Some peculiarities of the effect of molecular structure on the activation energy of radical gas-phase decay of aromatic nitro compounds

The relation of parameters of geometric and electronic structures to the activation energies of the radical gas-phase decomposition of nitro compounds is shown. A sufficiently simple and precise method for calculation of the activation energy is suggested.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2118–2122, December, 1994.     

ESR study of radical intermediates formed upon photooxidation of 4,4"-diazidodiphenyl

Radical intermediates (phenyl and phenylcarbonyl radicals) formed in photooxidation of 4,4"-diazidodiphenyl in benzene and toluene have been studied by ESR spectroscopy. These radicals are formed as a result of abstraction of a hydrogen atom from a solvent molecule by the triplet nitrene—dioxygen complex.     

Tautomerism of peroxyacetic acid derivatives: a quantum chemical study

The electronic and molecular structures and the relative stabilities of organic peracids X=C(R)-COOH and their cyclic tautomers, dioxiranes

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, with R = Me, CF₃; X = O, NH, were studied using the ab initio Hartree-Fock method and the density functional theory (B3LYP version) as well as at the MP2-MP4 Møller-Plesset levels of perturbation theory. Geometry optimization was performed by the UHF and B3LYP methods with the 6-31G** basis set and at the MP2/cc-pvtz level of theory. The acyclic form of the peracid is more stable than the cyclic dioxirane form irrespective of the nature of the substituent. The energy difference between these tautomers increases as the CF₃ and NH groups are replaced by Me and O, respectively. Parameters of the activation barrier to tautomeric conversion increase in parallel with enhancement of the electron-accepting properties of both substituents. The transition state of tautomeric interconversion, which is topologically similar to the acyclic structure of the carbonyl oxide derivative R(HX)C=O⁺-O^?, was found and characterized taking peroxyacetic acid as an example. The characteristic features of the transition state are an intramolecular “multicenter” H-bond and the charge distribution that is inconsistent with the canonical structure mentioned above. An appropriate reaction coordinate for the transformation of the quasi-tetrahedral dioxirane structure into a planar peroxyacetic acid structure is provided by the dihedral angle. Deprotonated anionic systems are characterized by much smaller differences between the relative stabilities of the open and closed forms of isomers and by much lower activation barriers to isomeric conversions.

     

Synthesis of a cobalt-based photoluminescent coordination complex to study quenching mechanisms of nitro compounds

Abstract

A cobalt metal coordination complex was synthesized using the bipyridyl spacer ligand along with a dicarboxylate as secondary ligand. The coordination complex was characterized using FTIR, ¹H NMR, elemental analysis, thermogravimetric analysis, and powder XRD. The structure shown from single-crystal XRD revealed that each Co(II) cation in the cluster can be described as trigonal bipyramidal. Luminescence properties of the complex were investigated using different solvents. The photoluminescence (PL) spectra depicted change in band gap for THF and DCM. The complex was further investigated for ligand-based photoluminescence properties and thus used for sensing of nitro compounds. The static and dynamic quenching mechanism of nitro compounds was identified through the Stern–Volmer model.     

Some peculiarities of the influence of molecular structure on the activation energy of radical gas-phase decomposition of aliphatic nitro compounds

Geometric parameters of nitro and fluoronitro derivatives of nitromethane, nitroethane, I-nitropropane, and I-nitrobutane (for which experimental data on kinetics of radical gas-phase decomposition are available) have been determined by the MINDO/3 method. Correlations between changes in logarithm of the activation energy and those in the lengths of C-N and C-H bonds as well as in the ionization potentials have been found. The major changes in the C-N- and C-H- bonds occur when bulky atoms and groups (N02, Cl, Br, and 1) are introduced into the molecules. Nonempirical calculations of energies of dissociation of the C-N bonds in the molecules of nitromethane and its halogen derivatives have been carried out. An equation was proposed which allows one to perform a high-accuracy determination of the activation energy for radical gas-phase decomposition of nitroalkanes using the coefficients of steric interactions in the molecules calculated by the methods of molecular mechanics.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2438–2444, October, 1996.     

Sandwich compounds of second-row elements: A quantum chemical study

The structures and stabilities of a number of neutral and charged sandwich-type boron, carbon, and nitrogen compounds designed based on the cyclophane cage and obeying the “electron octet” rule were studied by the B3LYP/6−311+G** density functional method. The possibility of targeted modification of the electronic structures of such compounds by varying the basal or bridging atomic groups was investigated. Dedicated to Academician O. M. Nefedov on the occasion of his 75th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1825–1835, November, 2006.     

Dependence of the extraction capacity of neutral bidentate organophosphorus compounds on their structure: a quantum chemical study

Correlations between the extraction capacities and molecular structures of organophosphorus compounds (reagents for extraction of transplutonium elements from spent nuclear fuel) were studied using a quantum chemical approach. The results of calculations are in qualitative agreement with experimental data. The approach proposed can be used for analysis of the extraction properties of all classes of organophosphorus compounds and also for prediction of the most efficient organophosphorus extragents with preset properties. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 609–615, April, 2006.     

Growing graphene sheets from reactions with methyl radicals: a quantum chemical study.

Hydrogen abstraction reactions by methyl radicals on the zigzag and armchair edges of perylene are studied by density functional theory (DFT) to explore various growth pathways that seem to be in line with experimental observations. The DFT approach is validated by comparing the results obtained from calculations with six different functionals with those obtained from correlated ab initio methods, thereby emphasizing the calculation of reaction barriers. A useful compromise between accuracy and computational cost is provided by DFT, and possible pathways are studied in detail at this level of calculation. Our computational study is carried out by ordering, as a first step, all of the isomers that arise from the abstraction of one or two H atoms from 1,12-dimethyl-1,12-dihydroperylene and 3,4-dimethyl-3,4-dihydroperylene with respect to their energies. Subsequently, only those pathways that connect low-energy isomers are investigated. The calculations reveal that the selected pathways are favored thermodynamically, and also that the reaction barriers are somewhat higher than the energy locally available for the respective reaction. Notably, in the case of 3,4-dimethyl-3,4-dihydroperylene, the first two reaction steps have no or only a very low reaction barrier. The final conclusion of our study is that a cascade of reactions is possible that leads to the growth of a graphene sheet on a graphite surface.     

Methodology of simple quantum chemical estimation of solid-phase heats of formation with constant intermolecular interactions over the series of compounds (by the example of quinones)

Heats of formation for some quinone compounds, including both 1,2-quinones and 1,4-quinones, are calculated. Linear correlations, uniform for all the compounds studied, which relate the experimentally calculated in the solid state and quantum chemically calculated gas phase standard heats of formation are found. The correlation coefficient values for all the dependences found exceed 0.98. The electron density distribution in 1,2-benzoquinone and 1,4-benzoquinone molecules is investigated using DFT at the B3LYP/6-311++G(3df,3pd) theory level using the NBO-analysis. Essential common features of the electronic distribution in the above molecules are stated, which serves as a premise for a constant contribution of the crystal field effect to the heats of formation of ortho-and para-quinones. On the whole, by the example of quinones we develop a methodology for a simple theoretical estimation of the solid phase (and also in any condensed state) heats of formation of chemical compounds under the condition of approximately constant intermolecular interactions in the series in question.     

A method and program for mass quantum chemical calculations of protein—ligand docking complexes

A new fast computational method for mass calculations of docking complexes by the AM1/PM3 semiempirical methods is proposed. The computation time is shortened by at least an order of magnitude compared to alternative schemes of quantum chemical calculations. The root-mean-square deviation of the AM1 calculated energies of formation of complexes from the results obtained by conventional diagonalization procedure is at most 0.4 kcal mol⁻¹. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 418–420, February, 2008.     

Polar and solvation effects in reactions of oxygen atoms and hydroxyl and alkoxyl radicals with oxygen-containing compounds

Experimental data on the activation energies of reactions of H-abstraction from oxygencontaining compounds by oxygen atoms and hydroxyl and alkoxyl radicals in the gas and liquid phases have been analyzed by means of the parabolic model of the transition state. The contribution of polar interaction to the activation energies of the reactions has been calculated. The contribution of solvation to the activation energy has been calculated by comparison of the reaction parameters of the respective reaction in the liquid and gas phases.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 38–42, January, 1994.     

Molecular structure and properties of oxime phosphonates: an FTIR and quantum chemical study

Peculiar features of the molecular structure, isomeric composition, and hydrogen bonding in new, potentially physiologically active γ-oxime alkylphosphonates were established. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1253–1258, July, 2007.     

The reactivity of ketyl and alkyl radicals in reactions with carbonyl compounds

A parabolic model of bimolecular radical reactions was used for analysis of the hydrogen transfer reactions of ketyl radicals: >C·OH+R¹COR²→>C=O+R¹R²C·OH. The parameters describing the reactivity of the reagents were calculated from the experimental data. The parameters that characterize the reactions of ketyl and alkyl radicals as hydrogen donors with olefins and with carbonyl compounds were obtained: >C·OH+R¹CH=CH₂→>C=O+R¹C·HCH₃; >R¹CH=CH₂+R²C·HCH₂R³→R²C·HCH₃+R²CH=CHR³. These parameters were used to calculate the activation energies of these transformations. The kinetic parameters of reactions of hydrogen abstraction by free radicals and molecules (adelhydes, ketones, and quinones) from the C−H and O−H bonds were compared. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2178–2184, November, 1998.     

Electronic structure of 1,3-dinitrobenzene radical anion: A multiconfigurational quantum chemical study

The structure of 1,3-dinitrobenzene radical anion in the doublet ground and lowest excited states was studied by ab initio multiconfiguration CASSCF methods. The results of calculations suggest the existence of one symmetrical and two asymmetrical structures of the radical anion. The energies of these structures were estimated. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2645–2647, December, 2005.     

Electronic structure and reactivity of guanylthiourea: A quantum chemical study

Electronic structure analysis of guanylthiourea (GTU) and its isomers has been carried out using quantum chemical methods. Two major tautomeric classes (thione and thiol) have been identified on the potential energy (PE) surface. In both the cases conjugation of pi‐electrons and intramolecular H‐bonds have been found to play a stabilizing role. Various isomers of GTU on its PE surface have been analyzed in two different groups (thione and thiol). The interconversion from the most stable thione conformer ( GTU‐1 ) to the most stable thiol conformer ( GTU‐t1 ) was found to take place via bimolecular process which involves protonation at sulfur atom of GTU‐1 followed by subsequent C? N bond rotation and deprotonation. The detailed analysis of the protonation has been carried out in gas phase and aqueous phase (using CPMC model). Sulfur atom (S1) was found to be the preferred protonation site (over N4) in GTU‐1 in gas phase whereas N4 was found to be the preferred site of protonation in aqueous medium. The mechanism of S‐alkylation reaction in GTU has also been studied. The formation of alkylated analogs of thiol isomers (alkylated guanylthiourea) is believed to take place via bimolecular process which involves alkyl cation attack at S atom followed by C? N bond rotation and deprotonation. The reactive intermediate RS(NH₂)C? N? C(NH₂)₂⁺ belongs to the newly identified ^⊕N(←L)₂ class of species and provides the necessary dynamism for easy conversion of thione to thiol. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Deactivation reactions in the modeled 2,2,6,6‐tetramethyl‐1‐piperidinyloxy‐mediated free‐radical polymerization of styrene: A comparative study with the 2,2,6,6‐tetramethyl‐1‐piperidinyloxy/acrylonitrile system

The competitiveness of the combination and disproportionation reactions between a 1‐phenylpropyl radical, standing for a growing polystyryl macroradical, and a 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO) radical in the nitroxide‐mediated free‐radical polymerization of styrene was quantitatively evaluated by the study of the transition geometry and the potential energy profiles for the competing reactions with the use of quantum‐mechanical calculations at the density functional theory (DFT) UB3‐LYP/6‐311+G(3df, 2p)//(unrestricted) Austin Model 1 level of theory. The search for transition geometries resulted in six and two transition structures for the radical combination and disproportionation reactions, respectively. The former transition structures, mainly differing in the out‐of‐plane angle of the N? O bond in the transition structure TEMPO molecule, were correlated with the activation energy, which was determined to be in the range of 8.4–19.4 kcal mol^?1 from a single‐point calculation at the DFT UB3‐LYP/6‐311+G(3df, 2p)//unrestricted Austin Model 1 level. The calculated activation energy for the disproportionation reaction was less favorable by a value of more than 30 kcal mol^?1 in comparison with that for the combination reaction. The approximate barrier difference for the TEMPO addition and disproportionation reaction was slightly smaller for the styrene polymerization system than for the acrylonitrile polymerization system, thus indicating that a β‐proton abstraction through a TEMPO radical from the polymer backbone could diminish control over the radical polymerization of styrene with the nitroxide even more than in the latter system. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 232–241, 2007     

Reactions of fullerene C60 with methyl methacrylate radicals: A density functional theory study

We have investigated the stepwise addition of four growing methyl methacrylate (MMA) radicals to C₆₀ fullerene, taking into account all possible types of the formed adducts. This reaction set is a reliable approximation for understanding the MMA polymerization process in the presence of C₆₀ fullerene. We have analyzed the structures of the fullerene-MMA adducts and energy parameters of their formation (heat effects and activation enthalpies). We found that up to three MMA growing radicals are favorably attached to C₆₀ as the fullerene-MMA trisadduct is a stable radical of the allyl type. It is inactive for further radical addition, and the elimination of the hydrogen atom from the growing MMA radical becomes preferable. The effects of steric factors and structures of the products of multiple growing MMA radical additions to C₆₀ on the radical polymerization of MMA in the presence of C₆₀ fullerene are considered.     

Cyclization tendencies in the free radical polymerization of allyl acrylate derivatives: A computational study

In this study density functional theory (DFT) has been used to model the elementary steps and rationalize the free radical polymerization kinetics in allyl methacrylate (AMA), allyl 2‐cyanoacrylate (ACA) and methyl α‐[(allyloxy)methyl]acrylate. The models used in this study have revealed the fact that while methyl α‐[(allyloxy)methyl]acrylate, cyclopolymerizes via 5‐membered rings, AMA and ACA do not. The cyclization tendency of methyl α‐[(allyloxy)methyl]acrylate is attributed to the similar hybridization (sp³) of C3 and C5 favoring a quasi cyclic structure for the reactive rotamer. On the other hand, the presence of the cyano (CN) group in ACA facilitates the initiation step as compared to AMA. The chain transfer reaction does not seem to play a major role in the monomers of interest. This study highlights the usage of quantum chemistry in determining the cyclization tendencies of allyl acrylate derivatives in their free radical polymerization reactions. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Ab initio study of reactions of hydroxyl radicals with chloro‐ and fluoro‐substituted methanes

Ab initio calculations at the unrestricted Hartree–Fock (UHF) level have been performed to investigate the hydrogen abstraction reactions of ^? OH radicals with methane and nine halogen‐substituted methanes (F, Cl). Geometry optimization and vibrational frequency calculations have been performed on all reactants, adducts, products, and transition states at the UHF/6‐31G* level. Single‐point energy calculations at the MP2/6‐31++G* level using the UHF/6‐31G* optimized geometries have also been carried out on all species. Pre‐ and postreaction adducts have been detected on the UHF/6‐31G* potential energy surfaces of the studied reactions. Energy barriers, ΔE^?, reaction energies, ΔE_r, reaction enthalpies, ΔH_r, and activation energies, E_a, have been determined for all reactions and corrected for zero‐point energy effects. Both E_a and ΔH_r come into reasonable agreement with the experiment when correlation energy is taken into account and when more polarized and diffuse basis sets are used. The E_a values, estimated at the PMP2/6‐31++G* level, are found to be in good agreement with the experimental ones and correctly reproduce the experimentally observed trends in fluorine and chlorine substitution effects. A linear correlation between E_a and ΔH_r is obtained, suggesting the presence of an Evans–Polanyi type of relationship. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 84: 426–440, 2001