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51.
52.
We present a new set of V–V rate coefficients for vibrational levels 0–5 in H2 at 300 K, measured using a stimulated Raman–spontaneous Raman pump/probe apparatus. The measured rate of the non-resonant process, H2(v = 1) + H2(v = 1) → H2(v = 0) + H2(v = 2), is consistent with the previously reported experimental value of Kreutz et al. However, semi-classical predictions of such non-resonant processes, using the identical inter-molecular potential and methodology to that given by Cacciatore and Billing, results in rates which are too slow, by a factor of approximately 3. For the “resonant” V–V process, H2(v = 1) + H2(v = 0) → H2(v = 0) + H2(v = 1), the semi-classical rate is found to be too slow by an even larger factor, of approximately 30, compared to the experimental rate, but consistent with the previously reported experimental result of Farrow and Chandler. Further, unlike the semi-classical model prediction in which the (1, 1 → 2, 0) process rate is predicted to exceed that of the (1, 0 → 0, 1) process, the experimental data shows it to be a factor of approximately 2.5 less, suggesting that semi-classical methods that treat the rotational motion classically are unsuitable for the highly anharmonic H2 molecule. The ratio of pure rotation and rotation–vibration Raman cross sections for scattering from levels 0 and 1 is also determined, with results which agree with calculations of Schwartz and LeRoy, but are somewhat larger than previous experimental results of Cureton. 相似文献
53.
Ksenia Shilyaeva Nils Elander Evgeny Yarevsky 《International journal of quantum chemistry》2007,107(6):1301-1315
Peaks in collision cross sections are often interpreted as resonances. The complex dilation method, as well as other methods relying on analytic continuation of the scattering formalism, can be used to clarify whether these structures are true resonances in the sense that they are poles of the S‐matrix and the associated Green function. The performance of the Mittag–Leffler expansion and T‐matrix Green function expansion methods are formally and computationally compared. The two methods are applied to two model potentials. Eigenenergies, s‐wave residues, and cross sections are computed with both methods. The resonance contributions to the cross sections are further analyzed by removing the residue contributions from the Mittag–Leffler and Green function expansion sums, respectively. It is suggested that the contribution of a resonance to a cross section should be defined through its S‐matrix residue. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 相似文献
54.
Triazene‐substituted arylboronic esters were prepared readily from the corresponding aryl magnesium derivatives and shown to function as a new class of donor–acceptor‐substituted coupling reagents. The selective functionalization of these aromatic derivatives led to a wide variety of terphenyl derivatives in which the original bifunctional unit (often further substituted with another functional group) formed the central aromatic ring. The functionalized terphenyl derivatives were formed in two efficient cross‐coupling steps from the triazene‐substituted boronic esters: Suzuki cross‐coupling with an aryl halide was followed by BF3?OEt2‐induced palladium‐catalyzed coupling of the diazonium salt generated in situ from the triazene with an arylboronic acid. 相似文献
55.
Angelika Basch Silvia Gross Namita Roy Choudhury Janis Matisons 《Journal of Sol-Gel Science and Technology》2005,33(1):39-45
New inorganic-organic hybrid materials were prepared by free-radical polymerization of methyl methacrylate (MMA) with methacrylate-substituted oxotantalum cluster [Ta4O4(OEt)8(OMc)4] and their properties evaluated. The cluster was prepared by the reaction of the parent alkoxide with methacrylic acid. Samples of the hybrid materials were produced with Ta-cluster to methyl methacrylate in the ratios of 1:50 and 1:100 and were characterized by thermal and spectroscopic techniques. The glass transition temperatures of the hybrid materials are shifted to higher temperatures than pure PMMA as a result of cross-linking of the polymer by the oxotantalum clusters. The increase in Tg is also observed from the dynamic mechanical analysis (DMA). Evidence of crosslinking between the Ta-cluster and PMMA is obtained from infrared spectroscopic study. Surface studies performed by X-ray photoelectron spectroscopy (XPS) provide information about the atomic concentrations of the surface and indicate tantalum bonded to oxygen. 相似文献
56.
Transition-metal-catalyzed carbon-carbon bond-forming reactions are among the most powerful methods in organic synthesis and play a crucial role in modern materials science and medicinal chemistry. Recent developments in the area of ligands and additives permit the cross-coupling of a large variety of reactants, including inexpensive and readily available sulfonyl chlorides. Their desulfitative carbon-carbon cross-coupling reactions (Negishi, Stille, carbonylative Stille, Suzuki-Miyaura, and Sonogashira-Hagihara-type cross-couplings and Mizoroki-Heck-type arylations) are reviewed together with carbon-carbon cross-coupling reactions with other organosulfur compounds as electrophilic reagents. 相似文献
57.
The solvents o-, m-, p-xylene, p-dichlorobenzene, 1,2,4-trichlorobenzene, and naphthalene were calibrated as condensates used in the thermoporosimetry technique. Exponential relationships were found connecting the pore radii R(p) (in nm) and the freezing-point depression of the swelling solvent deltaT (in degrees C) on the one hand and the apparent energy of crystallization W(a) (in J cm(-3)) and deltaT on the other hand: R(p) = t exp[-1/(c deltaT)]; W(a) = W0 exp(deltaT/f). Pore- or mesh-size distributions can be derived from differential scanning calorimetry results by using the following equation: dV(p)/dR(p) = k{[cY(T)deltaT2]/[W(a)R(p)]}. All the numerical parameters were determined. Polyethylene and polypropylene samples, cross-linked with high-energy electrons or gamma-rays, were submitted to thermoporosimetry study. Relative mesh-size distributions, which depend on the polymer/solvent pair, were calculated for these polyolefins with o-, m-, and p-xylene as solvent and were found to be in the same sequence as those of their degrees of swelling and the irradiation doses received. 相似文献
58.
Kremzow D Seidel G Lehmann CW Fürstner A 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(6):1833-1853
Oxidative insertion of [Pd(PPh3)4] or [Ni(cod)2]/PPh3 into the C-Cl bond of various 2-chloroimidazolinium- and other -amidinium salts affords metal-diaminocarbene complexes in good to excellent yields. This procedure is complementary to existing methodology in which the central metal does not change its oxidation state, and therefore allows to incorporate carbene fragments that are difficult to access otherwise. The preparation of a variety of achiral as well as enantiomerically pure, chiral metal-NHC complexes (NHC = N-heterocyclic carbene) and metal complexes with acyclic diaminocarbene ligands illustrates this aspect. Furthermore it is shown that oxidative insertion also paves a way to prototype Fischer carbenes of Pd(II). Since the required starting materials are readily available from urea- or thiourea derivatives, this novel approach allows for substantial structural variations of the ligand backbone. The catalytic performance of the resulting library of nickel- and palladium-carbene complexes has been evaluated by applications to prototype Suzuki-, Heck-, and Kumada-Corriu cross-coupling reactions as well as Buchwald-Hartwig aminations. It was found that even Fischer carbenes show appreciable catalytic activity. Moreover, representative examples of all types of neutral and cationic metal-carbene complexes formed in this study have been characterized by X-ray crystallography. 相似文献
59.
《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(1):308-320
Palladium‐catalyzed sixfold coupling of hexabromobenzene ( 20 ) with a variety of alkenylboronates and alkenylstannanes provided hexaalkenylbenzenes 1 in up to 73 % and 16 to 41 % yields, respectively. In some cases pentaalkenylbenzenes 21 were isolated as the main products (up to 75 %). Some functionally substituted hexaalkenylbenzene derivatives containing oxygen or sulfur atoms in each of their six arms have also been prepared (16 to 24 % yield). The sixfold coupling of the less sterically encumbered 2,3,6,7,10,11‐hexabromotriphenylene ( 24 ) gave the desired hexakis(3,3‐dimethyl‐1‐butenyl)triphenylene ( 25 ) in 93 % yield. The first successful cross‐coupling reaction of octabromonaphthalene ( 26 ) gave octakis‐(3,3‐dimethyl‐1‐butenyl)naphthalene ( 27 ) in 21 % yield. Crystal structure analyses disclose that, depending on the nature of the substituents, the six arms are positioned either all on the same side of the central benzene ring as in 1 a and 1 i , making them nicely cup‐shaped molecules, or alternatingly above and below the central plane as in 1 h and 23 . In 27 , the four arms at C‐1,4,6,7 are down, while the others are up, or vice versa. Upon catalytic hydrogenation, 1 a yielded 89 % of hexakis(tert‐butylethyl)benzene ( 23 ). Some efficient accesses to alkynes with sterically demanding substituents are also described. Elimination of phosphoric acid from the enol phosphate derived from the corresponding methyl ketones gave 1‐ethynyladamantane ( 3 b , 62 % yield), 1‐ethynyl‐1‐methylcyclohexane ( 3 c , 85 %) and 3,3‐dimethylpentyne ( 3 e , 65 %). 1‐(Trimethylsilyl)ethynylcyclopropane ( 7 ) was used to prepare 1‐ethynyl‐1‐methylcyclopropane ( 3 d ) (two steps, 64 % overall yield). The functionally substituted alkynes 3 f – h were synthesized in multistep sequences starting from the propargyl chloride 11 , which was prepared in high yields from the dimethylpropargyl alcohol 10 (94 %). The alkenylstannanes 19 were prepared by hydrostannation of the corresponding alkynes in moderate to high yields (42–97 %), and the alkenylboronates 2 and 4 by hydroboration with catecholborane (27–96 % yield) or pinacolborane (26–69 % yield). 相似文献
60.
Bistatic scattering by arbitrarily shaped objects with rough surface at optical and infrared frequencies 总被引:2,自引:0,他引:2
The scattering cross sections for arbitrarily shaped dielectric objects with rough surface are determined for optical and infrared frequencies using the Kirchhoff approximation. The formula of the coherent scattering cross section is derived, and numerical method of incoherent scattering cross section is given. As a specific example, the infrared laser scattering cross sections of rough spheres are calculated at 1.06 m. 相似文献