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141.
A new technique of searching the conformational space of transition states was used to explore the cause of the rate acceleration in the gem-dialkyl effect in intramolecular cyclization reactions. Several previous hypotheses were discarded and a new hypothesis was advanced based on this new data. This hypothesis, the “Facilitated Transition” hypothesis, states that increased steric hindrance reduces the overall activation energy by facilitating rotation through the transition state. The older “Reactive Rotamer” hypothesis was eliminated by generating all conformations of the starting materials using the
and
computer programs and demonstrating that no relationship is found between rate increase and a change in the concentration of “reactive rotamers”. 相似文献
142.
NH_2基态和激发态的SAC-CI和量子拓扑方法研究郑世钧,蔡新华,宋天乐,孟令鹏,中迁博,波田雅彦(河北师范学院化学系,石家庄,050091)(京都大学工程学院京都,日本)关键词SAC-CI方法,激发态,电子密度,拓扑分析用量子拓扑学方法研究激发态... 相似文献
143.
E. T. Denisov 《Russian Chemical Bulletin》2004,53(8):1602-1608
The transition state of addition of free radicals and atoms to multiple bonds is considered as a result of intersecting of two parabolic potential curves. One of them characterizes the stretching vibration of the attacked multiple bond, and another curve characterizes the stretching vibration of the bond formed in the transition state. The force constant of the latter is calculated by an empirical equation that correlates the force constant with the bond dissociation energy. In the framework of this model, the thermally neutral activation energy (E
e0) and the elongation of the attacked and formed bonds (r
e) in the transition state were calculated from the experimental data (activation energy (E
e) and enthalpy of reaction (H
e)) for the addition of an H atom and methyl, alkoxyl, aminyl, triethylsilyl, and peroxyl radicals to the C=C bond and the addition of H and CH3 to the C=O and CC bonds. Analysis of the data obtained showed that E
e0 depends linearly on the |H
e| + Ee sum, i.e., Ee0/kJ mol–1 = 14.2 + 0.61 · (Ee – H
e), and the bond elongation in the transition state for addition of the most part of radicals to ethylene and acetylene vary within (0.65–0.87)·10–10 m. The factors affecting the activation energy of the radical addition reactions are discussed.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1542–August, 2004. 相似文献
144.
Polystyrenes with different concentrations of side groups with cyano groups were prepared and complex dielectric constants were measured in the range of the glass transition temperature and the frequency range of 10–2–107 Hz.The GPC and DSC measurements showed that the molecular weight of these polystyrenes was about 10500 g/mole and the glass transition temperatures were 89.5°C for all samples.The dielectric relaxation spectra obtained for the side group polystyrene labels and also the chain-end polystyrene labels prepared before [9] were analyzed to find out the degree of coupling of the chain-end and side-group labels with the cooperative reorientation of the polymeric matrix. The analysis of the spectra was carried out using the analysis method developed by Mansour and Stoll [6].The results obtained showed that both end- and side-group labels are strongly coupled with the segmental reorientation and relax with relaxation times longer than that of the segments.The value of logf
m = (logf
m(label)) – logf
m(matrix)) was obtained from the recently designed comparison diagram suggested by Mansour and Stoll [6, 14]. The value of logf
m depends on the label length in the case of chain-end labels.It was surprising to find that the side groups relax slower than the segments by only 0.9 decades. These results obtained implied that the label relaxes through a multistep relaxation mechanism of the side and end groups and not through a diffusion mechanism of the whole chain. In addition, the effective lengths of the relaxing units were determined using the empirical equation obtained before in the case of rodlike molecules in polyisoprene [7]. 相似文献
145.
Simon G. Bott Ulrich Kynast Jerry L. Atwood 《Journal of inclusion phenomena and macrocyclic chemistry》1986,4(3):241-246
18-crown-6 reacts with TiCl3 in CH2Cl2 to form a complex in which the crown ether functions as a tridentate ligand. Addition of moist hexane affords a molecular complex in which the crown ether functions as a bidentate ligand. A water molecule is bonded directly to the titanium atom and is further hydrogen bonded to three of the oxygen atoms of the crown. The deep blue crystals of the CH2Cl2 adduct belong to the monoclinic space groupP21/n witha=13.481(8),b=8.021(5),c=21.425(9) Å, =97.32(5)°, and
calc = 1.51 g cm–3 forZ=4. Refinement led to a conventionalR value of 0.040 based on 873 observed reflections. The Ti–O bond distances for the crown oxygen atoms are 2.123(8) and 2.154(9) Å, while the oxygen atom of the water molecule is bonded at 2.072(8) Å. The octahedral coordination sphere of the titanium atom is completed by the three chlorine atoms at distances of 2.340(5), 2.352(4), and 2.373(4) Å.
Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82034 (10 pages). 相似文献
146.
Karla J. Moreno Antonio F. Fuentes Javier García-Barriocanal Jacobo Santamaría 《Journal of solid state chemistry》2006,179(3):928-934
Different compositions in a solid solution of general formula Dy2(Ti1−yZry)2O7, showing high oxygen ion conductivity, have been successfully prepared at room temperature via mechanochemical synthesis. Stoichiometric mixtures of the constituent oxides were dry milled together in a planetary ball mill by using zirconia vials and balls. Chemical changes in the powder mixtures as a function of composition and milling time were followed by X-ray diffraction and revealed that, in all cases and after milling for 19 h, the powder mixtures consisted of a single phase. Electrical properties were measured on sintered pellets as a function of frequency, temperature and zirconium content, revealing an increase in conductivity of more than one order of magnitude for y?0.4, which, as observed in the similar Y2(Ti1−yZry)2O7, has been related with the onset of disordering of the anion sublattice. Despite increasing structural disorder with increasing Zr content, conductivity remains almost constant for y>0.6, reaching a maximum value of ∼5×10−3 for Dy2Zr2O7 at 900 °C. 相似文献
147.
We present a theoretical study of dissociative photoionization of H2 via the 1Πu symmetry in the photon energy range 30–43 eV. Our results explain the various peaks observed in the experiments below 40 eV and illustrate, in particular, the important role of the 2Πu(2pπu) ionization threshold. At higher photon energies, they suggest the existence of new resonance structures that may be associated with doubly excited states of H2 lying above the third and fourth ionization thresholds (the Q3 and Q4 resonance series). From the calculated resonance parameters, we conclude that the dominant mechanism at high photon energies should involve the Q4 (and probably higher) doubly excited states of H2. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2002 相似文献
148.
Dr. Alexey S. Kiryutin Mikhail S. Panov Dr. Alexandra V. Yurkovskaya Prof. Dr. Konstantin L. Ivanov Prof. Dr. Geoffrey Bodenhausen 《Chemphyschem》2019,20(5):766-772
A study of long-lived spin order in chlorothiophene carboxylates at both high and low magnetic fields is presented. Careful sample preparation (removal of dissolved oxygen in solution, chelating of paramagnetic impurities, reduction of convection) allows one to obtain very long-lived singlet order of the two coupled protons in chlorothiophene derivatives, having lifetimes of about 130 s in D2O and 240 s in deuterated methanol, which are much longer than the T1-relaxation times (18 and 30 s, respectively, at a field =9.4 T). In protonated solvents the relaxation times become shorter, but the lifetime is still substantially longer than . In addition, long-lived coherences are shown to have lifetimes as long as 30 s. Thiophene derivatives can be used as molecular tags to study slow transport, slow dynamics and slow chemical processes, as has been shown in recent years. 相似文献
149.
M. Zakrzewski D. D. Klug J. A. Ripmeester 《Journal of inclusion phenomena and macrocyclic chemistry》1994,17(3):237-247
A high-pressure phase of the clathrate hydrate of tetrahydrofuran was prepared by freezing a liquid phase of overall composition THF · 7 H2O under a pressure of 3.0 kbar, or by pressurizing the solid structure II THF hydrate of 255K to 3.4 kbar. Unfortunately, the products recovered at 77K were always mixed phase materials as shown by X-ray powder diffraction. A number of diffraction lines could be indexed in terms of the cubic structure I hydrate with a slightly expanded lattice parameter, 12.08 Å, giving some support to Dyadin's idea that the high pressure phase transition involves a conversion of Structure II to Structure I. Other phases observed in the recovered product include Ice IX and amorphous materials. The reversion of the high pressure sample to the structure II hydrate was followed by differential scanning calorimetry. At ambient pressure, the high pressure sample converts slowly back to Structure II hydrate event at 77K.NRCC No. 35786. 相似文献
150.
Summary Time-dependent perturbation theory has been applied to calculate the doubly excited triplet statesNsns:3Se,Npnp:3De andNdnd:3Ge (N=2, 3, 4,n=N+1, ... ,5) for He, Li+, Be2+ and B3+. A time-dependent harmonic perturbation causes simulataneous excitation of both the electrons with a change of spin state. The doubly excited energy levels have been identified as the poles of an appropriately constructed linearized variational functional with respect to the driving frequency. In addition to the transition energies, effective quantum numbers of these doubly excited states have been calculated and analytic representations of their wave functions are obtained. These are utilized to estimate the Coulomb repulsion term for these states which checks the consistency of the wave functions. These wave functions may also be used for calculating other physical properties of the systems. 相似文献