Low and high temperature bromination of exocyclic dienes: high temperature bromination. Part 16

The electrophilic addition of bromine to an exocyclic diene, 5,6-dimethylenebicyclo[2.2.1]hept-2-ene, in CCl₄ at 0°C led to the formation of non-rearranged (73%) and rearranged products (27%). However, high temperature bromination of the exocyclic diene at 77°C suppressed the formation of the rearranged products. Similarly, bromination of 9,10-dimethylenetricyclo[6.2.1.0^2,7]undeca-2,4,6-triene at −10°C gave only the exo-1,2-addition product. Bromination at +5°C resulted in the formation of a mixture consisting of exo-1,2- and 1,4-addition products in a ratio of (1:4). High temperature bromination at 77°C resulted in the formation of the endo-1,2-addition product. Furthermore, it has been shown that the 1,4-addition product converts smoothly to the 1,2-addition product. The formation mechanism of the products is discussed and supported by calculations.     

High-Temperature Bromination XIII [1]: Bromination of Dimethyl 7-Oxabicyclo[2.2.1]hepta-2,5-diene-2,3dicarboxylate

The electrophilic addition of bromine to dimethyl 7-oxabicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxylate 10 at 0 and –60°C led in high yield to the formation of dibromide 11. However, high-temperature bromination of 10 in carbon tetrachloride at 77°C gave four products 12–15, three of them with nonrearranged skeleton. Similarly, bromination of the dibromides 14 and 15 at 77°C yielded the nonrearranged tetrabromides 13 and 16, respectively. The structure of the tetrabromide 13 was solved by single-crystal X-ray analysis. The formation mechanism is discussed and supported by calculations.     

Fragmentation mechanism for the loss of XCY (X and Y = O,OR, S) from 2-phenyl-1,3,4-oxadiazole-5-one and related sulfur-containing compounds

The fragmentation mechanism for loss of X?C?Y (X and Y = O or S) from 2-phenyl-1-3-4-oxadiazole-5-one and related sulfur-containing compounds begins with the breaking of the C? N bond of the heterocyclic ring. Then a X?C?Y molecule is ejected with the initial formation of a non-rearranged ion. The major part of ΔH_R⁰ is associated with the stabilization of this neutral fragment. The initial fragment ion is further rearranged before it decomposes.     

Synthesis of arenediynes via the vinylidenecarbene-acetylene rearrangement

A convenient method for the two-step synthesis of arenediynes from 1,2-arenedialdehydes is reported. Dibromomethylenation of dialdehydes under Corey-Fuchs conditions (CBr₄, Ph₃P, Zn) provides the tetrabromides in excellent yields. Treatment of the tetrabromides with n-BuLi or LDA affords 3,4-unsaturated 1,5-diynes, the key structural moiety present in several naturally occurring antitumour antibiotics, in varying yields. The key intermediates in these transformations appear to be vinylidenecarbenes or carbenoids, generated in situ via metal-halogen exchange and elimination.     

Reaction of thianthrene and phenoxathiin cation radicals with 2,3-dimethyl-2-butene. Chemical and electrochemical studies

Thianthrene cation radical tetrafluoroborate (Th*+ BF4-) has been found to add to 2,3-dimethyl-2-butene (DMB) at 0 degrees C and -15 degrees C. The adduct, 2,3-dimethyl-2,3-(5,10-thianthreniumdiyl)butane ditetrafluoroborate (12), was isolated at -15 degrees C, and its 1H NMR spectrum was recorded at that temperature. The adduct was stable in CD3CN solution at -15 degrees C but decomposed slowly at 0 degrees C and quickly at 23 degrees C, forming the salt of 2,4,4,5,5-pentamethyl-2-oxazoline (8) with loss of thianthrene (Th). These results explain why earlier attempts to prepare 12 and detect its formation at room temperature with NMR spectroscopy were not successful. Reaction of Th*+ with DMB was followed with cyclic voltammetry and was found to exhibit redox catalysis in which Th was regenerated. With the faster scanning techniques of cyclic voltammetry, the formation of 12 was detectable, with a reduction potential of about -1.0 V at 25 degrees C and 3 degrees C. The observed reduction potential was in harmony with reduction potentials of a number of other, stable monoadducts. Thus, the redox catalysis involved the rapid formation of 12 and its rapid decomposition into 8 and Th, the newly formed Th being responsible for the observed enhanced oxidation currents. In contrast, 8 appears to be formed directly by oxidation of DMB by PO*+PF6-.     

The role of free electrons in MALDI: electron capture by molecules of alpha-cyano-4-hydroxycinnamic acid

Low-energy (0-12 eV) electron attachment to molecules of a typical matrix substance used for matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS), namely alpha-cyano-4-hydroxicynnamic acid, has been investigated in the gas phase at different temperatures ranging from 140 degrees C to 260 degrees C by means of electron capture negative-ion mass spectrometry (ECNI MS). The yield of negative ions, formed by electron capture, was measured as a function of incident electron energy for four different temperatures. The long-lived parent molecular anion, [M]- (m/z 189), was observed in the negative-ion mass spectra of the substance under investigation. Its autodetachment lifetime was estimated to be approximately 600 micros. It was found that at 140 degrees C the main decay channel of the long-lived temporary molecular anion of alpha-cyano-4-hydroxicynnamic acid is a formation of the [M-COOH]-; fragment negative ion (m/z 144) with an intensity of 37.2% in percentage terms in respect of the total anion current. There are also [M-H]-, [M-CO2]- and [CN]- fragments in the spectra with intensities of about 7.7%, 21.6% and 3.1% at 140 degrees C. It was shown that the escape of the CO2 molecule from the parent molecular anion is a slow process. It takes [M]- about 10 micros to decay on carbon dioxide molecules and [M-CO2]- fragment anions. Increasing the temperature of the target molecule alters the negative-ion mass spectra of alpha-cyano-4-hydroxicynnamic acid significantly. A possible role for the findings in typical MALDI MS experiments is discussed.     

Kinetics and activation parameter analysis for the prebiotic oligocytidylate formation on Na(+)-montmorillonite at 0-100 degrees C

The kinetic analysis of the temperature dependence of the formation of oligocytidylate (oligo(C)) from the 5'-monophosphorimidazolide moiety of cytidine (ImpC) in the presence of Na (+)-montmorillonite (Na (+)-Mont) catalyst has been carried out at 0-100 degrees C. The rate constants for the formation of oligo(C), hydrolysis of ImpC with and without Na (+)-Mont and degradation of oligo(C) were determined. The apparent activation parameters were 30.8 +/- 3.9 kJ mol (-1) ( Ea), 28.3 +/- 4.0 kJ mol (-1) (Delta H++), and -231 +/- 13 J mol (-1) K (-1) (Delta S++) for the formation of the 2-mer; 45.6 +/- 2.9 kJ mol (-1) ( Ea), 43.0 +/- 3.0 kJ mol (-1) (Delta H++), -164 +/- 10 J mol (-1) K (-1) (Delta S++) for the 3-mer; and 45.2 +/- 0.6 kJ mol (-1) ( Ea), 42.7 +/- 0.7 kJ mol (-1) (Delta H++), -159 +/- 2 J mol (-1) K (-1) (Delta S++) for the 4-mer in the presence of Na (+)-Mont. An increasing trend for the rate constants for the formation of oligo(C) in the order 2-mer < 3-mer <4-mer was observed at high temperatures, which is consistent with that observed at low temperatures. These analyses implied for the first time that the associate formation between an activated nucleotide monomer and an elongating oligonucleotide prior to the phosphodiester bond formation during the elongation of an oligonucleotide on a clay surface would be based on the interaction between the two reactants at the phosphoester and/or ribose moieties rather than at the nucleotide bases. The hydrolysis rate of ImpC at 25-100 degrees C was 5.3-10.6 times greater in the presence of Na (+)-Mont than in its absence. Although the degradation of oligo(C) in the presence of Na (+)-Mont was slower than the formation of the 3-mer and longer oligo(C) on Na (+)-Mont, its yield decreased with temperature. This is mainly because the ratios of the rate constant of the 2-mer formation to those of ImpC hydrolysis and the 3-mer and 4-mer formation decrease with an increase in temperature, which is attributed to the enthalpy and entropy changes for the formation of the 2-mer. This trend resembles the case of the template-directed formation of oligo(G) on a poly(C) template but is different from the Pb (2+)-ion-catalyzed oligo(C) formation. According to the kinetics and activation parameter analyses regarding the clay reaction and other prebiotic polymerase models, the possible pathways for the oligonucleotide formation are discussed and compared.     

Temperature dependence of formation of nanorods and dots of iodine compounds on an H-terminated Si(111) surface in a concentrated HI solution

Immersion of atomically flat, H-terminated Si(111) surfaces in 7.6 M HI for 0.5 - 4 h caused spontaneous formation of nanosized clusters at the Si surface. X-ray photoelectron spectroscopy analysis suggested that the clusters were composed of silicon iodides (such as SiHxI4-x), produced most probably by Si etching with HI. Atomic force microscopy inspection revealed that the immersion at a low temperature below about 30 degrees C led to the formation of long rod-shaped clusters, oriented in the (112) direction or equivalents, whereas the immersion at a high temperature above 30 degrees C led to the formation of circular dot clusters, their size and shape changing abruptly at about 70 degrees C. It is shown experimentally that the formation of dot clusters at a high immersion temperature is explained on the basis of thermodynamics, whereas that of oriented rod clusters at a low temperature is explained by a kinetics-controlled mechanism.     

Acetone hydration in supercritical water: (13)C-NMR spectroscopy and Monte Carlo simulation

The (13)C-NMR chemical shift of acetone delta((13)C[Double Bond]O) was measured in aqueous solution at high temperatures up to 400 degrees C and water densities of 0.10-0.60 g/cm(3) for the study of hydration structure in the supercritical conditions. The average number N(HB) of hydrogen bonds (HBs) between an acetone and solvent waters and the energy change DeltaE upon the HB formation were evaluated from the delta and its temperature dependence, respectively. At 400 degrees C, N(HB) is an increasing function of the water density, the increase being slower at higher water densities. The acetone-water HB formation is exothermic in supercritical water with larger negative DeltaE at lower water densities (-3.3 kcal/mol at 0.10 g/cm(3) and -0.3 kcal/mol at 0.60 g/cm(3)), in contrast to the positive DeltaE in ambient water (+0.078 kcal/mol at 4 degrees C). The corresponding Monte Carlo simulations were performed to calculate the radial and orientational distribution functions of waters around the acetone molecule. The density dependence of N(HB) calculated at 400 degrees C is in a qualitative agreement with the experimental results. In the supercritical conditions, the HB angle in a neighboring acetone-water pair is weakly influenced by the water density, because of the absence of collective HB structure. This is in sharp contrast to the hydration structure in ambient water, where the acetone-water HB formation is orientationally disturbed by the tetrahedral HB network formation among the surrounding waters.     

Complexation of thorium(IV) with acetate at variable temperatures

The complexation between Th(IV) and acetate in 1.05 mol kg(-1) NaClO4 was studied at variable temperatures (10, 25, 40, 55 and 70 degrees C). The formation constants of five successive complexes, Th(Ac)j(4-j)+ where Ac = CH3COO- and j = 1-5, and the molar enthalpies of complexation were determined by potentiometry and calorimetry. Extended X-ray absorption fine structure spectroscopy (EXAFS) provided additional information on the complexes in solution. The effect of temperature on the stability of the complexes is discussed in terms of the electrostatic model.     

Low-temperature thermolysis behavior of tetramethyl- and tetraethyldistibines

The thermolysis behavior of tetramethyl- and tetraethyldistibine (Sb(2)Me(4) and Sb(2)Et(4)) was investigated using a mass spectrometer coupled to a tubular flow reactor under near-chemical vapor deposition (CVD) conditions. Sb(2)Me(4) undergoes a gas-phase disproportionation with an estimated activation energy of 163 kJ/mol. This reaction leads to the formation of methylstibinidine, SbMe, that reacts on the surface to produce antimony film and SbMe(3). Unfortunately, this clean decomposition pathway is limited to a narrow temperature range of 300-350 degrees C. At temperatures exceeding 400 degrees C, SbMe(3) decomposes following a radical route with a consequent risk of carbon contamination. In contrast, Sb(2)Et(4) disproportionates at the hot wall of the reactor. According to mass-spectrometric data, this reaction is significant starting at a temperature of 100 degrees C, with an apparent activation energy of 104 kJ/mol. Within the temperature range of 100-250 degrees C, the precursor decomposition leads to the formation of antimony films and SbEt(3), whereas different molecular reaction pathways are significantly activated above 250 degrees C. The use of Sb(2)Et(4) lowers the risk of carbon contamination compared to Sb(2)Me(4) at high temperature. Therefore, Sb(2)Et(4) is a promising CVD precursor for the growth of antimony films in the absence of hydrogen atmosphere in a wide temperature range.     

Phase behavior of CO2-expanded fluorinated microemulsions

The formation of CO2-expanded, fluorinated reverse microemulsions is demonstrated for the system of perfluoropolyether (PFPE) surfactant (ClPFPE-NH4, MW = 632) and PFPE oil (PFPE, MW = 580). The phase behavior of this system is examined as a function of temperature (25-45 degrees C), pressure, CO2 concentration, and water to surfactant molar ratios (W0 = 10 and 20). Visual observations of one-phase behavior consistent with reverse microemulsion formation are further supported by spectroscopic measurements that establish the existence of a bulk water environment within the aqueous core. Microemulsion formation is not observed in the absence of CO2 for this PFPE surfactant/PFPE oil system, and a CO2 content greater than 70 mol % is required to induce microemulsion formation. Over the range of water loadings and temperatures investigated, the lowest cloud point pressure is observed at 46 bar (5 wt % ClPFPE-NH4 in PFPE oil, W0 = 20, xCO2 = 0.7, T = 25 degrees C). In the regions where one-phase behavior is observed, the cloud point pressures increase with temperature, water loadings, and CO2 content. The driving forces of microemulsion formation in the CO2-expanded fluorinated solvent are discussed relative to traditional reverse microemulsions and CO2-continuous microemulsions.     

Energetics of cresols and of methylphenoxyl radicals

Combustion calorimetry studies were used to determine the standard molar enthalpies of formation of o-, m-, and p-cresols, at 298.15 K, in the condensed state as Delta(f)H(m) degrees (o-CH(3)C(6)H(4)OH,cr) = -204.2 +/- 2.7 kJ.mol(-1), Delta(f)H(m) degrees (m-CH(3)C(6)H(4)OH,l) = -196.6 +/- 2.1 kJ.mol(-1), and Delta(f)H(m) degrees (p-CH(3)C(6)H(4)OH,cr) = -202.2 +/- 3.0 kJ.mol(-1). Calvet drop calorimetric measurements led to the following enthalpy of sublimation and vaporization values at 298.15 K: Delta(sub)H(m) degrees (o-CH(3)C(6)H(4)OH) = 73.74 +/- 0.46 kJ.mol(-1), Delta(vap)H(m) degrees (m-CH(3)C(6)H(4)OH) = 64.96 +/- 0.69 kJ.mol(-1), and Delta(sub)H(m) degrees (p-CH(3)C(6)H(4)OH) = 73.13 +/- 0.56 kJ.mol(-1). From the obtained Delta(f)H(m) degrees (l/cr) and Delta(vap)H(m) degrees /Delta(sub)H(m) degrees values, it was possible to derive Delta(f)H(m) degrees (o-CH(3)C(6)H(4)OH,g) = -130.5 +/- 2.7 kJ.mol(-1), Delta(f)H(m) degrees (m-CH(3)C(6)H(4)OH,g) = -131.6 +/- 2.2 kJ.mol(-1), and Delta(f)H(m) degrees (p-CH(3)C(6)H(4)OH,g) = -129.1 +/- 3.1 kJ.mol(-1). These values, together with the enthalpies of isodesmic and isogyric gas-phase reactions predicted by the B3LYP/cc-pVDZ, B3LYP/cc-pVTZ, B3P86/cc-pVDZ, B3P86/cc-pVTZ, MPW1PW91/cc-pVTZ, CBS-QB3, and CCSD/cc-pVDZ//B3LYP/cc-pVTZ methods, were used to obtain the differences between the enthalpy of formation of the phenoxyl radical and the enthalpies of formation of the three methylphenoxyl radicals: Delta(f)H(m) degrees (C(6)H(5)O*,g) - Delta(f)H(m) degrees (o-CH(3)C(6)H(4)O*,g) = 42.2 +/- 2.8 kJ.mol(-1), Delta(f)H(m) degrees (C(6)H(5)O*,g) - Delta(f)H(m) degrees (m-CH(3)C(6)H(4)O*,g) = 36.1 +/- 2.4 kJ.mol(-1), and Delta(f)H(m) degrees (C(6)H(5)O*,g) - Delta(f)H(m) degrees (p-CH(3)C(6)H(4)O*,g) = 38.6 +/- 3.2 kJ.mol(-1). The corresponding differences in O-H bond dissociation enthalpies were also derived as DH degrees (C(6)H(5)O-H) - DH degrees (o-CH(3)C(6)H(4)O-H) = 8.1 +/- 4.0 kJ.mol(-1), DH degrees (C(6)H(5)O-H) - DH degrees (m-CH(3)C(6)H(4)O-H) = 0.9 +/- 3.4 kJ.mol(-1), and DH degrees (C(6)H(5)O-H) - DH degrees (p-CH(3)C(6)H(4)O-H) = 5.9 +/- 4.5 kJ.mol(-1). Based on the differences in Gibbs energies of formation obtained from the enthalpic data mentioned above and from published or calculated entropy values, it is concluded that the relative stability of the cresols varies according to p-cresol < m-cresol < o-cresol, and that of the radicals follows the trend m-methylphenoxyl < p-methylphenoxyl < o-methylphenoxyl. It is also found that these tendencies are enthalpically controlled.     

Functionalization and kinetic stabilization of the [4]paracyclophane system and aromaticity of its extremely bent benzene ring

Kinetic stabilization of the [4]paracyclophane skeleton by the introduction of substituents, which serve to sterically hinder reactions at the reactive bridgehead sites, and properties of the resultant [4]paracyclophanes are investigated in this study. Modification of the property of [4]paracyclophane by functionalization is also intended. [4]Paracyclophanes are designed to be derived from the corresponding Dewar benzene isomers via their photochemical aromatization, and the requisite 1,4-bridged Dewar benzenes bearing sterically demanding functional groups are prepared. Irradiation of these precursors under matrix isolation at 77 K leads to the formation of [4]paracyclophanes, which exhibit characteristic electronic absorption spectra. The half-lives of the generated species vary widely from less than 1 min at -90 degrees C to 0.5 h at -20 degrees C, depending on the type of substituents and the pattern of substitution. One of the derivatives, 24, is stable enough and its content in the irradiated mixture is high enough to permit the measurement of the (1)H NMR spectrum. The recorded spectrum, which is reproduced very well by theoretical calculations using the GIAO method at the hybrid HF-DFT (B3LYP/6-31+G*) level, suggests the sustenance of rather strong diatropicity in its severely bent benzene moiety. Calculations on the bent benzene whose geometry is constrained to that calculated for 24 support that aromaticity is retained to a significant extent as compared to that of planar benzene, as judged by the magnetic criteria of aromaticity, that is, diamagnetic susceptibility exaltation and nucleus-independent chemical shift. The reason for the retention of aromaticity despite the severe bending of the benzene ring is discussed. Cyclophane 24 is so strained that it exceeds the corresponding Dewar benzene precursor in energy and thermally reverts to the latter with a half-life of 15 +/- 5 min at -20 degrees C (DeltaG++ = 18.3 +/- 0.3 kcal mol(-1)).     

Temperature-dependent formation and photorepair of DNA damage induced by UV-B radiation in suspension-cultured tobacco cells

Two photoproducts of DNA damage, i.e. cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts (6-4PPs), induced by UV-B radiation in suspension-cultured tobacco cells were quantified by enzyme-linked immunosorbent assay (ELISA) with monoclonal antibodies. CPDs and 6-4PPs were induced in tobacco cells by UV-B radiation. Photorepair of CPDs was faster than that of 6-4PPs. UV-B radiation induces formation of CPDs and 6-4PPs even at 0 degrees C, but low temperature significantly decreases the UV-B-induced (in contrast to UV-C-induced) formation of CPDs and 6-4PPs. Low temperature also retarded the removal of CPDs and 6-4PPs under white light, and almost no photorepair of CPDs and 6-4PPs was detected at 0 degrees C. When purified DNA from tobacco cells grown in darkness was irradiated with UV-B, formation of CPDs and 6-4PPs took place at the same speed at different temperatures. It indicated that formation of CPDs and 6-4PPs induced by UV-B was temperature-independent in a non-cellular system. Based on our results for suspension-cultured tobacco cells, not only the photorepair but also UV-B-induced formation of CPDs and 6-4PPs are temperature-dependent.     

On the hydrolysis of the dioxouranium(VI) ion in oxalate solutions

The complex formation between the dioxouranium (VI) and the oxalate ions has been investigated by measuring the potential of a glass electrode, at 25.00 degrees C, in 1 and 3 M NaClO4, at lower acidities than 10(-4.5) M, in order to favour the formation of (mixed) ternary species. The upper limits of concentration of all the analytical parameters have been imposed by the modest solubility of Na2C2O4 in the ionic media. The experimental measurements at different ionic strengths have been treated by means of the computerised least square programme LETAGROP - ETITR. Ternary complexes of general composition (p, q, r) are formed according to reaction (1), in addition to the already reported binary complexes. pUO2(2+) + qH2O + rC2O4(2-) <==> (UO2)p(OH)q(C2O4)r(2p-q-2r)(+) + qH+ (1). The stoichiometric compositions of the ternary species are (1, 1, 1), (2, 4, 2), (2, 2, 4). Their formation constants, expressed in molality, obtained in the two ionic media, are listed below. [table: see text]. For reasons discussed in the present work in the last column only the value of the constant in 1 M ionic medium is reported for the species (2, 4, 2). Additional evidence on the stoichiometric composition of the species formed is afforded by the mass-spectrometric measurements, collected in solutions of known composition.     

Formation and stability of zinc(II) and cadmium(II) citrate complexes in aqueous solution at various temperatures

The citrate complexes of Zn(2+) and Cd(2+) have been investigated by pH titration at I = 0.1 M (KNO(3)) and 10, 25, 35 and 45 degrees . The species found were [Zn(cit)](-), [Zn(cit)H], [Zn(cit)(2)](4-) and [Zn(2)(cit)(2)H(-)(2)](4-), [Cd(cit)](-), [Cd(cit)H], [Cd(cit)(2)](4-) and [Cd(cit)H(-1)](2-). From the dependence of the formation constants on temperature, DeltaH degrees and DeltaS degrees values were calculated. Speciation in the Zn(2+)]- and Cd(2+)-citrate systems is discussed with particular attention to formation of polynuclear species. Some comparisons with literature data are made.     

Collisions of slow polyatomic ions with surfaces: dissociation and chemical reactions of C2H2+*, C2H3+, C2H4+*, C2H5+, and their deuterated variants C2D2+* and C2D4+* on room-temperature and heated carbon surfaces

Interaction of C2Hn+ (n = 2-5) hydrocarbon ions and some of their isotopic variants with room-temperature and heated (600 degrees C) highly oriented pyrolytic graphite (HOPG) surfaces was investigated over the range of incident energies 11-46 eV and an incident angle of 60 degrees with respect to the surface normal. The work is an extension of our earlier research on surface interactions of CHn+ (n = 3-5) ions. Mass spectra, translational energy distributions, and angular distributions of product ions were measured. Collisions with the HOPG surface heated to 600 degrees C showed only partial or substantial dissociation of the projectile ions; translational energy distributions of the product ions peaked at about 50% of the incident energy. Interactions with the HOPG surface at room temperature showed both surface-induced dissociation of the projectiles and, in the case of radical cation projectiles C2H2+* and C2H4+*, chemical reactions with the hydrocarbons on the surface. These reactions were (i) H-atom transfer to the projectile, formation of protonated projectiles, and their subsequent fragmentation and (ii) formation of a carbon chain build-up product in reactions of the projectile ion with a terminal CH3-group of the surface hydrocarbons and subsequent fragmentation of the product ion to C3H3+. The product ions were formed in inelastic collisions in which the translational energy of the surface-excited projectile peaked at about 32% of the incident energy. Angular distributions of reaction products showed peaking at subspecular angles close to 68 degrees (heated surfaces) and 72 degrees (room-temperature surfaces). The absolute survival probability at the incident angle of 60 degrees was about 0.1% for C2H2+*, close to 1% for C2H4+* and C2H5+, and about 3-6% for C2H3+.     

A theoretical study of reactivity and regioselectivity in the hydroxylation of adamantane by ferrate(VI)

The conversion of adamantane to adamantanols mediated by ferrate (FeO(4)(2)(-)), monoprotonated ferrate (HFeO(4)(-)), and diprotonated ferrate (H(2)FeO(4)) is discussed with the hybrid B3LYP density functional theory (DFT) method. Diprotonated ferrate is the best mediator for the activation of the C-H bonds of adamantane via two reaction pathways, in which 1-adamantanol is formed by the abstraction of a tertiary hydrogen atom (3 degrees ) and 2-adamantanol by the abstraction of a secondary hydrogen atom (2 degrees ). Each reaction pathway is initiated by a C-H bond cleavage via an H-atom abstraction that leads to a radical intermediate, followed by a C-O bond formation via an oxygen rebound step to lead to an adamantanol complex. The activation energies for the C-H cleavage step are 6.9 kcal/mol in the 1-adamantanol pathway and 8.4 kcal/mol in the 2-adamantanol pathway, respectively, at the B3LYP/6-311++G level of theory, whereas those of the second reaction step corresponding to the rebound step are relatively small. Thus, the rate-determining step in the two pathways is the C-H bond dissociation step, which is relevant to the regioselectivity for adamantane hydroxylation. The relative rate constant (3 degrees )/(2 degrees ) for the competing H-atom abstraction reactions is calculated to be 9.30 at 75 degrees C, which is fully consistent with an experimental value of 10.1.     

The thickness of a liquid layer on the free surface of ice as obtained from computer simulation

Molecular dynamic simulations were performed for ice I(h) with a free surface by using four water models, SPC/E, TIP4P, TIP4P/Ice, and TIP4P/2005. The behavior of the basal plane, the primary prismatic plane, and of the secondary prismatic plane when exposed to vacuum was analyzed. We observe the formation of a thin liquid layer at the ice surface at temperatures below the melting point for all models and the three planes considered. For a given plane it was found that the thickness of a liquid layer was similar for different water models, when the comparison is made at the same undercooling with respect to the melting point of the model. The liquid layer thickness is found to increase with temperature. For a fixed temperature it was found that the thickness of the liquid layer decreases in the following order: the basal plane, the primary prismatic plane, and the secondary prismatic plane. For the TIP4P/Ice model, a model reproducing the experimental value of the melting temperature of ice, the first clear indication of the formation of a liquid layer, appears at about -100 degrees C for the basal plane, at about -80 degrees C for the primary prismatic plane, and at about -70 degrees C for the secondary prismatic plane.