Theoretical study of the thermolysis reaction of β‐hydroxynitriles in the gas phase

The gas‐phase thermal decomposition of 3‐hydroxypropionitrile, 3‐hydroxybutyronitrile, and 3‐hydroxy‐3‐methylbutyronitrile has been studied at the MP2/6‐31G(d) level of theory at 683.15 K and 0.06 atm. Results based both in energy and structure data seem to indicate a favorable route of decomposition via a six‐membered cyclic transition state (similar to those suggested for thermal decomposition of other related compounds, such as β‐hydroxyketones, β‐hydroxyalkenes, and β‐hydroxyalkynes) rather than a four‐membered cyclic transition state or even a quasiheterolytic pathway. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003     

A theoretical study of the interactions between N,N-dimethylformamide and xylenes

The ab initio and density functional (DFT) methods were performed on binary systems of N,N-dimethylformamide (DMF) with xylenes (o-, or m-, or p-xylene), and seven stable configurations were obtained with no imaginary frequencies. To obtain the interaction energies of these complexes, single-point energy calculations with basis set superposition error (BSSE) correction were carried out at B3LYP/6-31G* and MP2/6-31G* levels. The structures, Chelpg (charges from electrostatic potentials using a grid-based method) charge distribution and bond characteristics of the mentioned complexes were calculated. The results indicated the presence of double C–H···O hydrogen bonds between DMF and xylenes in these complexes and the interaction energies of hydrogen bonding between DMF and xylene systems decreased in the following sequence: DMF–o-xylene: a1 > DMF–m-xylene: b1 > DMF–p-xylene: c1.     

Solvation Structure and Complexation of the Manganese(II) Ion in N,N-Dimethylpropionamide and N,N,N′,N′-Tetramethylurea Studied by Means of Titration Calorimetry and Raman Spectroscopy

Aprotic N,N-dimethylpropionamide (DMPA) and N,N,N′,N′-tetramethylurea (TMU) are both strong donor solvents and coordinate to metal ions through the carbonyl oxygen atom. These solvents show a different conformational aspect in the bulk phase, i.e., DMPA exists as either a planar cis or a nonplanar staggered conformer, while TMU exists in a single planar cis conformer. It has been established that the manganese(II) ion is solvated by five molecules in both solvents. Interestingly, although the planar cis conformer of DMPA is more favorable than the nonplanar staggered one in the bulk phase, the reverse is the case in the coordination sphere of the metal ion, i.e., a conformational change occurs upon solvation. To reveal the thermodynamic aspect of this conformational change, the complexation of Mn(II) with bromide ions in DMPA and TMU has been studied by titration calorimetry at 298 K. It was found that the Mn(II) ion forms mono-, di- and tri-bromo complexes in both solvents, and their formation constants, enthalpies and entropies were obtained. The Δ H∘₁ value for MnBr⁺ strongly depends on the solvent, i.e., it is positive (19.4 kJ-mol⁻¹) in DMPA and negative (−8.7 kJ-mol⁻¹) in TMU, whereas the Δ H^∘₂ and Δ H∘₃ values for the stepwise formation of MnBr₂ and MnBr₃⁻ are both small and negative. The enthalpy of transfer Δ_tH^∘ from DMPA to TMU, which is evaluated on the basis of the extrathermodynamic TATB assumption, is 25.5 kJ-mol⁻¹ for Mn²⁺ and −3.6 kJ-mol⁻¹ for MnBr⁺. These values indicate that the difference between the formation enthalpy of MnBr⁺ in the two solvents, Δ H^∘₁ (DMPA) – Δ H∘₁ (TMU), is mainly ascribed to the value of Δ_tH^∘(Mn²⁺). It is found that the metal ion is also five-coordinated in the monobromo complex, MnBr(DMPA)₄⁺ . The enthalpy for the conformational change of DMPA from its planar cis to the nonplanar staggered form is evaluated to be −11 and −5.5 kJ-mol⁻¹ for Mn(DMPA)₅^{2 +} and MnBr(DMPA)₄⁺, respectively. Note that these values are significantly smaller than the corresponding value (5.0 kJ-mol⁻¹) in the bulk phase. We thus conclude that, although steric hindrance among solvent molecules is reduced by replacing one DMPA of Mn(DMPA)₅^{2 +} with the relatively small bromide ion, DMPA molecules are still sterically hindered in the MnBr(DMPA)₄⁺ complex.     

Molecular structure of 6-(N,N-dimethylamino)fulvene from a joint gas-phase electron diffraction and quantum-chemical study

The molecular structure of 6-(N,N-dimethylamino)fulvene was studied by gas-phase electron diffraction and quantum-chemical methods (HF/6-31G(d), MP2/3Z, MP2/4Z, density functional theory with the B3LYP/6-31G(d) and PBE/3Z functionals). Pronounced flattening of the nitrogen atom and equalization of the intracyclic C—C bonds were found to be a consequence of the electron delocalization in the molecule.     

Ab initio study on the thermolysis of β-hydroxyolefins in gas phase

Thermolysis studies of β-hydroxyolefins in gas phase were realized using ab initio MP2 and DFT methods at the 6-31G^* levels to explore the possibility of determining a possible concerted process with a six-membered cyclic transition state (TS). Vibrational frequency calculations were carried out in order to confirm the stationary states, including TS structures. IRC calculations have been performed in all cases in order to verify that localized TS structures connect with the corresponding minimum stationary points associated with the reactant and products. With the aim of corroborating the postulated mechanism in the experimental study, we present a theoretical study in order to calculate the rate constants and the activation parameters. The results obtained are in accordance with the experimental conclusions.     

Photoinitiated processes in 1-p-tolylsulfonylazo-2,4,6,8-tetrakis(tert-butyl)phenoxazine

New photochromic compound 1-p-tolylsulfonylazo-2,4,6,8-tetrakis(tert-butyl)phenoxazine containing the intramolecular hydrogen bond NH...N and the corresponding model structures 2,4,6,8-tetrakis(tert-butyl)-1-(veratroylazo)phenoxazine and 2,4,6,8-tetrakis(tert-butyl)-N-acetyl-1-(p-tolylsulfonylazo)phenoxazine were synthesized and studied. The data obtained suggested the mechanism of the photoreaction resulting in the accumulation of betaine 1-hydroxy-2,4,6,8-tetrakis(tert-butyl)-10-tolylsulfonylphenoxazin-9-one. The photochromic transformations in the molecule under study are due to intramolecular proton phototransfer followed by E–Z-isomerization about the N–N bond and the formation of betaine 1-hydroxy-2,4,6,8-tetrakis(tert-butyl)-10-tolylsulfonylphenoxazin-9-one. The molecular and crystal structure of the photoproduct was studied by X-ray analysis.     

The maximum hardness and minimum polarizability principles as the basis for the study of reaction profiles

New and useful aspects of chemical reactivity as described by reactivity indexes and used in connection with the maximum hardness and minimum polarizability principles (MHP and MPP, respectively) are discussed and illustrated for two classical reactions in organic chemistry. They include the Beckmann rearrangement and the condensation reactions of -amino acids. The MPP appears as a more general rule than the MHP. Another relevant result is related to the usefulness of both empirical reactivity rules to predict the most probable reaction mechanism among two different pathways displaying very close values in activation energy (competitive pathways). This is illustrated for the condensation reaction of a series of -amino acids: while the accepted stepwise route follows both the MHP and MPP rules, the alternative concerted channel does not, yet its associated activation energy is slightly lower than that corresponding to the nonconcerted reaction mechanism.From the Proceedings of the 28th Congresco de Quimicos Teóricos de Expresión Latina (QUITEL 2002)     

Theoretical study on the mechanism of the gas-phase reaction of diborane(3) anion with carbon disulfide

The complex potential energy surface of the gas-phase reaction of HB(H)BH- with CS2 to give three low-lying products [B2H3S]- + CS, [BH2CS]- + HBS, and [BH3CS] + BS-, involving nine [B2H3CS2]- isomers and 12 transition states, has been investigated at the CCSD(T)/6-311++G(d,p)/B3LYP/6-311++G(d,p) level. Our calculations are in harmony with the recent experimental and theoretical results, and reveal some new bonding and kinetic features of this reaction system. Our theoretical results may help the further identification of the products [BH2CS]- + HBS and [BH3CS] + BS- and may provide useful information on the chemical behaviors of other electron-deficient boron hydride anions.     

Induced decomposition of di(tert-butyl)trioxide

Thermal decomposition of di(tert-butyl)trioxide (Bu^tOOOBu^t) in a wide range of concentrations was studied by visible and IR chemiluminescence. Induced decomposition of Bu^tOOOBu^t caused by its reaction with the peroxy radicals formed in the solvent (CH₂Cl₂) was found and investigated. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 924–927, May, 1997.     

Theoretical study of reaction mechanisms for the ketonization of vinyl alcohol in gas phase and aqueous solution

Theoretical ab initio calculations are done on different mechanisms for the conversion of vinyl alcohol to acetaldehyde, both in gas phase and in solution. Several basis sets are used in order to assess the accuracy of the results in gas phase and a continuum model of the solvent is employed to mimic reactions in water solution. The results indicate a catalytic action of water in hydrated clusters in gas phase, whereas in solution, and within the error limits of our calculations, both neutral water-chain and ionic mechanisms appear to be equally probable. Finally, the action of acids or bases is tested through the analysis of the reaction of vinyl alcohol with H₃O⁺ and HO^–. The results of the calculations are shown to be in qualitative agreement with the experimental facts when 6-31++G basis set is used but not when either STO-3G or 4-31G basis sets are employed.     

氧原子与二硫化碳反应的机理

用从头计算法、内禀反应坐标和电子密度拓扑分析方法研究了3P态氧原子与二硫化碳的反应.找到了分别形成CS SO,S OCS和S2 CO三个反应通道上的极小点和过渡态.采用UHF/631G进行几何构型优化,并在UMP2/631G水平上进行能量校正.三个反应通道上的稳定点和过渡态都经内禀反应坐标(IRC)跟踪得以确认,并用电子密度拓扑分析方法考察了反应过程中化学键的变化.计算结果表明,反应过程中所有稳定点和过渡态都具有Cs对称性,即对每个反应通道而言在整个反应过程中分子始终保持在同一平面内.在三个反应通道中,第一个反应通道O CS2→CS SO由于具有较小的活化能而更容易发生,与实验结果相一致.文中对反应机理进行了较详细的讨论.     

Theoretical study of neutral and protonated triple bonded molecules formed between C, N, Si, P, B and Al

A theoretical study of the possible protonation sites of simple molecules formed by C, N, Si, P, B and Al that present a triple bond between those atoms has been carried out. The calculations performed include MP2 and CCSD(T) methods with the aug-cc-pVTZ basis set. The nature of the protonated species has been analyzed with the Atoms In Molecules methodology. To Serafin, a free spirit and a good friend. Contribution to the Serafin Fraga Memorial Issue.     

IB 族金属-乙烯配合物[N{(Me)C(Ph)N}2]M-C2H4 (M=Cu,Ag, Au)的电子结构

采用从头算Hartree-Fock(HF),M??ller-Plesset微扰(MP2),二级近似耦合簇(CC2)和密度泛函理论(DFT)方法,对IB族金属-乙烯配合物LM-C2H4(L=[N{(Me)C(Ph)N}2];M=Cu,Ag,Au)的几何结构、电子结构以及LM与C2H4之间的结合能进行了理论研究.MP2、CC2和密度泛函方法对C2H4配位前后C=C键长的变化情况都给出了正确的描述.电子结构分析显示LM与C2H4之间主要以C2H4→LM"σ-给予"和LM→C2H4"π-反馈"方式协同成键,这种成键方式使C2H4配体π轨道上的电子密度下降,π*轨道上的电子密度增加,并使得C=C键长增加、键能下降,从而达到活化C=C键的目的.自然电荷布居和能量分解分析显示LM-C2H4中的"σ-给予"作用弱于"π-反馈"作用,若使用"σ-给予"作用强于"π-反馈"作用的M+-C2H4体系作为LM-C2H4的简化模型进行理论研究是不合适的.LM-C2H4中金属原子M的改变对C=C键长、C2H4电荷布居以及LM与C2H4之间的结合能等性质影响显著.LAu与LCu、LAg相比其接受和反馈电子的能力最强,使C2H4配体π轨道电子密度减少的程度和π*轨道电子密度增加的程度也最大,因此LAu对C2H4中C=C键的活化效果最好.螯合配体取代基供、吸电能力的改变对上述性质的影响则非常有限.     

Theoretical study of the walk rearrangement in perfluorotetramethyl (Dewar thiophene) exo-S-oxide

A theoretical study of the ‘walk’ rearrangement in bicyclo[2.1.0]pentene and perfluorotetramethyl (Dewar thiophene) exo-S-oxide has been carried out. Despite the differences between them, the results for both reactions show an enhancement of aromaticity in the transition state, which is consistent with a pericyclic behavior. NBO calculations show that the small activation energy for the second reaction can be interpreted in terms of a strong stabilization of the transition state by the exo-oxide substituent. So, the mechanism proposed in the past should be revised.     

Alkylation of salts ofN-(β-hydroxyalkyl)-N′-hydroxydiazeneN-oxides with alkyl halides and dimethyl sulfate

Salts ofN-(β-hydroxyalkyl)-N′-hydroxydiazeneN-oxides, RCH(OH)CH₂N(O)=NO⁻ M⁺ (R=Me, Prⁱ, or Bu^t; and M=Li, Na, K, Ag, NH₄, or Me₄N), were prepared. Their alkylation with alkyl halides R′X (X=Cl, Br, or I) and dimethyl sulfate was studied. Generally, alkylation afforded mixtures ofN-(β-hydroxyalkyl)-N′-alkoxydiazeneN-oxides RCH(OH)CH₂N(O)=NOR′ andO-alkyl-N-(β-hydroxyalkyl)-N-nitrosohydroxylamines RCH(OH)CH₂N(NO)OR′. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1996–2001, October, 1998.     

Quinonimines and aminoquinones, the reaction products of 3,6-di(tert-butyl)-o-benzoquinone with primary and secondary amines

Reactions of 3,6-di(tert-butyl)-o-benzoquinone with primary amines occur by the nucleophilic 1,4-addition mechanism and lead to the corresponding 2-hydroxy-p-quinonimines, which exist in solutions in equilibrium with tautomeric 4-amino-o-quinones. The thermodynamic parameters of this prototropic isomerism were determined by NMR spectroscopy. In the case of a secondary amine (piperidine), a derivative of 4-amino-o-quinone was obtained; the corresponding o-semiquinone complexes were studied in solution by ESR spectroscopy. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1151–1155, July, 2006.     

A kinetic stability study of <Emphasis Type="Italic">M</Emphasis>N<Subscript>5</Subscript> (<Emphasis Type="Italic">M</Emphasis>=Li,Na, K,and Rb)

A structure and kinetic stability study on some complexes with the general formula MN₅, where M are the alkali-metal atoms, Li, Na, K, and Rb, has been carried by using hybrid density functional methods. Complex B (C_2v) with two points of attachment to the N₅ ring is the most energetically favored for all metals considered here. Pyramidal structures A (C_5v) are kinetically unstable and they rapidly rearrange to the most stable planar structures B. At the QCISD(T)/6-311 + G*//B3LYP/6-311 + G* + ZPE (B3LYP/6-311 + G*) level, the decomposition barrier heights of LiN₅-B, NaN₅–B, KN₅-B, and RbN₅-B are predicted to be 19.9, 22.0, 22.5, and 23.0 kcal/mol, respectively. In addition, the rate constants of the decomposition reaction MN₅-B MN₃ + N₂ (M + Li, Na, K, and Rb) are also predicted using conventional transition state theory and canonical variational transition state theory, respectively.     

Theoretical study of PO and PO−

The diatomic systems, PO and PO^– are studied, using numerical Hartree-Fock (NHF) and coupled-cluster calculations. The latter employs a hybrid NHF and Slater orbital basis set. Highly accurate CCSD methods predict bond lengths accurate to <0.004 Å and frequencies to 60 cm^–1. In addition the electron affinity of PO is computed to be 0.89 eV compared to an experimental value of 1.09±0.01. Comparisons are made with SCF and MBPT(2) results for PO⁺ using conventional basis sets.Dedicated to Professor J. Koutecký on the occasion of his 65th birthdayGuggenheim Fellow     

A simultaneous TG–DTA study of the thermal decomposition of 2-hydroxybenzoic acid, 2-carboxyphenyl ester (salsalate)

Simultaneous thermogravimetry–differential thermal analysis (TG–DTA) and gas and liquid chromatography with mass spectrometry detection have been used to study the kinetics and decomposition of 2-hydroxybenzoic acid, 2-carboxyphenyl ester, commercially known as salsalate. Samples of salsalate were heated in the TG–DTA apparatus in an inert atmosphere (100 ml min⁻¹ nitrogen) in the temperature range 30–500 °C. The data indicated that the decomposition of salsalate is a two-stage process. The first decomposition stage (150–250 °C) had a best fit with second-order kinetics with E_a=191–198 kJ/mol. The second decomposition stage (300–400 °C) is described as a zero-order process with E_a=72–80 kJ/mol. The products of the decomposition were investigated in two ways:

(a)Salsalate was heated in a gas chromatograph at various isothermal temperatures in the range 150–280 °C, and the exit gas stream analyzed by mass spectrometry (GC–MS). This approach suggested that salsalate decomposes with the formation of salicylic acid, phenol, phenyl salicylate, and cyclic oligomers of salicylic acid di- and tri-salicylides.

(b)One gram samples of salsalate were heated in a vessel under nitrogen to 150 °C, and the residues were analyzed by liquid chromatography–mass spectrometry (LC–MS). The major compound detected was a linear tetrameric salicylate ester.

     

Theoretical study on the (O2-HF) complex

The potential energy surface of the (O₂-HF)⁺ complex has been investigated theoretically. Equilibrium structure has been calculated at the UCCSD/6-311++G(2d,2p) and UCCSD/aug-cc-pVTZ levels of theory. Two stable complexes, T-shape and collinear, have been found on the surface whose interaction energies are calculated to be −12.36 and −10.19 kcal mol⁻¹, respectively, at the UCCSD(T)/aug-cc-pVTZ level after correcting for the basis set superposition error (BSSE). Natural bond orbital (NBO) analysis revealed intermolecular charge transfers occur followed by intramolecular charge rearrangement. A large contribution from the n_F to π* _OO is found.