Maximum Bond Order Research Articles

Comprehensive Study on the Adsorption and Degradation of Dichlorodiphenyltrichloroethane on Bifunctional Adsorption-Photocatalysis Material TiO2/MCM-41 Using Quantum Chemical Methods.

The adsorption and degradation capacities of dichlorodiphenyltrichloroethane (DDT) on a photocatalyst composed of TiO2 supported on the mesoporous material MCM-41 (TiO2/MCM-41) were investigated using density functional theory and real-time density functional theory methods. The van der Waals interactions within the PBE functional were adjusted by using the Grimme approach. The adsorption of DDT was evaluated through analyses involving adsorption energy, Hirshfeld atomic charges, Wiberg bond orders, molecular electrostatic potential, noncovalent interaction analysis, and bond path analysis. The findings reveal that DDT undergoes physical adsorption on pristine MCM-41 or MCM-41 modified with Al or Fe due to the very small bond order (only about 0.15-0.18) as well as the change in total charge of DDT after adsorption is close to 0. However, it chemically adsorbs onto the TiO2/MCM-41 composite through the formation of Ti···Cl coordination bonds because the maximum bond order is very large, about 1.0 (it can be considered as a single bond). The adsorption process is significantly influenced by van der Waals interactions (accounting for approximately 30-40% of the interaction energy), hydrogen bonding, and halogen bonding. MCM-41 is demonstrated to concurrently function as a support for the TiO2 photocatalyst, creating a synergistic effect that enhances the photocatalytic activity of TiO2. Based on the computational results, a novel photocatalytic mechanism for the degradation of DDT on the TiO2/MCM-41 catalyst system was proposed.

The unusual quadruple bonding of nitrogen in ThN

Nitrogen has five valence electrons and can form a maximum of three shared electron-pair bonds to complete its octet, which suggests that its maximum bond order is three. With a joint anion photoelectron spectroscopy and quantum chemistry investigation, we report herein that nitrogen presents a quadruple bonding interaction with thorium in ThN. The quadruple Th≣N bond consists of two electron-sharing Th-N π bonds formed between the Th-6dxz/6dyz and N 2px/2py orbitals, one dative Th←N σ bond and one weak Th←N σ bonding interaction formed between Th-6dz2 and N 2s/2pz orbitals. The ThC molecule has also been investigated and proven to have a similar bonding pattern as ThN. Nonetheless, due to one singly occupied σ-bond, ThC is assigned a bond order of 3.5. Moreover, ThC has a longer bond length as well as a lower vibrational frequency in comparison with ThN.

Ligand Optimization of Exchange Interaction in Co(II) Dimer Single Molecule Magnet by Machine Learning.

Designing single-molecule magnets (SMMs) for potential applications in quantum computing and high-density data storage requires tuning their magnetic properties, especially the strength of the magnetic interaction. These properties can be characterized by first-principles calculations based on density functional theory (DFT). In this work, we study the experimentally synthesized Co(II) dimer (Co2(C5NH5)4(μ-PO2(CH2C6H5)2)3) SMM with the goal to control the exchange energy, ΔEJ, between the Co atoms through tuning of the capping ligands. The experimentally synthesized Co(II) dimer molecule has a very small ΔEJ < 1 meV. We assemble a DFT data set of 1081 ligand substitutions for the Co(II) dimer. The ligand exchange provides a broad range of exchange energies, ΔEJ, from +50 to -200 meV, with 80% of the ligands yielding a small ΔEJ < 10 meV. We identify descriptors for the classification and regression of ΔEJ using gradient boosting machine learning models. We compare one-hot encoded, structure-based, and chemical descriptors consisting of the HOMO/LUMO energies of the individual ligands and the maximum electronegativity difference and bond order for the ligand atom connecting to Co. We observe a similar overall performance with the chemical descriptors outperforming the other descriptors. We show that the exchange coupling, ΔEJ, is correlated to the difference in the average bridging angle between the ferromagnetic and antiferromagnetic states, similar to the Goodenough-Kanamori rules.

In vitro anti-leishmanial activities and structure-activity relationship analysis of new antimony(III) complexes

New fourteen antimony(III) complexes of general formula [SbLnCl3] (where n= 1 or 2, L =2-aminopyridine, 5-methyl-2-aminopyridine, 2-aminopyrimidine, 4,6-dimethoxy-2-aminopyrimidine, 2-benzyl-2-thiopseudeourea,2-guanidinobenzimidazole, 2-amino-5-thiol-4,6-dimethoxypyrimidine, 2-amino-5-(1H-tetrazol-5-ylthio)-4,6-dimethoxypyrimidine, N-2-pyrimidine, 2-piperidinecarboxamide, N-2-pyrimidine, 2-pyrrolidinecarboxamide, 2-amino-5-(1H-tetrazol-5-iltiyo)-4,6-dimethoxypyrimidine-2-pyrrolidinecarboxamide and N-2-pyrimidine, 5-chloro-2-thiophenecarboxamide, N-2-benzothiazol-2-pyrrolidinecarboxaamide, N,N-(1,2-phenyl)dipyrrolidine-2-carboxamide) have been synthesized and their anti-leishmanial activity have been assessed in vitro against Leishmania tropica promastigotes. The best complex, Sb(2-guanidinobenzimidazole)Cl3 is demostrated 3.16% growth inhibition at a concentration of 31.25 μg/mL. In general, antimony(III) complexes containing pyrimidine ligands has showed higher anti-leishmanial activity than antimony(III) complexes bearing pyridine ligands, and electron-donating substituents decrease the anti-leishmanial activity. All complexes have been optimised with DFT/B3LYP/LANL2DZ method in the gas phase. Several descriptors are tested to find a quantitative correlation between anti-leishmanial activity and structural properties of the complexes by best multiple linear regression method. Good correlations are obtained with minimum net atomic charge for a C atom and maximum bond order of a Cl atom. The developed QSAR equation is internally validated.

Observation of Four-Fold Boron-Metal Bonds in RhB(BO-) and RhB.

The maximum bond order between two main-group atoms was known to be three. However, it has been suggested recently that there is quadruple bonding in C2 and analogous eight-valence electron species. While the quadruple bond in C2 has aroused some debates, an interesting question is: are main-group elements capable of forming quadruple bonds? Here we use photoelectron spectroscopy and computational chemistry to probe the electronic structure and chemical bonding in RhB2O- and RhB- and show that the boron atom engages in quadruple bonding with rhodium in RhB(BO)- and neutral RhB. The quadruple bonds consist of two π-bonds formed between the Rh 4dxz/4dyz and B 2px/2py orbitals and two σ-bonds between the Rh 4dz2 and B 2s/2pz orbitals. To confirm the quadruple bond in RhB, we also investigate the linear Rh≡B-H+ species and find a triple bond between Rh and B, which has a longer bond length, lower stretching frequency, and smaller bond dissociation energy in comparison with that of the Rh≣B quadruple bond in RhB.

Toward Accurate Hydrogen Bonds by Scalable Quantum Monte Carlo.

Single-determinant (SD) fixed-node diffusion Monte Carlo (FNDMC) gains popularity as a benchmark method scalable to large noncovalent systems, although its accuracy limits are not yet fully mapped out. We report on an interesting example of significant SD FNDMC accuracy variations in middle-sized hydrogen-bonded dimer complexes, formic acid (FA) vs methanediol (MD), distinct by the maximum bond order (2 vs 1). While the traditional SD FNDMC schemes based on bias cancellation are capable of achieving benchmark (2%) accuracy for MD, this has not been the case for FA. We identify the leading systematic error source in energy differences and show that suitably designed Jastrow factors enable SD FNDMC to reach the reference accuracy for FA. This work clearly illustrates the varying accuracy of the present-day SD FNDMC at the 0.1 kcal/mol scale for a particular set of systems but also points out promising routes toward alleviation of these shortcomings, still within the single-reference framework.

Modeling the Transition State Structures of the Reductive-Half Reaction Active Site of Xanthine Oxidase Bound to Guanine Analogues: A Density Functional Theory Approach

Modeling and characterization the transition state structure of enzyme catalyzed reactions is essential. A DFT method employing B3LYP level of theory with 6-31G (d',p') basis set for non-metals and LanL2DZ basis set for molybdenum have been used. The bond orders of chemical fragments were calculated using AOmix softaware. The effect of chalcogen replacement, amine group and methyl group in the parent structure of xanthine bound to xanthine oxidase active site were compared. The transition state structure of model substrates (2AX, 2A6TP, 2A6SP and 2A6MP) bound to the truncated form of XO active site has been confirmed by the presence of one negative imaginary frequencies (s-1) (-60), (-140), (-230) and (-270), respectively. The corresponding normalized energy barriers (kcal/mol) from pre-transition state to the transition state, respectively, are (13.869), (21.753), (23.109) and (0.212). In this work, 2A6SP and 2A6TP substrates were found to be potential xanthine oxidase inhibitors. The large bond distances and minimum bond order for CRH-HRH bond, and small bond distances and maximum bond order for SMo-HRH bond at the transition state for chalcogen replaced 2AX confirms early transition state structure. Methyl substituted 2AX analog found to have post transition state structure. A potential xanthine oxidase inhibitor can be designed from purine family enzymes using DFT approach.

Quantum Study of Laser-Induced Initial Activation of Graphite-to-Diamond Conversion

Recently (Science 2009, 325, 181), femtosecond-resolved electron energy loss spectroscopy (FEELS) was used to map the structural changes of graphite upon laser irradiation, revealing the change from sp(2) to sp(3), i.e., diamond-like, hybridization. With a laser excitation energy of 2.39 eV and a fluence of 1.5 mJ/cm(2), the most pronounced changes were observed in approximately 180 fs, a time similar to the temporal resolution of the technique. The presence of the laser field turns the electronic wavefunction into a wavepacket whose quantum dynamics governs the onset of the structural rearrangement. Density functional theory calculations with a quantum propagator that include the laser field show that the charge density of graphite expands between the layers in an ultrafast process of the order of approximately 10 fs. Calculations as a function of the field/fluence further show different values of the maximum bond order reached at the stationary state. The experimentally used value is at the crossover between two regimes. It is tempting to associate the second regime with the electron organization necessary to achieve ablation or melting. The application of the model demonstrates its potential for examining the dynamical nature of the charge density and chemical bonding as it forms.

The charge capacitance of the chemical bond: Application to bonds containing metals

AbstractThe charge capacitance of metal containing complexes are studied. For molecules with multiple bonding between the metal atoms it is found that the charge capacitance is correlated to the maximum bond order, natural bond order, and to some extent the effective bond order. Furthermore the charge capacitance of some methylidene metal dihydride complexes are studied. These molecules have agostic interactions of varying strength, and it is concluded that this strength is very well reflected in the charge capacitances of the systems. In accordance with the definition of agostic interactions it is therefor concluded that the charge capacitance holds information about the strength of covalent interactions. The effect therefore on the agostic interactions upon substitution of one of the hydrogen atoms with fluorine in the methylidene metal complexes is studied, and found to reduce the agostic interactions. It is also demonstrated that there is an agostic interaction in an ArCrCrAr complex. The distance dependence of the charge capacitance is also discussed. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

The proposal of architecture for chemical splitting to optimize QSAR models for aquatic toxicity

One of the challenges in the field of quantitative structure-activity relationship (QSAR) analysis is the correct classification of a chemical compound to an appropriate model for the prediction of activity. Thus, in previous studies, compounds have been divided into distinct groups according to their mode of action or chemical class. In the current study, theoretical molecular descriptors were used to divide 568 organic substances into subsets with toxicity measured for the 96-h lethal median concentration for the Fathead minnow ( Pimephales promelas). Simple constitutional descriptors such as the number of aliphatic and aromatic rings and a quantum chemical descriptor, maximum bond order of a carbon atom divide compounds into nine subsets. For each subset of compounds the automatic forward selection of descriptors was applied to construct QSAR models. Significant correlations were achieved for each subset of chemicals and all models were validated with the leave-one-out internal validation procedure ( R cv 2 ≈ 0.80 ) . The results encourage to consider this alternative way for the prediction of toxicity using QSAR subset models without direct reference to the mechanism of toxic action or the traditional chemical classification.

Reaching the Maximum Multiplicity of the Covalent Chemical Bond

Maxing out at six: The maximum bond order that can be achieved between two equal atoms in the periodic system is six. The picture shows the potential energy curves for the diatoms Cr2, Mo2, and W2, where the latter two are sextuply bonded molecules (d=internuclear distance in atomic units).

Maximum bond order hybrid orbital calculations of C[dbnd]O bond stretching vibrational frequencies

A generalized relationship, which includes contributions of the s-characters of CO bond and the net atomic charges, is employed to elucidate CO bond stretching vibrational frequencies based on the maximum bond order hybrid orbital calculations at ab initio HF/6-31G∗ level of theory. It is demonstrated that the change of the s-characters is the most important factor for determining the change of CO stretching frequencies. The CO stretching frequencies calculated by using the explicit relationship obtained are in good agreement with the corresponding experimental frequencies.

MAXIMUM BOND ORDER HYBRID ORBITAL CALCULATIONS OF' THE S˭O STRETCHING FREQUENCIES FOR SULPHURYL AND THIONYL COMPOUNDS

A new generalized relationship, which includes the contributions of the net atomic charges and the orbital characters of the hybrid atomic orbitals forming the S˭O bond, is employed to elucidate the S˭O bond stretching Vibrational frequencies based on the maximum bond order hybrid orbital calculations. The S˭O stretching frequencies calculated by using the explicit relationship obtained are in good agreement with the corresponding experimental frequencies.

Maximum bond order hybrid orbital calculations of the P=S stretching frequencies for organophosphorus compounds

A novel, generalized correlation of the characteristic stretching frequencies with the atomic hybrids and net charges is employed to give a new, explicit relationship for calculating the P=S stretching frequencies by the use of the maximum bond order hybrid orbital procedure together with the extended Hückel molecular orbital calculation. The calculated numerical results show that the new relationship obtained in the present paper is quite satisfactory for the calculation of the characteristic P=S stretching frequencies for organophosphorus compounds.

MBOHO calculations of phosphorus-carbon nuclear spin-spin coupling constants

The novel generalized correlation of the nuclear spin-spin coupling constants with the atomic hybrids and net charges is employed to give a new relationship for calculating the directly bonded phosphorus-carbon coupling constants by use of the maximum bond order hybrid orbital procedure together with the extended Huckel molecular orbital calculation. The calculated coupling constants of phosphorus-carbon are all in good agreement with the experimental data, which shows that the new relationship obtained in the present paper is quite satisfactory for calculation of the phosphorus-carbon coupling constants.

A study of the correlativity between the ab initio maximum bond order hybrid orbital calculation results and the nuclear spin-spin coupling constants

The ab initio maximum bond order hybrid orbital (MBOHO) procedure has been performed with an STO-3G basis set in order to study the correlativity between the ab initio MBOHO calculation results and the nuclear spin-spin coupling constants. The generalized relationship which involves the contributions of not only the hybrid orbitals, but also the net atomic charges has been applied with the calculated s-character of the MBOHOs including only the valence atomic orbitals and the net atomic charges to obtain concrete relationships for calculation of the nuclear spin-spin coupling constants of various CH, CC, CN and CF bonds. The coupling constants evaluated by use of these concrete relationships are in good agreement with the experimental data, which shows that the ab initio MBOHO procedure and the generalized relationship including also the net atomic charges are feasible for elucidating the nuclear spin-spin coupling constants between the directly bonded atoms.

MBOHO calculations of carbon‐nitrogen nuclear spin‐spin coupling constants

AbstractThe novel generalized correlation of the nuclear spin‐spin coupling constants with the atomic hybrids and net charges was employed to give two new relationships for calculating the carbon‐nitrogen coupling constants by use of the maximum bond order hybrid orbital (MBOHO) procedure together with the simple CNDO/2 calculation. The calculated coupling constants of carbon‐nitrogen single and triple bonds are all in good agreement with the experimental data, which shows that the new relationships obtained are satisfactory for calculation of carbonnitrogen coupling constants.

Ab initio calculation of maximum bond order hybrid orbitals

The maximum bond order hybrid orbital (MBOHO) procedure is tested onab initio level by use of the density matrix in Lowdin orthogonalized atomic orbital basis. The direct MBOHO calculation based on the whole density matrix includes also the hybridization of the inner atomic orbitals, and the MBOHO calculation based on the valence orbital part of the density matrix considers only the hybridization of the valence atomic orbitals. The concrete MBOHO calculations based on theab initio calculation with STO-3G basis show that the components of the s atomic orbitals in MBOHOs and the maximum bond orders obtained from the two kinds of MBOHO calculations are very close to each other, and that the two kinds of MBOHOs all have the excellent correlativity with the nuclear spin-spin coupling constants.

Maximum bond order hybrid orbitals

Based on the simplified calculation scheme of the maximum bond order principle and the basic idea of the maximum overlap symmetry orbital method, a simple procedure is suggested for constructing systematically the bonding hybrid orbitals, called “maximum bond order hybrid orbitals”, for a given molecule from the first-order density matrix obtained from a molecular orbital calculation. As an example, the proposed procedure is performed for some typical small molecules by use of the density matrix obtained from CNDO/2 calculation. It is shown that the bonding hybrid orbitals constructed by using the procedure are extremely close to those by using the natural hybrid orbital procedure and in good agreement with chemical intuition, and that the proposed procedure can be performed more easily than the natural hybrid orbital procedure and can given simultaneously the values of the maximum bond order for all bonds in molecules.

Maximum bond order hybrid orbitals II. Correlativity with C-H and C-C spin-coupling constants

The excellent correlativity between the maximum bond order hybrid orbitals and the nuclear spin-spin coupling constants of directly bonded C-H and C-C is shown in the present paper. The maximum bond order hybrid orbital procedure is performed by use of the first-order density matrices obtained from CNDO/2 calculation to get the bonding hybrid orbitals and the corresponding maximum bond orders for a number of hydrocarbons and hetero-substituted hydrocarbons. The relations between the obtained calculation results and the experimental coupling constants are examined by using the basic relationships proposed by Muller and Pritchard, by Maksic et al. and by Gil, and summarized in the concrete relationships which are the most suitable for the maximum bond order hybrid orbital calculation. The obtained relationships combined with the maximum bond order hybrid orbital calculation is quite successful in predicting substituent effects on the C-H and C-C coupling constants in molecules which contain no substituents of the −I− type.