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.
Second group metal dimers can replace the carbon atom in benzene to form metallabenzene (C5H6M2) compounds. These complexes possess some aromatic character and promising hydrogen adsorption properties. In this study, we investigated the aromatic character of these compounds using aromaticity indices and molecular orbital analysis. To determine the nature of interactions between hydrogen and the metallic center, variation-perturbational decomposition of interaction energy was applied together with ETS-NOCV analysis. The results obtained suggest that the aromatic character comes from three π orbitals located mainly on the C5H5? fragment. The high hydrogen adsorption energy (up to 6.5 kcal mol?1) results from two types of interaction. In C5H6Be2, adsorption is controlled by interactions between the empty metal orbital and the σ orbital of the hydrogen molecule (Kubas interaction) together with corresponding back-donation interactions. Other C5H6M2 compounds adsorb H2 due to Kubas interactions enhanced by H2–π interactions.
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.
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.
We present a theoretical study on the detailed mechanism and kinetics of the H+HCN →H+HNC process. The potential energy surface was calculated at the complete basis set quantum chemical method, CBS-QB3. The vibrational frequencies and geometries for four isomers (H2CN, cis-HCNH, trans-HCNH, CNH2), and seven saddle points (TSn where n = 1 ? 7) are very important and must be considered during the process of formation of the HNC in the reaction were calculated at the B3LYP/6-311G(2d,d,p) level, within CBS-QB3 method. Three different pathways (PW1, PW2, and PW3) were analyzed and the results from the potential energy surface calculations were used to solve the master equation. The results were employed to calculate the thermal rate constant and pathways branching ratio of the title reaction over the temperature range of 300 up to 3000 K. The rate constants for reaction H + HCN → H + HNC were fitted by the modified Arrhenius expressions. Our calculations indicate that the formation of the HNC preferentially occurs via formation of cis–HCNH, the fitted expression is kPW2(T) = 9.98 × 10?22T2.41 exp(?7.62 kcal.mol?1/RT) while the predicted overall rate constant kOverall(T) = 9.45 × 10?21T2.15 exp(?8.56 kcal.mol?1/RT) in cm3 molecule?1s?1.
Graphical Abstract (a) Potential energy surface, (b) thermal rate constants as a function of temperature and (c) the branching ratios (%) of PW1, PW2, PW3 pathways involved in rm H + HCN → H + HNC process.
In order to study the effects of N-oxide on structure and performance, six categories of energetic compounds were systemically investigated. The results indicated that the C–C bonds in the rings were shortened, and the C–N bonds close to the N?→?O bond were elongated when N atoms was oxidized to form N?→?O bonds. N?→?O bonds can increase the densities of most categories of compounds, and the increment will increase with the number of N?→?O bonds. As to their detonation performances, almost all categories of compounds had an increased trend, except for some NO2-, NHNO2- and ONO2-substituted compounds. The contribution of 1,2,3,4-tetrazine and 1,2,4,5-tetrazine to performances was better than that of pyrazine and [1,2,5] oxadiazolo [3,4-b] pyrazine on the whole, and the groups, especially energetic groups, made a huge contribution to performance. When R was a NH2 or ONO2 group, all compounds had lower impact sensitivities, and thus represent candidates for novel energetic compounds. However, other than the sixth category of compounds, all compounds had higher impact sensitivities when R was a NO2 or NHNO2 group, and have little significance in application.
Graphical abstract To study the effects of N-oxide on the structure and performance of energetic compounds, and to propose theoretical guidance for the design of novel compounds, the six categories (94 species) listed in the figure were investigated systemically by density functional theory methods and some empirical formulae
Theoretical calculations for the first tri-iron-based extended metal atom chain (EMAC) molecule are reported. The studied triple-high-spin (S?=?6) complex exhibits ferromagnetic ordering (according to Ising and spin-projection approximations), which renders it unique among all previously prepared and theoretically calculated EMAC compounds. This ordering originates from the prevailing ferromagnetic nearest-neighbor interactions, while the magnetic superexchange between terminal Fe2+ sites is weaker and antiferromagnetic. Calculations indicate that this linear chain system based on a tri-iron core shows potential for the development of spin-frustrated behavior, which could be achieved through rational modification of the equatorial and axial ligands.
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 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 computational study of metal difluorides (MF2; M = Ca to Zn) and their interactions with carbon dioxide and water molecules was performed. The structural parameter values obtained and the results of AIM analysis and energy decomposition analysis indicated that the Ca–F bond is weaker and less ionic than the bonds in the transition metal difluorides. A deformation density plot revealed the stablizing influence of the Jahn–Teller effect in nonlinear MF2 molecules (e.g., where M= Sc, Ti, Cr). An anaysis of the metal K-edge peaks of the difluorides showed that shifts in the edge energy were due to the combined effects of the ionicity, effective nuclear charge, and the spin state of the metal. The interactions of CO2 with ScF2 (Scc3 geometry) and TiF2 (Tic2 geometry) caused CO2 to shift from its usual linear geometry to a bent geometry (η2(C=O) binding mode), while it retained its linear geometry (η1(O) binding mode) when it interacted with the other metal difluorides. Energy decomposition analysis showed that, among the various geometries considered, the Scc3 and Tic2 geometries possessed the highest interaction energies and orbital interaction energies. Heavier transition metal difluorides showed stronger affinities for H2O, whereas the lighter transition metal (Sc and Ti) difluorides preferred CO2. Overall, the results of this study suggest that fluorides of lighter transition metals with partially filled d orbitals (e.g., Sc and Ti) could be used for CO2 capture under moist conditions.
We designed three novel cage energetic anions by introducing ionic bridges containing NΘ, N(OΘ) and N(NΘNO2) into cis-2,4,6,8-tetranitro-1H,5H-2,4,6,8- tetraazabicyclo[3.3.0] octane (bicyclo-HMX or BCMHX). The properties of 21 energetic salts, based on cage anions and ammonium-based cations, were studied by density functional theory (DFT) and volume-based thermodynamics (VBT) calculations. Compared to the parent nonionic BCHMX, most title salts have lower predicted impact sensitivities, higher predicted densities, larger predicted heats of formation (HOFs) and better predicted detonation properties. In particular, 11 energetic salts not only exhibit excellent predicted energetic properties, superior to 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (CL-20), but also have lower predicted sensitivity than CL-20. The best salt had a predicted detonation velocity of 10.06 km s?1, a predicted detonation pressure of 48.54 GPa and a predicted sensitivity (h50) of 23.99 cm. By introducing ionic bridges into highly nitrated rings, or modifying the original bridge with ionic bridges, some highly nitrated cage compounds with both excellent performance and low sensitivity can be developed strategically.
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.
Phospholipase A2 (PLA2) is one of the key enzymes involved in the formation of inflammatory mediators. Inhibition of PLA2 is considered to be one of the efficient methods to control inflammation. In silico docking studies of 160 selected indole derivatives performed against porcine pancreatic PLA2 (ppsPLA2) suggested that, CID2324681, CID8617 (indolebutyric acid or IBA), CID22097771 and CID802 (indoleacetic acid or IAA) exhibited highest binding energies. In silico analysis was carried out to predict some of the ADME properties. The binding potential of these compounds with human non pancreatic secretory PLA2 (hnpsPLA2) was determined using molecular docking studies. In order to corroborate the in silico results, enzyme kinetics and isothermal titration calorimetric analysis of the two selected compounds, IAA and IBA were performed against ppsPLA2. From the analysis, it was concluded that IAA and IBA can act as competitive inhibitors to the enzyme and may be used as anti inflammatory agents.
Development of new energetic salts is the key factor in replacing low performance compounds in conventional formulations of high explosives as well as propellants. Ten salts based on the nitroformate anion and various nitrogen-rich cations were designed and their geometric optimizations carried out using the density functional method. With reasonable oxygen balance (from ?36 % to 0 %), heats of formation (47–624 kJ mol?1) and high densities (1.81–1.89 g cm?3), the detonation velocity (D) and pressure (P) values of salts were calculated as 8.62–9.36 km s?1 and 33.10–40.01 GPa, respectively. Lastly, the nitroformate salts studied in this work are of prospective interest as high performance explosives.
The σ-hole and π-hole of the protonated 2-halogenated imidazolium cation (XC3H4N2+; X = F, Cl, Br, I) were investigated and analyzed. The monomers of (CH3)3SiY(Y=F, Cl, Br, I), considered as the Lewis base, were combined with the σ-hole and π-hole of XC3H4N2+ to form the σ-hole and π-hole interactions in the bimolecular complexes (CH3)3SiY?·?·?·?XC3H4N2+ and (CH3)3SiY?·?·?·?C3(X)H4N2+(X/Y=F, Cl, Br, I), respectively. For both the σ-hole and π-hole interactions, the equilibrium geometries of complexes show regular changes according to the sequence of heavy sequence of the noncovalent interaction acceptors and donors. The electrostatic energy is the main contribution in the formation of both kinds of interactions, it has linear relations with the VS,max values of σ-hole and the V′S,max values of π-hole. Both the σ-hole and π-hole interactions belong to the closed-shell and noncovalent interactions. The π-hole interactions are stronger than the σ-hole interactions. For the π-hole interactions, the contribution percents of the dispersion energies are somewhat greater than those of the σ-hole interactions, while it is contrary for the polarization energy.
Self-assembly of melamine-cyanuric acid (MC) leads to urinary tract calculi and renal failure. The hydration effects on molecular geometry, the IR spectra, the frontier molecular orbital, the energy barrier of proton transfer (PT), as well as the stability of MC were explored by density functional theory (DFT) calculations. The intramolecular PT breaks the big π-conjugated ring of melamine or converts the p-π conjugation (:N-C'=O) to π-π conjugation (O=C-N=C') of cyanuric acid. The intermolecular PT varies the coupling between melamine and cyanuric acid from pure hydrogen bonds (Na…HNd and NH…O) to the cooperation of cation…anion electrostatic interaction (NaH+…Nd-) and two NH…O hydrogen bonds. Distinct IR spectra shifts occur for Na…HNd stretching mode upon PT, i.e., blue-shift upon intramolecular PT and red-shift upon intermolecular PT. It is expected that the PT would inhibit the generation of rosette-like structure or one-dimensional tape conformer for the MC complexes. Hydration obviously effects the local geometric structure around the water binding site, as well as the IR spectra of NH…O and N…HN hydrogen bonds. Hydration decreases the intramolecular PT barrier from ~45 kcal mol-1 in anhydrous complex to ~11.5 kcal mol-1 in trihydrated clusters. While, the hydration effects on intermolecular PT barrier is slight. The relative stability of MC varies slightly by hydration due to the strong hydrogen bond interaction between melamine and cyanuric acid fragments.
The catalytic pyrolysis pathways of carbonyl compounds in coal were systematically studied using density functional theory (DFT), with benzaldehyde (C6H5CHO) employed as a coal-based model compound and ZnO, γ-Al2O3, and CaO as catalysts. The results show that the products of both pyrolysis and catalytic pyrolysis are C6H6 and CO. However, the presence of any of the catalysts changes the reaction pathway and reduces the energy barrier, indicating that these catalysts promote C6H5CHO decomposition.
Graphical abstract The presence of catalysts changes the reaction pathway and the energy barrier decreases in the order Ea (no catalyst)> Ea (CaO)> Ea (γ-Al2O3)> Ea (ZnO), indicating that these catalysts promote C6H5CHO decomposition.
