Evaluation of bismuth-based dispersion energy donors - synthesis, structure and theoretical study of 2-biphenylbismuth(iii) derivatives.

A series of 2-biphenyl bismuth(iii) compounds of the type (2-PhC6H4)3-nBiXn [n = 0 (1); n = 1, X = Cl (2), Br (3), I (4), Me (5); n = 2, X = Cl (6), Br (7), I (8)] has been synthesized and analyzed with focus on intramolecular London dispersion interactions. The library of the compounds was set up in order to investigate the Biπ arene interaction by systematic variation of X. The structural analysis in the solid state revealed that the triarylbismuth(iii) compound 1 shows an encapsulation of the metal atom but the distances between the bismuth atom and the phenyl centroids amount to values close to or larger than 4.0 Å, which is considered to be a rather week dispersion interaction. In the case of monomeric diorganobismuth(iii) compounds 2-5 the moderate crowding effectively hinders the formation of intermolecular donor-acceptor interactions, but allows for intramolecular dispersion-type interactions with the 2-biphenyl ligand. In contrast, the structures of the monoorganobismuth compounds 6-8 show the formation of Bi-XBi donor-acceptor bonds leading to the formation of 1D ribbons in the solid state. These coordination bonds are accompanied by intermolecular dispersion interactions with BiPhcentroid distances < 4.0 Å. In solution the diorganobismuth(iii) halides 2-4 show a broadening of their NMR signals (H-8, H-8' and H-9, H-9' protons of the 2-biphenyl ligand), which is a result of dynamic processes including ligand rotation. For further elucidation of these processes compounds 2, 4 and 7 were studied by temperature-dependent NMR spectroscopy. Electronic structure calculations at the density functional theory and DLPNO-coupled cluster level of theory were applied to investigate and quantify the intramolecular London dispersion interactions, in an attempt to distinguish between basic intramolecular interactions and packing effects and to shed light on the dynamic behavior in solution.

The first microsolvation step for furans: New experiments and benchmarking strategies.

The site-specific first microsolvation step of furan and some of its derivatives with methanol is explored to benchmark the ability of quantum-chemical methods to describe the structure, energetics, and vibrational spectrum at low temperature. Infrared and microwave spectra in supersonic jet expansions are used to quantify the docking preference and some relevant quantum states of the model complexes. Microwave spectroscopy strictly rules out in-plane docking of methanol as opposed to the top coordination of the aromatic ring. Contrasting comparison strategies, which emphasize either the experimental or the theoretical input, are explored. Within the harmonic approximation, only a few composite computational approaches are able to achieve a satisfactory performance. Deuteration experiments suggest that the harmonic treatment itself is largely justified for the zero-point energy, likely and by design due to the systematic cancellation of important anharmonic contributions between the docking variants. Therefore, discrepancies between experiment and theory for the isomer abundance are tentatively assigned to electronic structure deficiencies, but uncertainties remain on the nuclear dynamics side. Attempts to include anharmonic contributions indicate that for systems of this size, a uniform treatment of anharmonicity with systematically improved performance is not yet in sight.

Balancing Donor-Acceptor and Dispersion Effects in Heavy Main Group Element π Interactions: Effect of Substituents on the Pnictogen⋠⋠⋠π Arene Interaction.

High-level ab initio calculations using the DLPNO-CCSD(T) method in conjunction with the local energy decomposition (LED) were performed to investigate the nature of the intermolecular interaction in bismuth trichloride adducts with π arene systems. Special emphasis was put on the effect of substituents in the aromatic ring. For this purpose, benzene derivatives with one or three substituents (R=NO2 , CF3 , OCHO, OH, and NH2 ) were chosen and their influence on donor-acceptor interaction as well as on the overall interaction strength was examined. Local energy decomposition was performed to gain deeper insight into the composition of the interaction. Additionally, the study was extended to the intermolecular adducts of arsenic and antimony trichloride with benzene derivatives having one substituent (R=NO2 and NH2 ) in order to rationalize trends in the periodic table. The analysis of natural charges and frontier molecular orbitals shows that donor-acceptor interactions are of π→σ* type and that their strength correlates with charge transfer and orbital energy differences. An analysis of different bonding motifs (Bi⋅⋅⋅π arene, Bi⋅⋅⋅R, and Cl⋅⋅⋅π arene) shows that if dispersion and donor-acceptor interaction coincide as the donor highest occupied molecular orbital (HOMO) of the arene is delocalized over the π system, the M⋅⋅⋅π arene motif is preferred. If the donor HOMO is localized on the substituent, R⋅⋅⋅π arene bonding motifs are preferred. The Cl⋅⋅⋅π arene bonding motif is the least favorable with the lowest overall interaction energy.

High-Level Ab Initio Calculations of Intermolecular Interactions: Heavy Main-Group Element π-Interactions.

This work reports high-level ab initio calculations and a detailed analysis on the nature of intermolecular interactions of heavy main-group element compounds and π systems. For this purpose we have chosen a set of benchmark molecules of the form MR3 , in which M=As, Sb, or Bi, and R=CH3 , OCH3 , or Cl. Several methods for the description of weak intermolecular interactions are benchmarked including DFT-D, DFT-SAPT, MP2, and high-level coupled cluster methods in the DLPNO-CCSD(T) approximation. Using local energy decomposition (LED) and an analysis of the electron density, details of the nature of this interaction are unraveled. The results yield insight into the nature of dispersion and donor-acceptor interactions in this type of system, including systematic trends in the periodic table, and also provide a benchmark for dispersion interactions in heavy main-group element compounds.

Nitrogen-Functionalized Hydrothermal Carbon Materials by Using Urotropine as the Nitrogen Precursor.

Nitrogen-containing hydrothermal carbon (N-HTC) materials of spherical particle morphology were prepared by means of hydrothermal synthesis with glucose and urotropine as precursors. The molar ratio of glucose to urotropine has been varied to achieve a continuous increase in nitrogen content. By raising the ratio of urotropine to glucose, a maximal nitrogen fraction of about 19 wt % could be obtained. Decomposition products of both glucose and urotropine react with each other; this opens up a variety of possible reaction pathways. The pH has a pronounced effect on the reaction pathway of the corresponding reaction steps. For the first time, a comprehensive analytical investigation, comprising a multitude of analytical tools and instruments, of a series of nitrogen-containing HTC materials was applied. Functional groups and structural motifs identified were analyzed by means of FTIR spectroscopy, thermogravimetric MS, and solid-state NMR spectroscopy. Information on reaction mechanisms and structural details were obtained by electronic structure calculations that were compared with vibrational spectra of polyfuran or polypyrrole-like groups, which represent structural motifs occurring in the present samples.

Computational Study of the Isomerization Reactions of Borirane.

Borirane is isoelectronic to the cyclopropyl cation, but is stable toward electrocyclic ring opening to 2-bora-propa-1,3-diyl A, the boron analogue of the allyl cation. A computational investigation using density functional theory (B3LYP) in combination with highly correlated electronic structure theory methods of the coupled-cluster [CCSD(T)] and multireference configuration interaction (MRCI) type in conjunction with basis sets of up to quadruple-ζ quality reveal that formation of A is endothermic by roughly 15 kcal mol-1 and that A collapses almost without barrier (0.2 kcal mol-1) to borirane. The vinylborane isomer B is more stable than borirane and its formation is associated with a barrier of 36-38 kcal mol-1. Methyl methylideneborane E, only slightly less stable than B, can only be accessed by pathways involving barriers of at least 60 kcal mol-1.

Evaluation of dispersion type metal···π arene interaction in arylbismuth compounds - an experimental and theoretical study.

The dispersion type Bi···π arene interaction is one of the important structural features in the assembly process of arylbismuth compounds. Several triarylbismuth compounds and polymorphs are discussed and compared based on the analysis of single crystal X-ray diffraction data and computational studies. First, the crystal structures of polymorphs of Ph3Bi (1) are described emphasizing on the description of London dispersion type bismuth···π arene interactions and other van der Waals interactions in the solid state and the effect of it on polymorphism. For comparison we have chosen the substituted arylbismuth compounds (C6H4-CH═CH2-4)3Bi (2), (C6H4-OMe-4)3Bi (3), (C6H3-t-Bu2-3,5)3Bi (4) and (C6H3-t-Bu2-3,5)2BiCl (5). The structural analyses revealed that only two of them show London dispersion type bismuth···π arene interactions. One of them is the styryl derivative 2, for which two polymorphs were isolated. Polymorph 2a crystallizes in the orthorhombic space group P212121, while polymorph 2b exhibits the monoclinic space group P21/c. The general structure of 2a is similar to the monoclinic C2/c modification of Ph3Bi (1a), which leads to the formation of zig-zag Bi-arenecentroid ribbons formed as a result of bismuth···π arene interactions and π···π intermolecular contacts. In the crystal structures of the polymorph 2b as well as for 4 bismuth···π arene interactions are not observed, but both compounds revealed C-HPh···π intermolecular contacts, as likewise observed in all of the three described polymorphs of Ph3Bi. For compound 3 intermolecular contacts as a result of coordination of the methoxy group to neighboring bismuth atoms are observed overruling Bi···π arene contacts. Compound 5 shows a combination of donor acceptor Bi···Cl and Bi···π arene interactions, resulting in an intermolecular pincer-type coordination at the bismuth atom. A detailed analysis of three polymorphs of Ph3Bi (1), which were chosen as model systems, at the DFT-D level of theory supported by DLPNO-CCSD(T) calculations reveals how van der Waals interactions between different structural features balance in order to stabilize molecular arrangements present in the crystal structure. Furthermore, the computational results allow to group this class of compounds into the range of heavy main group element compounds which have been characterized as dispersion energy donors in previous work.

Ring Enlargement of Three-Membered Boron Heterocycles upon Reaction with Organic π Systems: Implications for the Trapping of Borylenes.

New low-energy pathways for the reaction between substituted boriranes and borirenes with unsaturated hydrocarbons (ethyne or ethene) were discovered using density functional and coupled cluster theory. The interaction between the π bond of the hydrocarbon and the empty p orbital of the boron center leads to ring expansion of the three-membered to a five-membered boron heterocycle. The reactions are strongly exothermic and have low or even no barriers. They involve intermediates with a pentacoordinate boron center with two hydrocarbon molecules coordinating to boron akin to metal-olefin complexes. These borylene complexes are shallow minima on the potential energy surfaces. But significantly higher barriers for ring formation are computed for 1,5-cyclooctadiene and dibenzocyclooctatetraene complexes of borylenes, making these complexes likely detectable under appropriate experimental conditions. Our computational findings have implications for the interpretation of trapping experiments of thermally generated small borylenes with excess of small π systems. Because of very low barriers for reactions of three-membered boron heterocycles with π systems and the at least locally large excess of the latter under such conditions, formation of five-membered boron heterocycles should be considered.

Electronically Excited States of Borylenes.

Borylenes, RB, are elusive reactive intermediates. Still not much is known about their excited states from spectroscopic experiments, and existing knowledge is limited to diatomic borylenes only. The electronic structure and geometry of borylenes with diverse substituents on boron (where R = H, F, Cl, CH3, CF3, tBu, NH2, Ph, and SiMe3) were studied by means of computational chemistry. For this purpose, geometries of borylenes in their lowest singlet and triplet states were optimized at the B3LYP/def2-TZVP level of theory. Additionally, the influence of substitution on the energies of frontier molecular orbitals, HOMO-LUMO energy gaps, singlet-triplet energy splittings, and excitation energies was investigated. Two lowest vertical singlet-singlet excitations were computed using EOM-CCSD and TD-DFT (using hybrid B3LYP, and long-range separated CAM-B3LYP and ωB97X functionals) in combination with the aug-cc-pVTZ basis set. The electronic transitions involve excitations from nonbonding sp boron orbital (HOMO) mainly to empty p(B) orbitals and partially to the orbitals of the substituent, and are of n → π* type. The results can facilitate prospective identification of borylenes, e.g., in UV-vis matrix isolation or time-resolved spectroscopy experiments.

Computational study of van der Waals complexes between borylenes and hydrocarbons.

The addition of borylenes (RB) to prototypical carbon-carbon multiple bonds (ethyne, ethene) and the insertion into a C-H bond of methane involves weakly bound van der Waals complexes of the reaction partners according to computational chemistry methods. Geometries of all complexes were optimized using spin-component scaled second-order Møller-Plesset perturbation theory (SCS-MP2) in combination with a quadruple-ζ (def2-QZVP) basis set. Energies were further refined using the coupled-cluster (CCSD(T)) method in combination with basis sets up to quadruple-ζ quality (def2-QZVP and aug-cc-pVTZ). All of the complexes of borylenes studied correspond to shallow minima on their potential-energy surfaces. Borylene complexes with ethyne are the most stable and those with methane are the least stable ones. Aminoborylene complexes BNHR with ethyne and ethene are stabilized mainly by NH⋅⋅⋅π interactions. Symmetry-adapted perturbation theory (SAPT) was performed to analyze the nature of the interaction between borylene molecules and hydrocarbons. Most of the ethyne complexes are dominated by electrostatic interactions, whereas for most of the ethene and all of the methane complexes the interaction is mainly dispersive.

Occurrence of plant hormones in composts made from organic fraction of agri-food industry waste.

Utilizing the organic fraction of agri-food industry waste for fertilization represents one approach to waste management, with composting emerging as a popular method. Composts derived from this waste may contain plant hormones alongside primary macronutrients. This study aimed to evaluate the content of plant hormones in composts crafted from the organic fraction of agri-food industry waste. The presence of these substances was ascertained using liquid chromatography-mass spectrometry (LC-MS) analysis, applied to extracted samples from three composts produced in a bioreactor and three obtained from companies. The results indicate the presence of 35 compounds, which belong to six types of plant hormones: auxins, cytokinins, gibberellins, brassinosteroids, abscisic acid, and salicylic acid, in composts for the first time. The highest amount of plant hormones was noted in buckwheat husk and biohumus extract (35 compounds), and the lowest in hemp chaff and apple pomace (14 compounds). Brassinosteroids (e.g., brassinolide, 28-homobrassinolide, 24-epicastasterone, 24-epibrassinolide, and 28-norbrassinolide) and auxins (e.g., indolilo-3-acetic acid) are dominant. The highest concentration of total phytohormones was reported in biohumus extract (2026.42 ng g-1 dry weight), and the lowest in organic compost (0.18 ng g-1 dry weight).

Mechanisms for the formation of acenes from α-diketones by bisdecarbonylation.

The bisdecarbonylation of bridged α-diketones has turned into an important reaction for the photochemical generation of higher acenes, in particular under matrix isolation conditions. Here, a computational mechanistic analysis of the bisdecarbonylation of dibenzobicyclo[2.2.2]octadienedione 2 to anthracene 1 is presented. The study employed the B3LYP functional in conjunction with the 6-311+G** basis set for geometry optimization on the S0, S1, and T1 potential energy surfaces as well as coupled cluster [CCSD(T)] and second-order multireference perturbation theory (MRMP2) with the cc-pVDZ basis set for evaluation of energies of intermediates and transition states. The first step of the most favorable pathway on the T1 surface has a barrier of 12 kcal mol(­1) with respect to the T1 minimum 2-3B1 and involves cleavage of the C­C bond between the bridgehead and one carbonyl atom, C(bridge)­C(O), yielding a biradical intermediate (INT1-T). On the S1 surface, the analogous step only has a barrier of less than 4 kcal mol(­1). A conical intersection of the S1 with the S0 surface exists after the transition state and provides a means for relaxation to a biradical intermediate (INT1-S) on the S0 surface. The concerted loss of two CO molecules from INT1-S has only a very small barrier. Similarly, consecutive loss of two CO molecules from the triplet state of this diradical (INT1-T) to give triplet anthracene is more favorable than ejection of triplet ethylenedione. Thus, the features identified computationally on the S0, S1, and T1 potential energy surface agree with earlier experimental observations of a fast photobisdecarbonylation within 7 ns from the triplet and singlet states of 2 and a lack of triplet ethylenedione formation.

Reactivity of borylenes toward ethyne, ethene, and methane.

The electronic and geometric structure of various substituted borylenes BR (where R = H, F, Cl, Br, CH(3), Ph, NH(2), NHMe, and NMe(2)) in their lowest singlet and triplet electronic states was investigated by computational means using hybrid density functional (B3LYP) and second-order Møller-Plesset perturbation theories combined with 6-311+G** and cc-pVTZ basis sets. The reactivity of singlet borylenes towards prototypical saturated and unsaturated hydrocarbons was examined by the MP2 method in conjugation with the cc-pVTZ basis set and also by coupled cluster [CCSD(T)] computations in combination with the aug-cc-pVTZ basis set. To study the energetics and the mechanism of the addition reaction of borylenes to unsaturated CC bonds, ethyne and ethene are chosen as model compounds. The insertion reaction of borylene into a C-H bond of methane was also investigated. The addition reactions of borylenes to multiple C-C bonds are strongly exothermic. In case of the BH molecule the reactions proceed without barrier and are the most exothermic. For the insertion reaction of borylenes into methane, two approaches could be identified. Again, the smallest reaction barriers and highest reaction energies were computed for the BH insertion, while the highest barriers and the smallest exothermicities were obtained for the BF molecule. On the basis of frontier molecular orbital energies, barrier heights, reaction energies, and transition state geometries BH is the most electrophilic borylene, followed by BPh, while aminoborylenes and BF are the most nucleophilic ones among the investigated derivatives. Accordingly, reactions of BH have the smallest barriers (if there is one at all) and the largest reaction energies, while the reactions of BF have the highest barriers and the smallest reaction energies.