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1.
Pulsed IR radiation from a CO2 TEA laser has been shown to cause dehydrohalogenation of CF3CH3, CF2ClCH3 and C2H5Cl under collisionless conditions. IR emission has been observed and is attributed to HF2 and CF2CH22 from CF3CH3, and to HF2 and HCl2 from CF2ClCH3. 相似文献
2.
Using laser-induced fluorescence of ozone (to measure the rate of disappearance of O32) and NO2 titration (to determine O atom concentrations), we have determined bimolecular rate constants for the deactivation by O(3P atoms) of ozone in excited stretching and bending modes. These experiments do not distinguish between deactivation by (a) the exchange of vibrational and translational energy or (b) the chemical reaction O3 + O → 2O2. If the non-reactive pathway (a) is assumed to dominate, then O(3P) is 150 times more effective than O2 in deactivating O23. If chemical reaction (b) is dominant, the bimolecular rate constant for O23 + O(3P) is larger by a factor of 150–1500 than that for ground-state ozone. 相似文献
3.
Goren AC Hrovat DA Seefelder M Quast H Borden WT 《Journal of the American Chemical Society》2002,124(13):3469-3472
Time-dependent B3LYP/6-31G calculations have been performed at the optimized C(2) or C(2v) geometries of several substituted semibullvalenes (1(deloc)) and barbaralanes (2(deloc)), to compare the computed vertical electronic excitation energies with the temperature-dependent, long-wavelength absorptions that have been observed in the UV/vis spectra of some of these compounds by Quast and co-workers. The excellent agreement between the calculated vertical excitation energies and the observed values of lambda(max) provides strong support for the identification of the bishomoaromatic species 1(deloc) and 2(deloc) as the source of these absorptions. Furthermore, the CN stretching frequencies, computed for the C(2) geometry of 1,5-dimethyl-2,6-dicyano-4,8-diphenylsemibullvalene (1f(deloc)), fit the low-frequency absorptions seen in the IR spectrum of 1f, thus furnishing independent evidence that bishomoaromatic geometries of semibullvalenes have, in fact, been observed spectroscopically. B3LYP/6-31G calculations predict that 2,6-dicyano-4,8-diphenylsemibullvalene 1c has a C(2) equilibrium geometry (1c(deloc)) and that the long-wavelength UV/vis absorption (lambda(max) = 585 nm) and CN stretching frequencies (2192 and 2194 cm(-1)) computed for 1c(deloc) should serve to identify this bishomoaromatic semibullvalene when it is synthesized. 相似文献
4.
Unrestricted density functional theory (UB3LYP), CASSCF, and CASPT2 calculations have been employed to compute the relative energies of the C(s) and C(2v) geometries of several 1,5-disubstituted semibullvalenes. Substitution at these positions with R = F, -CH(2)-, or -O- affords semibullvalenes that are predicted to have C(2v) equilibrium geometries. Calculated singlet-triplet energy splittings and the energies of isodesmic reactions are used to assess the amount of bishomoaromatic character at these geometries. The results of these calculations show that employing strain to destabilize the C(s) geometries of semibullvalenes can lead to a significant decrease in the amount of bishomoaromatic stabilization of the C(2v) geometries, due to reduced through-space interaction between the two allyl groups. However, the C(2v) equilibrium geometries of the 1,5-disubstituted semibullvalenes with R = F and -RR- = -O- do benefit from stabilizing through-bond interactions between the two allyl groups. These interactions involve mixing of the bisallyl HOMO with the low-lying C-F or C-O sigma orbital combinations of the same symmetry. In contrast, for -RR- = -CH(2)-, through-bond interactions destabilize the bisallyl HOMO and are predicted to make the ground state of this semibullvalene a triplet. 相似文献
5.
Dehestani A Lam WH Hrovat DA Davidson ER Borden WT Mayer JM 《Journal of the American Chemical Society》2005,127(10):3423-3432
Osmium tetroxide is reduced by molecular hydrogen in the presence of ligands in both polar and nonpolar solvents. In CHCl3 containing pyridine (py) or 1,10-phenanthroline (phen), OsO4 is reduced by H2 to the known Os(VI) dimers L2Os(O)2(mu-O)2Os(O)2L2 (L2 = py2, phen). However, in the absence of ligands in CHCl3 and other nonpolar solvents, OsO4 is unreactive toward H2 over a week at ambient temperatures. In basic aqueous media, H2 reduces OsO4(OH)n(n-) (n = 0, 1, 2) to the isolable Os(VI) complex, OsO2(OH)4(2-), at rates close to that found in py/CHCl3. Depending on the pH, the aqueous reactions are exergonic by deltaG = -20 to -27 kcal mol(-1), based on electrochemical data. The second-order rate constants for the aqueous reactions are larger as the number of coordinated hydroxide ligands increases, k(OsO4) = 1.6(2) x 10(-2) M(-1) s(-1) < k(OsO4(OH)-) = 3.8(4) x 10(-2) M(-1) s(-1) < k(OsO4(OH)2(2-)) = 3.8(4) x 10(-1) M(-1) s(-1). The observation of primary deuterium kinetic isotope effects, k(H2)/k(D2) = 3.1(3) for OsO4 and 3.6(4) for OsO4(OH)-, indicates that the rate-determining step in each case involves H-H bond cleavage. Density functional calculations and thermochemical arguments favor a concerted [3+2] addition of H2 across two oxo groups of OsO4(L)n and argue against H* or H- abstraction from H2 or [2+2] addition of H2 across one Os=O bond. The [3+2] mechanism is analogous to that of alkene addition to OsO4(L)n to form diolates, for which acceleration by added ligands has been extensively documented. The observation that ligands also accelerate H2 addition to OsO4(L)n highlights the analogy between these two reactions. 相似文献
6.
Tracy L. Weston 《Research in Mathematics Education》2013,15(3):286-302
This study examined trainee teachers' mathematical knowledge in teaching (MKiT) over their final year in a US Initial Teacher Education (ITE) programme. This paper reports on an exploratory methodological approach taken to use the Knowledge Quartet to quantify MKiT through the development of a new protocol to code trainees' teaching of mathematics lessons. This approach extends Rowland's et al. work on the Knowledge Quartet (KQ). Justification for using the KQ to quantify MKiT, and the potential benefits such an attempt might provide those involved with ITE, are discussed. It is suggested that quantified MKiT data based on the Knowledge Quartet can be used to consider MKiT development in novice teachers in order to inform ITE programmes and form new theoretical loops between theory and practice in teacher education. 相似文献
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8.
Tinnefeld P Weston KD Vosch T Cotlet M Weil T Hofkens J Müllen K De Schryver FC Sauer M 《Journal of the American Chemical Society》2002,124(48):14310-14311
The photophysics of a dendrimer containing four chromophores are investigated at the single-molecule level. First, the multichromophoric character of single dendrimers' absorption is probed by modulating the linear polarization of the excitation beam. Subsequently, using circular polarization, the same dendrimers are excited, and their fluorescence transients are recorded. Using pulsed excitation in combination with the classical Hanbury-Brown and Twiss coincidence setup the presented data demonstrate that efficient singlet-singlet annihilation ensures that always only one photon is emitted even when several excitations are generated in an individual multichromophoric molecule. 相似文献
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