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91.
92.
The 35Cl NQR spectra of some cobalt(II) 2,4,6-trichlorophenolates, 2,6-dichlorophenolates, and 4-bromo-2,6-dichlorophenolates, including two of known crystal structures, have been investigated to study the nature of organochlorine-to-cobalt coordinate covalent bonding. As expected, the difference in frequency of the coordinated and the non-coordinated chlorines increases (to as high as 3 MHz, corresponding to thedonation of about 0.1 electron by the coordinated organochlorine) as the metal-chlorine bond distance decreases (to as short a distance as 262 pm). The frequency difference is found to be temperature-dependent (less at higher temperatures) if the difference at 77K exceeds about 1.3 MHz. 相似文献
93.
The dichloromethanesulfonates of silver and other +1-charged cations, M ?+?(Cl2CHSO $_{3}^{-})$ (M = Ag, Tl, Li, Na, K, Rb, Cs) were synthesized and studied by 35Cl NQR. Dichloromethanesulfonic acid was prepared by the methanolysis of dichloromethanesulfonyl chloride, and was then neutralized with the carbonates of the +1-charged cations to produce the corresponding dichloromethanesulfonate salt. This NQR study completed the investigation of the chloroacetates and chloromethanesulfonates of silver, Ag?+?(Cl x CH3???x SO $_{3}^{-})$ and Ag?+?(Cl x CH3???x CO $_{2}^{-})$ , and suggests (1) that the ability of organochlorine atoms to coordinate to silver decreases as the number of electron-withdrawing groups (Cl, SO $_{3}^{-}$ , CO $_{2}^{-})$ attached to the carbon atom increases; (2) that the unusually large NQR spectral width found among M ?+?(Cl2CHCO $_{2}^{-})$ salts is not present among M ?+?(Cl2CHSO $_{3}^{-})$ salts, and therefore is not generally characteristic of the dichloromethyl group in salts. 相似文献
94.
Roy W. Clark 《The Chemical Educator》2002,7(5):249-257
Because the presently used Lewis diagrams have their roots in static electron theories of the early twentieth century, this paper proposes substituting a double-arrow symbolism for the customary double dots. This means that students should be taught that a bond line (or stick as some call it) means a double arrow, not a double dot. Lone pairs are to be represented by double-curved arrows and later abbreviated to the old familiar double dot. There are obvious advantages to this new notation when atoms have different electronegativities. The double arrows can suggest the shifting of the bond pair equilibrium toward the most electronegative atom. The new notation is more compatible with VSEPR theory, and it seems to suggest the loci of largely imaginary MOs and AOs by the configuration of the arrows. With the help of some imagination, the new symbolism can clarify resonance in aromatic molecules and conjugated chains. The most radical suggestion is that resonance forms be abandoned in favor of conjugated blur bonds. The concept of blur bonds has the merit of being easily extendable to the explanation of metal bonding and electron conduction. 相似文献
95.
96.
Several 5,6-disubstituted-7-oxabicyclo[2.2.1]hept-2-enes (1-4) were synthesized on > or = 0.1 mol scale. The heat-induced retro Diels-Alder (rDA) decomposition of these derivatives was studied by thermal analysis, and the kinetics of the rDA were measured for 4. First-order rate constants (k = 1.91-14.2 x 10(-5) s(-1)), measured at four temperatures between 124 and 150 degrees C, were used to calculate Arrhenius activation parameters Ea (34.5 +/- 0.5 kcal/mol) and ln A (1.77 +/- 0.03 x 10(4)). The observed activation energy was significantly larger (by 9.5 kcal/mol) than that previously measured for the maleic anhydride adduct 1, and this was attributed to the difference in LUMO energies for the two dienophiles. Modeling of the activation parameters found for 4 with density functional theory (DFT) calculations for similar compounds 5 and 6 gave close quantitative correlations for deltaH double dagger, deltaG double dagger, deltaS double dagger. The rDA reactions studied were found to be entropy-driven. 相似文献
97.
A set of small radicals SiF, SiCl, F–CO, CN–O, O3H, NO3, CH2NC, CF3O, and O3 exhibit pronounced discrepancies between different experimental as well as experimental and calculated values of the respective enthalpies of formation ΔfHo(298.15). For stable molecules, this quantity is well established and reliable values are available. However, for free radicals and other short-lived intermediates, the situation is not nearly as favorable. Consequently, critical evaluation of thermodynamic properties of free radicals is necessary, both originating from experiment and computation. Calculated enthalpies of formation for the above systems are based on the ab initio methods G3MP2B3 and CCSD(T)–CBS (W1U) for which mean absolute deviations are known. 相似文献
98.
《The Journal of Mathematical Behavior》2001,20(1):89-107
One means of effecting innovation in calculus teaching and learning involves curricular change, but implementing a reform calculus curriculum does not mean necessarily that students go beyond rote learning and symbolic manipulation. In this paper, we document a process of self-reflection, during which the teacher-as-researcher engaged in analysis of students' written responses to prompts that were intended to inform the teacher about students' understanding of fundamental concepts. 相似文献
99.
Muhammad O. Ali John C. Lasseter Dr. Remigiusz Żurawiński Dr. Anna Pietrzak Dr. Jacek Pecyna Dr. Jakub Wojciechowski Prof. Andrienne C. Friedli Dr. Damian Pociecha Prof. Piotr Kaszyński 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(10):2616-2630
Two series of 1,12-bis-zwitterionic derivatives of the [closo-B12H12]2− anion ( B ), containing either two 4-alkoxypyridinium groups ( 1B[n]-p ) or one 4-alkoxypyridinium and one 4-pentylthianium groups ( 2B[n]-p ), were prepared and their structural (XRD, DFT), thermal, and photophysical properties were compared with those of the analogous derivatives of the [closo-B10H10]2− anion ( 1A[n]-p and 2A[n]-p ). Some 1,7-derivatives of B were isolated and investigated. Both series 1[n] and 2[n] exhibit nematic and crystalline polymorphism; the 12-vertex derivatives ( B ) have higher transition temperatures than those of the 10-vertex analogues ( A ). All compounds fluoresce with quantum yields higher for 1B (ΦF=0.37 for 1B[7]-p and ΦF=0.27 for 2B[7]-p ) than those for the 10-vertex analogues (ΦF=0.04 for 2A[5]-p ). DFT calculations demonstrate an order of magnitude lower first hyperpolarizability, β(−ω,ω,0), for 2B[7]-p than that for the 10-vertex analogue 2A[7]-p (1.7×10−30 vs. 18.9×10−30 esu at ω=0). 相似文献
100.
A graph is matching-covered if every edge of is contained in a perfect matching. A matching-covered graph is strongly coverable if, for any edge of , the subgraph is still matching-covered. An edge subset of a matching-covered graph is feasible if there exist two perfect matchings and such that , and an edge subset with at least two edges is an equivalent set if a perfect matching of contains either all edges in or none of them. A strongly matchable graph does not have an equivalent set, and any two independent edges of form a feasible set. In this paper, we show that for every integer , there exist infinitely many -regular graphs of class 1 with an arbitrarily large equivalent set that is not switching-equivalent to either or , which provides a negative answer to a problem of Lukot’ka and Rollová. For a matching-covered bipartite graph , we show that has an equivalent set if and only if it has a 2-edge-cut that separates into two balanced subgraphs, and is strongly coverable if and only if every edge-cut separating into two balanced subgraphs and satisfies and . 相似文献