New developments in the field of piezothermal analysis

The solid transitions of C₂₁, C₂₃ and C₂₅ n-paraffins are examined from a piezothermal point of view. The paper is divided into two parts. The first is a report of the main features of a piezothermal analyzer when pressure scanning allows the continuous record of the expansivity as a function of pressure up to 5 Kilobars. Small samples are required and the scanning speeds vary from 0.3 to 16 Kilobars per hour. The second part describes the experimental procedure appropriate for solid state determinations. The resulting piezothermograms are presented and entropies of transformation are determined. A model allows a crude statistical approach giving the entropies of transformation with the correct order of magnitude. Problems related with phase transformations under a shearing stress are considered.

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Zusammenfassung Feststoffumwandlungen bei C₂₁, C₂₃ und C₂₅ n-Paraffinen wurden unter dem Gesichtspunkt der Piezowärme untersucht. Im ersten Teil vorliegender Arbeit werden die Haupteigenschaften eines piezothermischen Analysators beschrieben, bei dem durch Druck-Scanning die kontinuierliche Aufzeichnung des Ausdehnungsvermögens als Punktion des Druckes bis zu 5 kbar ermöglicht wird. Es werden nur kleine Proben benötigt, die Scanning-Geschwindigkeit variiert zwischen 0,3 und 16 kbar/h. Der zweite Teil beschreibt das geeignete experimentelle Verfahren zur Feststoffzustandsbestimmung. Die erhaltenen Piezothermogramme werden dargestellt und die Entropien der Umwandlung bestimmt. Ein Modell liefert eine grobe statistische Näherung, die die Entropien der Umwandlung in der richtigen Grö\enordnung liefert. Probleme in Zusammenhang mit Phasenumwandlungen bei Scherbeanspruchungen werden betrachtet.

     

Thermodynamic properties of alkali halides. II. Enthalpies of dilution and heat capacities in water at 25°C

The enthalpies of dilution and volumetric specific heats of most alkali halides were measured in water at 25°C with flow microcalorimeters in the concentration range 0.01 to 1m. Apparent molal relative enthalpies _L, derived from the enthalpies of dilution, can be represented by a parametric equation in molality. Combining _L with osmotic data, excess entropies can be calculated. Excess free energies, enthalpies, and entropies are compared at 0.5m, and the observed trends are consistent with a model of structural interactions in aqueous alkali halide solutions. The apparent molal heat capacities _C were fitted with the equation _C= _C ^° +A_C(d₀m)^1/2+B _C m. The _C ^° are, in general, additive to better than 1 J-K^–1-mole^–1 and reflect mostly the structural part of ion-solvent interactions. Taking _C ^° (H⁺)=0, conventional ionic _C ^° are obtained. The parameterB _C for different pairs of ions follows approximately the same trends as the corresponding parameterB _V for apparent molal volumes and seems to reflect structural interactions between the ions.     

Switching the Statistical C3/C1 Ratio in the Threefold Aromatic Substitution of Tribenzotriquinacenes towards the C3 Isomer

Tribenzotriquinacene (TBTQ) is a bowl‐shaped molecule that has been widely used as a molecular building block in supramolecular and materials chemistry. Especially C₃‐symmetric threefold‐substituted TBTQs are interesting for these purposes. Until now a general and selective synthetic approach to those C₃‐symmetric products was lacking, mainly because the typically used electrophilic aromatic substitution reactions of the parent TBTQ hydrocarbons produce predominantly the C₁ isomer over the C₃ isomer (3:1 statistical ratio). Herein we introduce a threefold borylation of TBTQ with the C₃ isomer as the main product (2.6:1 C₃/C₁ ratio). The borylated TBTQ can be converted in good yields into other C₃‐symmetric TBTQs, thus allowing straightforward synthetic access to new building blocks for supramolecular and materials chemistry.     

Thermodynamics of saturated and unsaturated ketones C<Subscript>13</Subscript> and C<Subscript>18</Subscript> and the energy of specific intermolecular interactions

P-T dependences of the saturated vapor of pseudoionone, hexahydropseudoionone, and saturated and unsaturated ketones C₁₈ were studied using the statistical approach, and the enthalpies and entropies of its vaporization were calculated. The presence of monomer forms of the molecules of these compounds was established by studying the unsaturated vapor pressure. It was demonstrated that the role of isostructural methyl group in ketones is not related to the all-explaining steric effect concealing the real nature of the specific interaction. The energies of specific intermolecular interactions were determined in liquid symmetrical and unsymmetrical ketones.     

Thermodynamic properties of graphite-like nanostructures prepared by thermobaric treatment of fullerite C60

Temperature dependences of the heat capacities of disordered graphite-like nanostructures prepared by the thermobaric treatment of fullerite C₆₀ (p = 2 and 8 GPa, T = 1373 K) were measured in the temperature ranges from 7 to 360 K in an adiabatic vacuum calorimeter and from 330 to 650 K in a differential scanning calorimeter. At T < 50 K, the dependences obtained were analyzed using the Debye theory of the heat capacity of solids and its multifractal version. The fractal dimensions D were determined and some conclusions on the heterodynamic character of the structures studied were made. The thermodynamic functions C _p ^o T), H ^o(T) − H ^o(0), S ^o(T) − S ^o(0), and G ^o(T) − H ^o(0) were calculated in the temperature range from T → 0 to 610 (650) K. The thermodynamic properties of the graphite-like nanostructures studied and some carbon allotropes were compared. The standard entropies of formation Δ_f S ^o of the graphite nanostructures studied and diamond were calculated along with the standard entropies of the reactions of their synthesis from the face-centered cubic phase of fullerite C₆₀ and their interconversions at T = 298.15 K. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1940–1945, September, 2008.     

Thermodynamic properties of N-octyl- and N-dodecylnicotinamide chlorides in water

Densities, heat capacities and enthalpies of dilution at 25°C and osmotic coefficients at 37°C were measured for N-octyl- and N-dodecylnicotinamide chlorides in water over an extended concentration region. Partial molar volumes, heat capacities, relative enthalpies and nonideal free energies and entropies at 25°C were derived as a function of the surfactant concentration. For both surfactants, plots of volumes, enthalpies and free energies vs. concentration are regular whereas those of heat capacities and entropies present anomalies at about 0.8 and 0.1m for the octyl and dodecyl compounds, respectively. Changes in the slope of a plot of osmotic coefficients times molality vs. molality were also observed at these same concentrations. These peculiarities are ascribed to micelle structural transitions. The nonideal free energies do not seem to depend on the alkyl chain length when they are plotted vs. m/C _cmc . Also, a plot of the nonideal free energy vs. logm/C _cmc is roughly independent of the nature of the surfactant because of the constant activity of surfactants in micellar solutions. Nonideal free energies, enthalpies and entropies have been calculated at 15 and 35°C. At each concentration the nonideal free energy is temperature independent as a result of a compensatory effect between enthalpy and entropy. The thermodynamic functions of micellization were graphically evaluated on the basis of the pseudo-phase transition model. These data suggest that the nicotinamide group possesses less hydrophilic character than the ammonium group.     

Thermodynamic properties of crystalline polymeric C60 phases in the temperature region from O → 0 to 340 E

The temperature dependences of the heat capacity C _p° = f(T) were studied in an adiabatic vacuum calorimeter for the orthorhombic, tetragonal, and rhombohedral polymeric C₆₀ phases in the 7—340 K temperature interval with an error of 0.2%. Comparative analysis of C _p° of these phases formed by stacking of one-dimensional and two types of two-dimensional polyfullerenes C₆₀, was performed, and their fractal dimensionalities D were determined for temperatures below 50 K. The thermodynamic functions of the crystalline polymeric C₆₀ phases were calculated in the temperature region from O 0 to 340 K: C _p°(T), H°(T) — H°(0), S°(T) — S°(0), and G°(T) — H°(0). Assuming that S°(0) = 0, the standard entropies of formation _f S° of these phases from graphite at T = 298.15 K and standard pressure were calculated. In addition, the entropies of transformation of the initial face-centered cubic phase of fullerite C₆₀ in the crystalline polymeric C₆₀ phases and entropies of their interconversions under the same conditions were estimated. The thermodynamic characteristics of the polymeric C₆₀ phases were reviewed.     

Electronic structures and nonlinear optical properties of highly deformed halofullerenes C3v C60F18 and D3d C60Cl30

Electronic structures and nonlinear optical properties of two highly deformed halofullerenes C_3v C₆₀F₁₈ and D_3d C₆₀Cl₃₀ have been systematically studied by means of density functional theory. The large energy gaps (3.62 and 2.61 eV) between the highest occupied and lowest unoccupied molecular orbitals (HOMOs and LUMOs) and the strong aromatic character (with nucleus‐independent chemical shifts varying from ?15.08 to ?23.71 ppm) of C₆₀F₁₈ and C₆₀Cl₃₀ indicate their high stabilities. Further investigations of electronic property show that C₆₀F₁₈ and C₆₀Cl₃₀ could be excellent electron acceptors for potential photonic/photovoltaic applications in consequence of their large vertical electron affinities. The density of states and frontier molecular orbitals are also calculated, which present that HOMOs and LUMOs are mainly distributed in the tortoise shell subunit of C₆₀F₁₈ and the aromatic [18] trannulene ring of C₆₀Cl₃₀, and the influence from halogen atoms is secondary. In addition, the static linear polarizability and second‐order hyperpolarizability of C₆₀F₁₈ and C₆₀Cl₃₀ are calculated using finite‐field approach. The values of and for C₆₀F₁₈ and C₆₀Cl₃₀ molecules are significantly larger than those of C₆₀ because of their lower symmetric structures and high delocalization of π electrons. © 2010 Wiley Periodicals, Inc. J Comput Chem 2010     

The pyrolysis of CCl4 and C2Cl6 in the gas phase. Mechanistic modeling by thermodynamic and kinetic parameter estimation

A detailed radical reaction mechanism is proposed to describe the thermal reactions of CCl₄ and C₂Cl₆ in the gas phase quantitatively. A consistent set of activation energies and preexponential factors for all elementary reactions, in combination with enthalpies of formation and entropies for all species involved, is computer optimized to fit experimental pressure-rise curves and concentration profiles. For this purpose new experimental results on the pyrolysis of CCl₄ are used, together with published kinetic data on the pyrolysis of C₂Cl₆ (in the absence and in the presence of Cl₂). © 1996 John Wiley & Sons, Inc.     

Charge transfer in complexes of C60 and C70 in solutions and solid state

Electronic absorption spectra of complexes of C₆₀ and C₇₀ fullerenes with donors, tetrathiafulvalene and pyranylidene derivatives, were studied in solutions and in the solid state. Charge transfer bands were found in the 680–1300 nm range. The charge transfer energies (hv _ct) for the C₆₀ and C₇₀ complexes in solutions are close and almost independent of the solvent polarity. For the C₆₀ complexes in the solid state, the dependence ofhv _ct on the ionization potential (IP) of donors was found to behv _ct=0.82IP–3.93 eV. In the C₆₀ complexes in the solid state, thehv _ct values are 0.15–0.20 eV lower than those in the solution. The linear dependences ofhv _ct onIP of donors for the C₆₀ complexes lie 0.6–0.7 eV higher than those in the complexes with tetracyanoethylene (TCNE). This is associated with lower values of the electron affinity of C₆₀ and the energy of the electrostatic interaction in the fullerene complexes as compared to those of the TCNE complexes. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 478–483, March, 1999.     

Conformational stability and flexibility of the ala dipeptide in free space and water: Monte Carlo computer simulation studies

Monte Carlo determinations of the intramolecular thermodynamics of the Ala dipeptide in the C₇, C₅, α_R, and P_II conformations are reported. The calculations are carried out in the quasiharmonic approximation, with intramolecular entropies determined from the covariance matrix of the atomic displacements. The free energy of transition from C₇ to C₅, α_R, and P_II are found to be endergonic and dominated by the intrinsic energy of disrupting the intramolecular hydrogen bond in the C₇ conformation. These results are combined with previous estimates of the free energy of hydration of the Ala dipeptide in water computed from liquid state Monte Carlo simulations using the probability ratio method. The net free energy of C₇, α_R, and P_II are found to be similar, and it is thus reasonable to expect that all three forms are thermally populated at ambient temperature. The intermolecular carbonyl-water hydrogen bond in C₅, α_R, and P_II competes successfully with the intramolecular N? H…?O? C interaction in C₇.     

C3H4: Density functional study of interconversion of isomers

The B3LYP density functional has been used to calculate the reaction paths for the interconversion of the three stable isomers of C₃H₄, namely, propyne, allene, and cyclopropene, and the results compared with experimental data. It is found that the trans nonplanar isomer of vinylmethylene plays a central role in the interconversions. Other thermochemical parameters, like the entropies of reaction and activation, have also been calculated. The rate constants of the various competing reactions have been calculated using the Rice–Ramsperger–Kassel–Marcus theory. Kinetic isotope effects for the reactions have also been investigated to gain an insight into their mechanisms. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003     

Thermodynamic properties of some polymeric forms of fullerite C<Subscript>60</Subscript> at temperatures ranging from 0 to 320 K

The temperature dependences of the heat capacityC ⁰ _p of fullerites C₆₀ were studied at temperatures ranging from 5 to 320 K in an adiabatic vacuum calorimeter with an accuracy of 0.4–0.2%. The fullerite C₆₀ samples were prepared by treating the starting fullerite C₆₀ under 8 GPa at 920 and 1270 K and “quenched” by a sharp decrease in pressure to −10⁵ Pa and in temperature to ∼300 K. Fullerite C₆₀(8 GPa, 920 K), a crystalline polymer with layered structure formed by polymerized fullerene C₆₀ molecules, was obtained at 920 K and 8 GPa. Fullerite C₆₀(8 GPa, 1270 K), a three-dimensional polymer with a graphite-like structure formed by fragments of decomposed C₆₀ molecules and containing many C(sp³)−C(sp³) bonds, was obtained at 1270 K and 8 GPa. Both polymers are metastable polymeric phases. The anomalous character of the temperature dependence of the heat capacity was revealed in the 49–66 K range for the polymer formed at 1270 K. The thermodynamic functions of the substances under study were calculated for the 0–320 K region along with entropies of their formation from graphite. The entropies of transformation of the starting fullerite C₆₀ into metastable phases and that of intertransformation of phases were estimated. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 277–281, February, 2000.     

Preparation and study of hydrides of fullerenes C60 and C70

Fullerene hydrides were prepared by hydrogenation of fullerences C₆₀ and C₇₀ using proton transfer from 9,10-dihydroanthracene to fullerene and were studied by mass spectrometry (electron impact, field desorption), IR, UV, and¹H and¹³C NMR spectroscopy. The main product of the hydrogenation of C₆₀ is C₆₀H₃₆, which is sufficiently stable. Hydrogenation of fullerene C₇₀ gives a series of polyhydrides C₇₀H _n (n=36–46), and the main product is C₇₀H₃₆. The dehydrogenation of C₆₀H₃₆ by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone is not quantitative and results in the formation of fullerene derivatives along with C₆₀. The comparison of the IR and¹H and¹³C NMR spectral data for solid C₆₀H₃₆ with the theoretical calculations suggests that the fullerene hydride has aT-symmetric structure and contains four isolated benzenoid rings located at tetrahedral positions on the surface of the closed skeleton of the molecule. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya. No. 4, pp. 671–678, April, 1997.     

Reactions of excited fullerenes C60 and C70 studied by mass spectrometry

The transformation of the mass spectra of the laser-desorbed C₆₀ and C₇₀ samples with a successive increase in the laser power, resulting in an increase in the degree of excitation of C₆₀ (C₇₀) and in the number of the particles in the laser plume, was studied. Unusual metastable clusters (C₆₀ + C₂) and (C₇₀ + C₂) are formed even at a minimum laser power and begin to dissociate after 0.5 s following a short (3 ns) laser pulse. An increase in the laser power results in the appearance of peaks of metastable clusters C₆₂ (C₇₂) with the statistically normal lifetime without a delay of dissociation. A further increase in the laser power produces metastable clusters C_60k–2n and C_70k–2n (k = 2, 3) formed without a lag from the dimers and trimers of C₆₀ (C₇₀) by the ejection of a number of C₂ required for the stabilization of the C₂ molecules. The peak of C₇₀ appears simultaneously with the appearance of the (C₆₀)_2–2n peaks upon the laser desorption of pure C₆₀. These findings provide evidence for the growth of the excited fullerene clusters by coalescence and subsequent stabilization due to the ejection of a small fragment rather than by the implantation of C₂ into the fullerene framework. This mechanism of cluster growth should be taken into consideration in modeling fullerene formation in an electric arc reactor, because the clusters formed under these conditions have a substantial excess internal energy.     

Theoretical investigation of the heterofullerenes C59N and C69N and their dimers

The heterofullerenes C₅₉N and C₆₉N and their dimers are examined by the intermediate neglect of differential overlap (INDO/1 and INDO/S) models. The results confirm the stability of the (C₅₉N)₂ and (C₆₉)₂ (C_2h) isomers with a 6–6 closure fusion link between the two monomers. The nearest neighbor C of nitrogen, at the hexagon–hexagon fusion in C₅₉N and in C₆₉N where the N replaces one C in the pentagon of the fullerene has the largest electronic spin density, and this is where the two radicals of C₅₉N or C₆₉N link to form dimers. Also the dimers readily dissociate to free radical monomers to react regioselectively with other reagents. The electronic spectra of the radicals are similar to those of the dimers, which suggest essentially identical electronic structures. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 78: 422–436, 2000     

Dimerization of C60: Symmetry Considerations

Orbital‐symmetry analysis (OCAMS) of the dimerization of C₆₀ via [2+2] cycloaddition indicates that the reactant monomers should approach one another along a pathway in which C_2h symmetry is conserved. Point‐by‐point computations (AM1/UHF) confirm this prediction: a low‐energy pathway leads to a single‐bonded dimer 2 with C_2h symmetry. Closure to the stable D_2h dimer 1 is effected by relatively facile rotation about the single bond. A similar symmetry analysis was performed on a second isomer 3 with D_2h symmetry, the moieties of which are linked by two two‐atom chains. It raises the possibility that 3 , the so‐called `window' isomer, may be interconvertible with 1 along a pathway that retains C_i (S₂) symmetry. Although the computational results indicate that C₆₀ is in thermal equilibrium with its stable dimer 1 at moderate temperatures, the latter is not observed in the gas phase for thermodynamic reasons. According to THERMO computations (AM1/RHF), the equilibrium is shifted strongly towards the monomer pair at temperatures where vaporization of the solid C₆₀ is observed (>400°).     

Studies of equilibrium geometries and electronic spectra for C78O4

Equilibrium geometries and relative stabilities of 24 possible isomers for C₇₈O₄ based on C₇₈ (C_2v) were studied by intermediate neglect of differential overlap (INDO) calculations. It was indicated that the most stable geometry is 28,29,30,31,52,53,73,78‐C₇₈O₄, where three oxygen atoms are added to the same hexagon, through which the longest axis of C₇₈ (C_2v) goes, and the forth oxygen atom is added to the C(73)? C(78) bond intersected by the shortest axis of C₇₈ (C_2v), and epoxide structures are formed. Electronic spectra of C₇₈O₄ isomers were investigated based on the optimized geometries. The blue shift of the first absorption for 28,29,30,31,52,53,73,78‐C₇₈O₄ compared with that of C₇₈ (C_2v) was rationalized and nature of transition for the peaks discussed. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Chemistry of the Higher Fullerenes: Preparative isolation of C76 by HPLC and synthesis,separation, and characterization of Diels-Alder monoadducts of C70 and C76

HPLC Separation of higher fullerenes was compared on two different stationary phases, and the preparative isolation of pure C₇₆ is described. Higher-fullerene derivatives 1 and 2 were prepared by Diels-Alder reaction of C₇₀ and C₇₆ with an ortho-quinodimethane intermediate generated in situ. Three out of four possible isomeric C₇₀ monoadducts, i.e. 1a – c , and, for the first time, one isomeric C₇₆ monoadduct, i.e. 2c , could be isolated in pure form and characterized by ¹H-NMR, ¹³C-NMR, UV/VIS, and mass spectrometry. Five other C₇₆ isomers i.e., 2a , b , d – f were obtained in partially separated product fractions. Coalescence temperatures and energy barriers were determined for the cyclohexene-ring inversion in two of the isomeric C₇₀ derivatives. The structure of the C₇₀ adducts could be deduced unambiguously from symmetry considerations based on high- and low-temperature ¹H-NMR spectroscopy. A simple model for the qualitative evaluation of the local curvature of fullerene surfaces is presented and used for the prediction of addition sites in higher fullerenes. These predictions are compared to the experimental results mentioned above as well as to predictions resulting from π-bond-order considerations and from calculated pyramidalization angles.     

C45- und C50-Carotinoide. 2. Mitteilung. Synthese von optisch aktiven acyclischen C15-Endgruppen

C₄₅- and C₅₀-Carotenoids. Synthesis of Optically Active Acyclic C₁₅-End Groups The optically active C₁₅-end groups (S)- 12 , (S)- 13 and (R)- 14 were prepared from the C₁₂-synthon (S)- 11 in good chemical and optical yield. These C₁₅-end groups are suitable compounds for the synthesis of optically active C₄₅- and C₅₀-carotenoids.