Relativistic density functional theory finds that two isomers of a diuranium(III) complex of a polypyrrolic macrocycle (H4L) feature active sites on uranium moieties, allowing for their potential application in activating industrially and economically important small molecules. To address this, a series of adducts [(X)nU2(L)](2–m)+ (X?=?THF, I? and HI; n?=?1 and 2; m?=?0, 1 and 2) have been examined. The coordination from X to the exposed uranium(s) changes the general geometry and electronic structure slightly. Thermodynamic calculations reveal that iodine termination is energetically favored over THF/HI coordination.
Graphical abstract Scalar and spin-orbit coupling relativistic DFT calculation reveals that the active sites on the uranium moieties of [U2(L)]2+ lead to formation of adducts [(THF)nU2(L)]2+, [InU2(L)](2–n)+ and [(HI)nU2(L)]2+ (n?=?1 and 2). Coordination to the exposed uranium(s) changes geometrical and electronic properties slightly, but iodine termination is the most energetically favored.
A density functional theory (DFT) study of cct-As, ccc, and cct-CO isomers of the ruthenium dihydride complex RuH2(CO)2(AsMe2Ph)2 is reported (see Scheme for the labeling isomer 34 structures of RuH2(CO)2(AsMe2Ph)2). Complex geometries and relative energies of different isomers have been calculated with both B3LYP and M06-2X functionals. The results show that the B3LYP calculated Boltzmann populations of cct-As, ccc, and cct-CO isomers are 65.5, 34.2, and 0.3%, respectively. These are in better agreement with the experimental data than those calculated at the M06-2X level. However, the calculations of 1H NMR chemical shifts were found to be better described with M06-2X than with B3LYP or with HF level of theories. In addition, a transition state between the two most stable isomers was determined through DFT/(B3LYP or M06-2X) calculations.
For the first time, the structures, stabilities and electronic properties of alkaline-earth metal doped B44 fullerenes were investigated by means of density functional theory calculations. Our results reveal that M@B44 (M = Ca, Sr, Ba) possess endohedral configurations as their lowest energy structures, whereas the exohedral form is favored when metal is Be or Mg. The large binding energies and sizable HOMO–LUMO gap energies of Ca@B44, Sr@B44 and Ba@B44 suggest the considerable possibility to achieve these novel endohedral borofullerenes experimentally. Born-Oppenheimer molecular dynamics (BO-MD) simulations at various temperatures further confirmed the extreme dynamic stabilities of these endohedral complexes. Their bonding patterns were also analyzed in detail. Finally, we simulated their infrared absorption spectra and 11B nuclear magnetic resonance spectra to help future structural characterization.
Density functional theory (DFT) calculations were carried out to explore the geometric, spectroscopic, and electronic properties of three magic silver clusters Agn (n?=?8, 18, and 20) in detail. The computed results show that the global minima of these clusters are compact, near-spherical structures, while other low-lying isomers exhibit oblate or prolate shapes. Vertical ionization energies for the low-lying isomers were also computed and assigned with respect to available experimental values. Although several isomers were predicted to have similar energies, their electronic and vibrational signatures were quite distinctive, meaning that they could be used as fingerprint signals to distinguish between isomers. In addition, the electronic structures of these systems were explored using the phenomenological shell model. Calculations for the coinage metal clusters M20 (M?=?Cu, Ag, Au) indicated that the structures and properties of the Ag cluster are similar to those of the Cu cluster in that both Cu20 and Ag20 prefer a compact structure whereas Au20 prefers to adopt a tetrahedral form.
The cooperativity effects of the H-bonding interactions in HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane)???HMX???FA (formamide), HMX???HMX???H2O and HMX???HMX???HMX complexes involving the chair and chair–chair HMX are investigated by using the ONIOM2 (CAM-B3LYP/6–31++G(d,p):PM3) and ONIOM2 (M06-2X/6–31++G(d,p):PM3) methods. The solvent effect of FA or H2O on the cooperativity effect in HMX???HMX???HMX are evaluated by the integral equation formalism polarized continuum model. The results show that the cooperativity and anti-cooperativity effects are not notable in all the systems. Although the effect of solvation on the binding energy of ternary system HMX???HMX???HMX is not large, that on the cooperativity of H-bonds is notable, which leads to the mutually strengthened H-bonding interaction in solution. This is perhaps the reason for the formation of different conformation of HMX in different solvent. Surface electrostatic potential and reduced density gradient are used to reveal the nature of the solvent effect on cooperativity effect in HMX???HMX???HMX.
Performance of 18 DFT functionals (B1B95, B3LYP, B3PW91, B97D, BHandHLYP, BMK, CAM-B3LYP, HSEh1PBE, M06-L, mPW1PW91, O3LYP, OLYP, OPBE, PBE1PBE, tHCTHhyb, TPSSh, wB97xD, VSXC) in combinations with six basis sets (cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, aug-cc-pVTZ, IGLO-II, and IGLO-III) and three methods for calculating magnetic shieldings (GIAO, CSGT, IGAIM) was tested for predicting 1H and 13C chemical shifts for 25 organic compounds, for altogether 86 H and 88 C atoms. Proton shifts varied between 1.03 ppm to 12.00 ppm and carbon shifts between 7.87 ppm to 209.28 ppm. It was found that the best method for calculating 13C shifts is PBE1PBE/aug-cc-pVDZ with CSGT or IGAIM approaches (mae?=?1.66 ppm), for 1H the best results were obtained with HSEh1PBE, mPW1PW91, PBE1PBE, CAM-B3LYP, and B3PW91 functionals with cc-pVTZ basis set and with CSGT or IGAIM approaches (mae?=?0.28 ppm). We found that often larger basis sets do not give better results for chemical shifts. The best basis sets for calculating 1H and 13C chemical shifts were cc-pVTZ and aug-cc-pVDZ, respectively. CSGT and IGAIM NMR approaches can perform really well and are in most cases better than popular GIAO approach.
Graphical Abstract Mean absolute errors for 1H and 13C chemical shifts and computational times of neutral toluene molecule with aug-cc-pVDZ basis set and CSGT approach
A perfectly planar Al13+ cluster (CI) and a quasi-planar Al13+ cluster (CII) have been found for the first time. Both clusters have a triangular core surrounded by a set of ten Al atoms in the form of a ring. These cationic clusters have substantial aromatic character. The planar CI cluster has local antiaromatic patches within global aromatic sea. It is doubly aromatic having both σ and π aromatic character. The quasi-planar CII cluster is also aromatic but it has more σ-delocalization.
The dual role of the ionic liquid 1-butyl-3-methyl-imidazolium trifluoroacetic acid ([C4mim]TFA) as an extractant for thiophene (TH) and a catalyst for the oxidation of TH was explored at the molecular level by performing density functional theory (DFT) calculations. The calculated interaction energies demonstrated why [C4mim]TFA is a better extractant for thiophene sulfone (THO2) than for TH. Two pathways were proposed for the oxidation of TH to THO2 with [C4mim]TFA acting as a catalyst. In the dominant pathway, a peracid is formed which then oxidizes TH to the sulfoxide and sulfones. The presence of [C4mim]TFA was found to greatly reduce the barrier to the oxidative desulfurization (ODS) of TH using H2O2 as an oxidant.
Much effort has been devoted to investigating the molecular geometries, electronic structures, redox properties and nonlinear optical (NLO) properties of Ir complexes involving o-, m- or p-carborane groups by density functional theory (DFT) methods. Switchable second-order NLO properties were induced by redox processes involving these complexes, and it was found that mainly the coordination bonds of Ir complexes changed during the oxidation process. Our calculations revealed that oxidation reactions have a significant influence on the second-order NLO response owing to the change in charge transfer pattern. The βtot values of oxidized species are at least ～9 times larger for set I and ～5 times larger for set II than those of the corresponding parent complexes. Introduction of carborane groups into ppy (phenylpyridine) ligands can enhance the second-order NLO response by 1.2–?1.6 times by a metal-to-ligand charge transfer (MLCT) transition between the Ir atom and carborane. The βtot of complex 2 [(ppy)2Ir(phen)]+ (phen?=?phenanthroline) is 3.3 times larger than that of complex 1 (ppy)2Ir(acce) (acce?=?acetylacetonate), which is caused by ligand-to-ligand charge transfer (LLCT) between ppy ligands and the ancillary ligand. Therefore, it can be concluded that the second-order NLO response can be effectively enhanced by oxidation reactions.
A mechanistic investigation using Becke3LYP density functional theory (DFT) was carried out on the palladium-catalyzed amidition of bromobenzene and tBu-isocyanide. The whole catalytic cycle consists of five steps: oxidative addition, migratory insertion, anion exchange, reductive elimination, and hydrogen migration. The rate-determining step is oxidative addition, with a small Gibbs free energy of 14.6 kcal mol?1. In the migratory insertion step, tBu-isocyanide provides an important source of carboxy and amino groups to establish the amide group. For anion exchange, path 1a is suggested as the most favorable pathway with the help of the base, and water provides a source of oxygen which is perfectly in line with experimental observations. Finally, in the hydrogen migration step, we illustrate that the six-membered ring path is energetically favored due to the assisting influence of water. In addition, our calculations indicate that using dimethyl sulfoxide as a solvent does not change the rate-determining step.
In advanced oxidation processes (AOPs), the detailed degradation mechanisms of a typical explosive of 2,4-dinitrotoluene (DNT) can be investigated by the density function theory (DFT) method at the SMD/M062X/6-311+G(d) level. Several possible degradation routes for DNT were explored in the current study. The results show that, for oxidation of the methyl group, the dominant degradation mechanism of DNT by hydroxyl radicals (?OH) is a series of sequential H-abstraction reactions, and the intermediates obtained are in good agreement with experimental findings. The highest activation energy barrier is less than 20 kcal mol?1. Other routes are dominated by an addition-elimination mechanism, which is also found in 2,4,6-trinitrotoluene, although the experiment did not find the corresponding products. In addition, we also eliminate several impossible mechanisms, such as dehydration, HNO3 elimination, the simultaneous addition of two ?OH radials, and so on. The information gained about these degradation pathways is helpful in elucidating the detailed reaction mechanism between nitroaromatic explosives and hydroxyl radicals for AOPs.
Graphical Abstract The degradation mechanism of an important explosive, 2,6-dinitrotoluene (DNT), by the hydroxyl radical for advanced oxidation progresses
A topological analysis based on density functional electronic and spin densities of the bonding characteristics in a series of Fe, Ru, Os, Tc and Rh dimers and trimers bridged, respectively, by μ-1,8-naphthyridine (nap) and μ-2,2′-dipyridylamine (dpa) is presented. By this simple qualitative analysis, we were able to determine the electronic ground state and correlated bonding order for a number of complexes potentially involved in extended metal atom chains (EMAC). Furthermore, we showed in the Ru dimer that it was possible to control the spin state simply by changing the bonded counter-anion.
MP2/aug-cc-pVTZ calculations were performed for complexes linked by hydrogen bonds. Three types of proton donating species were taken into account: H2O, CCl3H, and H3O+. These calculations are supported by the natural bond orbital (NBO) method and the quantum theory of atoms in molecules (QTAIM) approach. Numerous correlations between parameters of H-bonded systems were found. The most important are those which show the response of the system on the H-bond formation; for example, the increase of polarization of the A-H bond correlates with the strength of the hydrogen bond. Similar relationships were found for the σ-hole bonds while the π-hole bonds do not follow the trends known for the hydrogen bonds.
A dispersion correction is introduced and tested for MNDO. The shift in electron density caused by the interaction between oscillating dipoles in the London picture of dispersion is mimicked by adding a small r?7-dependent attractive nucleus–electron potential to the core Hamiltonian. This potential results in a shift in electron density similar to that used by Feynman to explain dispersion. The resulting parameterized self-consistent and inherently multicenter treatment (MNDO-F) gives good results for CHNO compounds that do not exhibit hydrogen bonds, which MNDO cannot reproduce. This “Feynman” dispersion correction is also applicable to Hartree–Fock and density functional theory.
Ionic hydrocarbon compounds that contain hypercarbon atoms, which bond to five or more atoms, are important intermediates in chemical synthesis and may also find applications in hydrogen storage. Extensive investigations have identified hydrocarbon compounds that contain a five- or six-coordinated hypercarbon atom, such as the pentagonal-pyramidal hexamethylbenzene, C6(CH3)62+, in which a hexacoordinate carbon atom is involved. It remains challenging to search for further higher-coordinated carbon in ionic hydrocarbon compounds, such as seven- and eight-coordinated carbon. Here, we report ab initio density functional calculations that show a stable 3D hexagonal-pyramidal configuration of tropylium trication, (C7H7)3+, in which a heptacoordinate carbon atom is involved. We show that this tropylium trication is stable against deprotonation, dissociation, and structural deformation. In contrast, the pyramidal configurations of ionic C8H8 compounds, which would contain an octacoordinate carbon atom, are unstable. These results provide insights for developing new molecular structures containing hypercarbon atoms, which may have potential applications in chemical synthesis and in hydrogen storage.
Graphical abstract Possible structural transformations of stable configurations of (C7H7)3+, which may result in the formation of the pyramidal structure that involves a heptacoordinate hypercarbon atom.
The mechanistic details of N-heterocyclic olefin-catalyzed formation of cyclic carbonate from CO2 and propargylic alcohols were investigated by DFT calculations. Six mechanisms, four for the formation of five-membered cyclic carbonate (M-A, M-B, M-B’ and M-C), and two for six-membered cyclic carbonate (M-D and M-E), were fully investigated. The energy profiles in dichloromethane showed that M-B is the predominant reaction with the lowest barrier of 31.99 kcal mol?1, while M-C and M-D may be kinetically competitive to M-B. The very high activation energy of 45.37 kcal mol-1, 57.07 kcal mol-1 and 59.61 kcal mol?1 for M-A, M-B’ and M-E, respectively, suggest that they are of lesser importance in the overall mechanism.
Graphical abstract Formations of five-membered ring product and six-membered ring product are kinetically competitive, but five-membered ring product is thermodynamically more preferable.
This paper inquires the C60 capabilities to contain radio-iodide (131I2) molecules. The encapsulation conditions are investigated applying first principles method to simulate with geometric optimizations and molecular dynamics at 310 K and atmospheric pressure. We find that the n131I2@C60 system, where n?=?1, 2, 3…, is stable if the content does not exceed three molecules of radio-iodide. The application of density functional theory allows us to determine that, the nanocapsules content limit is related with the amount of charge that is transferred from the iodine 131I2 molecules to the carbon atoms in the fullerene surface. The Mulliken population analysis reveals that the excess of charge increases the repulsive forces between atoms and the bond length average in the C60 structure. The weakened bonds easily break and will critically damage the encapsulation properties. Additionally, we test the interaction nanocapsules with different amounts of radioactive iodine diatomic molecules content with calcium atoms, and find that only the fullerene containing one radioactive iodine diatomic molecule was able to interact with up to nine atoms of calcium without disrupting or cracking. Other fullerenes with two and three radio iodine diatomic molecules cannot resist the interaction with a single calcium atom without cracking or being broken.
Magnetic shielding constants for an isolated fullerene C60, cucurbituril CB[9], and the host-guest complex C60@CB[9] were calculated as a function of separation of the monomers. Our results in the gas phase and water indicate a significant variation of the magnetic properties for all atoms of the monomers in the complex and after liberation of fullerene C60 from the interior of the CB[9] cavity. The interaction between the two monomers results in a charge transfer that collaborates with a redistribution of electron density to deshield the monomers.
Natural bond orbital (NBO) analyses and dissected nucleus-independent chemical shifts (NICSπzz) were computed to evaluate the bonding (bond type, electron occupation, hybridization) and aromatic character of the three lowest-lying Si2CH2 (1-Si, 2-Si, 3-Si) and Ge2CH2 (1-Ge, 2-Ge, 3-Ge) isomers. While their carbon C3H2 analogs favor classical alkene, allene, and alkyne type bonding, these Si and Ge derivatives are more polarizable and can favor “highly electron delocalized”? and “non-classical”? structures. The lowest energy Si 2CH2 and Ge 2CH2 isomers, 1-Si and 1-Ge, exhibit two sets of 3–center 2–electron (3c-2e) bonding; a π-3c-2e bond involving the heavy atoms (C–Si–Si and C–Ge–Ge), and a σ-3c-2e bond (Si–H–Si, Ge–H–Ge). Both 3-Si and 3-Ge exhibit π and σ-3c-2e bonding involving a planar tetracoordinated carbon (ptC) center. Despite their highly electron delocalized nature, all of the Si2CH2 and Ge2CH2 isomers considered display only modest two π electron aromatic character (NICS(0)πzz=--6.2 to –8.9 ppm, computed at the heavy atom ring center) compared to the cyclic-C 3H2 (–13.3 ppm).
