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451.
TiCl4-promoted aldol reaction was carried out by adding TiCl4 to a solution of the aldol reaction substrates and (i-Pr)2NEt (DIPEA) in CH2Cl2. Compared to the conventional order of addition (sequentially adding TiCl4, DIPEA, and piperonal to the lactone 2 in CH2Cl2), this simplified procedure, gave a much cleaner reaction that could be executed on large scale and without cryogenic cooling. However this procedure provided no stereoselectivity. 相似文献
452.
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454.
Molecular dynamics simulations were performed on methane clathrate hydrates at ambient conditions. Thermal expansion results over the temperature range 60-300 K show that the unit cell volume increases with temperature in agreement with experiment. Power spectra were obtained at 273 K from velocity autocorrelation functions for selected atoms, and normal modes were assigned. The spectra were further classified according to individual atom types, allowing the assignment of contributions from methane molecules located in small and large cages within the structure I unit cell. The symmetric C-H stretch of methane in the small cages occurs at a higher frequency than for methane located in the large cages, with a peak separation of 14 cm(-1). Additionally, we determined that the symmetric C-H stretch in methane gas occurs at the same frequency as methane in the large cages. Results of molecular dynamics simulations indicate the use of power spectra obtained from the velocity autocorrelation function is a reliable method to investigate the vibrational behavior of guest molecules in clathrate hydrates. 相似文献
455.
Sremaniak LS Whitten JL Truitt MJ White JL 《The journal of physical chemistry. B》2006,110(42):20762-20764
Ab initio calculations at the Hartree-Fock self-consistent field/single determinant (SCF) and configuration interaction multi-determinant (CI) expansion levels have been used to show that isobutane primary C-H bond activation occurs via direct protium exchange with the zeolite surface via a weakly hydrogen-bonded complex. The calculated 15 kcal/mol activation barrier agrees with the 13.7 kcal/mol value from a recently reported experimental study (J. Am. Chem. Soc. 2006, 128, 1847-1852). Overall, the mechanism described in this contribution demonstrates that weak C-H to O hydrogen bonding leads to complexes at the zeolite acid site that can facilitate C-H bond activation. 相似文献
456.
Varinder K. Aggarwal Guangyu Fang Christoforos G. Kokotos Jeffery Richardson Matthew G. Unthank 《Tetrahedron》2006,62(49):11297-11303
A substantially improved synthesis of synthetically useful chiral sulfide 1 is described. Starting from (+)-10-camphorsulfonic acid, the chiral sulfide was synthesised on large scale in five steps and 56% overall yield. Significant improvements include the use of Bu3P in place of Ph3P for the reduction of the chlorosulfonyl group to the thiol, allowing removal of phosphine oxide by aqueous extraction and improvements in the photochemistry using either a modified batch reactor or a single pass continuous flow reactor. 相似文献
457.
Jeffery D. Densmore James S. Warsa Robert B. Lowrie 《Journal of computational physics》2010,229(10):3691-3705
A Monte Carlo method for simulating Compton scattering in high energy density applications has been presented that models the photon–electron collision kinematics exactly [E. Canfield, W.M. Howard, E.P. Liang, Inverse Comptonization by one-dimensional relativistic electrons, Astrophys. J. 323 (1987) 565]. However, implementing this technique typically requires an explicit evaluation of the material temperature, which can lead to unstable and oscillatory solutions. In this paper, we perform a stability analysis of this Monte Carlo method and develop two time-step limits that avoid undesirable behavior. The first time-step limit prevents instabilities, while the second, more restrictive time-step limit avoids both instabilities and nonphysical oscillations. With a set of numerical examples, we demonstrate the efficacy of these time-step limits. 相似文献
458.
Rodney L. Willer Robson F. Storey Mark Frisch Jeffery R. Deschamps 《Journal of heterocyclic chemistry》2012,49(1):227-231
The crystal structures of the two compounds reported to be 4‐aminofurazan‐3‐carboxylic acid have been determined. The compound reported by Sheremetev et al. (J Heterocycl Chem 2005, 42, 519) is the actual 4‐aminofurazan‐3‐carboxylic acid. The compound reported by Meyer (Org Prep Proced Int 2004, 36, 361) is the interesting complex formed from a molecule of the acid and a molecule of the potassium salt of the acid. J. Heterocyclic Chem., (2012). 相似文献
459.
Cowan MG Olguín J Narayanaswamy S Tallon JL Brooker S 《Journal of the American Chemical Society》2012,134(6):2892-2894
Triply switchable [Co(II)(dpzca)(2)] shows an abrupt, reversible, and hysteretic spin crossover (T(1/2)↓ = 168 K, T(1/2)↑ = 179 K, and ΔT(1/2) = 11 K) between the high-spin (HS) and low-spin (LS) states of cobalt(II), both of which have been structurally characterized. The spin transition is also reversibly triggered by pressure changes. Moreover, in a third reversible switching mechanism for this complex, the magnetic properties can be switched between HS cobalt(II) and LS cobalt(III) by redox. 相似文献
460.
Beata Sweryda-Krawiec Jeffery L. Coffer Shubhra Gancopadhahay 《Journal of Cluster Science》2002,13(4):637-645
Visible light emission emanating from nanostructured porous silicon is sensitive to adsorption of a number of small molecules, suggesting its utility as a chemical sensor. In this work we investigate the mechanistic origin of the diminution of the luminescence by n-propyl amine adsorption and restoration by trifluoroacetic acid, most sensitively by an examination of the changes in observed lifetime of the porous Si luminescence. This is quantified in terms of a stretched exponential model that is consistent with a distribution of luminescent centers in this nanophase material. 相似文献