Corannulene as a Lewis base: computational modeling of protonation and lithium cation binding.

A computational modeling of the protonation of corannulene at B3LYP/6-311G(d,p)//B3LYP/6-311G(d,p) and of the binding of lithium cations to corannulene at B3LYP/6-311G(d,p)//B3LYP/6-31G(d,p) has been performed. A proton attaches preferentially to one carbon atom, forming a sigma-complex. The isomer protonated at the innermost (hub) carbon has the best total energy. Protonation at the outermost (rim) carbon and at the intermediate (bridgehead rim) carbon is less favorable by ca. 2 and 14 kcal mol(-)(1), respectively. Hydrogen-bridged isomers are transition states between the sigma-complexes; the corresponding activation energies vary from 10 to 26 kcal mol(-)(1). With an empirical correction obtained from calculations on benzene, naphthalene, and azulene, the best estimate for the proton affinity of corannulene is 203 kcal mol(-)(1). The lithium cation positions itself preferentially over a ring. There is a small energetic preference for the 6-ring over the 5-ring binding (up to 2 kcal mol(-)(1)) and of the convex face over the concave face (3-5 kcal mol(-)(1)). The Li-bridged complexes are transition states between the pi-face complexes. Movement of the Li(+) cation over either face is facile, and the activation energy does not exceed 6 kcal mol(-)(1) on the convex face and 2.2 kcal mol(-)(1) on the concave face. In contrast, the transition of Li(+) around the corannulene edge involves a high activation barrier (24 kcal mol(-)(1) with respect to the lowest energy pi-face complex). An easier concave/convex transformation and vice versa is the bowl-to-bowl inversion with an activation energy of 7-12 kcal mol(-)(1). The computed binding energy of Li(+) to corannulene is 44 kcal mol(-)(1). Calculations of the (7)Li NMR chemical shifts and nuclear independent chemical shifts (NICS) have been performed to analyze the aromaticity of the corannulene rings and its changes upon protonation.     

An ab initio and density functional theory study of the dimethylzinc-hydrogen selenide adduct: (CH3)2Zn:SeH2

The molecular structure (equilibrium geometry) and binding energy of the dimethylzinc (DMZn)-hydrogen selenide (H₂Se) adduct, (CH₃)₂Zn:SeH₂, have been computed with ab initio molecular orbital and density functional theory (DFT) methods and, where possible, compared with experimental results. The structure of the precursors DMZn and H₂Se are perturbed to only a small extent upon adduct formation. (CH₃)₂Zn:SeH₂ was found to be 3 kcal mol⁻¹ less stable than the precursors at the B3LYP/6-311 + G(2d,p)//B3LYP/6-311 + G(2d,p) level of computation, indicating that the (CH₃)₂Zn:SeH₂ adduct is unlikely to be a stable gas-phase species under chemical vapour deposition conditions. Further calculations at the B3LYP/6-311 + G(2d,p)//B3LYP/6-311 + G(2d,p) level of computation suggest that the 1:2 adduct species, (CH₃)₂Zn:(SeH₂)₂, is much less stable than the 1:1 adduct and consequently the precursors by 19 kcal mol⁻¹.     

Thermal decomposition of a series of 1,2-diazetines

A homologous series of tricyclic diazetines (6a-c), differing by the number of methylene groups in the saturated bridges of the fused carbon bicycles, was synthesized. The DeltaH++ of decomposition for each of the diazetines to afford N2 and the corresponding alkene was determined experimentally: 6a, 31.7; 6b, 39.3; 6c, 38.8 kcal/mol. The ground-state strain energy of each diazetine was estimated utilizing computationally obtained DeltaHf's for each of the experimentally investigated diazetines as well as several other diazetines whose DeltaH++'s had been previously reported in the literature. The sum of the ground-state strain energies and DeltaH++'s of decomposition for all of the diazetines was nearly constant, with an average value of 59 kcal/mol, suggesting that all of the diazetines decompose via the same mechanism. Generally, the higher the ground-state strain energy of the diazetine, the less the DeltaH++ for decomposition. The decomposition transition states for 6a-c and 7 were modeled computationally at the RB3LYP/6-311+G(3df,2p)//UB3LYP/6-31+G(d,p) level. The agreement of the experimentally determined DeltaH++ values with transition-state energies obtained computationally supports the reaction mechanism originally proposed by Yamabe that the elimination process occurs by an unsymmetrical, yet concerted, transition state with strong biradical character.     

Reactivity of alkyl versus silyl peroxides. The consequences of 1, 2-silicon bridging on the epoxidation of alkenes with silyl hydroperoxides and bis(trialkylsilyl)peroxides

The bond dissociation energies for a series of silyl peroxides have been calculated at the G2 and CBS-Q levels of theory. A comparison is made with the O-O BDE of the corresponding dialkyl peroxides, and the effect of the O-O bond strength on the activation barrier for oxygen atom transfer is discussed. The O-O bond dissociation enthalpies (DeltaH(298)) for bis (trimethylsilyl) peroxide (1) and trimethylsilyl hydroperoxide (2) are 54.8 and 53.1 kcal/mol, respectively at the G2 (MP2) and CBS-Q levels of theory. The O-O bond dissociation energies computed at G2 and G2(MP2) levels for bis(tert-butyl) peroxide and tert-butyl hydroperoxide are 45.2 and 48.3 kcal/mol, respectively. The barrier height for 1,2-methyl migration from silicon to oxygen in trimethylsilyl hydroperoxide is 47.9 kcal/mol (MP4//MP2/6-31G). The activation energy for the oxidation of trimethylamine to its N-oxide by bis(trimethylsilyl) peroxide is 28.2 kcal/mol (B3LYP/6-311+G(3df,2p)// B3LYP/6-31G(d)). 1,2-Silicon bridging in the transition state for oxygen atom transfer to a nucleophilic amine results in a significant reduction in the barrier height. The barrier for the epoxidation of E-2-butene with bis(dimethyl(trifluoromethyl))silyl peroxide is 25.8 kcal/mol; a reduction of 7.5 kcal/mol relative to epoxidation with 1. The activation energy calculated for the epoxidation of E-2-butene with F(3)SiOOSiF(3) is reduced to only 2.2 kcal/mol reflecting the inductive effect of the electronegative fluorine atoms.     

Theoretical study of the conformational energy hypersurface of cyclotrisarcosyl

The multidimensional Conformational Potential Energy Hypersurface (PEHS) of cyclotrisarcosyl was comprehensively investigated at the DFT (B3LYP/6-31G(d), B3LYP/6-31G(d,p) and B3LYP/6-311++G(d,p)), levels of theory. The equilibrium structures, their relative stability, and the Transition State (TS) structures involved in the conformational interconversion pathways were analyzed. Aug-cc-pVTZ//B3LYP/6-311++G(d,p) and MP2/6-31G(d)//B3LYP/6-311++G(d,p) single point calculations predict a symmetric cis-cis-cis crown conformation as the energetically preferred form for this compound, which is in agreement with the experimental data. The conformational interconversion between the global minimum and the twist form requires 20.88 kcal mol-1 at the MP2/6-31G(d)//B3LYP/6-311++G(d,p) level of theory. Our results allow us to form a concise idea about the internal intricacies of the PEHSs of this cyclic tripeptide, describing the conformations as well as the conformational interconversion processes in this hypersurface. In addition, a comparative analysis between the conformational behaviors of cyclotrisarcosyl with that previously reported for cyclotriglycine was carried out     

Theoretical study of the equilibrium structure, vibrational spectrum, and thermochemistry of the peroxynitrate CF2BrCFBrOONO2

The results of a theoretical study of the molecular structure and conformational mobilities of the peroxynitrate CF(2)BrCFBrOONO(2) and its radical decomposition product CF(2)BrCFBrOO are reported in this paper. The most stable structures were calculated from ab initio G3(MP2)B3 and G4(MP2) methods and from density functional theory at the B3LYP/6-311+G(d) and B3LYP/6-311+G(3df) levels of theory. The equilibrium conformation of CF(2)BrCFBrOONO(2) indicates that the bromine atoms lie in position anti to each other and possess a COON dihedral angle of 114°. A quantum statistical analysis shows that about 40% of the internal rotors can freely rotate at room temperature. Our best values for the standard enthalpies of formation of CF(2)BrCFBrOONO(2) and CF(2)BrCFBrOO at 298 K obtained from isodesmic reactions at the G3(MP2)//B3LYP/6-311+G(3df) level of theory are -144.7 and -127.0 kcal mol(-1). From these values and the enthalpy of formation of the NO(2) radical, a CF(2)BrCFBrOO-NO(2) bond dissociation enthalpy of 26.0 ± 2 kcal mol(-1) was estimated.     

2-Pyridinethiol/2-pyridinethione tautomeric equilibrium. A comparative experimental and computational study

The gas phase and solvent dependent preference of the tautomerization between 2-pyridinethiol (2SH) and 2-pyridinethione (2S) has been assessed using variable temperature Fourier transform infrared (FTIR) experiments, as well as ab initio and density functional theory computations. No spectroscopic evidence (nu(S)(-)(H) stretch) for 2SH was observed in toluene, C(6)D(6), heptane, or methylene chloride solutions. Although, C(s)() 2SH is 2.61 kcal/mol more stable than C(s)() 2S (CCSD(T)/cc-pVTZ//B3LYP/6-311+G(3df,2p)+ZPE), cyclohexane solvent-field relative energies (IPCM-MP2/6-311+G(3df,2p)) favor 2S by 1.96 kcal/mol. This is in accord with the FTIR observations and in quantitative agreement with the -2.6 kcal/mol solution (toluene or C(6)D(6)) calorimetric enthalpy for the 2S/2SH tautomerization favoring the thione. As the intramolecular transition state for the 2S, 2SH tautomerization (2TS) lies 25 (CBS-Q) to 30 kcal/mol (CCSD/cc-pVTZ) higher in energy than either tautomer, tautomerization probably occurs in the hydrogen bonded dimer. The B3LYP/6-311+G(3df,2p) optimized C(2) 2SH dimer is 10.23 kcal/mol + ZPE higher in energy than the C(2)(h)() 2S dimer and is only 2.95 kcal/mol + ZPE lower in energy than the C(2) 2TS dimer transition state. Dimerization equilibrium measurements (FTIR, C(6)D(6)) over the temperature range 22-63 degrees C agree: K(eq)(298) = 165 +/- 40 M(-)(1), DeltaH = -7.0 +/- 0.7 kcal/mol, and DeltaS = -13.4 +/- 3.0 cal/(mol deg). The difference between experimental and B3LYP/6-311+G(3df,2p) [-34.62 cal/(mol deg)] entropy changes is due to solvent effects. The B3LYP/6-311+G(3df,2p) nucleus independent chemical shifts (NICS) are -8.8 and -3.5 ppm 1 A above the 2SH and 2S ring centers, respectively, and the thiol is aromatic. Although the thione is not aromatic, it is stabilized by the thioamide resonance. In solvent, the large 2S dipole, 2-3 times greater than 2SH, favors the thione tautomer and, in conclusion, 2S is thermodynamically more stable than 2SH in solution.     

Laser flash photolysis of 1,2-diketopyracene and a theoretical study of the phenolic hydrogen abstraction by the triplet state of cyclic alpha-diketones

Laser flash photolysis (LFP) studies, atoms in molecules (AIM) studies, and density functional theory (DFT) calculations have been performed in order to study the mechanism of the hydrogen abstraction by alpha-diketones in the presence of phenols. Laser irradiation of a degassed solution of 1,2-diketopyracene in acetonitrile resulted in the formation of a readily detectable transient with absorption at 610 nm, but with very low absorptivity. This transient decays with a lifetime of around 2 micros. The quenching rate constant for substituted phenols, kq, ranged from 1.10x10(8) L mol-1 s-1 (4-cyanophenol) to 3.87x10(9) L mol-1 s-1 (4-hydroxyphenol). The Hammett plot for the reaction of the triplet of 1,2-diketopyracene with phenols gave a reaction constant rho=-0.9. DFT calculations (UB3LYP/6-311++G**//UB3LYP/6-31G*) of the triplet complex ketone-phenol revealed that hydrogen transfer has predominantly occurred and that the reaction with alpha-diketones are generally 7 kcal/mol less endothermic than the respective reactions of the monoketones. These results together with the geometries obtained from the DFT calculations, natural bond order (NBO) analysis, and AIM results indicate that hydrogen abstraction for alpha-diketones is facilitated by the electrophilicity of the ketone, instead of neighboring group participation by the second carbonyl group.     

Structural elucidation of nitro-substituted five-membered aromatic heterocycles utilizing GIAO DFT calculations

The GIAO (Gauge Including Atomic Orbitals) DFT (Density Functional Theory) method is applied at the B3LYP/6-31+G(d,p)//B3LYP/6-31+G(d), B3LYP/6-311++G(d,p)//B3LYP/6-31+G(d), B3LYP/6-311+G (2d,p)//B3LYP/6-31+G(d) and B3LYP/6-311++G(d,p)//B3LYP/6-311++G(d,p) levels of theory for the calculation of proton and carbon chemicals shifts and coupling constants for 25 nitro-substituted five-membered heterocycles. Difference (1D NOE) spectra in combination with long-range gHMBC experiments were used as tools for the structural elucidation of nitro-substituted five-membered heterocycles. The assigned NMR data (chemical shifts and coupling constants) for all compounds were found to be in good agreement with theoretical calculations using the GIAO DFT method. The magnitudes of one-bond (1JCH) and long-range (nJCH, n>1) coupling constants were utilized for unambiguous differentiation between regioisomers of nitro-substituted five-membered heterocycles.     

Cheletropic decomposition of cyclic nitrosoamines revisited: the nature of the transition states and a critical role of the ring strain

The cheletropic decompositions of 1-nitrosoaziridine (1), 1-nitroso-Delta(3)-pyrroline (2), 7-nitroso-7-azabicyclo[2.2. 1]hepta-2,5-diene (3), and 6-nitroso-6-azabicyclo[2.1.1]hexa-4-ene (4) have been studied theoretically using high level ab initio computations. Activation parameters of the decomposition of nitrosoaziridine 1 were obtained experimentally in heptane (DeltaH()(298) = 18.6 kcal mol(-)(1), DeltaS()(298) = -7.6 cal mol(-)(1) K(-)(1)) and methanol (20.3 kcal mol(-)(1), 0.3 cal mol(-)(1) K(-)(1)). Among employed theoretical methods (B3LYP, MP2, CCD, CCSD(T)//CCD), the B3LYP method in conjunction with 6-31+G, 6-311+G, and 6-311++G(3df,2pd) basis sets gives the best agreement with experimental data. It was found that typical N-nitrosoheterocycles 2-4 which have high N-N bond rotation barriers (>16 kcal mol(-)(1)) extrude nitrous oxide via a highly asynchronous transition state with a planar ring nitrogen atom. Nitrosoaziridine 1, with a low rotation barrier (<9 kcal mol(-)(1)) represents a special case. This compound can eliminate N(2)O via a low energy linear synperiplanar transition state (DeltaH()(298) = 20.6 kcal mol(-)(1), DeltaS()(298) = 2.5 cal mol(-)(1) K(-)(1)). Two higher energy transition states are also available. The B3LYP activation barriers of the cheletropic fragmentation of nitrosoheterocycles 2-4 decrease in the series: 2 (58 kcal mol(-)(1)) > 3 (18 kcal mol(-)(1)) > 4 (12) kcal mol(-)(1). The relative strain energies increase in the same order: 2 (0 kcal mol(-)(1)) < 3 (39 kcal mol(-)(1)) < 4 (52 kcal mol(-)(1)). Comparison of the relative energies of 2-4 and their transition states on a common scale where the energy of nitrosopyrroline 2 is assumed as reference indicates that the thermal stability of the cyclic nitrosoamines toward cheletropic decomposition is almost entirely determined by the ring strain.     

Inductive effects in radicals calculated from DFT energies; substituted bicyclo[2.2.2]octan-1-yloxy radicals

Energies of a series of 4-substituted 1-oxybicyclo[2.2.2]octan-1-yloxy radicals with 18 various substituents were calculated within the framework of the DFT theory at the levels UB3LYP/6-311+G(d,p)//UB3LYP/6-311+G(d,p) and UB3LYP/6-311++G(2df,p)//UB3LYP/6-311+G(d,p) and compared with similar series of the parent alcohols, their deprotonated and protonated forms calculated at the levels B3LYP/6-311+G(d,p)//B3LYP/6-311+G(d,p) and B3LYP/6-311++G(2df,p)//B3LYP/6-311+G(d,p). The two levels are of the same performance and both are sufficient for molecules of this type according to comparison with scarce experimental gas-phase acidities and basicities. The substituent effects were analyzed in terms of isodesmic equations. In addition to strong dependence on the substituent inductive effect, a slight dependence on the electronegativity of the first atom of the substituent was proven in certain cases. In all aspects, there is no qualitative difference between the effects on radicals and on similar closed shell species. Radicals behave as slightly electron deficient; the substituent effect is weaker than that on the ions but stronger than on neutral molecules.     

Conformational studies in the cyclohexane series. 2. Phenylcyclohexane and 1-methyl-1-phenylcyclohexane

The structures and relative energies of the conformers of phenylcyclohexane, and 1-methyl-1-phenylcyclohexane have been calculated at theoretical levels including HF/6-31G, B3LYP/6-311G, MP2/6-311G, MP2/6-311(2df,p), QCISD/6-311G, and QCISD/6-311G(2df,p). The latter gives conformational enthalpy (DeltaH degrees ), entropy (DeltaS degrees ), and free energy (DeltaG degrees ) values for phenylcyclohexane that are in excellent agreement with the experimental data. The calculations for 1-methyl-1-phenylcyclohexane find a free energy difference of 1.0 kcal/mol at -100 degrees C, favoring the conformation having an axial phenyl group, that is in only modest agreement with the experimental value of 0.32 +/- 0.04 kcal/mol. The origin of the phenyl rotational profiles for the conformers of phenylcyclohexane and 1-methyl-1-phenylcyclohexane is discussed.     

Complexation of Phenol and Thiophenol by Amine N‐Oxides: Isothermal Titration Calorimetry and ab Initio Calculations

To develop a new solvent‐impregnated resin (SIR) system for removal of phenols from water, the complex formation of dimethyldodecylamine N‐oxide (DMDAO), trioctylamine N‐oxide (TOAO), and tris(2‐ethylhexyl)amine N‐oxide (TEHAO) with phenol (PhOH) and thiophenol (PhSH) is studied. To this end we use isothermal titration calorimetry (ITC) and quantum chemical modeling (on B3LYP/6‐311G(d,p)‐optimized geometries: B3LYP/6‐311+G(d,p), B3LYP/6‐311++G(2d,2p), MP2/6‐311+G(d,p), and spin component scaled (SCS) MP2/6‐311+G(d,p); M06‐2X/6‐311+G(d,p)//M06‐2X/6‐311G(d,p), MP2 with an extrapolation to the complete basis set limit (MP2/CBS), as well as CBS‐Q). The complexes are analyzed in terms of structural (e.g., bond lengths) and electronic elements (e.g., charges). Furthermore, complexation and solvent effects (in benzene, toluene, and mesitylene) are investigated by ITC measurements, yielding binding constants K, enthalpies ΔH⁰, Gibbs fre energies ΔG⁰, and entropies ΔS⁰ of complex formation, and stoichiometry N. The ITC measurements revealed strong 1:1 complex formation between both DMDAO–PhOH and TOAO–PhOH. The binding constant (K=1.7–5.7×10⁴ M ^?1) drops markedly when water‐saturated toluene was used (K=5.8×10³ M ^?1), and π–π interaction with the solvent is shown to be relevant. Quantum mechanical modeling confirms formation of stable 1:1 complexes with linear hydrogen bonds that weaken on attachment of electron‐withdrawing groups to the amine N‐oxide moiety. Modeling also showed that complexes with PhSH are much weaker than those with PhOH, and in fact too weak for ITC determination. CBS‐Q incorrectly predicts equal or even higher binding enthalpies for PhSH than for PhOH, which invalidates it as a benchmark for other calculations. Data from the straightforward SCS‐MP2 method without counterpoise correction show very good agreement with the MP2/CBS values.     

Membrane-assisted solvent extraction of seven phenols combined with large volume injection-gas chromatography-mass spectrometric detection

Membrane-assisted solvent extraction (MASE) was applied for the determination of seven phenols (phenol, 2-chlorophenol, 2,4-dimethylphenol, 2,4-dichlorophenol, 4-chloro-3-methylphenol, 2,4,6-trichlorophenol and pentachlorophenol) with log Kow (octanol-water-partition-coefficient) between 1.46 (phenol) and 5.12 (pentachlorophenol) in water. The extraction solvents cyclohexane, ethyl acetate and chloroform were tested and ethyl acetate proved to be the best choice. The optimisation of extraction conditions showed the necessity of adding 5 g of sodium chloride to each aqueous sample to give a saturated solution (333 g/L). The pH-value of the sample was adjusted to 2 in order to convert all compounds into their neutral form. An extraction time of 60 min was found to be optimal. Under these conditions the recovery of phenol, the most polar compound, was 11%. The recoveries of the other analytes ranged between 42% (2-chlorophenol) and 98% (2,4-dichlorophenol). Calibration was performed using large volume injection (100 microL injection volume). At optimised conditions the limits of detection were between 0.01 and 0.6 microg/L and the relative standard deviation (n = 3) was on average about 10%. After the method optimisation with reagent water membrane-assisted solvent extraction was applied to two contaminated ground water samples from the region of Bitterfeld in Saxony-Anhalt, Germany. The results demonstrate the good applicability of membrane-assisted solvent extraction for polar analytes like phenols, without the necessity of derivatisation or a difficult and time-consuming sample preparation.     

Homoaromaticity in carbene intermediates

Analyses of isodesmic reactions for singlet 7-carbenanorbornene (4S), 8-carbena-endo-tricyclo[3.2.1.0(2,4)]octane (5S), 3-carbenabicyclo[3.1.0]hexane (3S), 2-carbenanorbornene 6S, and 2-carbenabicyclooctadiene 7S at the B3LYP/6-311+G(3df, 2p)//B3LYP/6-31G level provide stabilization energies of 13.83, 13. 50, 3.00, -2.22, and -3.01 kcal/mol, respectively. The C7 carbene in 4S and the C8 carbene center in 5S are strongly bent toward the double bond and cyclopropane ring, respectively, in contrast to their related triplets, 4T and 5T and parent hydrocarbons. The geometric change for 3S compared to 3T or to parent bicyclo[3.1. 0]hexane is minimal. Comparison of the stability of 6S with 2-carbenanorbornane and the geometry of 6S with that of 6T and also with the singlet and triplet 2-carbenanorbornane suggests very modest bridging. The stabilization energy and geometry of 2-carbenabicyclooctadiene 7 resemble antihomoaromatic bicyclooctadienyl cation 9 rather than the related homoaromatic bicyclooctadienyl anion 8. The diamagnetic susceptibility exhaltations (Deltachi) of 3, 4, 5, and 7, calculated at the B3LYP/6-311+G(2d,p)//B3LYP/6-31G level, are -0.7, 22.7, 26.0, and -10.3 cgs-ppm, respectively. The singlet-triplet energy differences, DeltaE(TS), for carbenacyclohexane, carbenacyclopentane, 3, 4, 5, 6, 7, and 2-carbenabicyclo[3.2.1]oct-3-ene at the B3LYP/6-311+G(3df, 2p)//B3LYP/6-31G level are 3.4, 10.3, 8.8, 27.1, 25.9, 12.7, -4.0, and -0.9 kcal/mol, respectively.     

Hydrogen Bonding in Phosphine Oxide/Phosphate–Phenol Complexes

To develop a new solvent‐impregnated resin (SIR) system for the removal of phenols and thiophenols from water, complex formation by hydrogen bonding of phosphine oxides and phosphates is studied using isothermal titration calorimetry (ITC) and quantum chemical modeling. Six different computational methods are used: B3LYP, M06‐2X, MP2, spin component‐scaled (SCS) MP2 [all four with 6‐311+G(d,p) basis set], a complete basis set extrapolation at the MP2 level (MP2/CBS), and the composite CBS‐Q model. This reveals a range of binding enthalpies (ΔH) for phenol–phosphine oxide and phenol–phosphate complexes and their thio analogues. Both structural (bond lengths/angles) and electronic elements (charges, bond orders) are studied. Furthermore, solvent effects are investigated theoretically by the PCM solvent model and experimentally via ITC. From our calculations, a trialkylphosphine oxide is found to be the most promising extractant for phenol in SIRs, yielding ΔH=?14.5 and ?9.8 kcal mol^?1 with phenol and thiophenol, respectively (MP2/CBS), without dimer formation that would hamper the phenol complexation. In ITC measurements, the ΔH of this complex was most negative in the noncoordinating solvent cyclohexane, and slightly less so in π–π interacting solvents such as benzene. The strongest binding is found for the dimethyl phosphate–phenol complex [?15.1 kcal mol^?1 (MP2/CBS)], due to the formation of two H‐bonds (P?O???H‐O‐ and P‐O‐H???O‐H); however, dimer formation of these phosphates competes with complexation of phenol, and would thus hamper their use in industrial extractions. CBS‐Q calculations display erroneous trends for sulfur compounds, and are found to be unsuitable. Computationally relatively cheap SCS‐MP2 and M06‐2X calculations did accurately agree with the much more elaborate MP2/CBS method, with an average deviation of less than 1 kcal mol^?1.     

Thermolysis of alkyl sulfoxides and derivatives: a comparison of experiment and theory.

Gas-phase activation data were obtained for model sulfoxide elimination reactions. The activation enthalpy for methyl 3-phenylpropyl sulfoxide is 32.9 +/- 0.9 kcal/mol. Elimination by methyl vinyl sulfoxide to form acetylene has an enthalpic barrier of 41.6 +/- 0.8 kcal/mol and that of 3-phenylpropyl methanesulfinate to form hydrocinnamaldehyde is 34.6 +/- 0.6 kcal/mol. Calculations at the MP2/6-311+G(3df,2p)//MP2/6-31G(d,p) level for simplified models of these reactions provide barriers of 32.3, 40.3, and 32.7 kcal/mol, respectively. A series of other compounds are examined computationally, and it is shown that the substituent effects on the sulfoxide elimination reaction are much more straightforward to interpret if DeltaH data are available in addition to the usually determined DeltaH++. The activation enthalpy of the reverse addition reaction is also subject to structural variation and can usually be rationalized on the basis of nucleophilicity of the sulfur or polarity matching between the sulfenic acid and olefin derivative.     

Energetic preferences for alpha,beta versus beta,gamma unsaturation

Density functional theory has been applied at the B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) level to examine the energetics of alpha,beta- versus beta,gamma-unsaturation for some common organic functional groups. Specifically, the relative stabilities of allyl-X (H2C=CHCH2X) and 1-propenyl-X (H3CCH=CHX) isomers have been computed for X = methyl, vinyl, phenyl, formyl, acetyl, methoxy, methylthio, methylsulfinyl, methylsulfonyl, sulfamoyl, and methoxysulfonyl, and the results are compared to available experimental data. The intrinsic preference of 3 kcal/mol for the 1-propenyl isomer when X = CH3 is exceeded by 2-4 kcal/mol for first-row conjugating groups, but it is not met for the sulfur-containing groups. In particular, alpha,beta-unsaturation is favored by less than 1 kcal/mol for the sulfone and sulfonamide analogues, while it is preferred by 8 kcal/mol for the vinyl-substituted case. Detailed structural results and torsional energy profiles are also reported.     

Accurate prediction of heats of formation by a combined method of B3LYP and neural network correction

Recently, we proposed the X1 method which combines the B3LYP/6‐311+G(3df,2p)//B3LYP/6‐311+G(d,p) method with a neural network correction for an accurate yet efficient prediction of heats of formation (Wu and Xu, J Chem Phys 2007, 127, 214105). In this contribution, we discuss in detail how to set up the X1 neural network. We give examples, showing how to apply the X1 method and how the applicability of X1 can be extended. The overall mean absolute deviation of the X1 method from experiment for the 488 heats of formation is 1.52 kcal/mol compared with 9.44 kcal/mol for the original B3LYP results. © 2008 Wiley Periodicals, Inc. J Comput Chem 2009     

An organic solvent-free microwave-assisted extraction of some priority pollutants of phenols in lake sediments.

Alkaline water was used for the microwave-assisted extraction of some priority pollutants of phenols in sediments, i.e. phenol (Ph), 2-chlorophenol (2CP), 2,4-dichlorophenol (2,4DCP), 4-chlorophenol (4CP), 4-dinitrophenol (4NP) and pentachlorophenol (PCP). This organic solvent-free extraction procedure was optimized by studying the parameters such as pH, volume of the alkaline water, extraction pressure and time. Under the optimized conditions, the recoveries of phenols were in the range of 80% to 110%. The extracts were then cleaned-up and concentrated by microcolumn solid phase extraction (SPE) and determined by gas chromatography-flame ionization detection system. The relative standard deviation of the overall-method for most phenol determinations was about 5.0% (n = 6). The proposed method, which needs little volume (1 mL) of ethanol for SPE, has been applied to determine these phenols in sediment samples, and the analytical results are in good agreement with those achieved by Soxhlet extraction.