Molecular structure,conformational mobility,vibrational spectra,and thermochemistry of peroxyacetic acid: an ab initio and density functional study

The structure of the peroxyacetic acid (PAA) molecule and its conformational mobility under rotation about the peroxide bond was studied by ab initio and density functional methods. The free rotation is hindered by the trans-barrier of height 22.3 kJ mol^–1. The equilibrium molecular structure of AcOOH (C _s symmetry) is a result of intramolecular hydrogen bond. The high energy of hydrogen bonding (46 kJ mol^–1 according to natural bonding orbital analysis) hampers formation of intermolecular associates of AcOOH in the gas and liquid phases. The standard enthalpies of formation for AcOOH (–353.2 kJ mol^–1) and products of radical decomposition of the peroxide — AcO^· (–190.2 kJ mol^–1) and AcOO^· (–153.4 kJ mol^–1) — were determined by the G2 and G2(MP2) composite methods. The O—H and O—O bonds in the PAA molecule (bond energies are 417.8 and 202.3 kJ mol^–1, respectively) are much stronger than in alkyl hydroperoxide molecules. This provides an explanation for substantial contribution of non-radical channels of the decomposition of peroxyacetic acid. The electron density distribution and gas-phase acidity of PAA were determined. The transition states of the ethylene and cyclohexene epoxidation reactions were located (E _a = 71.7 and 50.9 kJ mol^–1 respectively).     

Kinetics of the thermal decomposition of [Co(NH3)6]2(C2O4)3·4H2O

The thermal decomposition of [Co(NH₃)₆]₂(C₂O₄)₃·4H₂O was studied under isothermal conditions in flowing air and argon. Dissociation of the above complex occurs in three stages. The kinetics of the particular stages thermal decomposition have been evaluated. The R_N and/or A_M models were selected as those best fitting the experimental TG curves. The activation energies,E, and lnA were calculated with a conventional procedure and by a new method suggested by Kogaet al. [10, 11]. Comparison of the results have showed that the Arrhenius parameters values estimated by the use of both methods are very close. The calculated activation energies were in air: 96 kJ mol^–1 (R_1.575, stage I); 101 kJ mol^–1 (Ain1.725 stage II); 185 kJ mol^–1 (A _2.9, stage III) and in argon: 66 kJ mol^–1 (A _1.25, stage I); 87 kJ mol^–1 (A _1.825, stage II); 133 kJ mol^–1 (A _2.525, stage III).     

Theoretical Study of Gas-Phase Acidities of Selected Angiotensin-Converting Enzyme Inhibitors

The structure and gas-phase acidities of six members of large angiotensin-converting enzyme (ACE) inhibitor family [cilazaprilat, silanediol, fosinoprilat, AcSDKP, angiotensin_I (terminal part), and RXP_407] have been studied using the ONIOM Becke3LYP/6-311+G(d,p):HF/3-21G^∗ method. The investigated ACE inhibitors are weak acids with calculated acidity of about 1270–1650 kJ mol⁻¹. Of acids studied the highest gas-phase acidity (1273 kJ mol⁻¹) possesses experimental ACE inhibitor RXP_407. This drug, according to the computed pK_a value (3.2), is also in water solution the most acidic compound of the ACE inhibitors investigated.     

Mechanism of formation and monomolecular decomposition of aci-nitromethanes: a quantum-chemical study

A quantum-chemical study of the reactions of formation of aci-nitromethane (aci-NM) and aci-dinitromethane (aci-DNM) and their decomposition with elimination of water was carried out. The methods employed were the ab initio RHF method with inclusion of electron correlation at the MP2 level of theory and the Dunning—Hay double zeta basis set augmented with polarization d-functions on heavy-element atoms, the DFT approach at the B3LYP level, and the semiempirical PM3 method. The formation of aci-NM and aci-DNM was found to be the limiting stage of the mechanism under study. For DNM, the barrier to reaction is substantially lower than for NM. The estimates of the heights of the barriers to formation found from density functional calculations at the B3LYP/6-311++G(df,p) level (258 kJ mol^–1 for aci-NM and 218.5 kJ mol^–1 for aci-DNM) are thought to be the most reliable.     

Structure and thermal stability of difluoroamino compounds in the liquid state

The kinetics of decomposition of 15 difluoroamino compounds with NF₂ groups at primary, secondary, and tertiary carbon atoms in the liquid state was investigated. Activation energies (E _a) for all of the compounds were in the interval 100–120 kJ · mol^–1. The reaction rate does not depend on the electronic effects of the substituents and decreases only in the case of steric shielding of the NF₂ group. ForN-difluorobenzylamine it was shown that the gas-phase elimination of HF is characterized byE = 176 kJ·mole^–1, while the rate of decomposition in a solution depends on the dielectric constant of the medium. Based on the results obtained, a mechanism for liquid-phase decomposition, which involves heterolysis of the N-F bonds, is suggested.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 336–338, February, 1994.     

Anab initio investigation of rotation and inversion barriers in formylphosphine

Rotation about N–CO bonds in amides has been extensively investigated, but a corresponding barrier to rotation about the P–CO bond in an acylphosphine has yet to be observed. In the present 4-31Gab initio study of formylphosphine, rotation barriers of 9.6 and 13.5 kJ mol^–1 and a phosphorus pyramidal inversion barrier of 108.0 kJ mol^–1 are predicted. A comparison of STO-3G and STO-3G^* barriers suggests that polarization functions are not needed to describe rotation in this system.

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Eine ab initio Untersuchung von Rotations- und Inversions-Barrieren in Formylphosphin

Zusammenfassung Die Rotation um N–CO-Bindungen in Amiden wurde bisher intensiv untersucht, eine entsprechende Rotationsbarriere für Drehungen um die P–CO-Bindung in Acylphosphinen wurde jedoch nicht beobachtet. Eine 4-31Gab initio-Untersuchung an Formylphosphin ergibt Rotationsbarrieren von 9,6 und 13,5 kJ mol^–1 und eine pyramidale Inversionsbarriere von 108,0 kJ mol^–1 als Voraussage. Ein Vergleich der STO-3G und STO-3G^* Barrieren legt nahe, daß Polarisationsfunktionen für die Beschreibung der Rotation in diesen Systemen nicht nötig sind.

     

The thermal dehydration and decomposition of Ba[Cu(C2O4)2(H2O)]·5H2O

The thermal behaviour of Ba[Cu(C₂O₄)₂(H₂O)]·5H₂O in N₂ and in O₂ has been examined using thermogravimetry (TG) and differential scanning calorimetry (DSC). The dehydration starts at relatively low temperatures (about 80°C), but continues until the onset of the decomposition (about 280°C). The decomposition takes place in two major stages (onsets 280 and 390°C). The mass of the intermediate after the first stage corresponded to the formation of barium oxalate and copper metal and, after the second stage, to the formation of barium carbonate and copper metal. The enthalpy for the dehydration was found to be 311±30 kJ mol^–1 (or 52±5 kJ (mol of H₂O)^–1). The overall enthalpy change for the decomposition of Ba[Cu(C₂O₄)₂] in N₂ was estimated from the combined area of the peaks of the DSC curve as –347 kJ mol^–1. The kinetics of the thermal dehydration and decomposition were studied using isothermal TG. The dehydration was strongly deceleratory and the -time curves could be described by the three dimensional diffusion (D3) model. The values of the activation energy and the pre-exponential factor for the dehydration were 125±4 kJ mol^–1 and (1.38±0.08)×10¹⁵ min^–1, respectively. The decomposition was complex, consisting of at least two concurrent processes. The decomposition was analysed in terms of two overlapping deceleratory processes. One process was fast and could be described by the contracting-geometry model withn=5. The other process was slow and could also be described by the contracting-geometry model, but withn=2.The values ofE _a andA were 206±23 kJ mol^–1 and (2.2±0.5)×10¹⁹ min^–1, respectively, for the fast process, and 259±37 kJ mol^–1 and (6.3±1.8)×10²³ min^–1, respectively, for the slow process.Dedicated to Prof. Menachem Steinberg on the occasion of his 65th birthday     

Relative Stabilities and Molecular Structures of Isomeric Dihalobenzenes. A DFT Study

The molecular structures, total energies, and other computational data of benzene, and its monoand dihalogenated derivatives (halogen = F, Cl, Br) have been studied by DFT calculations. The main aim of the study was to estimate the relative stabilities (energies) of the ortho, meta, and para isomers of the six series of dihalobenzenes investigated. The computational data show that the ortho isomers always have the highest, and the meta isomers usually, but not always, the lowest total energies. Thus, 1,2-difluorobenzene is ca. 16.6 kJ mol^–1, and 1,4-difluorobenzene 2.5 kJ mol^–1 less stable than 1,3-difluorobenzene. Among the other isomeric dihalobenzenes, the differences in stability are less pronounced. For the dibromo-, dichloro-, and bromochlorobenzenes, the para compounds are calculated to be slightly (0.2–0.4 kJ mol^–1) more stable than their meta isomers. In addition to the thermochemical aspect of the study, the computational molecular structures of the halobenzenes are compared with available experimental data and discussed in terms of the substituent-induced deformation of the ideal geometry of the benzene ring. The computational electric dipole moments, especially for the fluorine-containing compounds, compare favorably with the respective experimental (gas-phase) values.     

High levelab initio stabilization energies of benzene

Summary G2 theory is shown to be reliable for calculating isodesmic and homodesmotic stabilization energies (ISE and HSE, respectively) of benzene. G2 calculations give HSE and ISE values of 92.5 and 269.1 kJ mol^–1 (298 K), respectively. These agree well with the experimental HSE and ISE values of 90.5±7.2 and 268.7±6.3 kJ mol^–1, respectively. We conclude that basis set superposition error corrections to the enthalpies of the homodesmotic or isodesmic reactions are not necessary in calculations of the stabilization energies of benzene using G2 theory. The calculated values of the enthalpies of formation of such molecules containing multiple bonds such as benzene ands-trans 1,3-butadiene, which are found from the enthalpies of isodesmic and homodesmotic reactions rather than of atomization reactions, demonstrate good performance of G2 theory. Estimates of theH _f ^o value for benzene from the G2 calculated enthalpies of homodesmotic reaction (2) and isodesmic reaction (3) are 80.9 and 82.5 kJ mol^–1 (298 K), respectively. These are very close to the experimentalH _f ^o value of 82.9±0.3 kJ mol^–1. TheH _f ^o value ofs-trans 1,3-butadiene calculated using the G2 enthalpy of isodesmic reaction (4) is 110.5 kJ mol^–1 and is in excellent agreement with the experimentalH _f ^o value of 110.0±1.1 kJ mol^–1.     

Molecular structure and decomposition mechanism of peracetic acid esters AcOOR (R = Me,Bu<Superscript>t</Superscript>)

The molecular structure and conformational mobility of methyl and tert-butyl esters of peracetic acid AcOOR (R = Me (1), Bu^t (2)) were studied by the ab initio MP4(SDQ)//MP2(FC)/6-31G(d,p) method and density functional B3LYP/6-31G(d,p) approach. The B3LYP calculated equilibrium conformations of the molecules are characterized by the C-O-O-C torsion angles of 93.6° (1) and 117.0° (2). Structural features of the molecules under study and a distortion of tetrahedral bond configuration at the C_α atom were explained using the natural bonding orbital approach. The standard enthalpies of formation of AcOOMe (−328.5 kJ mol⁻¹) and AcOOBu^t (−440.4 kJ mol⁻¹) were determined using the G2 and G2(MP2) computational schemes and the isodesmic reaction approach. The transition state of AcOOMe decomposition into AcOOH and formaldehyde was calculated (E _a = 122.8 kJ mol⁻¹). The thermal effects of homolytic decomposition of the peroxy esters following a concerted mechanism (Me^· + CO₂ + ^·OR) and simple homolysis of the peroxide bond (AcO^· + ^·OR) were found to be 97.5±0.3 and 155.1±0.3 kJ mol⁻¹, respectively. At temperatures below 400 K, the most probable decomposition mechanism of peroxy esters 1 and 2 involves simple homolysis of the O-O bond.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2021–2027, October, 2004.     

Standard Enthalpies of Formation of 2,6-Di-tert-butyl4-methylphenol and 3,5-Di-tert-butylphenol and Their Phenoxy Radicals

The standard (p^o = 0.1 MPa) enthalpies of formation of 2,6-di-tert-butyl-4-methylphenol and 3,5-di-tert-butylphenol in the gaseous phase, –315.5 ± 4.4 kJ mol^–1 and –312.7 ± 4.6 kJ mol^–1, respectively, were derived from the standard enthalpies of combustion, in oxygen, at 298.15 K, measured by static bomb combustion calorimetry, and from the standard enthalpies of sublimation, at 298.15 K, measured by Calvet microcalorimetry. The O—H bond dissociation enthalpies in those compounds were determined in benzene by photoacoustic calorimetry, leading to the standard enthalpies of formation of the gaseous phenoxy radicals: –189 ± 8 kJ mol^–1 and –154 ± 6 kJ mol^–1, respectively. These results were used to calculate enthalpies of substituent redistribution reactions, which are proposed as a method to estimate new data for substituted phenols.     

Thermodynamic characteristics of thermal dissociation of platinum tetrachloride

The pressure of thermal dissociation of platinum tetrachloride by the first step PtCl₄(s) = PtCl₃(s) + 0.5 Cl₂(g) was measured by the static method with a quartz membrane-gauge zero-pressure manometer. An approximating equation for the dissociation pressure vs. temperature was found. The enthalpy (52160±880 J mol⁻¹) and entropy (72.1±1.6 J mol⁻¹ K⁻¹) of dissociation were calculated. The heat of formation found for platinum tetrachloride (−246.3±1.3 kJ mol⁻¹) at 298.15 K agrees well with the value obtained by the calorimetric method (−245.6±1.9 kJ mol⁻¹).__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2028–2031, October, 2004.     

Tensimetric investigation of the CrCl<Subscript>3</Subscript>—Cl<Subscript>2</Subscript> system in the temperature range of 600–1200 K

The sublimation pressure of chromium trichloride was measured by the static method with a quartz membrane-gauge manometer in the temperature range of 875–1230 K. An approximating equation for the sublimation pressure vs. temperature was found. The enthalpy (259.4±4 kJ mol^–1) and the entropy (224.2±3.5 J mol^–1 K^–1) of sublimation at 298 K were calculated. For the process 2 CrCl₃(g) + Cl₂(g) = 2 CrCl₄(g), the following values were obtained: _r H°₂₉₈ = –207.1±11.6 kJ mol^–1 and _r S°₂₉₈ = –173.6±10 5 J mol^–1 K^–1.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1561–1564, August, 2004.     

Non-isothermal Studies on Mechanism And Kinetics of Thermal Decomposition of Cobalt(II) Oxalate Dihydrate

Thermal decomposition of CoC₂O₄⋅2H₂O was studied using DTA, TG, QMS and XRD techniques. It was shown that decomposition generally occurs in two steps: dehydration to anhydrous oxalate and next decomposition to Co and to CoO in two parallel reactions. Two parallel reactions were distinguished using mass spectra data of gaseous products of decomposition. Both reactions run according toAvrami–Erofeev equation. For reaction going to metallic cobalt parameter n=2 and activation energy is 97±14 kJ mol^–1. It was found that decomposition to CoO proceeds in two stages. First stage (0.12<α_II<0.41) proceeds according to n=2, with activation energy 251±15 kJ mol^–1 and second stage (0.45<α_II<0.85) proceeds according to parameter n=1 and activation energy 203±21 kJ mol^–1. This revised version was published online in August 2006 with corrections to the Cover Date.     

Modeling the Proton Transport in Orthoperiodic and Orthotelluric Acids and Their Salts

Orthoperiodic and orthotelluric acids, their salts MIO₆H₄ (M = Li, Rb, Cs) and CsH₅TeO₆, and dimers of the salt · acid type are calculated within density functional theory B3LYP and basis set LanL2DZ complemented by the polarizationd,p-functions. According to calculations, the salt · acid dimerization is energetically favorable for compounds MIO₆H₄ · H₅IO₆ (M = Rb, Cs) and CsIO₆H₄ · H₆TeO₆. The dimerization energy is equal to 138–146 kJ mol^–1. With relatively small activation energies equal to 4 kJ mol^–1 (M = Li) and 11 kJ mol^–1 (M = Rb, Cs), possible is rotation of octahedron IO₆ relative to the M atom in monomers of salt molecules. The proton transfer along an octahedron occurs with activation energies of 63–84 kJ mol^–1. The activation energy for the proton transfer between neighboring octahedrons of the type salt · acid acid · salt equals 8–17 kJ mol^–1. Quantum-chemical calculations nicely conform to x-ray diffraction and electrochemical data.     

Thermal decomposition and kinetic data of Cd(BF4)2·6H2O

The thermal dehydration and decomposition of Cd(BF₄)₂·6H₂O were studied by means of DTA, TG, DSC and X-ray diffraction methods and the end products of the thermal decomposition were identified. The results of thermal analysis show that the compound is fused first, then it is dehydrated until Cd(BF₄)₂·3H₂O is obtained, which has not been described in the literature so far. The enthalpy of phase transition is H _ph.tr.=115.6 kJ mol^–1 Separation of the compound is difficult since it is highly hygroscopic. Then, dehydration and decomposition take place simultaneously until CdF₂ is obtained which is proved by X-ray diffraction. On further increasing the temperature, CdF₂ is oxidized to CdO and the characteristic curve assumes a linear character.Based on TG data, kinetic analyses were carried out separately for both parts of the curve: first until formation of the trihydrate and then — until formation of CdF₂. The formal kinetic parameters are as follows:for the first phase:E ^*=45.3 kJ mol^–1; rate equationF=^2/3; correlation coefficient 0.9858 for the second phase:E ^*=230.1 kJ mol^–1; rate equationF=(1–)^2/3[1-(1–)^1/3]^–1; correlation coefficient 0.9982.     

Activation Parameters for the Formation and Decay of Partial-Charge-Transfer Exciplexes of 9-Cyanoanthracene, 1,12-Benzperylene, and Pyrene

The factors affecting the rate of formation and decay of exciplexes with partial charge transfer, which form in the kinetic region of photoinduced electron transfer (G ^* _et > –0.2 eV), were studied. The rate of formation of exciplexes is controlled mainly by the diffusion of reactants and the low steric factor (0.15–1.0). The activation enthalpy and entropy for the exciplex formation (9–13 kJ mol^–1 and –(12–28) J mol^–1 K^–1) are close to the activation enthalpy and entropy of diffusion, respectively. Charge transfer in an exciplex and polarization of the medium generally occur after passing the transition state. In contrast, the activation enthalpy of exciplex decay (its conversion into the reaction products) is close to zero (±6 kJ mol^–1) and the activation entropy is strongly negative –(80–130) J mol^–1 K^–1.     

Influence of packing of the reactants and products in solution on the volume parameters of the Diels-Alder reaction

The possible causes of a dramatic difference in the volume parameters (by up to 15–20 cm³ mol^–1) of the Diels-Alder reactions involving bulky and small reactants are discussed. The partial molar volumes of anthracene and 9,10-dihydroanthracene and the heats of their solution in six solvents were determined.Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 10, 2004, pp. 1674–1680.Original Russian Text Copyright © 2004 by Kiselev, Kashaeva, Potapova, Shikhab, Konovalov.This revised version was published online in April 2005 with a corrected cover date.     

Thermogravimetric and differential scanning calorimetric investigations of manganese–glycine interactions

Thermogravimetric (t.g.) and differential scanning calorimetric (d.s.c.) data have been used to study metal–amino acid interactions in adducts of general formula MnCl₂ · ngly (gly = glycine, n = 0.7, 2.0, 4.0 and 5.0). All the prepared adducts exhibit only a one step mass loss associated with the release of glycine molecules, except for the 0.7gly adduct, which exhibits two glycine mass loss steps. From d.s.c. data, the enthalpy values associated with the glycine mass loss can be calculated: MnCl₂ · 0.7gly = 409 and 399 kJ mol^–1, MnCl₂ · 2.0gly = 216 kJ mol^–1, MnCl₂ · 4.0gly = 326 kJ mol^–1 and MnCl₂ · 5.0gly = 423 kJ mol^–1, respectively. The enthalpy associated with the ligand loss, plotted as function of the number of ligands for the n = 2.0, 4.0 and 5.0 adducts, gave a linear correlation, fitting the equation: H (ligand loss)/kJ mol^–1 = 67 × (number of ligands, n) + 76. A similar result was achieved when the enthalpy associated with the ligand loss was plotted as a function of the _a(COO^–) bands associated with the coordination through the carboxylate group, 1571, 1575 and 1577 cm^–1, respectively, for the n = 2.0, 4.0 and 5.0 adducts, giving the equation H (ligand loss) /kJ mol^–1 = 33.5 × _a(COO^–) /cm^–1 – 52418.5. This simple equation provides evidence for the enthalpy associated with the ligand loss being very closely related to the electronic density associated with the metal–amino acid bonds.     

The N—H and O—H bond dissociation energies in 4-hydroxydiphenylamine and its phenoxyl and aminyl radicals

The N—H and O—H bond dissociation energies in 4-hydroxydiphenylamine Ph—NH—C₆H₄—OH (D _NH= 353.4, D _OH=339.3 kJ mol^–1) and its semiquinone radicals D _NH(Ph—NH—C₆H₄—O^·) = 273.6, D _OH(Ph—N^·—C₆H₄—OH) = 259.5 kJ mol^–1 were first estimated using the parabolic model and experimental data (rate constants) on two elementary reactions with participation of N-phenyl-1,4-benzoquinonemonoimine (2). One of the reactions, namely, that of 2 with aromatic amines, was studied in this work using a specially developed method.