Unimolecular Decomposition Mechanism Research Articles

Theoretical Investigations on the Thermal Decomposition Mechanism of 5-Hydroxy-6-hydroperoxy-5,6-dihydrothymidine in Water

The main primary product of DNA oxidation by free radicals is 5-hydroxy-6- hydroperoxy-5,6-dihydrothymidine (5-OH-6-OOH-DHT), whose further degradation can yield the other mutagenic products and amplify the spectrum of DNA damage. In this study, to illustrate the thermal stability of 5-OH-6-OOH-DHT in DNA, the decomposition mechanism of 5-OH-6-OOH-DHT was identified based on the cis-(5R,6S) diastereomer. Optimized structures for all of the stationary points in the gas phase were investigated at the B3LYP/6-31+G(d,p) level of theory. Four pathways were characterized. The decomposition mechanism of 5-OH-6-OOH-DHT was proposed to involve either dehydration for paths A and B or the cleavage of a glycosidic bond for paths C and D. Moreover, to simulate the title reaction in aqueous solution, a water-mediated mechanism and cluster-continuum model, based on the SCRF/CPCM model, were taken into account. The results indicate that the most favorable reaction pathways for paths A and B both involve a sort of eight-membered ring transition structure formed by two (path A) or one (path B) auxiliary water molecules, suggesting that the thermal decomposition of 5-OH-6-OOH-DHT can be significantly facilitated by water molecules. Path A is the most feasible mechanism reported for the decomposition of 5-OH-6-OOH-DHT in the aqueous solution, which is slightly more favorable than path B. However, the unimolecular decomposition mechanisms (paths C and D) both have high-energy barriers and are largely endothermic, suggesting that the cleavage of the N-glycosidic bond via unimolecular decomposition is thermodynamically and kinetically unfavorable. These studies have shed light on the chemical properties of 5-OH-6-OOH-DHT in free radical reactions and thereby have provided new insights into the complex mechanism of oxidative DNA damage.

Theoretical prediction regarding structural and thermodynamical characteristics of stable CH3PO2 isomers and unimolecular decomposition mechanisms of species CH3P(O)2, CH3OPO, and CH2P(O)OH

The detailed isomerization and dissociation reaction potential energy profile of the CH(3)PO(2) system was established at the UCCSD(T)/6-311++G(3df,2p)//UB3LYP/6-311++G(d,p) level of theory. Seventy minimum isomers were located and connected by 93 optimized interconversion transition states. Furthermore, 32 isomers with high kinetic stability were predicted to be possible candidates for further experimental detection. The bonding nature of the suggested stable isomers was analyzed while their molecular properties including heats of formation, adiabatic ionization potentials, and adiabatic electronic affinities were calculated at the G2, G2(MP2), G3, and CBS-Q levels. Based on the isomerization and dissociation potential energy surface, possible unimolecular decomposition mechanisms and pathways of the low-lying molecules CH(3)P(=O)(2), CH(3)O-P=O, and CH(2)=P(=O)OH were discussed. Furthermore, the transition state theory rate constants of the primary unimolecular dissociation channels were also calculated.

Unimolecular Decomposition of 5-Aminotetrazole and its Tautomer 5-Iminotetrazole: New Insight from Isopotential Searching

Aminotetrazole compounds have become attractive ingredients in gas generating compositions, solid rocket propellants, and green pyrotechnics. Therefore, a fundamental understanding of their thermal decomposition mechanisms and thermodynamics is of great interest. In this study, the specular reflection isopotential searching method was used to investigate the unimolecular decomposition mechanisms of 5-iminotetrazole (5-ITZ), 1H-5-aminotetrazole (1H-5-ATZ), and 2H-5-aminotetrazole (2H-5-ATZ). Subsequent thermochemical analysis of the unimolecular decomposition pathways was performed at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311++G(3df,3pd) level of theory. Based upon the relative reaction barriers predicted in this study, the initial gaseous products of 5-ITZ unimolecular decomposition are HN(3) and NH(2)CN (calculated activation barrier equal to 199.5 kJ/mol). On the other hand, the initial gaseous products of 1H-5-ATZ and 2H-5-ATZ unimolecular decomposition are predicted to be N(2) and metastable CH(3)N(3) (calculated activation barriers equal to 169.2 and 153.7 kJ/mol, respectively). These predicted unimolecular decomposition products and activation barriers are in excellent agreement with thermal decomposition experiments performed by Lesnikovich et al. [Lesnikovich, A. I.; Ivashkevich, O. A.; Levchik, S. V.; Balabanovich, A. I.; Gaponik, P. N.; Kulak, A. A. Thermochim. Acta 2002, 388, 233], in which the apparent activation barriers were measured to be approximately 200 and 150 kJ/mol, respectively, for 5-ITZ and 1H-5-ATZ/2H-5-ATZ.

Ab initio studies of the thermal decomposition pathways of 1-bromo-3,3,3-trifluoropropene

The complex potential energy surface for the unimolecular isomerization and dissociation of 1-bromo-3,3,3-trifluoropropene (CF3CHCBrH), including 12 CF3CHCBrH isomers, 33 interconversion transition states and 13 major dissociation products, is probed theoretically at the CCSD/aug-cc-pVDZ//B3LYP/6-311++G(d, p) level of theory. The geometries, vibrational frequencies and relative energies for various stationary points are determined. Based on the calculated CCSD/aug-cc-pVDZ potential energy surface, the possible 1-bromo-3,3,3-trifluoropropene unimolecular decomposition mechanism is discussed. Eight distinct decomposition pathways of 1-bromo-3,3,3-trifluoropropene (BTP) are investigated at the CCSD/aug-cc-pVDZ//B3LYP/6-311++G(d, p) theory level. The reaction enthalpies and energy barriers are determined. From the energetics, the most feasible decomposition pathways for 1-bromo-3,3,3-trifluoropropene are those that lead to CF3CCH + HBr and CF2CCBrH + HF. Considering the extreme high flame temperature, the decomposition pathways are competitive. The BTP dissociation occurs via concerted molecular eliminations.

Decomposition mechanisms of dinitrotoluene

AbstractTo develop a viable detection system for 2,4‐dinitrotoluene (DNT), its unimolecular decomposition mechanisms were studied by using the density functional theory. Three decomposition mechanisms were investigated: (1) homolytic cleavage; (2) migration of one of the O atoms in the NO2 groups to its neighboring C atoms; (3) migration of one of the H atoms in the methyl group to a neighboring O atom in the NO2 group. Only homolytic cleavage of the CN bond yields NO2 radicals with a total energy of 61 kcal/mol or more. Initial migration of the O atom to its second‐nearest‐neighbor C atom requires about 56 kcal/mol, but subsequent homolysis produces NO fragments with less energy ∼15 kcal/mol. NO formation from both NO2 groups in 2,4‐DNT is possible with a total energy of ∼63 kcal/mol. The H‐migration has the lowest energy barrier of 39 kcal/mol among the three initial decomposition mechanisms. Removing O directly from 2,4‐DNT requires ∼90 kcal/mol, while NO2 radical decomposes to NO and O with ∼73 kcal/mol. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Ab initio studies of isomerization and dissociation reactions of peroxyacetyl nitrate (PAN)

The complex potential energy surface for the unimolecular isomerization and decomposition of peroxyacetyl nitrate (PAN, CH3C(O)OONO2), including 11 isomers, 45 interconversion transition states and 18 major dissociation products, is probed theoretically at the G2MP2//B3LYP/6-311G(2d,2p) level of theory. The geometries and relative energies for various stationary points are determined. Based on the calculated G2MP2 potential energy surface, the possible unimolecular decomposition mechanism of PAN is proposed. It is shown that the most feasible decomposition channels of PAN are those leading to 2CH3C(O)OO+NO2, 2CH3C(O)O+2NO3, and 2CH3+CO2+2NO3, respectively. Among them, the formations of the products of the first two channels are produced by the homolytic O–N and O–O bond ruptures of PAN with the bond dissociation energies of 32.3 and 33.9 kcal/mol, respectively, while the last one is initiated by the concerted C–C and O–O bond fissions via a barrier of 36.5 kcal/mol. Our results suggest that besides the first two decomposition pathways which have been reported by the literature, the last concerted bond fission dissociation channel via a well-defined transition state is also feasible, which has been confirmed by using CBS-Q and CBS-QB3 methods. Therefore, PAN dissociation occurs mainly via the homolytic O–N or O–O bond ruptures or via the concerted C–C and O–O bond fissions while the first channel is more favorable than the last two channels.

Theoretical study of the unimolecular decomposition mechanisms of energetic TNAD and TNAZ explosives

AbstractCalculation methods based on hybrid Density Functional Theory (DFT) with the basis sets of the B3LYP/6‐31+G(d)//B3LYP/4‐31G(d) method and the differential overlap (INDO) program were used to derive reasonable decomposition mechanisms of 1,4,5,8‐tetranitro‐1,4,5,8‐tetraazadecalin (TNAD) and 1,3,3‐trinitroazetidine (TNAZ) explosives. All possible decomposition species and transition states, including intermediates and products, were identified and their corresponding enthalpy of formation and Gibbs free energy of formation were obtained using polyparametric modification equations. INDO bond energy calculation results reveal the weakest bonding site for reference and determine where cleavage can occur easily. This work is concerned mainly with eliminating HONO (cis or trans form). The activation energy for trans‐form HONO elimination is lower than that of cis‐form HONO elimination in the initial steps of both TNAD and TNAZ decomposition, being 18.5 kJ/mol and 33.3 kJ/mol, respectively. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

A Quantum Chemical Study of the Unimolecular Decomposition Mechanisms of Zinc Dialkyldithiophosphate Antiwear Additives

Density functional theory calculations were applied to study a set of isomerization and decomposition reactions that zinc dialkyldithiophosphate (ZDDP) engine oil antiwear additives may take part in. The products of these reactions comprise a set of chemical species that can lead to the formation of poly(thio)phosphate films that are thought to be responsible for the antiwear protection offered by ZDDPs. The specific reactions examined involved either intramolecular alkyl group transfer within ZDDP or the intramolecular elimination of olefins from this molecule. A series of substituents were employed to examine how the nature of the substituent on the ZDDP molecule affects the thermodynamic and mechanistic details of these reactions, which in turns may have ramifications regarding various aspects of antiwear film formation. It was found that qualitative and quantitative aspects of these reactions were markedly different when hydrogen atoms were used as substituents instead of alkyl groups. The details of ...

Studies on Unimolecular Metastable Decomposition of Several Organic Compounds

The unimolecular metastable decomposition mechanism of several organic compounds, which have small molecular weights, was investigated in this study by means of mass-analyzed ion kinetic energy (MIKE) spectrometry, D-labeling technique and molecular orbital (MO) calculations. Accurate mass determination is also described by using a multiple sprayers Nano-ESI arrangement on a double-focusing magnetic sector instrument in this thesis. For example, ‘Unimolecuar HCl Loss from the Molecular Ions of Chlorophenols, a “Ring-walk” Mechanism for a Chlorine Atom’ is described in this abstract. The loss of hydrogen chloride (HCl) is one of the main fragmentation processes in the molecular of o, m-, and p-chlorophenols in the metastable time window. By investigation of D-labeled compounds, it could be established that the eliminated HCl molecule contains only the hydroxyl hydrogen atom, not other benzene ring hydrogen atoms. This process is rationalized by the so-called “ring-walk” mechanism for a chlorine atom.

Theoretical Study of the CH3NO2 Unimolecular Decomposition Potential Energy Surface

The complex potential energy surface for the unimolecular isomerization and dissociation of nitromethane (CH3NO2), including 10 CH3NO2 isomers, 46 interconversion transition states, and 16 major dissociation products, is probed theoretically at the G2MP2//B3LYP/6-311++G(2d,2p) level of theory. The geometries and relative energies for various stationary points are determined and are in good agreement with the available experimental values. Based on the calculated G2MP2 potential energy surface, the possible nitromethane unimolecular decomposition mechanism is discussed. It is shown that the most feasible decomposition channels for CH3NO2 are those that lead to 2CH3 + 2NO2, 2CH3O + 2NO, H2CO + HNO, and HCNO + H2O, respectively. Among them, 2CH3 and 2NO2 are produced by the direct C−N bond rupture of nitromethane, while the formation of the latter three products is initiated by CH3NO2 rearranging first to methyl nitrite or to aci-nitromethane. The C−N bond dissociation energy for nitromethane is calculated t...

Thermal Decomposition of 5-Nitraminotetrazole of 3-Azolylamino-2-polyfluorobenzoylacrylates

Thermal decomposition of 5-nitraminotetrazole in solid phase and diluted solutions in aprotic solvents was investigated. The kinetic and thermodynamic parameters of the process were established. A mechanism of unimolecular decomposition was assumed including isomerization into the nitroguanyl azide followed by decomposition of he azide function.

A Theoretical Investigation of the Decomposition Mechanism of Pyridyl Radicals

We carried out a detailed quantum mechanical study of the unimolecular decomposition mechanism of pyridine. The critical structures of all reasonable reaction pathways were optimized by density functional theory using the B3LYP functional and 6-31G** basis set. Relative energies were evaluated based on single-point QCISD(T)/cc-pVDZ energies. In agreement with general belief and pervious theoretical studies, the calculated results indicate that C−H bond scission in pyridine preferentially produces the o-pyridyl radical. Also in agreement with the accepted mechanism, the calculations indicate that ring-opening via C−N bond cleavage in o-pyridyl radical is more favorable than C−C bond cleavage, as the former has a significantly lower activation barrier and the resulting open-chain cyano radical is more stable than other linear C5NH4 radicals. The calculated activation energy for the formation of cyanovinylacetylene + H from the open-chain cyano radical is the lowest, compared to the other channels considered...

Reaction of dimethylethylamine alane and ammonia on Si(100) during the atomic layer growth of AlN: static SIMS, TPSIMS, and TPD

Dimethylethylamine alane [DMEAA; AlH 3:N(CH 3) 2(CH 2CH 3)] has been used as an Al source in the chemical vapor deposition of AlN. In the atomic layer growth mode, ammonia and DMEAA interact selectively by nucleophilic displacement. In the first part of this study, the surface adsorption and reaction processes are characterized with static secondary-ion mass spectrometry and temperature-programmed secondary-ion mass spectrometry. The secondary ion emission from DMEAA-covered Si surface shares similar general characteristics with the gas phase cracking pattern. The secondary ion emission spectrum is interpreted according to a unimolecular ion decomposition mechanism and is used as the fingerprint for the presence of molecular DMEAA. During the surface reaction between DMEAA and ammonia, the intensity of the fingerprint peaks diminish, representing the departure of the amine ligand. The thermal stability of DMEAA and its decomposition behavior on Si are also examined.

A theoretical study of the unimolecular decomposition of N-chloro-α-amino acids in aqueous solution

The substituent effects on the barrier heights for the unimolecular decomposition of the anionic form of different N-chloro-α-amino acids have been analyzed. The positions of the transition structures on the potential energy surfaces in the singlet and triplet electronic states have been analyzed by means of a More O'Ferrall–Jencks diagram. The selected model systems were: N-chloroglycine, N-chloroalanine, N-chloro-2-amino-butyric acid, N-chlorosarcosine, N-chloro-N-ethylglycine, N-chloro-2-amino-isobutyric acid and N-chloro-N-methylalanine. The present study has been carried out taking into account the solvent effects by means of the self-consistent reaction field method. The calculations has been carried out with 6-31G* basis set and the electron correlation energy has been included at CISD level. The results complement our earlier works (J. Phys. Chem. 100 (1996) 3561; J. Phys. Org. Chem. 9 (1996) 371), in which the unimolecular decomposition mechanism of the N-chloroglycine and other N-chloroamino acids was described as an asynchronous concerted process with an antiperiplanar conformation for the corresponding transition structures. The main change with respect to the gas phase results is an enhancement of the asynchronicity of the decomposition process. In addition, an increase of the size and number of substituents on the Cα atom, and to a lesser extent on N atom, decreases the barrier height. N-substitution effects control the perpendicular effects along the reaction pathway.

Ab Initio Study of RDX Decomposition Mechanisms

The mechanism of the gas phase unimolecular decomposition of hexahydro-1,3,5,-trinitro-1,3,5,-triazine (RDX) has been investigated using first principles gradient-corrected density functional theory. We have calculated the potential energy profile for two previously suggested dissociation channels: (I) N−NO2 bond rupture, and (II) concerted ring fission to three methylenenitramine molecules. The activation barriers for channels I and II are predicted to be 34.2 and 52.5 kcal/mol at the B-PW91/cc-pVDZ level, respectively. We have performed a simple transition state theory analysis which indicates that the prefactors for channel I and II are roughly 7 × 1017 and 1 × 1017 s-1, respectively. Thus, our results suggest that path I is the dominant channel in gas phase thermal RDX decomposition.

A comparative QCISD(T), DFT and MCSCF study of the unimolecular, decomposition of the N-chloro-α-glycine anion in gas phase

Stationary points were localized and characterized on potential energy surfaces for the unimolecular decomposition of the anionic form of N-chloro-α-glycine in its singlet and triplet electronic states by means of QCISD(T), DFT and MCSCF methods. The present study predicts that the unimolecular decomposition mechanism takes place in the singlet electronic state through a concerted and slightly asynchronous process and the transition structure has an antiperiplanar conformation. A comparison of the structures for stationary points calculated with different methods yields similar geometries and the components of transition vector are weakly dependent on the computing method.

Unimolecular Decomposition of the Anionic Form ofN-Chloro-α-glycine. A Theoretical Study

The molecular mechanism for the unimolecular decomposition of the anionic form of N-chloro-α-glycine in its singlet and triplet electronic states has been theoretically characterized by using analytical gradients and AM1 and PM3 semiempirical procedures as well as the ab initio RHF and UHF methods at 4-31G, 6-31G*, and 6-311+G* basis set levels. Correlation effects were included by using the perturbational approach at the MP2/6-31G* level. The global potential energy surfaces have been further studied, and the stationary points were localized and characterized. The geometry, electronic structure, and the transition vector associated with the transition structures have been analyzed. The dependence of these properties upon theoretical methods is analyzed and discussed. The topological analysis of the fundamental singlet and triplet PESs shows the existence of a quadratic flat region that plays a fundamental role in the description of the nature of the molecular mechanism. The decomposition process takes place in the singlet electronic state. The geometry and electronic structure for TSs in singlet and triplet states correspond to antiperiplanar conformations, possessing a productlike character. The unimolecular decomposition mechanism is a concerted and slightly asynchronous process.

Ab Initio Study on the Unimolecular Decomposition Mechanisms and Spectroscopic Properties of CH3OF

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTAb Initio Study on the Unimolecular Decomposition Mechanisms and Spectroscopic Properties of CH3OFYitzhak Apeloig and Karsten AlbrechtCite this: J. Am. Chem. Soc. 1995, 117, 37, 9564–9569Publication Date (Print):September 1, 1995Publication History Published online1 May 2002Published inissue 1 September 1995https://doi.org/10.1021/ja00142a027RIGHTS & PERMISSIONSArticle Views87Altmetric-Citations19LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-Alerts Get e-Alerts

Subpicosecond resolution studies of the H+CO2→CO+OH reaction photoinitiated in CO2–HI complexes

Time resolved studies of the title reaction have been carried out by photodissociating the HI moiety within weakly bound CO2–HI complexes. The HOCO° intermediate decomposes via a unimolecular decomposition mechanism, and the emerging hydroxyl radicals are monitored with subpicosecond temporal resolution by using laser-induced fluorescence. The measured rates are in good agreement with several theoretical predictions: Rice, Ramsperger, Kassel, and Marcus (RRKM calculations; classical trajectory simulations on the best available potential energy surface; and recent quantum scattering calculations.

Picosecond resolution measurements of NO 3 unimolecular decomposition. The NO + O 2 channel

The title reaction has been studied with picosecond resolution at energies near threshold. Expansion cooled nitrate radicals were photoexcited to admixtures of 2E′ and ground state 2A′ 2 levels, ensuring a unimolecular decomposition mechanism. NO was monitored by LIF. The dissociation rate is ≈ 1 × 10 9 s −1 just above the 16800 cm −1 NO+O 2 barrier and increases rapidly with energy, reaching 5.5 × 10 9 s −1 at 17040 cm −1. A marked signal decrease at higher energies is attributed to NO 2+O dominating above its thermodynamic threshold. These observations support three-center NO+O 2 formation via 2A′ 2.