Frontier Molecular Orbital Interactions Research Articles

An efficient synthesis of novel triazoles incorporating barbituric motifs via [3+2] cycloaddition reaction: An experimental and theoretical study

In this work, the synthesis of novel triazole derivatives with barbituric motifs in good yields is described. The alkyne was prepared through the Knoevenagel reaction of barbituric derivatives with ortho and para O-propargylated hydroxybenzaldehyde. The mechanism and regioselectivity of this [3+2] cycloaddition reaction were investigated using the density functional theory at the B3LYP/6-31+G(d) level of theory. The computational studies revealed that a di-copper catalyzed stepwise mechanism, involving six-membered ring intermediate, is the preferred pathway. The regioselectivity was explained in terms of frontier molecular orbital (FMO) interactions, local and global electrophilicity and nucleophilicity indices. Accordingly, the favored interactions for di-copper acetylide are in good agreement with the observed regioselectivity, while completely opposite results were obtained for a possible uncatalysed reaction.

Reaction mechanism of hydrogen cyanide catalyzed by gas‐phase titanium

AbstractTo explore the details of the reaction mechanism of Ti atom with HCN, the reactive site and reactivity have been predicted first, the potential energy surfaces have been systematically studied at different theoretical levels. Four different reaction pathways and product distribution are discussed by means of the activation strain model and Curtin–Hammett principle. In addition, the structures, bonding properties and the frontier molecular orbital interaction diagrams of main stationary points were analyzed by atoms in molecules and natural bond orbital. The results show that for this system, there are four reaction pathways, in which path b (HCN＋Ti→IM1→TS1→IM2→T2b→IM4) is the most favorable pathway.

A theoretical view on FeO2+-mediated H[sbnd]H bond activation

The gas-phase reaction of activation HH bond of H2 by FeO2+(6A1/4B2) has been investigated using DFT at the CCSD(T)//B3LYP/6-311G(2d, p) level. The calculation results show that the reaction is a TSR and belongs to the fourth case of the Hammond postulate. The involved crossing between the potential energy surfaces is discussed and the CPs and MECPs are located. In addition, the orbital interaction for ion-molecule complexes 4IM1 and 6IM1 have been examined by FMO analysis. Finally, the frontier molecular orbital interaction analysis about 4TS1 and 6TS1 were used to gain useful information about the HH σ bond activation by FeO2+.

Copper hydroxide nanostructure-modified carbon ionic liquid electrode as an efficient voltammetric sensor for detection of metformin: a theoretical and experimental study

The electrocatalytic oxidation of metformin (MET) was investigated at Cu(OH)2 nanoparticle-modified carbon ionic liquid electrode (Cu(OH)2/CILE). This electrode exhibited excellent characteristic for the electrocatalytic oxidation of metformin at the potential of +0.6 V with good sensitivity and selectivity. The presence of Cu(OH)2 nanostructures in the composite electrode leads to the appearance of oxidation peak of MET. Under optimal experimental conditions, the peak current response increased linearly with metformin concentration over the range of 1 µM–4 mM. The detection limit of the method is 0.5 µM. Moreover, the closer look was taken at the electronic properties of MET and its Cu (II) complexes such as frontier molecular orbital (HOMO and LUMO) and binding interaction energies using density functional theory. Effect of pH was also investigated at B3LYP/6-311++g** level. Theoretical results confirmed the experimental evidences of Cu (II) complexation. Therefore, Ease of preparation, wide linear range, low overpotential, high sensitivity and selectivity provide the possibility of applying this method for the detection of MET in biological samples.

Molecular recognition of 4′-Nitrobenzo-15-crown-5 by bis(benzimidazolium)propane borontetrafluoride in acetonitrile

ICT based ratiometric sensing due to H-bonding interaction among three different crown ethers (C): Dibenzo-24-crown-8 (DB24C8 or C1), Benzo-15-crown-5 (B15C5 or C2) and 4′-Nitrobenzo-15-crown-5 (4′–NB15C5 or C3) along with the axle bis(benzimidazolium)propane borontetrafluoride (BBIM-propane) (3a–3d) have been studied. The association were initially ascertained from isosbestic formation and later corroborated by iso-emissive formation where C3 fails to establish iso-emissive. Stoichiometry of adducts were 1:1 both in the ground as well as in excited state. The threading or external association was finally distinguished by Monte Carlo simulation and frontier molecular orbital interaction.

A theoretical study of C–H bond of CH4 activation catalyzed by VO2+ in gas phase

The mechanism of C–H bond of CH4 activation catalyzed by VO2+(1A1/3A′) has been investigated by using density functional theory (DFT) at the CCSD/6-311G(2d,p)//B3LYP/6-311G(2d,p) level. According to our calculation results, the four channels have been well-established and the reactivity patterns on the singlet and triplet PESs are almost similar. And the reaction proceeds in three distinct manners yielding the [V(OCH2)++H2O] products, the [V(OH2)++CH2O] products, the [OV(OCH2)++H2] products and the [VO++CH3OH] products. Specially, the crossing points (CPs) between the different PESs have been located by means of the intrinsic reaction coordinate (IRC) approach used by Yoshizawa et al., and corresponding minimum energy crossing points (MECPs) that we obtained by the mathematical algorithm proposed by Harvey et al. has also been employed. In addition, the frontier molecular orbital interaction analysis about 1TS1, 3TS1, 1TS4 and 3TS4 are used to gain useful information about the C–H bond of CH4 activation by VO2+.

Activation-strain analysis reveals unexpected origin of fast reactivity in heteroaromatic azadiene inverse-electron-demand diels-alder cycloadditions.

Heteroaromatic azadienes, especially 1,2,4,5-tetrazines, are extremely reactive partners with alkenes in inverse-electron-demand Diels-Alder reactions. Azadiene cycloaddition reactions are used to construct heterocycles in synthesis and are popular as bioorthogonal reactions. The origin of fast azadiene cycloaddition reactivity is classically attributed to the inverse frontier molecular orbital (FMO) interaction between the azadiene LUMO and alkene HOMO. Here, we use a combination of ab initio, density functional theory, and activation-strain model calculations to analyze physical interactions in heteroaromatic azadiene-alkene cycloaddition transition states. We find that FMO interactions do not control reactivity because, while the inverse FMO interaction becomes more stabilizing, there is a decrease in the forward FMO interaction that is offsetting. Rather, fast cycloadditions are due to a decrease in closed-shell Pauli repulsion between cycloaddition partners. The kinetic-thermodynamic relationship found for these inverse-electron-demand cycloadditions is also due to the trend in closed-shell repulsion in the cycloadducts. Cycloaddition regioselectivity, however, is the result of differences in occupied-unoccupied orbital interactions due to orbital overlap. These results provide a new predictive model and correct physical basis for heteroaromatic azadiene reactivity and regioselectivity with alkene dieneophiles.

Kinetic study of the Diels-Alder reaction of Li⁺@C₆₀ with cyclohexadiene: greatly increased reaction rate by encapsulated Li⁺.

We studied the kinetics of the Diels-Alder reaction of Li(+)-encapsulated [60]fullerene with 1,3-cyclohexadiene and characterized the obtained product, [Li(+)@C60(C6H8)](PF6(-)). Compared with empty C60, Li(+)@C60 reacted 2400-fold faster at 303 K, a rate enhancement that corresponds to lowering the activation energy by 24.2 kJ mol(-1). The enhanced Diels-Alder reaction rate was well explained by DFT calculation at the M06-2X/6-31G(d) level of theory considering the reactant complex with dispersion corrections. The calculated activation energies for empty C60 and Li(+)@C60 (65.2 and 43.6 kJ mol(-1), respectively) agreed fairly well with the experimentally obtained values (67.4 and 44.0 kJ mol(-1), respectively). According to the calculation, the lowering of the transition state energy by Li(+) encapsulation was associated with stabilization of the reactant complex (by 14.1 kJ mol(-1)) and the [4 + 2] product (by 5.9 kJ mol(-1)) through favorable frontier molecular orbital interactions. The encapsulated Li(+) ion catalyzed the Diels-Alder reaction by lowering the LUMO of Li(+)@C60. This is the first detailed report on the kinetics of a Diels-Alder reaction catalyzed by an encapsulated Lewis acid catalyst rather than one coordinated to a heteroatom in the dienophile.

DFT study of the mechanism and stereoselectivity of the 1,3-dipolar cycloaddition between pyrroline-1-oxide and methyl crotonate

A theoretical study of the regio- and stereoselectivities of the 1,3-dipolar cycloaddition reaction between methyl crotonate and pyrroline-1-oxide has been carried out using density functional theory (DFT) at the B3LYP/6-31G(d) level of theory. The reaction has been followed by performing transition state optimization, calculations of intrinsic reaction coordinate and activation energies; the molecular mechanism of the reactions is concerted and asynchronous. The regio- and exo/endo-selectivity have been explained in terms of frontier molecular orbital interactions, local and global electrophilicity and nucleophilicity indices and an analysis of the Wiberg bond indices in the transition state. The FMO analysis and DFT-based reactivity indices showed that the regioselectivity of this reaction is controlled by the HOMOdipole–LUMOdipolarophile interaction. The activation parameters indicated favoured endo approach along the meta-pathway in agreement with the experimental results. The molecular mechanism of the 1,3-dipolar cycloaddition reaction between pyrroline-1-oxide and methyl crotonate is theoretically investigated by DFT method at the B3LYP/6-31G* level. The transition states corresponding to the possible stereoisomeric pathways along regioisomeric reaction channels were searched, localized and optimized. The cycloaddition is favored along the reaction channel with the meta-endo adduct as major diastereomer in agreement with experiment.

Why Triple Bonds Protect Acenes from Oxidation and Decomposition

An experimental and computational study on the impact of functional groups on the oxidation stability of higher acenes is presented. We synthesized anthracenes, tetracenes, and pentacenes with various substituents at the periphery, identified their photooxygenation products, and measured the kinetics. Furthermore, the products obtained from thermolysis and the kinetics of the thermolysis are investigated. Density functional theory is applied in order to predict reaction energies, frontier molecular orbital interactions, and radical stabilization energies. The combined results allow us to describe the mechanisms of the oxidations and the subsequent thermolysis. We found that the alkynyl group not only enhances the oxidation stability of acenes but also protects the resulting endoperoxides from thermal decomposition. Additionally, such substituents increase the regioselectivity of the photooxygenation of tetracenes and pentacenes. For the first time, we oxidized alkynylpentacenes by using chemically generated singlet oxygen ((1)O(2)) without irradiation and identified a 6,13-endoperoxide as the sole regioisomer. The bimolecular rate constant of this oxidation amounts to only 1 × 10(5) s(-1) M(-1). This unexpectedly slow reaction is a result of a physical deactivation of (1)O(2). In contrast to unsubstituted or aryl-substituted acenes, photooxygenation of alkynyl-substituted acenes proceeds most likely by a concerted mechanism, while the thermolysis is well explained by the formation of radical intermediates. Our results should be important for the future design of oxidation stable acene-based semiconductors.

Solvent and Zn2+ Binding Effects on the Photo-physical Properties of (Dibenzoylmethanato)boron Difluoride

Absorption and emission spectroscopic studies of (dibenzoylmethanato)boron difluoride (1bf) in various polar and non-polar, protic and aprotic solvents are reported. The solvatochromic shifts of the spectral bands were examined in terms of solvent properties, including donor and acceptor numbers, followed by multilinear regression in which several solvent parameters were simultaneously analyzed. This π-conjugated positively charged system exhibits excellent solvatochromism. Variations in the electronic absorption spectral characteristics of 1bf were studied in solution in the presence of zinc perchlorate. Absorption spectral studies indicate stable complex formation between the zinc ion and 1bf in the ground state in aprotic dipolar benzonitrile rather than in protic polar solvent methanol. Zinc ion binding of 1bf was theoretically rationalized through frontier molecular orbital interaction.

A theoretical study of HH σ bond activation catalyzed by VO2+ in gas phase

AbstractThe mechanism of HH σ bond activation catalyzed by VO(1A1/3A′) has been investigated by using density functional theory at the B3LYP/6‐311G(2d, p) level and the single‐point energy calculations were done at the CCSD/6‐311G (2d, p)//B3LYP/6‐311G(2d, p) level of theory using the geometries along the minimum energy pathway. According to our calculation results, the different reaction mechanisms were found for the singlet and triplet potential energy surfaces (PESs). Specially, the crossing points (CPs) between the different PESs have been located by means of the intrinsic reaction coordinate approach used by Yoshizawa et al, and corresponding minimum energy CPs that we obtained by the mathematical algorithm proposed by Harvey et al. has also been employed. In addition, the orbital interaction for ion‐molecule complexes 1IM1 and 3IM1 have been examined by fragment molecular orbital analysis. Finally, the frontier molecular orbital interaction analysis about 3TS1 and 3TS2 were used to gain useful information about the HH σ bond activation by VO. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012

Mechanism of methylacetylene bisselenation catalyzed by palladium complex from density functional study

The reaction mechanism of Pd(0)-catalyzed methylacetylene bisselenation reaction is investigated by using the density functional method. The overall reaction mechanism involves the oxidative addition, insertion, and reductive elimination steps. The regioselectivity has been investigated for the methylacetylene insertion into Pd-Se bond of both cis and trans palladium complexes. It is found that the methylacetylene insertion into Pd-Se bond of the trans palladium complex using the substituted carbon atom attached to selenyl group is preferred among the four pathways of methylacetylene insertion processes. The electronic mechanisms on the methylacetylene insertion into Pd-Se bond are discussed in terms of the Frontier molecular orbital interactions. In addition, the influence of carbon monoxide on methylacetylene bisselenation was studied and found that the methylacetylene coordination and insertion into Pd-Se bond take place first generating the Pd-C bond, followed by CO insertion into the Pd-C bond.

ChemInform Abstract: Solvent Effect as the Result of Frontier Molecular Orbital Interaction. Part 6. The Diels-Alder Reactions of an N-Acyl-o-quinone Monoimide Behaving as Diene or Dienophile.

ChemInform Abstract: Solvent Effect as the Result of Frontier Molecular Orbital Interaction. Part 6. The Diels-Alder Reactions of an N-Acyl-o-quinone Monoimide Behaving as Diene or Dienophile.

ChemInform Abstract: Tandem Cyclization-Cycloaddition Reaction of Rhodium Carbenoids. Studies Dealing with the Geometric Requirements of Dipole Formation.

The carbenoid intermediate derived by the treatment of several 1-diazobutanediones with rhodium(II) acetate undergoes ready transannular cyclization onto the neighboring keto group to give five-membered ring carbonyl ylides. The dipole derived from ethyl 4-diazo-2-methyl-3-oxobutyrate was found to undergo a rapid proton transfer, producing 5-ethoxy-4-methyl-3-(2H)-furnone. When the position adjacent to the diazo carbonyl group is blocked with two substituent groups, however, smooth 1,3-dipolar cycloaddition occurs. The observed regioselectivity can be nicely accomodated in terms of frontier molecular orbital (FMO) theory. A type II FMO interaction is involved since carbonyl ylides possess one of the smallest HOMO-LUMO energy gaps of common 1,3-dipoles. The rhodium(II)-catalyzed reaction of 1-diazo-6-phenyl-2,6-hexanedione afforded a mixture of products. In addition to the expected cycloadduct, a product derived from the bimolecular addition of the rhodium carbenoid to benzene was obtained

ChemInform Abstract: Solvent Effect as the Result of Frontier Molecular Orbital Interaction. Part 7. The Retro-Diels-Alder Reaction.

ChemInform Abstract: Solvent Effect as the Result of Frontier Molecular Orbital Interaction. Part 7. The Retro-Diels-Alder Reaction.

DFT study of allylic and aliphatic alcohols reactivity: Transesterification and alkylation reactions

The reactions of allylic and aliphatic alcohols with ethyl acetoacetate have been investigated at the DFT/B3LYP level using 6-311G(d) basis set. Analysis results of frontier molecular orbital (FMO) interactions, chemical potential μ and electrophilicity index ω according to Fukui and Pearson respectively, provide a good prediction of experimental results. Alkylation or transesterification reaction is predicted reliably by the calculations.

Quinoline alkaloids as intercalative topoisomerase inhibitors

Quinoline alkaloids are abundant in the Rutaceae, and many have exhibited cytotoxic activity. Because structurally related antitumor alkaloids such as camptothecin and fagaronine are known to function as intercalative topoisomerase poisons, it is hypothesized that cytotoxic Stauranthus alkaloids may also serve as intercalative topoisomerase inhibitors. To test this hypothesis theoretically, ten Stauranthus quinoline alkaloids were examined for potential intercalation into DNA using a molecular docking approach. Four of the alkaloids (stauranthine, skimmianine, 3',6'-dihydroxy-3',6'-dihydrostauranthine, and trans-3',4'-dihydroxy-3',4'-dihydrostauranthine) were able to intercalatively dock consistently into DNA. In order to probe the intermolecular interactions that may be responsible for intercalation of these quinoline alkaloids, density functional calculations have been carried out using both the B3LYP and M06 functionals. M06 calculations indicated favorable pi-pi interactions between either skimmianine or stauranthine and the guanine-cytosine base pair. Furthermore, the lowest-energy face-to-face orientation of stauranthine with guanine is consistent with favorable dipole-dipole orientations, favorable electrostatic interactions, and favorable frontier molecular orbital interactions. Likewise, the lowest-energy face-to-face orientation of stauranthine with the guanine-cytosine base pair reveals favorable electrostatic interactions as well as frontier molecular orbital interactions. Thus, not only can quinoline alkaloids dock intercalatively into DNA, but the docked orientations are also electronically favorable.

A Density Functional Theory Study on the Ring-Opening Polymerization of D-Lactide Catalyzed by a Bifunctional-Thiourea Catalyst

The thiourea-catalyzed methanolysis of d-lactide, a model system for the initiation and propagation of the organocatalyzed ring-opening polymerization (ROP) of lactide, has been studied by performing density functional theory calculations. Both the catalyzed and uncatalyzed reactions are explored along two possible pathways: one involves the stepwise addition–elimination pathway and the other is related to the concerted pathway. It is found that the reaction without the presence of the catalyst is difficult because the barrier involved is as high as 176 kJ mol–1. With the aid of a thiourea catalyst, the barrier is reduced to 88 kJ mol–1 with a preference for the stepwise addition–elimination mechanism over the concerted one. The role of the catalyst has been rationalized by analyzing the frontier molecular orbital interactions between the catalyst and substrates and by performing natural population analysis. Finally, another mechanism involving acyl transfer is discussed for the thiourea-catalyzed ROP.

Theory of 1,3-Dipolar Cycloadditions: Distortion/Interaction and Frontier Molecular Orbital Models

Quantum chemical calculations of activation barriers and reaction energies for 1,3-dipolar cycloadditions by the high-accuracy CBS-QB3 method reveal previously unrecognized quantitative trends in activation barriers. The distortion/interaction model of reactivity explains why (1) there is a monotonic decrease of approximately 6 kcal/mol in the activation energy along the series oxides, imine, and ylide for the diazonium, nitrilium, and azomethine betaine classes of 1,3-dipoles; (2) nitrilium and azomethine betaines with the same trio of atoms have almost identical cycloaddition barrier heights; (3) barrier heights for the cycloadditions of a given 1,3-dipole with ethylene and acetylene have the same activation energies (mean absolute deviation of 0.6 kcal/mol) in spite of very different reaction thermodynamics (Delta DeltaH(rxn) range = 14-43 kcal/mol) and frontier molecular orbital (FMO) energy gaps. The energy to distort the 1,3-dipole and dipolarophile to the transition state geometry, rather than FMO interactions or reaction thermodynamics, controls reactivity for cycloadditions of 1,3-dipoles with alkenes or alkynes. A distortion/interaction energy analysis was also carried out on the transition states for the cycloadditions of diazonium dipoles with a set of substituted alkenes (CH2CHX, X = OMe, Me, CO 2Me, Cl, CN) and reveals that FMO interaction energies between the 1,3-dipole and the dipolarophile differentiate reactivity when transition state distortion energies are nearly constant.