Kinetics of Chlorohydrination of Allyl Chloride

The chlorohydrination of allyl chloride with chlorine in water was studied at 20–80°C. The effect of the concentration of chloride ions within the range 0–3.6 mol/l on the selectivity of formation of glycerol dichlorohydrins was studied. An equation that relates the selectivity and the concentration of Cl^–was derived, which adequately describes experimental data. The schemes of parallel and consecutive reactions occurring in the system were suggested. The ratios between the rate constants of the following reactions were found: the reactions of chlorine with water and allyl chloride dissolved in water (k ₁/k ₄= 4.1 × 10^–4), the reaction of allyl chloride with hypochlorous acid and the decomposition of hypochlorous acid (k ₂/k ₃= 1.7 × 10³), and the reactions of the allyl chloride–chlorine complex with a water molecule and Cl^–(k ₅/k ₆= 2.9 × 10^–2).     

Kinetic and equilibrium study of substitution reactions oftrans-tetracyanodioxorhenate(V) ions with monodentate nucleophiles

Summary The kinetics of the substitution reactions of the protonated froms oftrans-tetracyanodioxorthenate(V) with thiourea (TU),N-methylthiourea (NMTU),N, N-dimethylthiourea (NNDMTU) and hydrazoic acid (HN₃) were studied. The results were compared with those obtained for similar reactions of [WO₂(CN)₄]^4–. This study showed that the diprotonated form [ReO(H₂O)(CN)₄]^– is the only species reactive towards substitution reactions (and not the [ReO(OH)(CN)₄]^2– ion) and that only the aqua ligand in [ReO(H₂O)(CN)₄]^– is substituted by the incoming group. A dissociative mechanism is proposed for the substitution reactions between [ReO(H₂O)(CN)₄]^– and the monodentate nucleophiles. The i.r. data for these Re^v complexes are reported and discussed in terms of the relativetrans influence of the various monodentate ligands.     

Kinetics and Mechanism of the Autocatalytic Oxidation of L-Asparagine in a Moderately Concentrated Sulfuric Acid Medium

The reaction kinetics of the Autocatalytic Oxidation of L-asparagine by permanganate ions has been investigated in moderately strong acid medium using the spectrophotometric technique. In all cases studied, an autocatalytic effect due to Mn²⁺ ions formed as a reaction product was observed. Both catalytic and noncatalytic processes were determined to be first order with respect to the permanganate ions while a first and a fractional order with respect to the amino acid for noncatalytic and catalytic reactions were obtained, respectively. The overall rate equation for this process may be written asd[MO₄ ^–]/dt= k´₁[MnO₄ ^–]+k´₂[MnO₄ ^–][Mn⁺²],where k´₁ and k´₂ are rate pseudoconstants for noncatalytic and catalytic reactions, respectively. The influence of some factors such as temperature and reactant concentration on the rate constants has been studied, and the activation parameters have been calculated. Reaction mechanisms satisfying observations for both catalytic and noncatalytic routes have been presented.     

Synthesis and structural characterisation of the oxo-rhenium(V) complexes with spirophosphorane derived bidentate ligand (PO). Crystal structure of trans-ReOCl2[OCMe2CMe2OP(OCMe2CMe2O)](PPh3)

The reactions of ReO(OEt)Cl₂L₂, L=py, PPh₃ or ReOCl₃(Me₂S)(OPPh₃), with spirohydrophosphorane HP(OCMe₂CMe₂O)₂ – abbreviated here as HPO – in toluene yield ReOCl₂(PO)L complexes, L=py (1), PPh₃ (2) and OPPh₃ (3), respectively. The encountered bidentate phosphite pinacolato (OCMe₂CMe₂O)POCMe₂CMe₂O⁻ ligand (PO⁻) is afforded by means of a spirophosphorane ring-opening reaction. All the pink–violet compounds 1–3 were characterised by NMR, IR and UV–Vis spectroscopies. The structure of trans-ReOCl₂(PO)PPh₃ (2) was determined crystallographically. The rhenium atom adopts distorted octahedral geometry with a trans multiply bonded terminal oxo ligand (Re–O_t=1.698(2) Å) trans to pinacolate oxygen (Re–O=1.880(2) Å). Two phosphorus atoms as well as two chlorides are mutually in a trans arrangement.     

The preparation and characterisation of chromium(III) complexes of C-meso-5, 12-dimethyl-1, 4, 8, 11-tetraazacyclotetradecane (LM). Aquation and base hydrolysis kinetics ofcis- andtrans-[CrLMCl2]+

Summary The reaction of [CrCl₃(DMF)₃] with C-meso-5, 12-dimethyl-1, 4, 8, 11-tetra-azacyclotetradecane(L_M) in DMF gives a mixture ofcis-[CrL_MCl₂]Cl (ca. 90%) andtrans-[CrL_MCl₂]Cl (ca. 10%). These complexes are readily separated, as thecis-isomer is insoluble in warm methanol while thetrans-isomer is soluble. Using the dichlorocomplexes as precursors it has been possible to prepare a range ofcis-[CrL_MX₂]⁺ complexes (X=Br^–, NO ₃ ^– , N ₃ ^– , NCS^– and X₂=bidentate oxalate) and alsotrans-[CrL_MX₂]⁺ complexes (X=Br^–, H₂O or NCS^–). The spectroscopic properties and detailed stereochemistry of the complexes are discussed.The aquation and base hydrolysis kinetics ofcis- andtrans-[CrL_MCl₂]⁺ have been studied at 25° C. Base hydrolysis of thecis-complex is extremely rapid with K_OH =1.46×10⁵ dm³ mol^–1 at 25° C. This unusual reactivity appears to be associated with thetrans II stereochemistry of thesec-NH centres of the macrocycle. Base hydrolysis of thetrans complex with thetrans III chiral nitrogen stereochemistry is quite normal with k_OH =1.1 dm³ mol^–1 s^–1 at 25° C.     

Driving forces in substitution reactions of octahedral complexes: The influence of the competitive effect

The reaction of cis-[RuCl₂(P–P)(N–N)] type complexes (P–P = 1,4-bis(diphenylphosphino)butane or (1,1′-diphenylphosphino)ferrocene; N–N = 2,2′-bipyridine or 1,10-phenantroline) with monodentate ligands (L), such as 4-methylpyridine, 4-phenylpyridine and benzonitrile forms [RuCl(L)(P–P)(N–N)]⁺ species. Upon characterization of the isolated compounds by elemental analysis, ³¹P{¹H} NMR and X-ray crystallography it was found out that the type of the L ligand determines its position in relation to the phosphorus atom. While pyridine derivatives like 4-methylpyridine and 4-phenylpyridine coordinate trans to the phosphorus atom, the benzonitrile ligand (bzCN), a good π acceptor, coordinates trans to the nitrogen atom. A ³¹P{¹H} NMR experiment following the reaction of the precursor cis-[RuCl₂(dppb)(phen)] with the benzonitrile ligand shows that the final position of the entering ligand in the complex is better defined as a consequence of the competitive effect between the phosphorus atom and the cyano-group from the benzonitrile moiety and not by the trans effect. In this case, the benzonitrile group is stabilized trans to one of the nitrogen atoms of the N–N ligand. A differential pulse voltammetry experiment confirms this statement. In both experiments the [RuCl(bzCN)(dppb)(phen)]PF₆ species with the bzCN ligand positioned trans to a phosphorus atom of the dppb ligand was detected as an intermediate complex.     

Vibrational Spectra, Assignment, Conformational Stability and Ab Initio/DFT Calculations for 1-Nitropropane

The infrared spectra (4000–400 cm^{– 1}) of solid and the Raman spectra (3500–30 cm^{– 1}) of liquid and solid 1-nitropropane, CH₃CH₂CH₂NO₂, have been registered. Both the trans and gauche conformers have been identified in the fluid phase, while the trans form remains in the stable solid. Temperature dependence (190–230K) of the liquid 1-nitropropane Raman spectra has been carried out. From these data, the enthalpy difference was determined to be 870 ± 105 J-mol^–1, with the gauche conformer being the more stable rotamer. Ab initio and DFT calculations at different levels of approximation (HF, MP2, B3LYP, B3PW91) gave optimized geometries, harmonic force fields, and vibrational frequencies for the trans and gauche conformers. All the calculations (except the B3PW91/6-31G* level) predicted gauche as the low-energy conformer. Theoretical force constants are analyzed for formulating constraints in the molecular force field model of 1-nitropropane.     

Far-Infrared Spectra, Conformational Stability, Barriers to Internal Rotation, Ab Initio Calculations, r0 Structural Parameters, and Vibrational Assignment of Ethyl Methyl Ether

The far-infrared spectra (350–35 cm^–1) of gaseous ethyl methyl ether-d ₀ and ethyl methyl-d ₃-ether have been recorded at a resolution of 0.10 cm^–1. For the d ₀ species, the fundamental asymmetric torsion of the more stable trans conformer (two methyl moieties are trans to one another) has been observed at 115.40 cm^–1 with four upper state transitions falling to lower frequency, whereas, for the gauche form, it has been observed at 93.56 cm^–1 with two excited states falling to lower frequency. the corresponding series for the d ₃ species start from 106.00 and 87.10 cm^–1, respectively. From these data, the asymmetric torsional potential coefficients for the d ₀ species have been determined to be: V ₁ = 572 ± 30; V ₂ = 85 ± 8; V ₃ = 619 ± 30; V ₄ = 175 ± 18, and V ₆ = –28 ± 3 cm^–1. The trans to gauche and gauche to gauche barriers were calculated to be 958 cm^–1 (11.5 kJ/mol) and 631 cm^–1 (7.55 kJ/mol), respectively, with an energy difference of 550 ± 6 cm^–1 (6.58 ± 0.07 kJ/mol). Utilizing three conformer pairs, variable temperature studies (–105 to –150°C) of the infrared spectra of the d ₀ sample dissolved in liquid krypton gave an enthalpy difference of 547 ± 28 cm^–1 (6.54 ± 0.33 kJ/mol) with the trans conformer the more stable rotamer. It is estimated that there is only 4% of the gauche conformer present at ambient temperatures. The structural parameters, conformational stabilities, barriers to internal rotation, and fundamental vibrational frequencies, which have been determined experimentally, are compared to those obtained from ab initio gradient predictions from RHF/6-31G* and with full electron correlation at the MP2 level with three different basis sets. The adjusted r ₀ structural parameters have been obtained for the trans conformer from combined ab initio MP2/6-311+G** predictions and previously reported microwave rotational constants. The reported distances should be accurate to 0.003 Å and the angles to 0.5°. These results are compared to the corresponding quantities obtained for some similar molecules.     

Size-dependent reactivity of cobalt cluster ions with nitrogen monoxide: Competition between chemisorption and decomposition of NO

Reactions of NO molecules on cobalt cluster ions were studied in a beam-gas geometry by using a tandem mass spectrometer. Single-particle collision reactions of Co_mNO⁺ (m = 3–10) with NO were found to proceed in such a manner that NO decomposition dominates at m = 4–6 with the maximum reaction cross section at m = 5 and chemisorption dominates in m ≥ 7. On the other hand, in two-particle collision reactions of Co_n⁺ (n = 2–10) with NO, NO decomposition at n ≥ 5 and chemisorption of two NO molecules with Co atoms loss at n ≥ 8 were found to proceed. These results indicate that the size-dependency of the multiple collision reactions originates from secondary attacking of an NO molecule to primary products of the initial single collision reactions. The DFT calculation supports the scheme that both the decomposition and chemisorption of two-particle collision reactions proceed via a common intermediate, Co_mN₂O₂⁺, in which the two NO molecules are dissociatively chemisorbed on the cobalt cluster ion, and the size-dependency of the two-particle collision reactions is explained in terms of the structure of this reaction intermediate.     

Catalyzed Peptide Bond Formation in the Gas Phase. Role of Bivalent Cations and Water in Formation of 2-Aminoacetamide from Ammonia and Glycine and in Dimerization of Glycine

The formation of 2-aminoacetamide from ammonia and glycine and N-glycylglycine from two glycine molecules with and without Mg²⁺, Cu²⁺, and Zn²⁺ cations as catalysts have been studied as model reactions for peptide bond formation using the B3LYP functional with 6–311+G(d,p) and 6–31G(d) basis sets. The B3LYP method was also used to characterize the nine gas–phase complexes of neutral glycine, its amide (2-aminoacetamide), and N-glycylglycine with Lewis acids Mg²⁺, Cu²⁺, and Zn²⁺, respectively. Further, the gas-phase hydration of metal-coordinated complexes of glycine, 2-aminoacetamide, and N-glycylglycine was also investigated. Finally, the effect of water on the structure and reactivity of the metal coordinated complexes was determined. Enthalpies and Gibbs energies for the stationary points of each reaction have been calculated to determine the thermodynamics of the reactions investigated. A substantial decrease in reaction enthalpies and Gibbs energies was found for glycine–ammonia and glycine–glycine reactions coordinated by Mg²⁺, Cu²⁺, and Zn²⁺ ions compared to those of the uncoordinated 2-aminoacetamide bond formation. The formation of a dipeptide is a more exothermic process than the creation of simple 2-aminoacetamide from glycine. The energetic effect of the transition metal ions Cu²⁺ and Zn²⁺ is of similar strength and more pronounced than that of the Mg²⁺ cation. The basicity order of the amides investigated shows the order: NH₂CH₂CO₂H < NH₂CH₂CONH₂ < NH₂CH₂CONHCH₂CO₂H. Interaction enthalpies and Gibbs energies of metal ion–amide complexes increase as Mg²⁺2+2+. In both reactant (glycine) and reaction products (2-aminoacetamide, N-glycylglycine) dihydration caused considerable reduction (about 200–500 kJ-mol^–1) of the strength of the bifurcated metal–amide bonds. Solvent effects also reduce the reaction enthalpy and Gibbs energy of reactions under study.     

Identification of the cobalt(II) chloride ‘mixed’ solvate Co4Cl8(H2O)4(MeCN)6: the ionic structure trans-[Co(H2O)2(MeCN)4] trans-[Co(H2O)2(MeCN)2(CoCl4)2]

The structure of the title compound features mononuclear octahedral Co^II cations, trans-[Co(H₂O)₂(MeCN)₄]²⁺, and trinuclear anions, trans-[Co(H₂O)₂(MeCN)₂(CoCl₄)₂]^2–; the latter centrosymmetric units contain a central octahedral Co(H₂O)₂(MeCN)₂ moiety with two tetrahedral [CoCl₄]^2– ligands. These two large ions are held in a network of intra- and inter-molecular hydrogen bonding.     

Solvent and chelation effects on the photoreduction of cobalt(III)–amine complexes in aqueous–organic solvent media

The photoreduction of trans-[Co(NH₃)₄Cl₂]⁺, trans-[Co(en)₂Cl₂]⁺, [Co(dien)Cl₃], [Co(trien)Cl₂]⁺, and [Co(tetren)Cl]²⁺, ions has been studied using a low pressure Hg vapour lamp as light source (254 nm) in aqueous–organic solvents [0–30% (v/v) MeOH or 1,4-dioxane]. Quantum yields for Co^II production by redox decomposition have been determined in all the cases, and increase considerably with the increase in concentration of MeOH or 1,4-dioxane in the binary solvent mixtures under investigation. A plot of log(quantum yield) versus the Grunwald–Winstein Parameter, Y, which is a measure of solvent ionizing power, shows that a different blend of general and specific solvent interacts with the solute. This kind of specific solvent interaction on the reactant/excited state has been analysed using multiple regression: viz. Krygowski–Fawcett and Kamlet–Taft equations. Reasons for the difference in reactivity with chelation are also discussed.     

Kinetics and mechanisms of the reactions of thiosulphato-amine complexes of cobalt(III). Part I. Isomerization and base hydrolysis ofcis andtrans isomers of dithiosulphatobis(ethylenediamine) cobalt(III) ion in aqueous solution

Summary Investigations were carried out on the isomerization and base hydrolysis ofcis andtrans forms of dithiosulphatobis-(ethylenediamine)cobalt(III) ions. Thecis form isomerizes to thetrans form in neutral aqueous medium, rates being 1.15, 2.30 and 4.0×10^–5s^–1, respectively at 42, 50 and 58 °C. Thetrans complex isomerizes to thecis form in basic solution only, the rate varying with pH in a sigmoid pattern. In presence of OH^–, an acid-base equilibrium of the complex ion sets in, but only the basic form takes part in the isomerization reaction. Hydrolysis of thecis isomer proceeds through a base-dependent path only, but that of thetrans isomer proceeds both through base-dependent and base-independent paths. The mechanisms are associative in nature. Thetrans form reacts faster thancis in all cases.     

Sorption of cesium on montmorillonite and effects of salt concentration

The sorption behavior of cesium on montmorillonite type clay was studied by using radioactivity measurements. Concentrations of Cs⁺ ions ranged from 10^–6 to 10^–2M. Cesium retention reduced with increasing salt concentration which was varied between 10^–4 and 10^–1M. Selectivity coefficients K_Cs–Na for the exchange between Cs and Na were calculated for different equivalent fractions of Cs on the solid phase. Using theK _Cs–Na values, free energy change was found to be 7.8 kJ/mol. The data could be fitted to a Freundlich isotherm, and empirical Freundlich parameters enabled the generation of a site distribution function. By fitting the data to the Dubinin-Radushkevich (D-R) isotherm, a mean energy of sorption of 8.6 kJ/mole was calculated which corresponds to the energy of ion exchange reactions. The values of energy changes calculated by using two different methods were in good agreement.     

The Association Constants of H<Superscript><Emphasis Type="Italic">+</Emphasis></Superscript> and Ca<Superscript><Emphasis Type="Italic">2 +</Emphasis></Superscript> with 2-Keto-D-Gluconate in Aqueous Solutions

2-Keto-D-gluconate (kG) is naturally produced in soils, sediments and rock faces through the microbial oxidation of glucose. Studies have qualitatively shown kG to enhance the dissolution of soil minerals. However, quantitative information, such as the log K values for the formation of metal–kG complexes, are not available. This paper presents the results of potentiometric titration studies that employ H⁺ and Ca²⁺ ion selective electrodes (ISEs) to determine the conditional ion association constants (log Q values) for the protonation and deprotonation of kG and the formation of Ca–kG complexes. The experimentally-determined log Q values were then converted to the corresponding ion association constants (the zero ionic strength condition; log K values) by employing a modified Davies equation for charged species and the Setchenów equation for neutral species. The log K values were determined by potentiometric titrations at constant kG concentration, varied ionic strengths, 25 or 22 ^∘C, and in the absence of CO₂. The computer model GEOCHEM-PC was used to determine the aqueous speciation of ions other than kG and the computer model FITEQL was used to estimate conditional log Q values for reactions in the various chemical models. Based on our evaluations, equilibrium constants for the following reactions were determined: H⁺+ kG^– ⇌ HkG⁰, log K_a1 = (3.00 ± 0.06), kG^–⇌ H_–1kG^2–+ H⁺, log K_a–1 = –(11.97 ± 0.41), and Ca²⁺+ kG^–⇌ CakG⁺, log K₁₀₁ = (1.74 ± 0.04).     

Nature of the Chemical Bond and the Mutual Influence of Ligands in Co(III) Dioximinates

The results of MWG calculations of the electronic structures of real Co(III) complexes [Co(HD)₂L¹L²] ⁿ were used to analyze the electron density distribution and to determine the charges on atoms and configurations, where nis the charge of the complex and HD is the acid residue of dimethylglyoxime (H₂D); L¹= NH₃at L²= NH₃, Cl^–, Br^–, or I^–and L¹= L²= Cl^–; and L¹= H₂O or NO^– ₂at L²= NO^– ₂, with self-consistency over all atoms of the system and over d, s, and pconfigurations of cobalt. The mutual influence of the ligands (trans- and cis-) was shown to be determined by the atomic charges and bond orders on the axial coordinate and in the equatorial plane of the complex. The following order of the trans-effect was proposed: I^–> Br^–> Cl^–> NO^– ₂> NH₃> H₂O. The effects of the electronic factors on distorsion and conformational processes in the complexes were discussed.     

Base hydrolysis of mono-alkylsubstituted chloropentaamminecobalt(III) complexes

Summary The preparation of the series ofcis- andtrans-[Co(NH₃)₄(RNH₂)Cl]²⁺ complexes (withcis, R = Me orn-Pr andtrans, R = Me, Et,n-Pr,n-Bu ori-Bu) is described. The u.v-visible spectra indicate a decrease of the ligand field on increasing chain length. Infrared spectra show an enhanced Co-Cl bond strength compared to the pentaammine. Partial molar volumes of the complex cations do not reveal steric compression. From proton exchange studies in D₂O it follows that [Co(NH₃)₅Cl]²⁺ and thecis- andtrans-[Co(NH₃)₄-(CH₃NH₂)C1]²⁺ complexes exchange the amine protons on the grouptrans to the chloro faster than those on thecis. A coordinated methylamine group exchanges its amine protons slower than a corresponding NH₃ group in the parent pentaammine, but the methyl introduction accelerates the exchange of the other NH₃ groups. The aquation of thetrans-alkylamine complexes (studied at 52° C) is acceleratedca. 10 times compared to the parent pentaammine, irrespective of the nature of the alkyl group. Thecis complexes do not show this acceleration of aquation. In base hydrolysis (studied at 25° C) thecis complexes are the most reactive (a factor 20 over the parent ion). Thecis/trans product ratio in base hydrolysis and the competition ratio in the presence of azide ions were calculated from the 500 MHz¹H n.m.r. spectra, which display distinctly different alkyl resonances for each individual complex. Thecis ions react under stereochemical retention of configuration; thetrans compounds give 10±1%trans tocis rearrangement. The ionic strength (4 mol dm^–3) and the pH do not affect this result. The same product ratio is obtained in methanol-water and DMSO-water mixtures. Ammoniation in liquid ammonia gives the same ratios as in base hydrolysis, base-catalyzed solvolysis in neat methylamine gives stereochemical retention for both thecis- andtrans-methylamine ion. The product competition ratio (Co-N₃)/(Co-OH₂) for thecis compounds and the bulkier amines (R =n- andi-Bu), 15–25% at 1 mol dm^–3N ₃ ^– , isca. twice that of thetrans compounds and the pentaammine. The results are interpreted in the classical conjugate base mechanism, and discussed in the context of current ideas about stereochemistry of base hydrolysis.Prof. C. R. Píriz Mac-Coll from Uruguay is a guest at the Free University of Amsterdam.     

Kinetics of ammoniation of some halobis(ethylenediamine)-rhodium(III) perchlorates in liquid ammonia

Summary The ammoniation ofcis-[Rh(en)₂Cl₂] · (ClO₄) in liquid NH₃ was studied at constant ionic medium of 0.20 m perchlorate in the 0 to 35° range. The complex reacts in two distinct steps to givecis-[Rh(en)₂(NH₃)₂] · (ClO₄)₃, with the intermediate formation ofcis-[Rh(en)₂(NH₃)Cl] · (ClO₄)₂. Both steps follow a conjugate-base mechanism. Activation parameters were obtained for the acid-base preequilibrium and the rate-determining step. The entropies of activation for the rate-determining step are 0 and –42 JK^–1mol^–1 for the first and second ammoniations respectively. These values are considerably lower than those found for the cobalt(III) analogues. The entropy changes for the acid-base equilibria are –84 and –36 JK^–1mol^–1 respectively, which is less negative than those values found for the cobalt(III) analogues. Trans-[Rh(en)₂I₂] · (ClO₄) ammoniates totrans-[Rh(en)₂(NH₃)I] · (ClO₄)₂. The contribution of spontaneous ammoniation to the overall reaction oftrans-[Rh(en)₂I₂] · (ClO₄) is negligible, so the uniqueness oftrans-[Co(en)₂Cl₂] · (ClO₄) among cobalt(III) complexes in this respect is not reproduced for thetrans-dihalotetraamine structure in rhodium(III) complexes. A comparison of cobalt(III) and rhodium(III) amines with respect to activation parameters and the influence of formal charge of the metal complex on reactivity indicates a more associative type of activation for rhodium(III).     

Crown-ether styryl dyes

The surface enhanced Raman scattering (SERS) spectra of styryl dyes containing a crown-ether group and a heteroaromatic residue with sulfoalkyl (1a) or alkyl (1b)N-substituent and of their complexes with Mg²⁺ cations were recorded in the 10^–4–10^–8 mol L^–1 concentration range. A model for the interaction of compoundsla,b with a silver surface during their adsorption on an electrochemically treated electrode was suggested. Fastcis-trans relaxation of the adsorbed molecules1a,b and complexes (1a,b)Mg²⁺ was found. It was shown that at [1a] = 10^–5 mol L^–1 and moderate molar ratios (C _Mg/[1a] = 3/1 to 9/1) in acetonitrile solutions, (trans-1a)Mg²⁺ complexes are joined into head-to-tail type dimers. An excess of Mg²⁺ cations (C_Mg/[1a] > 100) leads to dissociation of the dimers yielding (trans-1a)(Mg²⁺)₂ complexes. The formation of dimers from complexes (trans-1a)Mg²⁺ is accompanied by a substantial distortion of the planar structure oftrans-1a. This may be an important factor influencing the efficiency of photocycloaddition involving dimers of (trans-1a)Mg²⁺.For part 15, see Ref. 1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2429–2436, December, 1995.The work was carried out with financial support of the Russian Foundation for Basic Research (Project No. 94-03-08760) and, to some extent, of INTAS (Grant No. 93-1829) and of the International Science Foundation (Grant No. MHAOOO).     

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).