The pressure-dependence of the dimerization of 1,3-butadiene: Experimental and theoretical volumes of activation and reaction

Abstract

The volume of activation found for the [4+2]-Diels-Alder cyclodimerization of 1,3-butadiene leading to 4-vinylcyclohexene turned out to be substantially lower than that found for the competing [2+2]-cyclodimerization leading to trans-divinylcyclobutane (ΔΔ₀ ^#= ?12cm³mol^?1). The [4+2]-cyclodimerization of Z, Z-1,4-dideuterio-l,3-butadiene shows only 3 % loss of stereochemistry at 1 bar and <1 % at 6.8–8.0 kbar. These findings show good evidence for a stereospecific pericylic Diels-Alder mechanism competing with a small amount of nonstereospecific stepwise reaction which is almost completely suppressed at high pressure. The volumes of activation and reaction for the various dimerization pathways of 1,3-butadiene are calculated by a Monte-Carlo computer simulation. The good agreeement between experimental and simulated data confirms the hypothesis that configurational effects (e.g. different packing of cyclic and acyclic states) are important for the explanation of activation and reaction volumes.     

High pressure studies of elementary chemical dynamics

Abstract

Studies of chemical reactions over large density ranges provide a simple access to a variety of static and dynamic interactions between reactants and solvents. The interpretation of the observed “activation volumes” by transition state theory is shown to be inadequate. Kramers-Smoluchowski theory provides a more suitable starting point of the analysis. However, multidimensional barrier and cluster effects also have to be taken into account. Viscoelastic effects describe the transition into the solid phase. Photoisomerization studies of diphenylpolyenes illustrate barrier crossing processes. The effects of cage dynamics and diffusion control are demonstrated with bromine and iodine photolysis experiments.     

Spin glass behavior in the new amorphous alloys M2SnTe4 (M=Cr,Mn, Fe)

The amorphous alloys M₂SnTe₄ (M=Cr, Mn, Fe) are prepared by a new method involving the oxidation of main group polyanions (Zintl anions) by transition metal cations in solution at or below room temperature. The M₂SnTe₄ materials undergo a transition to a spin glass state at 12 K ? T_f ? 20 K and were characterized by dc magnetization, ⁵⁷Fe and ¹¹⁹Sn Mossbauer and x-ray diffraction measurements as well as the behavior of the remanent magnetization.     

Li and Nb codoping in dielectric properties and phase transitions of KTaO3: Reentrant dipolar glass and long-range dipole ordering

Abstract

The highly polarizable perovskite-type oxide, KTaO₃ doped simultaneously with Li⁺ and Nb⁵⁺ (K_1?xLi_xTa_1?yNb_yO₃, KLTN), reveals unexpected properties and ordering effects. Studies of the dielectric permittivity ?'(T, f) (10—300K, 100Hz-1 MHz) for x = 0.0014 and y = 0.024 show collective dipolar ordering effects with a transition from paraelectric into a mixed phase (coexisting dipole-glass-like and long-range ordered ferroelectric phases) taking place near 39 K. At 15 K another phase transition into a reentrant dipolar glass-like state is observed. Such a sequence of transitions and the existence of a reentrant glass state are unknown for electrical dipolar systems.     

Interplay of structural, magnetic and transport properties in thelayered Co-based perovskite LnBaCo 2 O 5 (Ln = Tb, Dy, Ho)

The oxygen deficient cobaltites LnBaCo₂O₅ (Ln = Tb, Dy, Ho) exhibit two successive crystallographic transitions at T _N ∼340 K and at T _CO ∼210 K. Whereas the first transition (P4/mmm to Pmmm) is related to the long-range antiferromagnetic ordering of the Co ions (spin ordering), the second transition (Pmmm to Pmmb) corresponds to the long-range ordering of the Co²⁺ and Co³⁺ species (charge ordering) occurring in 1:1 ratio in the structure. The charge ordered (CO) state was directly evidenced by the observation of additional superstructure peaks using neutron and electron diffraction techniques. The CO state was also confirmed indirectly from refinement of high resolution neutron diffraction data as well as from resistivity and DSC measurements. From the refined saturated magnetic moment values only, ∼3.7 and ∼2.7 , the electronic configuration of the Co ions in LnBaCo₂O₅ remains conjectural. Two pictures, with Co³⁺ ions either in intermediate spin state ( t ⁵ _2g e ¹ _g ) or in high spin state ( t ⁴ _2g e ² _g ), describe equally well our experimental data. In both cases, the observed magnetic structure can be explained using the qualitative Goodenough-Kanamori rules for superexchange. Finally, in contrast to the parent Ln = Y compound [Vogt et al. , Phys. Rev. Lett. 84, 2969 (2000)], we do not report any spin transition in LnBaCo₂O₅ (Ln = Tb, Dy, Ho). Received 13 December 2000     

DFT study on abstraction reaction mechanism of oh radical with 2‐methoxyphenol

Reaction mechanism of 2‐methoxyphenol (2MP) (guaiacol) with OH radical has been performed using density functional theory methods BH&HLYP and MPW1K method with 6‐311++G(d,p) basis set. Single‐point energy calculations were done using CCSD(T)/6‐311++G(d,p). The theoretical results reveal that the hydrogen abstraction from methoxy group is found to be the dominant reaction channel with an energy barrier of 9.31 kcal/mol. Also, time‐dependent density functional theory calculations have been performed using BH&HLYP/6‐311++G(d,p) level of theory, and the results reveal that the reactions occur in ground state than the excited state. The results of reaction force profile indicate that structural rearrangements are most influential with high percentage than the relaxation process. The calculated theoretical rate constants (12.19 × 10^?11 cm³ molecule^?1 s^?1) are in good agreement with the experimental rate constant. The atmospheric lifetime of 2‐methoxyphenol with respect to OH radicals is 2.27 hours, which implies that OH radical plays an important role in the degradation of 2MP. The Wiberg bond index of the abstraction reaction reveals that the bond order is concerted, partially synchronic. The reactant‐like transition state satisfies Hammond postulate, which eventually results in an exothermic reaction, and the product‐like transition state reveals in endothermic nature.     

Pressure-induced activation and pressure-tolerance of mutated thermolysin at 119

Abstract

The pressure activation of thermolysin substitution at the 119th site was studied for four mutants and the wild type (Q119Q. Q119N, Q119R, Q119E and Q119D). The highest activation recorded over 30 fold and the activation volumes (ΔV?) were about -75ml/mol for Q119Q, Q119N and Q119R, while Q119E and Q119D showed only a10 fold activation and ΔV? of around -6Oml/mol. The pressure-tolerance of these enzymes were investigated through the in situ observation of their intrinsic fluorescence. Q119E and Q119D showed smaller ΔG _app and ΔG _app of transition than the wild type.     

The effect of pressure on pericyclic reactions

Abstract

Evidence will be presented that packing coefficients calculated from van der Waals and partial molar volumes can be correlated with the transition state geometry of pericyclic and diradical processes.     

Induced ferromagnetism and colossal magnetoresistance by Ir-doping in Pr 1 - x Ca x MnO 3

The doping of the manganese site by iridium (up to 15%) in the small A cation manganites Pr_1-xCa_xMnO₃ ( 0.4 ? x ? 0.8), has been investigated as a new method to suppress charge-ordering and induce CMR effects. Ir doping leads to ferromagnetism and to insulator to metal transitions, with high transition temperatures reaching 180 K and CMR ratio in 7 T as large as 10⁴. The efficiency with which iridium induces ferromagnetism and CMR is compared to previous results obtained with other substitutions (Ru, Rh, Ni, Cr...). The ionic radius of the foreign cations and their mixed-valencies are found to be the main parameters governing the ability to collapse the charge-ordered state. Received 14 May 2001 and Received in final form 2 July 2001     

Li(p,pd)He reaction at 100 MeV

An angular correlation experiment of the ⁶Li(p, pd)⁴He reaction at 100 MeV has been performed using a symmetric and coplanar geometry and covering an angular range from 45.5 to 57°. In addition to the conventional ΔE-E telescope systems for the detection of both protons and deuterons, a 14° analyzing magnet was employed to select deuterons of a given momentum range. The results for the ground state transition are consistent with those obtained at other incident energies. The angular distribution for the transition to the 20 MeV excited state of the recoiling α-particle was also obtained and the overall magnitude of the differential cross section is at least an order of magnitude smaller than that of the ground state transition. The plane wave impulse approximation (PWIA) was used to analyze the experimental results. A form factor was generated using the Saxon-Woods potential as the d-α inter-cluster potential. Satisfactory agreement between theory and experiment was obtained. A value of 0.45 ±0.08 was extracted for the deuteron clustering probability on ⁶Li. The exchange and the optical distortion effects were also discussed.     

The (18O,16O) reaction on even tin isotopes

The two-neutron transfer reaction^ASn(¹⁸O,¹⁶O)^A+2Sn has been measured for the isotopes A=112, 116, 118, 120, 122 and 124 at bombarding energies of 57 and 60 MeV together with the elastic scattering. Angular distributions have been analysed for the transitions to the ground state and to the first excited 2⁺ state. The observed ground state transition is strongly enhanced. The theoretical DWBA analysis is performed with a finite range 2n-transfer form factor including recoil correction. The calculated cross section reproduces the observed systematic change over all isotopes. The absolute cross sections are normalized by a factor of 4.7 and 7.5, depending on the two different sets of 2n-wave functions used in the analysis. The results confirm the prediction of the pairing model that the transition strengths of a neutron pair between the ground states of even tin isotopes are the same.     

Intrinsic magnetostriction and phase transitions

A new possibility of first-order magnetic phase transitions in simple systems, based on the concept of intrinsic volume-magnetostriction Ω_s ≡ ? V_n/V_n, is suggested. The net volume V_n is defined as the difference between the total lattice volume and the sum of the individual atomic volumes. A magneto-volume equation of state for the net volume: Ω_s = Q₀η^s with s < 2, as well as the dependence of the effective exchange integral on the long-range order parameter η, are postulated. The value s ? 1.5 accounts well for the experimental values of the critical exponents β and δ, as well as for the deviation from linearity of the Arrott—Belov plots obtained for the ferromagnetic transition metals. The model leads to an order—order transition just below the order—disorder transition, thus providing an explanation for the two experimentally found temperatures, the forromagnetic Curie temperature and the paramagnetic Curie—Weiss temperature.     

Computational study on mechanisms and pathways of the atmospheric NH2 + IO reaction

ABSTRACT

The potential-energy surfaces of the amino radical (NH₂) with IO reaction have been studied at the CCSD(T)/cc-pVTZ//MP2/6-311++G(d,p) level. Two kinds of pathways are revealed, namely H-abstraction and addition/elimination. Rice–Ramsperger–Kassel–Marcus theory and transition state theory are employed to calculate the overall and individual rate constants over a wide range of temperatures and pressures. It is predicted that, at atmospheric pressure with N2 as bath gas, the formation of P1 (HI?+?HNO) is the dominant pathways at 200–700?K, while the direct H-abstraction leading to P3 (³NH?+?HOI) takes over the reaction at a temperature above 700?K. At the high-pressure limit, IM1 [IONH₂] formed by collisional stabilisation is dominant at 200–700?K; the direct H-abstraction resulting in P3 (³NH?+?HOI) plays an important role at higher temperatures. However, the total rate constants are independence on the pressure; however, the individual rate constants are sensitive to pressure. The atmospheric lifetime of NH₂ in IO is around one week. TD-DFT computations imply that IM1 [IONH₂], IM1A [IONH₂′], IM2 [IN(H₂)O], IM3 [OINH₂], IM4 [HOINH], tra-IM5 [tra-HON(H)I] and cis-IM5 [cis-HON(H)I] will photolyze under the sunlight.     

Phase coexistence in proton glass

Abstract

Proton glasses are crystals of composition M_1?x(NW₄)_xW₂A0₄, where M = K,Rb,Cs, W = H,D, A = P, As. For x = 0 there is a ferroelectric (FE) transition, while for x-1 there is an antiferroelectric (AFE) transition. In both cases, the transition is from a paraelectric (PE) state of tetragonal structure with dynamically disordered hydrogen bonds to an ordered state of orthorhombic structure. For an intermediate x range there is no transition, but the hydrogen rearrangements slow down, and eventually display nonergodic behavior characteristic of glasses. We and others have shown from spontaneous polarization, dielectric permittivity, nuclear magnetic resonance, and neutron diffraction experiments that for smaller x there is coexistence of ferroelectric and paraelectric phases, and for larger × there is coexistence of antiferroelectric and paraelectric phases. We present a method for analytically describing this coexistence, and the degree to which this coexistence is spatial and/or temporal. We discuss also the experimental determination of these coexistence parameters.     

Thermo-elastic property of Ca(OH)2 portlandite

An equation of state (EoS) for Ca(OH)₂ portlandite has been obtained through measurements of pressure and temperature dependence of volume by means of in-situ X-ray observation. The bulk modulus and its pressure derivative at zero pressure calculated using third-order Birch-Murnaghan's equation of state is 33.1 GPa and 4.2 at 300 K, respectively. The unit cell parameters and the volumes have been also determined at 573 K and 673 K. Temperature derivatives of the bulk modulus and its pressure derivative have been calculated to be ?0.022 GPa/K and 0.0072 K^?1, respectively. Thermal expansion coefficient of portlandite has been calculated from the EoS. The pressure dependence of entropy has been obtained from the present thermo-elastic parameters.     

Growth of he bubbles in al during annealing

Abstract

The temperature dependent growth of He bubbles in Al films implanted at room temperature to various He concentrations is investigated by electron energy-loss spectroscopy (EELS) and transmission electron microscopy (TEM). EELS reveals even the weakest changes in He density within the bubbles by measuring the pressure shift of the He 1¹S₀?2¹P₁ transition. This is applied to investigate the mechanisms driving the growth of bubbles in the temperature range 20°C ? T ? 500°C. For T?200°C indication is found that bubbles relax by emission of interstitial dislocation loops. At higher temperatures bubble migration and coalescence under absorption of thermal vacancies is evident. The final state is characterized by large cavities filled with He at low pressure as evidenced by the detection of the atomic He series.     

Theoretical study on structures and stability of SiC3S isomers

Various levels of calculations are carried out to explore the potential energy surfaces (PES) of singlet and triplet SiC₃S, a molecule of potential interest in interstellar chemistry. At the DFT/B3LYP/6-311G(d) level, a total of 57 minimum isomers and 92 interconversion transition states are located. The structures of the most relevant isomers and transition states are further optimized at the QCISD/6-311G(d) level followed by CCSD(T)/6-311?+?G(2df) single-point energy calculations. At the QCISD level, the lowest-lying isomer is the chain-like SiCCCS ³ 1 (0.0?kcal/mol) with a great kinetic stability of 54.1?kcal/mol. In addition, ring isomers CC-cCSSi ¹ 9 (19.8?kcal/mol), S-cCCCSi ¹ 12 (30.4?kcal/mol), S-cCCSiC ¹ 18 (9.4?kcal/mol), S-cSiCCC ¹ 21 (34.4?kcal/mol) and cage-like isomer cage-SiSCCC ¹ 23 (51.8?kcal/mol) also possess considerable kinetic stability (more than 10.0?kcal/mol). As a result, these six isomers are predicted to be possible candidates for future experimental and astrophysical detection. The bond natures and possible formation pathways in interstellar space of the SiCCCS are discussed. The predicted structure and spectroscopic properties for it are expected to be informative for the identification of SiC₃S and even larger SiC _n S species either in laboratory or in space.     

An exotic type of nuclear isomeric transition

A new type of nuclear isomeric transition similar to double beta decay, the β^-? process, is proposed. Under the full ionization of atoms, this process can compete with ordinary types of nuclear isomeric transitions in which the excited metastable state is deexcited via a gamma transition.