?". In this work, we tested the applicability of ANI-1ccx and ANI-nr NN atomistic potentials for the global minima optimization of carbon clusters Cn (n = 3-10). We showed that with the introduction of the cluster connectivity restriction and consequent DFT or ab initio calculations, ANI-1ccx and ANI-nr can be considered as robust PES pre-samplers that can capture the GM structure even for large clusters such as C20.

Bismuth Infrared Star: Being at a Glance.

Bismuth polycations have garnered significant attention from researchers due to their extraordinary and counter-intuitive structures and stoichiometries. Despite extensive experimental and theoretical investigations, understanding of the bonding in such clusters remains insufficient. An AdNDP bonding analysis was conducted to elucidate the bonding characteristics using both homoatomic and heteroatomic bismuth clusters with various stoichiometries. Analysis of the calculated nucleus-independent chemical shift data confirmed the aromatic nature of these species. Universal bonding patterns were identified that can be applied to a range of homoatomic and heteroatomic bismuth clusters. Additionally, calculations of absorbance and fluorescence spectra were performed to gain insights into the near-infrared emission and establish a potential correlation between absorbance and the identified bonding patterns.

Analysis of the Ability of C6H5I to Phosphoresce.

The heavy atom effect is a widely utilized strategy to enhance the phosphorescence intensity of organic molecules. Among the preferred choices, heavy halogen atoms such as bromine (Br) or iodine (I) have gained popularity. However, the incorporation of these heavy atoms can introduce challenges due to the presence of unstable excited states that undergo dissociation upon excitation. This study focuses on investigating the excited states of the C6H5I molecule, with particular emphasis on the intricate interplay of spin-orbital coupling effects, employing multireference ab initio methodologies. The absence of phosphorescence in the C6H5I molecule is attributed to the efficient energy transfer to dissociative excited states and the near-zero spin-orbital coupling between stable triplet sub-levels and the ground singlet state. To gain insights into the explicit dynamics of the excited states, the research employed Ehrenfest dynamics driven by real-time time-dependent density functional theory (TDDFT). Furthermore, the study explored the complete active space compositions and various post-CASSCF (complete active space self-consistent field) approaches.

Global Minimum Search and Bonding Analysis of Tl2 Hx and Tl3 Hy (x=0-6; y=0-5) Series.

A remarkable distinction between boron and carbon hydrides lies in their extremely different bonding patterns and chemical reactivity, resulting in diverse areas of application. Particularly, carbon, characterized by classical two-center - two-electron bonds, gives rise to organic chemistry. In contrast, boron forms numerous exotic and non-intuitive compounds collectively called non-classical structures. It is reasonable to anticipate that other elements of Group 13 exhibit their own unusual bonding patterns; however, our knowledge of the hydride chemistry for other elements in Group 13 is much more limited, especially for the heaviest stable element, thallium. In this work, we performed a conformational analysis of Tl2 Hx and Tl3 Hy (x=0-6, y=0-5) series via Coalescence Kick global minimum search algorithm, DFT, and ab initio quantum chemistry methods; we investigated the bonding pattern using the AdNDP algorithm, thermodynamic stability, and stability toward electron detachment. All found global minimum structures are classified as non-classical structures featuring at least one multi-center bond.

On the existence of CO32- microsolvated clusters: a theoretical study.

Microsolvated clusters of multiply charged anions play a crucial role in atmospheric chemistry and some of them were previously registered experimentally. At the same time, there are no experimental observations of [CO3·(H2O)n]2-. The reasons for this may be related to the thermodynamical or kinetical instability of microsolvated CO32- toward autoionization or autoprotonation processes. In this study we theoretically investigate the potential stability of the [CO3·(H2O)n]2- microsolvated clusters from both perspectives - thermodynamic and kinetic - and we claim they are stable toward autoionization and kinetically semi-stable toward autoprotonation. In addition, the behaviour of CO32- anions in bulk water solvent was analysed to highlight important precautions for synthetic purposes.

Exploring the Structure and Bonding of Group 13 Homogeneous and Heterogeneous Hydrides with the X2H4 Stoichiometry: A Theoretical Investigation.

We investigated the structure evolution of the Group 13 hydrides on the example of X2H4 (X = B, Al, Ga, In, Tl) and BAlH4, AlGaH4, GaInH4, and InTlH4 stoichiometries via density functional theory (DFT) and ab initio quantum chemistry methods performing Coalescence Kick (CK) global minimum search and AdNDP chemical bonding analysis. We found that all global minimum structures possess multicenter electron bonds. The difference between the structures of X2H4 stoichiometry for boron and aluminum is much more significant than that between other pairs of Al-Ga, Ga-In, and In-Tl. The evolution of Group 13 hydride structure involves the gradual prevalence of classical 2c-2e bonds over multicenter bonds for heavier elements. The found structural features of the heterogeneous hydrides are in total agreement with those of homogeneous hydrides and with the trends common for the periodic table that allows us to investigate the structural evolution of Group 13 hydrides more completely.

Photoelectron Spectroscopy and Theoretical Study of Di-Copper-Boron Clusters: Cu2B3- and Cu2B4.

Copper has been found to be able to mediate the formation of bilayer borophenes. Copper-boron binary clusters are ideal model systems to probe the copper-boron interactions, which are essential to understand the growth mechanisms of borophenes on copper substrates. Here, we report a joint photoelectron spectroscopy and theoretical study on two di-copper-doped boron clusters: Cu2B3- and Cu2B4-. Well-resolved photoelectron spectra are obtained, revealing the presence of a low-lying isomer in both cases. Theoretical calculations show that the global minimum of Cu2B3- (C2v, 1A1) contains a doubly aromatic B3- unit weakly interacting with a Cu2 dimer, while the low-lying isomer (C2v, 1A1) consists of a B3 triangle with the two Cu atoms covalently bonded to two B atoms at two vertexes. The global minimum of Cu2B4- (D2h, 2Ag) is found to consist of a rhombus B4 unit covalently bonded to the two Cu atoms at two opposite vertexes, whereas in the low-lying isomer (Cs, 2A'), one of the two Cu atoms is bonded to two B atoms.

Overlapping electron density and the global delocalization of π-aromatic fragments as the reason of conductivity of the biphenylene network.

Recently fabricated 2D biphenylene network is an astonishing solid-state material, which possesses unique metal-like conductive properties. At the same time, two-dimensional boron nitride network (2D-BN)-an isoelectronic and structural analogue of biphenylene network, is an insulator with a wide direct bandgap. This study investigates the relationship between the electronic properties and chemical bonding patterns for these species. It is shown that the insulating 2D-BN network possesses a strong localization of electron density on the nitrogen atoms. In turn, for a carbon-containing sheet, we found a highly delocalized electron density and an appreciable overlap of pz orbitals of neighboring C6 rings, which might be a reason for the conductive properties of the material.

Tinning the carbon: hydrostannanes strike back.

Carbon possesses an important ability to be in a valence state of IV, which is essential for organic chemistry and all carbon-based life forms. In turn, tin is usually observed in the valence state of II, although it is a carbon group element. This creates an open question about the possibility of the existence of tin-based "organic" molecules. In this work, we investigate hydro-tin compounds Sn2Hx (x = 1-6) and Sn3Hy (y = 1-8) via DFT and ab initio quantum chemistry methods, studying their global minimum geometry, thermodynamic stability, and chemical bonding patterns. We show that hydrogen-saturated stoichiometries (Sn2H6 and Sn3H8) are exact analogs of hydrocarbons, while unsaturated stoichiometries are characterized by multi-center bonds, aromaticity, and different valence states of tin. In addition, a refined procedure of global geometry minimization based on simulated annealing and ab initio molecular dynamics is proposed.

Superalkali Coated Rydberg Molecules.

A series of complexes of Na, K, NH4, and H3O with [bpy.bpy.bpy]cryptand, [2.2.2]cryptand, and spherical cryptand were investigated via DFT and ab initio methods. We found that by coating Rydberg molecules with the "organic skin" one could further decrease their ionization potential energy, reaching the values of ∼1.5 eV and a new low record of 1.3 eV. The neutral cryptand complexes in this sense possess a weakly bounded electron and may be considered as very strong reducing agents. Moreover, the presence of an organic cage increases the thermodynamic stability of Rydberg molecules making them stable toward the proton detachment.

Sn368-: a 2.7 nm naked aromatic tin rod.

In this work, we synthesize naked tin cluster anion Sn368-, representing the first example of pure Sn nanowire assembled through oxidative coupling reactions of a super atomic cluster Sn94-. Theoretical analysis confirm the presence of aromaticity for each Sn9 unit showing four adjacent aromatic subunits bridged by parallel Sn-Sn bonds.

Symmetry collapse due to the presence of multiple local aromaticity in Ge244.

Understanding the structural changes taking place during the assembly of single atoms leading to the formation of atomic clusters and bulk materials remains challenging. The isolation and theoretical characterization of medium-sized clusters can shed light on the processes that occur during the transition to a solid-state structure. In this work, we synthesize and isolate a continuous 24-atom cluster Ge244-, which is characterized by X-ray diffraction analysis and Energy-dispersive X-ray spectroscopy, showing an elongated structural characteristic. Theoretical analysis reveals that electron delocalization plays a vital role in the formation and stabilization of the prolate cluster. In contrast with carbon atoms, 4 s orbitals of Ge-atoms do not easily hybridize with 4p orbitals and s-type lone-pairs can be localized with high occupancy. Thus, there are not enough electrons to form a stable symmetrical fullerene-like structure such as C24 fullerene. Three aromatic units with two [Ge9] and one [Ge6] species, connected by classical 2c-2e Ge-Ge σ-bonds, are aligned together forming three independent shielding cones and eventually causing a collapse of the global symmetry of the Ge244- cluster.

Theoretical Investigation of Geometries and Bonding of Indium Hydrides in the In2Hx and In3Hy (x = 0-4,6; y = 0-5) Series.

Boron hydrides have been an object of intensive theoretical and experimental investigation for many decades due to their unusual and somewhat unique bonding patterns. Despite boron being a neighboring element to carbon, boron hydrides almost always form non-classical structures with multi-center bonds. However, we expect indium to form its interesting molecules with non-classical patterns, though such molecules still need to be extensively studied theoretically. In this work, we investigated indium hydrides of In2Hx (x = 0-4,6) and In3Hy (y = 0-5) series via DFT and ab initio quantum chemistry methods, performing a global minimum search, chemical bonding analysis, and studies of their thermodynamical stability. We found that the bonding pattern of indium hydrides differs from the classical structures composed of 1c-2e lone pairs and 2c-2e bonds and the bonding pattern of earlier investigated boron hydrides of the BnHn+2 series. The studied stoichiometries are characterized by multi-center bonds, aromaticity, and the tendency for indium to preserve the 1c-2e lone pair.

Extending machine learning beyond interatomic potentials for predicting molecular properties.

Machine learning (ML) is becoming a method of choice for modelling complex chemical processes and materials. ML provides a surrogate model trained on a reference dataset that can be used to establish a relationship between a molecular structure and its chemical properties. This Review highlights developments in the use of ML to evaluate chemical properties such as partial atomic charges, dipole moments, spin and electron densities, and chemical bonding, as well as to obtain a reduced quantum-mechanical description. We overview several modern neural network architectures, their predictive capabilities, generality and transferability, and illustrate their applicability to various chemical properties. We emphasize that learned molecular representations resemble quantum-mechanical analogues, demonstrating the ability of the models to capture the underlying physics. We also discuss how ML models can describe non-local quantum effects. Finally, we conclude by compiling a list of available ML toolboxes, summarizing the unresolved challenges and presenting an outlook for future development. The observed trends demonstrate that this field is evolving towards physics-based models augmented by ML, which is accompanied by the development of new methods and the rapid growth of user-friendly ML frameworks for chemistry.

Occurrence of Double Bond in π-Aromatic Rings: An Easy Way to Design Doubly Aromatic Carbon-Metal Structures.

A chemical bonding of several metallabenzenes and metallabenzynes was studied via an adaptive natural density partitioning (AdNDP) algorithm and the induced magnetic field analysis. A unique chemical bonding pattern was discovered where the M=C (M: Os, Re) double bond coexists with the delocalized 6c-2e π-bonding elements responsible for aromatic properties of the investigated complexes. In opposition to the previous description where 8 delocalized π-electrons were reported in metallabenzenes and metallabenzynes, we showed that only six delocalized π-electrons are present in those molecules. Thus, there is no deviation from Hückel's aromaticity rule for metallabenzynes/metallabenzenes complexes. Based on the discovered bonding pattern, we propose two thermodynamically stable novel molecules that possess not only π-delocalization but also retain six σ-delocalized electrons, rendering them as doubly aromatic species. As a result, our investigation gives a new direction for the search for carbon-metal doubly aromatic molecules.

DFT Study of Microsolvated [NO3·(H2O)n]- (n = 1-12) Clusters and Molecular Dynamics Simulation of Nitrate Solution.

Investigation of the process of the NO3- anion solvation is central to understanding the chemical and physical properties of its aqueous solutions. The importance of this topic can be seen in atmospheric chemistry, as well as in nuclear waste processing research. In this work, we used a particle swarm optimization technique driven by density functional theory to sample the potential energy surface of various microsolvated [NO3·(H2O)n]- (n = 1-12) clusters. We found that the charge transfer plays a crucial role in the stabilization of the investigated species. Moreover, by conducting ab initio molecular dynamics simulations, we showed that at low concentrations (∼0.2 M) the NO3- species tend to be located on the surface of water solution. We also observed that the contact ion pair K+-NO3- undergoes a fast dissociation and each of the ions is solvated separately. As a result, from our calculations, we expect that at low concentration there could be oppositely signed concentration gradients for NO3- and K+ ions in a thin water film.

Photoelectron Spectroscopy of Size-Selected Bismuth-Boron Clusters: BiBn- (n = 6-8).

Because of its low toxicity, bismuth is considered to be a "green metal" and has received increasing attention in chemistry and materials science. To understand the chemical bonding of bismuth, here we report a joint experimental and theoretical study on a series of bismuth-doped boron clusters, BiBn- (n = 6-8). Well-resolved photoelectron spectra are obtained and are used to understand the structures and bonding of BiBn- in conjunction with theoretical calculations. Global minimum searches find that all three BiBn- clusters have planar structures with the Bi atom bonded to the edge of the planar Bn moiety via two Bi-B σ bonds as well as π bonding by the 6pz orbital. BiB6- is found to consist of a double-chain B6 with a terminal Bi atom. Both BiB7- and BiB8- are composed of a Bi atom bonded to the planar global minima of the B7- and B8- clusters. Chemical bonding analyses reveal that BiB6- is doubly antiaromatic, whereas BiB7- and BiB8- are doubly aromatic. In the neutral BiBn (n = 6-8) clusters, except BiB6 which has a planar structure similar to the anion, the global minima of both BiB7 and BiB8 are found to be half-sandwich-type structures due to the high stability of the doubly aromatic B73- and B82- molecular wheel ligands.