Possibility of non‐adiabatic level crossing by DFT study of tautomerism and potential energy surfaces in of 3‐hydroxy‐5‐(pyrimidin‐2‐yl)‐2H‐pyrrol‐2‐one and its tautomer

3‐Hydroxy‐5‐(pyrimidin‐2‐yl)‐2H‐pyrrol‐2‐one (HYPO, T1) and 2‐hydroxy‐5‐(pyrimidine‐2‐yl)‐3H‐pyrrole‐3‐one (HYPO, T2) have designed in this research to study potential energy curves for their dynamic motions and possibility of crossing between levels. Study of tautomerism shows that T1 tautomer is more stable than T2 (about 5.83 kJ/mol). Dynamic study of possible motions show rate constants (highest possible) equal to 8.82 M/s for tautomerism, 1.70 × 10⁹ M/s for relative rotation of ring (rr) and 3.67 × 10⁶ M/s for rotation of OH bond (br). Moreover, variations of orbital populations, NBO charges, hybridations, and acceptor–donor interactions in IRC steps have been investigated to study the possibility of non‐adiabatic crossing between tautomerism and ring rotation potential energy curves. The data showed that in spite of the fact that these two potentials share three common points, these two potential curves cannot have non‐adiabatic crossing because of different symmetries and a large difference between their barrier energies. Copyright © 2010 John Wiley & Sons, Ltd.     

UV‐induced transformations of matrix‐isolated 1,3,4‐thiadiazole‐2‐thiones

Monomers of 5‐mercapto‐1,3,4‐thiadiazole‐2‐thione (bismuthiol) were studied using an experimental matrix‐isolation technique as well as by carrying out theoretical quantum chemical calculations. The calculations, performed using the quadratic configuration interaction method with single and double excitations (QCISD)/6‐31++G(d,p)//DFT(B3LYP)/6‐311++G(2d,p), predict that the thione–thiol tautomer of bismuthiol should be significantly (by more than 19 kJ mol^?1) more stable than other tautomeric forms. Accordingly, only the signatures of the thione–thiol tautomer were observed in the FT‐IR spectrum of bismuthiol, recorded directly after deposition of an Ar matrix. UV (λ > 320 nm) irradiation induced the conversion of the thione–thiol tautomer into the dithiol form. Analogous investigations were carried out for two related compounds: 5‐methyl‐1,3,4‐thiadiazole‐2‐thione and 5‐methylthio‐1,3,4‐thiadiazole‐2‐thione. For these two species, only the thione tautomeric forms were observed after deposition of Ar matrices. These tautomers were predicted (by QCISD calculations) to be more stable (by at least 19 kJ mol^?1) than other tautomeric forms. Upon UV irradiation, the most stable thione forms of these compounds were transformed into the corresponding thiol tautomers. Direct observation of the thione → thiol phototautomeric processes provides a clear proof that intramolecular proton transfer reaction can occur in molecules, such as bismuthiol, in spite of the increased NH···S distance, in comparison to other phototautomerizing species studied so far. All the isomers of the studied compounds (substrates and products of the photoreactions) were identified by comparison of their IR spectra with the spectra calculated at the DFT(B3LYP)/6‐311++G(2d,p) level of theory for possible isomeric structures. Copyright © 2009 John Wiley & Sons, Ltd.     

Catalysis by hydrogen chloride in the gas‐phase elimination kinetics of 2‐phenyl‐2‐propanol and 3‐methyl‐1‐buten‐3‐ol

A homogeneous, molecular, gas‐phase elimination kinetics of 2‐phenyl‐2‐propanol and 3‐methyl‐1‐ buten‐3‐ol catalyzed by hydrogen chloride in the temperature range 325–386 °C and pressure range 34–149 torr are described. The rate coefficients are given by the following Arrhenius equations: for 2‐phenyl‐2‐propanol log k₁ (s^?1) = (11.01 ± 0.31) ? (109.5 ± 2.8) kJ mol^?1 (2.303 RT)^?1 and for 3‐methyl‐1‐buten‐3‐ol log k₁ (s^?1) = (11.50 ± 0.18) ? (116.5 ± 1.4) kJ mol^?1 (2.303 RT)^?1. Electron delocalization of the CH₂?CH and C₆H₅ appears to be an important effect in the rate enhancement of acid catalyzed tertiary alcohols in the gas phase. A concerted six‐member cyclic transition state type of mechanism appears to be, as described before, a rational interpretation for the dehydration process of these substrates. Copyright © 2007 John Wiley & Sons, Ltd.     

Joint theoretical and experimental study of the gas‐phase elimination kinetics of tert‐butyl ester of carbamic,N, N‐dimethylcarbamic,N‐hydroxycarbamic acids and 1‐(tert‐butoxycarbonyl)‐imidazole

The gas‐phase elimination kinetics of the title compounds were carried out in a static reaction system and seasoned with allyl bromide. The working temperature and pressure ranges were 200–280 °C and 22–201.5 Torr, respectively. The reactions are homogeneous, unimolecular, and follow a first‐order rate law. These substrates produce isobutene and corresponding carbamic acid in the rate‐determining step. The unstable carbamic acid intermediate rapidly decarboxylates through a four‐membered cyclic transition state (TS) to give the corresponding organic nitrogen compound. The temperature dependence of the rate coefficients is expressed by the following Arrhenius equations: for tert‐butyl carbamate logk₁ (s^?1) = (13.02 ± 0.46) – (161.6 ± 4.7) kJ/mol(2.303 RT)^?1, for tert‐butyl N‐hydroxycarbamate logk₁ (s^?1) = (12.52 ± 0.11) – (147.8 ± 1.1) kJ/mol(2.303 RT)^?1, and for 1‐(tert‐butoxycarbonyl)‐imidazole logk₁ (s^?1) = (11.63 ± 0.21)–(134.9 ± 2.0) kJ/mol(2.303 RT)^?1. Theoretical studies of these elimination were performed at Møller–Plesset MP2/6‐31G and DFT B3LYP/6‐31G(d), B3LYP/6‐31G(d,p) levels of theory. The calculated bond orders, NBO charges, and synchronicity (Sy) indicate that these reactions are concerted, slightly asynchronous, and proceed through a six‐membered cyclic TS type. Results for estimated kinetic and thermodynamic parameters are discussed in terms of the proposed reaction mechanism and TS structure. Copyright © 2007 John Wiley & Sons, Ltd.     

Conformational analysis of 3,3‐dimethyl‐3‐silathiane, 2,3,3‐trimethyl‐3‐silathiane and 2‐trimethylsilyl‐3,3‐dimethyl‐3‐silathiane—preferred conformers,barriers to ring inversion and substituent effects

The first conformational analysis of 3‐silathiane and its C‐substituted derivatives, namely, 3,3‐dimethyl‐3‐silathiane 1 , 2,3,3‐trimethyl‐3‐silathiane 2 , and 2‐trimethylsilyl‐3,3‐dimethyl‐3‐silathiane 3 was performed by using dynamic NMR spectroscopy and B3LYP/6‐311G(d,p) quantum chemical calculations. From coalescence temperatures, ring inversion barriers ΔG^≠ for 1 and 2 were estimated to be 6.3 and 6.8 kcal/mol, respectively. These values are considerably lower than that of thiacyclohexane (9.4 kcal/mol) but slightly higher than the one of 1,1‐dimethylsilacyclohexane (5.5 kcal/mol). The conformational free energy for the methyl group in 2 (?ΔG° = 0.35 kcal/mol) derived from low‐temperature ¹³C NMR data is fairly consistent with the calculated value. For compound 2 , theoretical calculations give ΔE value close to zero for the equilibrium between the 2 ‐Me_ax and 2 ‐Me_eq conformers. The calculated equatorial preference of the trimethylsilyl group in 3 is much more pronounced (?ΔG° = 1.8 kcal/mol) and the predominance of the 3 ‐SiMe_{3 eq} conformer at room temperature was confirmed by the simulated ¹H NMR and 2D NOESY spectra. The effect of the 2‐substituent on the structural parameters of 2 and 3 is discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Theoretical studies on structure and performance of [1,2,5]‐oxadiazolo‐[3,4‐d]‐pyridazine‐based derivatives

Based on energetic compound [1,2,5]‐oxadiazolo‐[3,4‐d]‐pyridazine, a series of functionalized derivatives were designed and first reported. Afterwards, the relationship between their structure and performance was systematically explored by density functional theory at B3LYP/6‐311 g (d, p) level. Results show that the bond dissociation energies of the weakest bond (N–O bond) vary from 157.530 to 189.411 kJ · mol^?1. The bond dissociation energies of these compounds are superior to that of HMX (N–NO_2, 154.905 kJ · mol^?1). In addition, H1, H2, H4, I2, I3, C1, C2, and D1 possess high density (1.818–1.997 g · cm^?3) and good detonation performance (detonation velocities, 8.29–9.46 km · s^?1; detonation pressures, 30.87–42.12 GPa), which may be potential explosives compared with RDX (8.81 km · s^?1, 34.47 GPa ) and HMX (9.19 km · s^?1, 38.45 GPa). Finally, allowing for the explosive performance and molecular stability, three compounds may be suggested as good potential candidates for high‐energy density materials. Copyright © 2016 John Wiley & Sons, Ltd.     

Homo‐Diels–Alder reaction of a very inactive diene,bicyclo[2,2,1]hepta‐2,5‐diene,with the most active dienophile, 4‐phenyl‐1,2,4‐triazolin‐3,5‐dione. Solvent,temperature, and high pressure influence on the reaction rate

Solvent, temperature, and high pressure influence on the rate constant of homo‐Diels–Alder cycloaddition reactions of the very active hetero‐dienophile, 4‐phenyl‐1,2,4‐triazolin‐3,5‐dione (1), with the very inactive unconjugated diene, bicyclo[2,2,1]hepta‐2,5‐diene (2), and of 1 with some substituted anthracenes have been studied. The rate constants change amounts to about seven orders of magnitude: from 3.95^.10^?3 for reaction (1+2) to 12200 L mol^?1 s^?1 for reaction of 1 with 9,10‐dimethylanthracene (4e) in toluene solution at 298 K. A comparison of the reactivity (ln k₂) and the heat of reactions (?_r‐nH) of maleic anhydride, tetracyanoethylene and of 1 with several dienes has been performed. The heat of reaction (1+2) is ?218 ± 2 kJ mol^?1, of 1 with 9,10‐dimethylanthracene ?117.8 ± 0.7 kJ mol^?1, and of 1 with 9,10‐dimethoxyanthracene ?91.6 ±0.2 kJ mol^?1. From these data, it follows that the exothermicity of reaction (1+2) is higher than that with 1,3‐butadiene. However, the heat of reaction of 9,10‐dimethylanthracene with 1 (?117.8 kJ mol^?1) is nearly the same as that found for the reaction with the structural C=C counterpart, N‐phenylmaleimide (?117.0 kJ mol^?1). Since the energy of the N=N bond is considerably lower (418 kJ/bond) than that of the C=C bond (611 kJ/bond), it was proposed that this difference in the bond energy can generate a lower barrier of activation in the Diels–Alder cycloaddition reaction with 1. Linear correlation (R = 0.94) of the solvent effect on the rate constants of reaction (1+2) and on the heat of solution of 1 has been observed. The ratio of the volume of activation (?V^≠) and the volume of reaction (?V_r‐n) of the homo‐Diels–Alder reaction (1+2) is considered as “normal”: ?V^≠/?V_r‐n = ?25.1/?30.95 = 0.81. Copyright © 2012 John Wiley & Sons, Ltd.     

Self‐association,tautomerism and E–Z isomerization of isatin–phenylsemicarbazones – spectral study and theoretical calculations

The self‐association and tautomerism of (E)‐isatin‐3‐4‐phenyl(semicarbazone) Ia and (E)‐N‐methylisatin‐3‐4‐phenyl(semicarbazone) IIa were investigated in solvents of various polarity. In weakly interacting non‐polar solvents, such as CHCl₃ and benzene, phenylsemicarbazone concentrations above 1×10^?5 mol dm^?3 result in the formation of dimers or higher aggregates of E‐isomers Ia and IIa . This aggregate formation prevents room temperature E–Z isomerization of Ia and IIa to more stable Z‐isomers. In contrast to the situation in non‐polar solvents, E–Z isomerization from the monomeric form of phenylsemicarbazone Ia and IIa E‐isomers occurs in highly interactive polar solvents including MeOH and DMF only at temperatures above 70 °C. Moreover, decrease in phenylsemicarbazone concentration below 1×10^?4 mol dm^?3 in these highly solute–solvent interacting systems leads to aggregate dissociation, and a new hydrazonol tautomeric form with a high degree of conjugation predominates in these solutions. Theoretical calculations confirm obtained experimental results. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis and rotation barriers in 2, 6‐Di‐(o‐anisyl) anisole

Variable temperature ¹H NMR spectroscopic studies of 2, 6‐di(o‐anisyl) anisole show syn and anti atropisomers at low temperature. The barrier for interconverting these isomers by rotation about the aryl‐aryl bond, found by fitting the experimental data, is 41.2 kJ/mol. Copyright © 2012 John Wiley & Sons, Ltd.     

Conformational analysis of 4,4‐dimethyl‐1‐(trifluoromethylsulfonyl)‐1,4‐azasilinane and 2,2,6,6‐tetramethyl‐4‐(trifluoromethylsulfonyl)‐1,4,2,6‐oxazadisilinane

4,4‐Dimethyl‐1‐(trifluoromethylsulfonyl)‐1,4‐azasilinane 1 and 2,2,6,6‐tetramethyl‐4‐(trifluoromethylsulfonyl)‐1,4,2,6‐oxazadisilinane 2 were studied by variable temperature dynamic ¹H, ¹³C, ¹⁹F NMR spectroscopy and theoretical calculations at the DFT (density functional theory) and MP2 (Møller‐Plesset 2) levels of theory. Both kinetic (barriers to ring inversion) and thermodynamic data (frozen conformational equilibria) could be obtained for the two compounds. The computations revealed two minima on the potential energy surface for molecules 1 and 2 corresponding to the rotamers with the CF₃SO₂ group directed ‘inward’ and ‘outward’ the ring, the latter being 0.2–0.4 kcal/mol (for 1 ) and 1.1 kcal/mol (for 2 ) more stable than the former. The vibrational calculations at the DFT and MP2 levels of theory give the values of the free energy difference ΔG^o for the ‘inward’ ‘outward’ equilibrium consistent with those determined from the experimentally measured ratio of the rotamers. The structure of crystalline compound 2 was ascertained by X‐ray diffraction analysis. Copyright © 2011 John Wiley & Sons, Ltd.     

Conformational and structural studies of n‐propylamine from temperature dependent Raman and far infrared spectra of xenon solutions and ab initio calculations

The Raman and infrared spectra (4000 to 50 cm^–1) of the gas, liquid or solution, and solid have been recorded of n‐propylamine, CH₃CH₂CH₂NH₂. Variable temperature (−60 to −100 °C) studies of the Raman (1175 to 625 cm^–1) and far infrared (600 to 10 cm^–1) spectra dissolved in liquid xenon were carried out. From these data, the five possible conformers were identified and their relative stabilities obtained with enthalpy difference relative to trans–trans (Tt) for trans–gauche (Tg) of 79 ± 9 cm^–1 (0.9 ± 0.1 kJ/mol); for Gg of 91 ± 26 cm^–1 (1.08 ± 0.3 kJ/mol); for Gg′ of 135 ± 21 cm^–1 (1.61 ± 0.2 kJ/mol); for Gt of 143 ± 11 cm^–1 (1.71 ± 0.1 kJ/mol). The percentage of the five conformers is estimated to be 18% for the Tt, 24 ± 1% for Tg, 23 ± 3% for Gg, 18 ± 1% for Gg′ and 18 ± 1% for Gt at ambient temperature. The conformational stabilities have been predicted from ab initio calculations utilizing several different basis sets up to aug‐cc‐pVTZ from both second‐order Møller–Plesset (MP2, full) and density functional theory calculations by the Becke, three‐parameter, Lee–Yang–Parr method. Vibrational assignments were provided for the observed bands for all five conformers, which are supported by MP2(full)/6‐31G(d) ab initio calculations to predict harmonic force constants, wavenumbers, infrared intensities, Raman activities and depolarization ratios for both conformers. Estimated r₀ structural parameters were obtained from adjusted MP2(full)/6‐311+G(d,p) calculations. The results are discussed and compared with the corresponding properties of some related molecules. Copyright © 2012 John Wiley & Sons, Ltd.     

Kinetics of the gas‐phase homogeneous unimolecular elimination of selected ethyl esters of 2‐oxo‐carboxylic acids

The gas‐phase elimination kinetics of selected ethyl esters of 2‐oxo‐carboxylic acid have been studied over the temperature range of 270–415 °C and pressures of 37–114 Torr. The reactions are homogeneous, unimolecular, and follow a first‐order rate law in a seasoned static reaction vessel, with an added free radical suppressor toluene. The observed overall and partial rate coefficients are expressed by the following Arrhenius equations:

• Ethyl oxalyl chloride
• log k_overall (s^?1) = (13.22 ± 0.45) ? (179.4 ± 4.9) kJ mol^?1 (2.303 RT)^?1
• Ethyl piperidineglyoxylate
• log k_(CO2) (s^?1) = (12.00 ± 0.30) ? (191.2 ± 3.9) kJ mol^?1 (2.303 RT)^?1
• log k_(CO) (s^?1) = (12.60 ± 0.09) ? (210.7 ± 1.2) kJ mol^?1 (2.303 RT)^?1
• log k_t(overall) (s^?1) = (12.22 ± 0.26) ? (193.4 ± 3.4) kJ mol^?1 (2.303 RT)^?1
• Ethyl benzoyl formate
• log k_(CO2) (s^?1) = (12.89 ± 0.72) ? (203.8 ± 9.0) kJ mol^?1 (2.303 RT)^?1
• log k_(CO) (s^?1) = (13.39 ± 0.31) ? (213.3 ± 3.9) kJ mol^?1 (2.303 RT)^?1
• log k_t(overall) (s^?1) = (13.24 ± 0.60) ? (205.8 ± 7.6) kJ mol^?1 (2.303 RT)^?1

The kinetic and thermodynamic parameters of these reactions, together with those reported in the literature, lead to consider three different mechanistic pathways of elimination. Copyright © 2010 John Wiley & Sons, Ltd.     

Effect of strain on gas‐phase basicity of (E)‐1‐methyl‐2‐(1‐methyl‐2‐adamantylidene)adamantane

Steric strain, caused by forced deformation of double bond geometry, has significant impact on alkene reactivity, as was shown by a study of the gas‐phase basicity of (E)‐1‐methyl‐2‐(1‐methyl‐2‐adamantylidene)adamantane. The Gibbs free energy of the strain effect in this compound was 42 kJ/mol. This analysis was made based on adamantylideneadamantane as a congeneric reference compound with a planar double bond. It was concluded that gas‐phase basicity could serve as a valuable alternative indicator for quantification of steric strain in alkenes. Copyright © 2015 John Wiley & Sons, Ltd.     

Theoretical investigations on 4,4′,5,5′‐tetranitro‐2,2′‐1H,1′H‐2,2′‐biimidazole derivatives as potential nitrogen‐rich high energy materials

The ―NH₂, ―NO₂, ―NHNO₂, ―C(NO₂)₃ and ―CF(NO₂)₂ substitution derivatives of 4,4′,5,5′‐tetranitro‐2,2′‐1H,1′H‐2,2′‐biimidazole were studied at B3LYP/aug‐cc‐pVDZ level of density functional theory. The crystal structures were obtained by molecular mechanics (MM) methods. Detonation properties were evaluated using Kamlet–Jacobs equations based on the calculated density and heat of formation. The thermal stability of the title compounds was investigated via the energy gaps (?E_{LUMO ? HOMO}) predicted. Results show that molecules T5 (D = 10.85 km·s^?1, P = 57.94 GPa) and T6 (D = 9.22 km·s^?1, P = 39.21 GPa) with zero or positive oxygen balance are excellent candidates for high energy density oxidizers (HEDOs). All of them appear to be potential explosives compared with the famous ones, octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetraazocane (HMX, D = 8.96 km·s^?1, P = 35.96 GPa) and hexanitrohexaazaisowurtzitane (CL‐20, D = 9.38 km·s^?1, P = 42.00 GPa). In addition, bond dissociation energy calculation indicates that T5 and T6 are also the most thermally stable ones among the title compounds. Copyright © 2014 John Wiley & Sons, Ltd.     

Trans‐2‐Aminocyclohexanol derivatives as pH‐triggered conformational switches

A series of trans‐2‐aminocyclohexanol derivatives have been explored as powerful conformational pH triggers. On protonation of the amino group, a conformer with equatorial position of ammonio and hydroxy groups becomes predominant because of an intramolecular hydrogen bond and electrostatic interactions. The energy of these interactions was estimated to be above 10 kJ/mol and in some models exceeded 20 kJ/mol (strong enough to twist a ring in tert‐butyl derivatives). As a result of this conformational flip, all other substituents are forced to change their orientation. If the substituents are designed to perform certain geometry‐dependent functions, for example, as cation chelators or as lipid tails, such acid‐induced transition may be used to control the corresponding molecular properties. The pH sensitivity of conformational equilibria was explored by ¹H nuclear magnetic resonance spectroscopy (NMR), and the titration curves were used for estimation of the pK_a values of protonated compounds that varied from 2.6 to 8.5 (in d₄‐methanol) depending on the structure of amino group. Thus, trans‐2‐aminocyclohexanols can be also used as conformational pH indicators in organic solvents.     

N‐protonated and O‐protonated tautomers of 1‐azabicyclo[3.3.1]nonan‐2‐one: observation of individual 13C‐NMR carbonyl peaks and comparisons with protonated tautomers of planar and other distorted lactams

An earlier study fit calculated dynamic ¹³C‐NMR spectra in trifluoroacetic acid (TFA) (with added sulfuric acid) to slow exchange between N‐protonated and O‐protonated tautomers of 1‐azabicyclo[3.3.1]nonan‐2‐one. The present study reports simultaneous observation of both carbonyl ¹³C peaks in 40% sulfuric acid/60% TFA at ?40 °C. This furnishes the only example in which experimental carbonyl ¹³C chemical shifts may be compared with a neutral lactam (in TFA or CDCl₃) with its N‐protonated and O‐protonated derivatives. The seemingly anomalous upfield chemical shifts (experimental and computational) of the ¹³C carbonyl peaks in this N‐protonated lactam (and other twisted N‐protonated lactams) relative to the free bases are compared with data for unstrained protonated lactams and amides. The results are rationalized through conventional resonance structures. Copyright © 2012 John Wiley & Sons, Ltd.     

Kinetics and mechanism of gas‐phase pyrolysis of ylides. Part 3.1 Thermal reactivity of α‐carbonyl‐ and thiocarbonyl‐stabilized methylenetriphenylphosphoranes

Fourteen ketone/thione‐stabilized triphenylphosphonium methylides were subjected to conventional gas‐phase and flash vacuum pyrolysis (FVP). The kinetics of the first‐order thermal gas‐phase reactions of all these compounds were investigated over 360–653 K temperature range. The values of the Arrhenius log A and energy of activation of these ylides averaged 11.52 ± 0.34 s^?1 and 133.20 ± 3.14 kJ mol^?1, respectively. The products of sealed‐tube (static) and FVP were analyzed and compared. A mechanism is proposed to account for the products of reaction. The rate constants [k (s^?1)] of the substrates at 500 K were calculated and used to substantiate the proposed mechanism of pyrolysis, and to rationalize the thermal gas‐phase reactivities of the ylides under study. Copyright © 2010 John Wiley & Sons, Ltd.     

A theoretical prediction on the ground‐state complexes bound by metal ions to thymine base isomers

Stability orderings of 150 stable complexes formed by metal ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, and Zn²⁺) and 13 stable thymine tautomers in both solvent and gas phases are obtained, and the optimal binding site for a metal ion in a specific thymine tautomer is identified. Results indicate that the complex with the canonical thymine tautomer (T1) is more stable than those with the rare ones, and the monodentate complex M–T1o4(o2) are their ground‐state form in the solvent phase. The ground‐state thymine complexes bound by Ca²⁺, Mg²⁺, or Zn²⁺ become bidentate M–T3o4lo2,n3, which is derived from a rare thymine tautomer T3o4l, whereas those bound by Na⁺ and K⁺ are still monodentate complexes M–T1o4(o2), however, in the gas phase. The differences in stability are discussed in detail from the binding strength of metal ions, relative energy of the corresponding thymine tautomers, and solution effect. Copyright © 2011 John Wiley & Sons, Ltd.     

Alkane oxidation by the system ‘tert‐butyl hydroperoxide–[Mn2L2O3][PF6]2 (L = 1,4,7‐trimethyl‐1,4,7‐triazacyclononane)–carboxylic acid’

The kinetics of cyclohexane (CyH) oxygenation with tert‐butyl hydroperoxide (TBHP) in acetonitrile at 50 °C catalysed by a dinuclear manganese(IV) complex 1 containing 1,4,7‐trimethyl‐1,4,7‐triazacyclononane and co‐catalysed by oxalic acid have been studied. It has been shown that an active form of the catalyst (mixed‐valent dimeric species ‘Mn^IIIMn^IV’) is generated only in the interaction between complex 1 and TBHP and oxalic acid in the presence of water. The formation of this active form is assumed to be due to the hydrolysis of the Mn? O? Mn bonds in starting compound 1 and reduction of one Mn^IV to Mn^III. A species which induces the CyH oxidation is radical tert‐BuO ^. generated by the decomposition of a monoperoxo derivative of the active form. The constants of the equilibrium formation and the decomposition of the intermediate adduct between TBHP and 1 have been measured: K = 7.4 mol^?1 dm³ and k = 8.4 × 10^?2 s^?1, respectively, at [H₂O] = 1.5 mol dm^?3 and [oxalic acid] = 10^?2 mol dm^?3. The constant ratio for reactions of the monomolecular decomposition of tert‐butoxy radical (tert‐BuO ^. → CH₃COCH₃ + CH) and its interaction with the CyH (tert‐BuO ^. + CyH → tert‐BuOH + Cy ^. ) was calculated: 0.26 mol dm^?3. One of the reasons why oxalic acid accelerates the oxidation is due to the formation of an adduct between oxalic acid and 1 (K ≈ 10³ mol^?1 dm³). Copyright © 2007 John Wiley & Sons, Ltd.     

Enthalpies of formation and isomerization of cis‐ and trans‐decalin,and their oxa‐analogs by G3(MP2)//B3LYP calculations

G3(MP2)//B3LYP calculations have been carried out on trans‐ and cis‐decalin, and their mono‐, di‐, tri‐, and tetraoxa‐analogs. The main purpose of the work was to obtain enthalpies of formation for these compounds, and to study the relative stabilities of the cis–trans and positional isomers of the various (poly)oxadecalins. Comparison of the computational enthalpies of formation with the respective experimental ones, known only for the decalins and 1,3,5,7‐tetraoxadecalins, shows that in both cases the computational values are more negative than the experimental ones, the deviations being ?5 to ?7 kJ mol^?1 for the decalins and ?12 to ?17 kJ mol^?1 for the 1,3,5,7‐tetraoxadecalins. The respective computational enthalpies of cis–trans isomerization, however, are in excellent to satisfactory agreement with the experimental data. The cis–trans enthalpy differences vary from +11.0 kJ mol^?1 for decalin to ?15.4 kJ mol^?1 for 1,4,5,8‐tetraoxadecalin. Low relative enthalpy values were also calculated for the cis isomers of 1,8‐dioxadecalin (?3.7 kJ mol^?1), 1,3,6‐trioxadecalin (?4.6 kJ mol^?1), 1,3,8‐trioxadecalin (?9.7 kJ mol^?1), 1,4,5‐ trioxadecalin (?5.6 kJ mol^?1), 1,3,5,8‐tetraoxadecalin (?7.3 kJ mol^?1), and 1,3,6,8‐tetraoxadecalin (?14.5 kJ mol^?1). Copyright © 2009 John Wiley & Sons, Ltd.