Frontier Orbital Characteristics Research Articles

Synthesis and characterization of novel quinaldic acid based Ni, Fe, and Cr Complexes: A computational and experimental study

Complexes ([Ni(quin-2-c)2(H2O)2]•2H2O, [Fe(quin-2-c)2(H2O)2], [Cr(quin-2-c)2(H2O)2] were synthesized from aqueous solutions of metal salts, quinaldic acid, and semicarbazide. In the crystal structure of the first complex, the nickel atom is chelated by the deprotonated quinaldic acid molecule through the oxygen atom of the carboxyl group, and the heterocyclic nitrogen atom, as well as two water molecules, are coordinated. The other two water molecules are located in the outer sphere. In the structure of the iron and chromium complexes, the metal atoms have a similar complex formation but do not have water molecules in the outer sphere. In all compounds, quinaldic acid acts as a bidentate ligand to form five-membered metallocycles. The electronic structure properties were investigated through theoretical modeling using the B3LYP method at the 6–31 + G(d,p) computational level. Density functional theory (DFT) was employed in quantum chemistry simulations to refine proposed structures and explore the characteristics of frontier orbitals via analysis of frontier molecular orbitals (FMOs). These computations facilitated the identification of specific sites on the surface of molecules by generating molecular electrostatic potential maps and provided insights into the structural and reactive properties. Hirshfeld surface analysis was conducted to examine intermolecular interactions, revealing a prevalence of H⋅⋅⋅H interactions.

Synthesis, NMR, IR, Raman spectra and DFT calculations of 1‐octyl‐1,4‐diazabicyclo [2.2.2] octan‐1‐ium bis(trifluoromethylsulfonyl)imide

In this study, a novel [C8DABCO+] [(CF3SO2)2N−] ionic liquid (IL) was prepared by an alkylation reaction and then characterized it by experimental 1H-, 13C-, and 19F-NMR, IR and Raman spectroscopies together with B3LYP/6-311G** calculations in gas phase and aqueous solution to confirm its structure. DSC and TGA measurements show that it can be classified as a thermally stable ionic liquid. Three different structures (I), (II) and (III) were proposed for the IL with Cis and Trans positions of CF3 groups of anion evidencing better correlations among experimental and predicted spectra for structure (III). Strong S-O···H interactions were predicted in the gas phase while in solution the C-F···H distance is shorter (2.654 Å) than the S-O···H ones (2.827 Å). The analysis of MK charges reveals the presence of positive charges on quaternary N23 atoms both in vaccum and in solution; conversely positive Mulliken charges reside on O49 and O50 in the gas phase, while negative ones are found in solution. Molecular Electrostatic potential (MEP) surfaces support the electrophilic sites on quaternary N23 while the nucleophilic sites are observed on O, N and F atoms of the anion. NBO and AIM calculations reveal the characteristics of cation-anion interactions in the two media, as supported by the different f(νN-C), f(νSO2), f(δSO2) and f(δCH2) force constants values. Gap values suggest that [C8DABCO+] [(CF3SO2)2N−] is less reactive than [C8DABCO+] [Br−] while the characteristics of frontier orbitals justify the cation-anion interactions and support the important role of 1,4-diazabicyclo [2.2.2] octane (DABCO) in the properties of [C8DABCO+] [(CF3SO2)2N−] IL in both media. The complete assignments of 174 vibration modes expected for [C8DABCO+] [(CF3SO2)2N−] IL are reported together with the scaled force constants.

Single-molecule anisotropic magnetoresistance at room temperature: Influence of molecular structure

Organic molecules featuring weak spin-orbit and hyperfine interactions show promise in the construction of effective spintronic devices. However, the impact of molecular structures on anisotropic magnetoresistance at the single-molecule level remains to be explored. In this work, we report systematic investigations on the construction and magnetoresistance of spintronic single molecule junctions of aliphatic dicarboxylic acids HOOC−(CH2)n−COOH (n=1−4) and conjugated benzenedicarboxylic acids HOOC-(C6H4)n-COOH (n=1-3) binding to Fe electrodes by employing electrochemically assisted jump-to-contact STM break junction technique (JTC-STM-BJ) under a constant of external magnetic field. Results show that single-molecule tunneling anisotropic magnetoresistance (AMR) of alkanedicarboxylic acids is independent of the molecular length and can reach as large as 44% (average), which is about twice that of benzenedicarboxylic acids. The almost same constant βN value but different contact conductance Gc with the applied magnetic field indicate that AMR arise from interface effect and spin injection. Besides Br substituent on the benzene ring can almost double the AMR by changing the spatial distribution of molecular charge density and frontier orbital characteristics. The present work shows that molecular structures play important roles in electron transport through single-molecule spintronic junctions.

Understanding the Activity of Single-Atom Catalysis from Frontier Orbitals.

The d-band center and charge states are often used to analyze the catalytic activity of noble or transition metal surfaces and clusters, but their applicability for single-atom catalysts (SACs) is unsure. This work suggests that the spatial structure and orientation of frontier orbitals which are closest to the Fermi level of SACs play a vital role. Taking adsorption of several molecules and CO oxidization on C_{3}N-supported single-atom Au as examples, we demonstrate that adsorption and catalytic activities are well correlated with the characteristics of frontier orbitals. This work provides an effective guidance for understanding the performance of single-atom catalysts.

Reaction model and thermodynamic properties between sulfur-containing active groups and oxygen during coal self-heating

To further study the mechanism of coal self-heating, the reaction sequences and thermodynamic properties between sulfur-containing groups and oxygen during coal self-heating were analyzed. The benzyl mercaptan and diphenyl sulfide were selected as typical sulfur-containing structures existing in coal. Their structural parameters, frontier orbital characteristics, and thermodynamic parameters were analyzed through quantum chemistry calculation and their detailed reaction sequences with oxygen were proposed. The results indicate that the thiol structure in coal can easily react with oxygen at low temperatures and release large amounts of heat (146.70 kJ/mol) during coal self-heating, providing active free radicals and energy for subsequent chain reactions of coal spontaneous combustion. The oxidation reaction between the thioether structure and oxygen cannot occur at room temperature. With the accumulation of heat, thioether gradually becomes active and reacts with oxygen to form sulfoxide and release an enormous amount of heat (248.09 kJ/mol), which can be further oxidized to sulfone with an increase in temperature. The reaction models of thiol and thioether groups during coal self-heating were proposed, which involves eight main reaction sequences (R1∼R8). It indicates that the reactions of thiol and thioether groups play crucial roles during the evolution of coal self-heating, with a slow oxidation stage at low temperatures and an accelerated oxidation stage at high temperatures.

Reaction Mechanism and Thermodynamic Properties of Aliphatic Hydrocarbon Groups during Coal Self-Heating

To further understand the development of coal self-heating, the reaction sequences and thermal properties of aliphatic hydrocarbon groups during coal self-heating were analyzed. The structural parameters, frontier orbital characteristics, molecular orbital, and perturbation energy of aliphatic hydrocarbons and oxygen were analyzed by the quantum chemistry method. Then, the reaction pathways of aliphatic hydrocarbon groups and the corresponding reaction model were proposed. The results indicate that the reactions of aliphatic hydrocarbon groups include three kinds, i.e., the hydrogen capture by oxygen, reaction between aliphatic hydrocarbon radicals and the hydroxyl radical, and reaction between aliphatic hydrocarbon radicals and oxygen. The main reactions include the reaction between carbon free radicals and oxygen (E1), the reaction between aliphatic hydrocarbon and the hydroxyl radical (E2), the reaction between methyne and oxygen (E3), and other spontaneous reactions caused by E1 and E3 (E2). There is ...

Preparation of phosphorus-doped carbon nanospheres and their electrocatalytic performance for O2 reduction

Phosphorus-doped carbon nanospheres without any metal residues were synthesized and characterized. The results revealed that the doping phosphorus atoms could significantly improve the electrocatalytic activity of graphitic carbon for the oxygen-reduction reaction (ORR) both in acidic and alkaline media, and the materials exhibited high electrocatalytic activity, long-term stability, and excellent tolerance to crossover effects especially in alkaline media. Quantum mechanics calculations with the density functional theory demonstrated that the changes in charge density and energetic characteristics of frontier orbitals for the P-doped graphene sheet could facilitate the ORR.

Excited-State Energy Transfer Processes in Phenylene- and Biphenylene-Linked and Directly-Linked Zinc(II) and Free-Base Hybrid Diporphyrins

The photoinduced energy transfer processes in 1,4-phenylene-, 1,3-phenylene, 1,2-phenylene, and 4,4‘-biphenylene-linked and directly-linked Zn(II)-free base porphyrin heterodimers in THF were investigated by femtosecond transient absorption spectroscopy. The energy transfer rates were compared between TPP-type and OEP-type heterodimers respectively as A2u-HOMO and A1u-HOMO subunits, for evaluating the relative contribution of the through-bond and through-space interactions. The rate difference becomes smaller with a decrease of spacer, more than 10 for 1,4-bis(phenylethynyl)phenylene and 1,4-diphenylethynylene, 4 for 4,4‘-biphenylene-linked heterodimer, and 3 for 1,3- and 1,4-phenylene-linked spacers. In the meso−meso directly-linked case, the energy transfer rates are the same ((0.55 ps)-1) for 5,5,15,15-tetrakis(3,5-bis(octyloxy)phenyl)-substituted and 5,5,15,15-tetrakis(pentafluorophenyl)-substituted heterodimers, featuring only a minor influence of the frontier orbital characteristics on the energy transfer rate. The energy transfer rates are identical (0.55 ps)-1 for the directly-linked meso−meso heterodimers substituted with 3,5-bis(octyloxy)phenyl and pentafluorophenyl groups regardless of the difference in the HOMO orbital symmetry characteristics, suggesting the predominant Coulombic interaction for the energy transfer in these close proximity porphyrin dimers. In the case of 1,2-phenylene-linked heterodimers, the choice of the peripheral substituents can lead to a state-to-state rapid energy transfer with a rate of (0.55 ps-1) for the TPP-type model or a delocalized excimer-like diporphyrin excited state for the OEP-type model. Collectively, these results indicate that even for the covalently-linked models the relative contribution of the through-space Coulombic interaction becomes increasingly important upon the decrease of the center-to-center separation. Especially, the fast and efficient energy transfer occurring in the directly-linked heterodimer illustrates that this porphyrin unit can be utilized as a good candidate for energy transfer functional arrays in molecular photonic devices.

A theoretical study of the addition of silyl radicals to olefinic monomers

In this paper, the high reactivity of silyl macroradicals toward double bonds of olefinic compounds has been explained by means of quantum-mechanical calculations through their frontier orbital characteristics. In this way, the main orbital interaction corresponds to the overlapping between the SOMO of the disilyl radical and the LUMO of the olefin. In order to obtain more accurate results of differential reactivity, an orbitalic SOMO-HOMO interaction should be included in addition to the main SOMO-LUMO one. Also, we theoretically studied the regioselectivity of the addition of silyl radicals to double bonds obtaining similar results as for carbon centered radicals where the reaction takes place on the less hindered carbon of the olefin. Regarding to the geometrical and electronic parameters, it has been shown that carbon radicals have a sp 2 geometry and a negative charge on the radical center whilst silyl radicals have a sp 3 goemetry and a positive charge. Both factors contribute to the enhanced reactivity of silyl radicals with respect to carbon ones.

Reactivity of radicals from 1‐(N,N‐dimethylamino)ethylene‐2‐derivatives in the acrylic photopolymerization

Relative reactivities of radicals derived from several 1-(N,N-dimethylamino)ethylene-2-derivatives towards the polymerization of butyl acrylate have been calculated. It has been observed that the reactivity of the radicals increases as the electron withdrawing character of the groups attached in the β position to the N atom increases. The experimental efficiency factor (f) has been found to be proportional to the σF value defined in the Taft equation. The dependence of the reactivities with the nature of the radicals has also been explained through their frontier orbital characteristics. The presence of heteroatoms next to the radical center could modify, in some cases, the accuracy of these calculations, although the inclusion of more sophisticated quantum-mechanical methods does solve this drawback. Moreover, the enthalpy calculation of the possible radicals formed from a certain molecule gives a good approach to realize about the existence of anomalous behaviors. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 2889–2899, 1997

Reactivity of radicals from 1-(N,N-dimethylamino)ethylene-2-derivatives in the acrylic photopolymerization

Relative reactivities of radicals derived from several 1-(N,N-dimethylami-no) ethylene-2-derivatives towards the polymerization of butyl acrylate have been calculated. It has been observed that the reactivity of the radicals increases as the electron withdrawing character of the groups attached in the β position to the N atom increases. The experimental efficiency factor (f) has been found to be proportional to the σ F value defined in the Taft equation. The dependence of the reactivities with the nature of the radicals has also been explained through their frontier orbital characteristics. The presence of heteroatoms next to the radical center could modify, in some cases, the accuracy of these calculations, although the inclusion of more sophisticated quantummechanical methods does solve this drawback. Moreover, the enthalpy calculation of the possible radicals formed from a certain molecule gives a good approach to realize about the existence of anomalous behaviors.

Quantum molecular modeling of hyaluronan

Molecular modeling of the secondary structure of hyaluronan is presented on the basis of the following molecular models: GlcA, GlcNAc, GlcA 1 → 3 GlcNAc, GlcNAc 1 → 4 GlcA, GlcNAc 1 → 4 GlcA 1 → 3 GlcNAc and GlcA 1 → 3 GlcNAc 1 → 4 GlcA. The optimized geometries of the monosaccharides obtained using AM1 and ab initio STO-3G methods are in good agreement with the crystallographic data. The flexibility of the β1 → 3 and β1 → 4 linkages was studied using AM1 and ab initio HF/3-21G methods. Our study shows that in all cases the β1 → 3 linkage is more flexible than the β1 → 4 linkage. The minimal energy conformations of the di- and trisaccharides studied correspond to a compact conformation for the β1 → 3 linkage and to standard conformations in agreement with the left-handed helices of sodium hyaluronate or calcium hyaluronate. The electrostatic properties and the frontier orbital characteristics obtained at the HF/3-21G level are discussed.

Biosynthesis of Nitric Oxide—Quantum Chemical Modelling of Nω‐Hydroxy‐l‐arginine Formation

AbstractThe electronic structure (charge distribution, bond indices, character of the frontier orbitals) and geometry (bond distances and angles) of L‐arginine and N‐methyl‐L‐arginine were determined by means of the INDO procedure. The method was also adopted to model the conversion of L‐arginine into N‐hydroxy‐L‐arginine in biological systems. This revealed that the approach of diatomic O species does not result in reaction, whereas the approach of either an O atom or an O2‐ ion leads to insertion of oxygen and formation of hydroxy‐L‐arginine. The insertion of oxygen between the nitrogen and hydrogen atoms leads to more stable products than insertion into the C–H bond. The same results were obtained for N‐methyl‐L‐arginine, and are consistent with the hypothesis that the inhibitive effect of N‐substitution in L‐arginine is of no importance for the first step in the biosynthesis of NO (hydroxylation process). The mechanistic considerations based on the charge distribution and frontier orbital characteristics led to the conclusion that the most probable mechanism of L‐arginine hydroxylation consists in electrophilic attack of [FeO]3+ at the Nω‐H bond, initiated by the reduction of L‐arginine+, followed by insertion of oxygen and product oxidation.

Conformational effects on the electronic structure and chemical reactivity of lignin modelp-quinone methides and benzyl cations

The electronic structure and its dependence on the conformation in typical lignin model intermediates, β-arylethericp-quinone methide {2-methoxy-4-[2-(2-methoxyphenoxy)]propylidene-2,5-cyclohexadien-1-one} and related benzyl cation has been determined on the basis of semi-empirical quantum chemical calculations. Electrostatic but not frontier orbital characteristics of lignin intermediates were demonstrated to be dependent on the conformation. Conformationally induced electrostatic non-equivalence of two possible routes of nucleophile approach to the reaction center may be the main cause of stereoselectivity of the reactions of quinone methides (but not related cations) with nucleophiles. Quinone methides and related cations also differ in their intramolecular charge transfer properties.

Reactivity of Radicals Derived from Dimethylanilines in Acrylic Photopolymerization

Relative reactivities of radicals derived from several p-dimethylanilines toward the polymerization of butyl acrylate have been calculated. It was observed that the reactivity of the radicals increases as the electron-withdrawing character of the groups attached in the para position increases. The experimental efficiency factor (f) has been found to be proportional to the σ p value defined in the Hammett equation. The dependence of the reactivities on the nature of the radicals has also been explained through their frontier orbital characteristics. The high similarity of the behavior of f versus σ p and f versus E SOMO led us to conclude that the simple perturbative molecular orbital calculations can be used to predict the relative reactivity of initiating radicals with similar structure toward the polymerization of acrylic monomers

Regioselectivity of cytochrome P-450 catalyzed hydroxylation of fluorobenzenes predicted by calculated frontier orbital substrate characteristics

In the present study, a hypothesis is presented for the prediction of the regioselectivity of cytochrome P-450 catalyzed hydroxylation of fluorobenzenes. The regioselectivity of the in vivo hydroxylation of fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,2,3-triluorobenzene, and 1,2,4-triflurobenzene could be predicted within 6% accuracy on the basis of the substrate's frontier orbital characteristics for electrophilic attack. The in vivo regioselectivity of the hydroxylation of fluorobenzene was not significantly influenced by changes in the cytochrome P-450 enzyme pattern. This implies that the regioselectivity is not predominantly determined by the juxtaposition of the relatively small substrates in the active sites of the cytochrome P-450s catalyzing the reaction. Additional in vitro experiments using 1,2-difluorobenzene as the model substrate demonstrated that minor factors influencing the regioselectivity and possibly responsible for the 6% deviation from the calculated values in in vivo experiments might be (i) the influence of biotransformation routes occurring in vivo but not of importance in in vitro microsomal incubations and (ii) a small variation due to influences of the contribution of various cytochrome P-450 enzymes. On the basis of the results obtained, it is concluded that the aromatic hydroxylation of fluorobenzenes proceeds through an initial electrophilic attack of (FeO)3+ on the aromatic substrate, and not through initial electron abstraction followed by attack of the (FeO)2+ species on the substrate radical cation. The fact that the regioselectivity observed could be predicted and/or explained by the site of initial (FeO)3+ attack also argues against epoxides as important intermediates in the formation of phenol metabolites from fluorobenzenes.

Quantum chemical study of aromatic diamine and dianhydride reactivities in acylation reactions

Semi-empirical calculations of isolated diamines and model anhydrides and of their interaction energy, ΔE, have been made. Reactivity indices of diamines and dianhydrides (atom charges, characteristics of frontier orbitals) have been found. Analysis of the ΔE components has shown that chemical structure of diamines affects mainly electrostatic and charge transfer contributions to ΔE. Chemical structure of diahnydrides influences most significantly charge transfer component. Results of the ΔE calculation have substantiated the choice of reactivity indices.