Rate of the retro-Diels-Alder dissociation of 1,2,3,6-tetrahydropyridine over a wide temperature range

Rate constants for the retro-Diels-Alder dissociation of 1,2,3,6-tetrahydropyridine, to 1,3-butadiene and methanimine, have been measured over 650–1450 K. To cover this range, three separate techniques were used at three laboratories: laser schlieren and single pulse shock tube methods, and a comparative rate flow system technique. The derived rate constants are extrapolated to the high-pressure limit with an RRKM model parameterized to fit the falloff observed in the laser-schlieren measurements. The resulting high-pressure rate constants show a reduction in activation energy of about 10 kcal/mol, comparing the isoelectronic cyclohexene, but little change in A-factor. There is an apparent increase in activation energy of 4 kcal/mol over the temperature range of these experiments, which is just outside probable error. Such a rise in activation energy is in marked contrast to the drop usually seen in simple bond fission, and may reflect a change from a concerted to a stepwise, biradical mechanism at high temperatures.     

A study of sensitized photooxygenation of 3,4-dihydro-2H-pyran

The photooxygenation of 3,4-dihydro-2H-pyran sensitized by cyanoanthracenes (9,10-dicyanoanthracene and 9-cyanoanthracene) in different solvents (CH₃CN, CH₃Cl_2,C₆H₆ and CCl₄) has been studied in this paper. The products, product distribution as well as solvent isotope effect are the same as those in the reaction of singlet oxygen. Fluorescence quenching, exciplex formation and free energy change also reveal that electron transfer occurs between sensitizer-excited singlet state and substrate and then singlet oxygen is formed subsequently as the reactive intermediate in the process.     

Thermodynamic quantities for the acid dissociation of alkylammonium ions in water over a wide temperature range

Enthalpies of protonation in aqueous solution have been determined from calorimetric measurements between 0 and 100°C for methylpentylamine, butyldimethylamine, and quinuclidine; between 0 and 125°C for di-iso-propylamine; and between 0 and 75°C for 1-azacycloheptane. Values ofpK_a at 25°C have been determined from pH measurements using a glass electrode for di-iso-propyl-, methylpentyl-, and butyldimethylamine. The calorimetric results are used together with values ofpK_a at 25°C to derive expressions which allow the calculation of accuratepK_a values over a large temperature range. The heat capacity and entropy changes for the protonation of aqueous alkylamines found in the present and in previous studies are discussed in terms of differences in hydrophobic solvation of the alkyl groups in the neutral amines and the ions.     

The H+NO recombination reaction over a wide temperature range

Rate coefficients of the title reaction have been measured in a high‐temperature photochemistry (HTP) reactor using Ar as the bath gas. H atoms were generated by flash photolysis of NH₃ and their relative concentrations were monitored by resonance fluorescence. The data are best fitted by k(295–905 K) = 6.5 × 10^?34 (T/K)^0.206 exp(780K T) cm⁶ molecule^?2 s^?1, with ±2σ precision values varying from 16 to 36% and corresponding suggested accuracy levels of 29–42%. Using a literature value for the relative collision efficiencies of N₂ and Ar indicates that for N₂ as the third body the above rate coefficient expression should be multiplied by 1.6. This leads to good agreement with two recent near 1000 K measurements. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 374–380, 2003     

A triarylboron-based fluorescent thermometer: sensitive over a wide temperature range

Feeling blue: the luminescence of a triarylboron compound has a high quantum yield (at least 0.64) over a wide temperature range (-50 to +100 °C) and changes from green to blue as the temperature is increased. The luminescence color was determined by the population of the two distinct excited-state conformations-a local excited state (high temperature) and a twisted intramolecular charge-transfer state (low temperature).     

The thermal decomposition of 4-methyl-3,6-dihydro-2H-pyran

The thermal decomposition of the title compound has been studied in the gas phase in the temperature range of 584–634 K. The decomposition was found to be a first-order homogeneous process yielding 2-methylbuta-1,3-diene and formaldehyde. The rate constants obtained at 11 temperatures within the quoted range fitted the Arrhenius equation The decomposition is probably unimolecular and concerted.     

Determination of nitrogen diffusivities in liquid cyclohexane over a wide temperature range

The diffusivity of nitrogen in liquid cyclohexane has been measured at temperatures from 21 °C to 140 °C and at pressures of 10 – 15 atm, using a high pressure wetted sphere absorber apparatus. The diffusivity increased from 4.0 × 10⁻⁵ cm² s⁻¹ to 2.8 × 10⁻⁴ cm² s⁻¹ over this temperature range.Rippling of the film at the higher flow rates restricted the flow rate range which could be utilized for the diffusivity determinations. The measured absorption rates for flows in which rippling occurred were up to 15% higher on a 20 mm sphere and up to 40% higher on a 30 mm sphere. This limited the suitability of the wetted sphere absorber for this type of work.     

Dielectric constants of fluid mixtures over a wide range of temperature and density

A new expression is presented for estimating the dielectric constant of a fluid mixture as a function of temperature, density and composition. The estimated dielectric constants (and their derivatives) are required for phase-equilibrium calculations, based on an equation of state, for systems containing electrolytes and nonelectrolytes. The new expression holds for the entire range of fluid densities, from zero to liquid-like densities. Mixing of components is performed on a volume-fraction basis at constant temperature and constant reduced density. For polar components where data are not available at the temperature and/or reduced density of interest, the well-characterized behavior of water is used to extrapolate the available pure-component data. The importance of using the correct density of the mixture is shown. Using one adjustable parameter for each nonideal binary subsystem, predicted results can be significantly improved.     

Reaction of 2-methyl-5,6-dihydro-2H-pyran with dichlorocarbene

Upon treatment of 2-methyl-5,6-dihydro-2H-pyran with dichlorocarbene there are formed products of addition to the double bond and insertion at the C-H bond giving cis- and trans-7,7-dichloro-2-methyl-3-oxabicyclo[4.1.0]heptane and 2-dichloromethyl-2-methyl-5,6-dihydro-2H-pyran.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 190–191, February, 1986.     

Specific features of the reaction of 3,4-dihydro-2H-pyran with 2-(chlorosulfanyl)pyridine 1-oxide

Russian Chemical Bulletin -     

Additive estimates of the thermodynamic properties of triene hydrocarbons over a wide temperature range

An increment method for calculating the thermodynamic properties of aliphatic triene hydrocarbons is suggested. The method is based on substituting procedures and additive inclusion of 1,4-and 1,5-carbon-carbon intramolecular interactions. Parameter values for estimating the enthalpies of formation of gaseous alkatrienes at 298.15 K were found. A procedure for determining the thermodynamic properties of conjugated alkatrienes at arbitrary temperatures was developed. This procedure is based on the additive determination of the coefficients of polynomials that describe the temperature dependences of properties. The parameters for determining the isobaric heat capacities, entropies, and reduced enthalpies and Gibbs energies of conjugated triene hydrocarbons in the gas phase were found. Calculations reproduce experimental data with accuracy at the level of measurement errors. A thermodynamic analysis of the rearrangements of 2,6-dimethyloctatrienes (alloocimene isomers) was performed. The compositions of equilibrium mixtures of 14 aliphatic terpenes were determined at five gas phase temperatures from 298.15 to 1000 K.     

The reaction of benzotriazoles with 3,4-dihydro-4H-pyrane and 2-acetoxymethyl-3,4-dihydro-2H-pyrane

From the reaction of benzotriazoles with 2,3-dihydro-4H-pyrane and 2-acetoxymethyl-3,4-dihydro-2H-pyrane the corresponding 1-(2-tetrahydropyranyl)benzotriazole and cis-and trans-1-(6-acetoxymethyl-2-tetrahydropyranyl)benzotriazole derivatives were obtained. The structures and conformations of these compounds were confirmed by UV and NMR spectra.     

Calorimetric studies of poly(ethylene terephthalate) stretching over a wide temperature range

Heat effects and structural transformations in amorphous crystallizable poly(ethylene terephthalate) (PET) during uniaxial stretching accompanied by neck formation, have been investigated by calorimetric and x-ray methods over a wide range of temperatures and deformation rates. At small deformation (not exceeding 1–2%) and at temperatures below the glass transition temperature of the polymer, PET behaves as an elastic body. Upon stretching at a constant rate, constant heat power is absorbed, heat effects during loading and unloading coincide completely, and no hysteresis is observed. At large deformations (of the order of 50%), cold drawing develops in this temperature range. The internal energy change in cold drawing is zero within experimental error. A periodic heat release during the self-oscillation regime of drawing PET corresponds to periodic changes in stress, in the rate of the neck formation, and in the appearance of the sample. The temperature limits of the region where crystallization resulting from an uniaxial drawing of the polymer is possible, have been determined, and the heat effect of this phase transition has been measured. Orientation crystallization develops only from 70 to 94°C. These limits are insensitive to changes in deformation rate within one decimal order. The structure of PET in this temperature range has been investigated. The heat of phase transition of orientation crystallization of PET has been determined from the relationship between the measured values of the internal energy change during this process and the limiting degree of crystallinity for the stretched samples. This heat proves to be 5.5 ± 0.1 cal/g.     

2-Methyl-5,6-dihydro-2H-pyran in the simmons-smith reaction

2-Methyl-5,6-dihydro-2H-pyran reacts with the Simmons-Smith reagent to give a mixture of 2-ethyl-5,6-dihydro-2H-pyran, cis- and trans-2-methyl-3-oxabicyclo-[4.1.0]heptanes, and 2-methyl-2,5,6,7- or 3-methyl-2,3,6,7-tetrahydrooxepine in a ratio of 2251.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 449–450, April, 1987.     

In situ ESR studies over wide temperature range for conducting polymers

In this work, in situ variable temperature electron spin resonance (ESR) measurements were realized for the first time to verify the spin nature of conducting polymers at controlled potentials. By using mixed solvents consisting of water and propanol in 1:1 volume ratio the temperature window was widened to 100 °C (−40 to +60 °C) for the study of polyaniline. According to the temperature dependence of ESR intensities obtained with this technique, it can be clearly seen that polyaniline in HCl solutions took on the Curie spin behavior at low doping level (0.0 V vs AgCl/Ag), and turned to the Pauli spin behavior at high doping level (0.4 V where conductivity reached about its maximum).