UHF-AM1 studies on 1,n(n=2,3,4,5) intramolecular H-atom transfer reactions in alkyl radicals

The transition states of intramolecular 1,n(n=2,3,4,5) H-atom transfer reactions from/to primary (p), secondary (s) or tertiary (t) carbon atoms were studied at the level of the semiempirical quantum-chemical method AM1 with the UHF approximation. The activation and reaction enthalpies were calculated and compared with data available in the literature. The calculated average differences between the activation enthalpies forpp-sp, pp-tp andss-ts are 8.9±2.6, 18.0±5.5 and 9.7±1.3 kJ mol^–1, respectively. The differences show a small decrease with increasing ring size.     

Thermochemistry of heteroatomic compounds

The vaporization enthalpies (ΔH _vap) of 97 primary, secondary, and tertiary alkylphosphines and alkyl(aryl)phosphines with different spatial structures were calculated using the Trouton and Wadso equations and the first-order topological solvation index¹χ^s. The contributions of the H₂P and HP groups and the phosphorus atom to the vaporization enthalpies of primary, secondary, and tertiary phosphines, respectively, were calculated. The results obtained can be used in calculations of ΔH _vap for related phosphorus compounds. For Part 16, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 32–37, January, 2000.     

Effect of the solvent on the thermodynamic parameters of conformational transformations: Pseudo effects of the medium

The vaporization enthalpies (ΔH _vap) of 97 primary, secondary, and tertiary alkylphosphines and alkyl(aryl)phosphines with different spatial structures were calculated using the Trouton and Wadso equations and the first-order topological solvation index¹χ^s. The contributions of the H₂P and HP groups and the phosphorus atom to the vaporization enthalpies of primary, secondary, and tertiary phosphines, respectively, were calculated. The results obtained can be used in calculations of ΔH _vap for related phosphorus compounds. For Part 16, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 32–37, January, 2000.     

Thermochemistry of heteroatomic compounds 22. Enthalpies of combustion and formation of alkyl(aryl)arsines and arsenites in the condensed and gaseous phases

The enthalpies of combustion (ΔH _comb) of five primary, secondary, and tertiary alkyl(aryl)arsines in the condensed state were calculated using the equation ΔH _comb = −385.8–110.3N, where N is the number of bond-forming electrons. The dependence presented is used for the calculation of the enthalpies of combustion of full esters and amidoesters of arsinous acid of noncyclic and cyclic structures. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1042–1043, May, 2007.     

Microcalorimetric Study of the Oscillating Extraction System

The power–time curves of the oscillating extraction system were determined at different temperatures for the extraction of hydrochloric acid and acetic acid with primary amine N₁₉₂₃ (R–CH(NH₂)–R¹), R, R ¹ represent alkyl of C_9–11 in chloroform using the titration microcalorimetric method. The apparent activation energy was calculated from the induction period (t _in), the first oscillation period (t ^p.1) and the second oscillation period (t _p.2). This revised version was published online in August 2006 with corrections to the Cover Date.     

Transition-Metal Free Electrophilic Aminations of Polyfunctional O-2,4,6-Trimethylbenzoyl Hydroxylamines with Zinc and Magnesium Organometallics

We reported a new electrophilic amination of various primary, secondary and tertiary alkyl, benzylic, allylic zinc and magnesium organometallics with O-2,4,6-trimethylbenzoyl hydroxylamines (O-TBHAs) in 52–99 % yield. These O-TBHAs displayed an excellent long-term stability and were readily prepared from various highly functionalized secondary amines via a convenient 3 step procedure. The amination reactions showed remarkable chemoselectivity proceeding without any transition-metal catalyst and were usually complete after 1–3 h reaction time at 25 °C. Furthermore, this electrophilic amination also provided access to enantioenriched tertiary amines (up to 88 % ee) by using optically enriched secondary alkylmagnesium reagents of the type s-AlkylMgCH₂SiMe₃.     

On the dichotomy of the SN2/ET reaction pathways: an apparent SN2 reactivity in the reaction of naphthalene dianion with alkyl fluorides

Dilithium naphthalene (Li₂C₁₀H₈) displays a S_N2 reactivity profile in its reaction with alkyl fluorides (n-, s- and t-octyl fluoride). S_N2 seems to be the dominant mechanism operating with primary alkyl fluorides, which presumably turns into competition with ET as we move to secondary and tertiary alkyl fluorides. Significantly, lithium naphthalene (LiC₁₀H₈) seems to have also an important nucleophilic component when reacting with alkyl fluorides, in contrast to the previously proposed general ET process valid for all alkyl halides. These results explain the observed distribution of products and are reinforced by a complete analysis of the products originated by the reaction with 6-halohexenyl radical probes, whose main alkylation products are described here for the first time.     

Ab initio study on alkyl radical decomposition and alkyl radical addition to olefins

The β bond dissociation of alkyl radicals and their reverse reactions, the addition of alkyl radicals to olefins were studied by G3MP2 level of theory to obtain a consistent kinetic data set. Both reaction families can be classified depending on the type of radical formed by β bond scission, namely the CH₃, primary, secondary tertiary radical formed. The kinetics of the reaction classes were described by only a limited number of Arrhenius parameters. The unified A factor of 10^13.7 s⁻¹ was found for all β bond dissociations. The Arrhenius activation energies are 125, 121, 113 and 103 kJ mol⁻¹, for methyl, primary, secondary, and tertiary radicals, respectively. The activation energies of 32, 25 and 18 kJ mol⁻¹ are calculated for the terminal addition of primary (including methyl), secondary, and tertiary radicals to olefins, respectively. The biologically important nonterminal radical additions to olefins have higher barriers of 37, 31 and 35 kJ mol⁻¹, respectively. At room temperature both strongly exothermic additions can compete with H-atom abstraction. New groups for Benson’s group additivity rules were defined to describe activation parameters for the β bond dissociation reactions. The group values were calculated by using the ab initio heats of formation of transition state structures.     

Understanding the Subtleties of Frustrated Lewis Pair Activation of Carbonyl Compounds by N‐Heterocyclic Carbene/Alkyl Gallium Pairings

This study reports the use of the trisalkylgallium GaR₃ (R=CH₂SiMe₃), containing sterically demanding monosilyl groups, as an effective Lewis‐acid component for frustrated Lewis pair activation of carbonyl compounds, when combined with the bulky N‐heterocyclic carbene 1,3‐bis(tert‐butyl)imidazol‐2‐ylidene (ItBu) or 1,3‐bis(tert‐butyl)imidazolin‐2‐ylidene (SItBu). The reduction of aldehydes can be achieved by insertion into the C=O functionality at the C2 (so‐called normal) position of the carbene affording zwitterionic products [ItBuCH₂OGaR₃] ( 1 ) or [ItBuCH(p‐Br‐C₆H₄)OGaR₃] ( 2 ), or alternatively, at its abnormal (C4) site yielding [aItBuCH(p‐Br‐C₆H₄)OGaR₃] ( 3 ). As evidence of the cooperative behaviour of both components, ItBu and GaR₃, neither of them alone are able to activate any of the carbonyl‐containing substrates included in this study NMR spectroscopic studies of the new compounds point to complex equilibria involving the formation of kinetic and thermodynamic species as implicated through DFT calculations. Extension to ketones proved successful for electrophilic α,α,α‐trifluoroacetophenone, yielding [aItBuC(Ph)(CF₃)OGaR₃] ( 7 ). However, in the case of ketones and nitriles bearing acidic hydrogen atoms, C?H bond activation takes place preferentially, affording novel imidazolium gallate salts such as [{ItBuH}⁺{(p‐I‐C₆H₄)C(CH₂)OGaR₃}^?] ( 8 ) or [{ItBuH}⁺{Ph₂C=C=NGaR₃}^?] ( 12 ).     

Stereospecific [2+2] cycloaddition reactions of diphosphorus with alkenes

[2+2] Cycloaddition reactions of P₂ with alkenes were predicted to have concerted paths, that is, pseudoexcitation, distorted 2π_s+2π_s, and 2π_s+2π_a processes without any interventions of intermediates. The pseudoexcitation and/or distorted 2π_s+2π_s paths with retention of configuration of alkenes are kinetically preferred to the 2π_s+2π_a path with inversion of configuration. The reactions were predicted from the appreciable difference in the calculated enthalpies of activation to be stereospecific.     

Determination of chromatographic dissociation constants of some carbapenem group antibiotics and quantification of these compounds in human urine

The dissociation constant values (_s^spK_a) of some carbapenem group drugs (ertapenem, meropenem, doripenem) in different percentages of methanol–water binary mixtures (18, 20 and 22%, v/v) were determined from the mobile phase pH dependence of their retention factor. Evaluation of these data was performed using the NLREG program. From calculated pK_a values, the aqueous pK_a values of these subtances were calculated by different approaches. Moreover, the correlation established between retention factor and the pH of the water–methanol mobile phase was used to determine the optimum separation conditions. In order to validate the optimized conditions, these drugs were studied in human urine. The chromatographic separation was realized using a Gemini NX C₁₈ column (250 × 4.6 mm i.d., 5 µm particles) and UV detector set at 220 and 295 nm. Copyright © 2013 John Wiley & Sons, Ltd.     

Absolute Rate Constants for the Addition of the 1‐(tert‐Butoxy)carbonylethyl Radical to Alkenes in Solution

Absolute rate constants and some of their Arrhenius parameters are reported for the addition of the 1‐[(tert‐butoxy)carbonyl]ethyl radical (MeC . HCO₂Me₃) to several mono‐ or 1,1‐disubstituted alkenes in acetonitrile as obtained by time‐resolved electron spin resonance spectroscopy. At 295 K, the rate constants range from 470 M ⁻¹ s⁻¹ (but‐1‐ene) to 2.4⋅10⁵ M ⁻¹ s⁻¹ (1,1‐diphenylethene), the experimental activation energies range from 26.8 kJ/mol (but‐1‐ene) to 14.7 kJ/mol (styrene), and the frequency factors obey on the average log (A/M ⁻¹ s⁻¹)=7.9±0.5. The rate constants of the secondary 1‐[(tert‐butoxy)carbonyl]ethyl radical are close to the geometric mean of those of the related primary [(tert‐butoxy)carbonyl]methyl and the tertiary 2‐(methoxycarbonyl)propan‐2‐yl radicals. The activation energies for addition of these three carboxy‐substituted alkyl radicals are mainly governed by the addition enthalpy but are also substantially lowered by ambiphilic polar effects. The results support a previously derived predictive analysis, and relations to rate constants of acrylate polymerizations are discussed.     

Thermochemical investigations of some electrolytes in water-acetamide mixtures at 298.15 K

The enthalpies of solution of NaCl, KCl, KBr, KNO₃ and CsI have been measured in mixtures of acetamide with water at 298.15 K in the miscibility range. The standard enthalpies of solutions (ΔH⁰) of the investigated electrolytes have been evaluated The corresponding enthalpies of solvation (ΔH_s⁰) and enthalples of transfer (ΔH_t⁰) from water to water-acetamide mixtures have been calculated The dependence of the standard enthalpies of solution and enthalpies of transfer of the investigated electrolytes on the solvent composition is discussed     

Electron-pair radial density functions

We introduce and discuss a generalized electron-pair radial density function G(q; a) that represents the probability density for the electron-pair radius |r ₁+ar ₂| to be q, where a is a real-valued parameter. The density function G(q; a) is a projection of the two-electron radial density D ₂(r ₁, r ₂) along lines r ₁ + ar ₂ ± q = 0 in the r ₁ r ₂ plane onto a point in the qa plane, and connects three densities S(s), D(r), and T(t), defined independently in the literature, as a smooth function of a: For an N-electron (N ≥ 2) system, S(s) = G(s; + 1), D(r) = 2G(r; 0)/(N − 1), and T(t) = G(|t|;−1)/2, where S(s) and T(t) are the electron-pair radial sum and difference densities, respectively, and D(r) is the single-electron radial density. Simple illustrations are given for the helium atom in the ground 1s² and the first excited 1s2s ³S states.     

Two polymorphs of chlorido(cyclohexyldiphenylphosphine)gold(I)

The title compound, [AuCl(C₁₈H₂₁P)], a monomeric two‐coordinate gold(I) complex, has been characterized at 100 K as two distinct monoclinic polymorphs, one from a single crystal, (Is), and one from a pseudo‐merohedrally twinned crystal, (It). The molecular structures in the two monoclinic [P2₁/n for (Is) and P2₁/c for (It)] polymorphs are similar; however, the packing arrangements in the two lattices differ considerably. The structure of (It) is pseudo‐merohedrally twinned by a twofold rotation about the a* axis.     

Propagation rate coefficients of isobornyl acrylate,tert‐butyl acrylate and 1‐ethoxyethyl acrylate: A high frequency PLP‐SEC study

Pulsed laser polymerization (PLP) coupled to size exclusion chromatography (SEC) is considered to be the most accurate and reliable technique for the determination of absolute propagation rate coefficients, k_p. Herein, k_p data as a function of temperature were determined via PLP‐SEC for three acrylate monomers that are of particular synthetic interest (e.g., for the generation of amphiphilic block copolymers). The high‐T_g monomer isobornyl acrylate (iBoA) as well as the precursor monomers for the synthesis of hydrophilic poly(acrylic acid), tert‐butyl acrylate (tBuA), and 1‐ethoxyethyl acrylate (EEA) were investigated with respect to their propagation rate coefficient in a wide temperature range. By application of a 500 Hz laser repetition rate, data could be obtained up to a temperature of 80 °C. To arrive at absolute values for k_p, the Mark‐Houwink parameters of the polymers have been determined via on‐line light scattering and viscosimetry measurements. These read: K = 5.00 × 10⁵ dL g⁻¹, a = 0.75 (piBoA), K = 19.7 × 10⁵ dL g⁻¹, a = 0.66 (ptBA) and K = 1.53 × 10⁵ dL g⁻¹, a = 0.85 (pEEA). The bulky iBoA monomer shows the lowest propagation rate coefficient among the three monomers, while EEA is the fastest. The activation energies and Arrhenius factors read: (iBoA): log(A/L mol⁻¹ s⁻¹) = 7.05 and E_A = 17.0 kJ mol⁻¹; (tBuA): log(A/L mol⁻¹ s⁻¹) = 7.28 and E_A = 17.5 kJ mol⁻¹ and (EEA): log(A/L mol⁻¹ s⁻¹) = 6.80 and E_A = 13.8 kJ mol⁻¹. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6641–6654, 2009     

Preparation,Characterization and Standard enthalpy of formation of Sr2CeO4.

Sr₂CeO₄ has been prepared by sol-combustion and co-precipitate routes and the resulting products have been characterized by XRD analysis. The molar enthalpies of solution of Sr₂CeO₄(s), Sr(NO₃)₂(s) and Ce(NO₃)₃·6H₂O(s) in 0.150 dm^–3 of (4.41 mol dm–3 H₂O₂+4.23 mol dm^–3 of HNO₃) solvent as well as the molar enthalpies of solution of Sr₂CeO₄(s), SrCl₂(s) and CeCl₃(s) in 0.150 dm³ of (1.47 mol dm^–3 H₂O₂+3.05 mol dm^–3 of HClO₄) solvent have been measured using an isoperibol type calorimeter. From these results and other auxiliary data, the standard molar enthalpy of formation of Sr₂CeO₄ has been derived to be –2277.3±3.1 kJ mol^–1 at 298.15 K. This is the first reported thermodynamic data on this compound.     

Dissolution Thermodynamics of 1,2,3-Triazole Nitrate in Water

The enthalpies of dissolution of 1,2,3-triazole nitrate in water were measured using a RD496-2000 Calvet microcalorimeter at four different temperatures under atmospheric pressure. Differential enthalpies (Δ_dif H) and molar enthalpies (Δ_diss H) of dissolution were determined. The corresponding kinetic equations that describe the dissolution rate at the four experimental temperatures are \fracdadt / s ^{- 1} = 10 ^{- 3.75}( 1 - a)^0.96\frac{d\alpha}{dt} / \mathrm{s}^{ - 1} =10^{ - 3.75}( 1 - \alpha)^{0.96} (T=298.15 K), \fracdadt /s ^{- 1} = 10 ^{- 3.73}( 1 - a)^1.00\frac{d\alpha}{dt} /\mathrm{s}^{ - 1} = 10^{ - 3.73}( 1 - \alpha)^{1.00} (T=303.15 K), \fracdadt / s ^{- 1} = 10 ^{- 3.72}( 1 - a)^0.98\frac{d\alpha}{dt} / \mathrm{s}^{ - 1} = 10^{ - 3.72}( 1 - \alpha)^{0.98} (T=308.15 K) and \fracdadt / s ^{- 1} = 10 ^{- 3.71}( 1 -a)^0.97\frac{d\alpha}{dt} / \mathrm{s}^{ - 1} = 10^{ - 3.71}( 1 -\alpha)^{0.97} (T=313.15 K). The determined values of the activation energy E and pre-exponential factor A for the dissolution process are 5.01 kJ⋅mol⁻¹ and 10^−2.87 s⁻¹, respectively.     

Pyrolytic production of Se (3P) from carbon diselenide. I. Equilibrium

Thermodynamic characteristics of the dissociation of carbon diselenide are examined. Results of a shock-tube study of CSe₂ pyrolysis are combined with previously unreported photoionization spectroscopy data on CSe₂ and with a critical examination of the available literature on Se(g), Se₂(g), CSe(g), CSe₂(g), and CSe₂(t). Based on the best currently available information, recommendations are made for the values of enthalpies, entropies, heat capacities, and derived functions. In particular, the most likely value for ΔH₀ (CSe₂(g) → CSe(g) + Se(g, ³P)) is +83 kcal/mol, and for K_p at 2000°K it is 0.018 atm.     

Thermochemistry of heteroatomic compounds

The enthalpies of combustion (H _comb) of 13 primary, secondary, and tertiary alkylphoshines in the condensed state were calculated using the equation H _comb = –860.7 – 107.0N, where N is the number of valent (bond-forming) electrons. This equation can be used for the calculation of enthalpies of combustion and formation of phosphoric acid esters.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1693–1694, August, 2004.