共查询到20条相似文献,搜索用时 562 毫秒
1.
Anna Rita Campanelli Fabio Ramondo Aldo Domenicano István Hargittai 《Structural chemistry》1999,10(1):29-40
The molecular structure and conformation of p-bis(trimethylsilyl)benzene have been investigated by gas-phase electron diffraction, ab initio MO calculations at the HF/6-31G*, MP2(f.c.)/6-31G*, and B3LYP/6-31G* levels, and MM3 molecular mechanics calculations. The calculations indicate the syn- and anti-coplanar conformations, with two
bonds in the plane of the benzene ring, to be energy minima. The perpendicular conformations, with two
bonds in a plane orthogonal to the ring plane, are transition states. The two coplanar conformers have nearly the same energy with a low interconversion barrier, 0.3–0.5 kJ mol–1. The calculated lengths of the
and
bonds differ by only a few thousandths of an angstrom, in agreement with electron diffraction results from molecules containing either
or
bonds. The geometrical distortion of the benzene ring in p-bis(trimethylsilyl)-benzene may be described by superimposing independent distortions from each of the two SiMe3 groups. The electron diffraction intensities from a previous study (Rozsondai, B.; Zelei, B.; Hargittai, I. J. Mol. Struct.
1982, 95, 187) have been reanalyzed, imposing constraints from the theoretical calculations, and using a model based on a 1:1 mixture of the two coplanar conformers. The effective torsion angles of the SiMe3 groups may indicate nearly free rotation. Important geometrical parameters from the present electron diffraction analysis are
, and
. While the mean bond lengths are virtually the same from the previous and present analyses, the new ipso angle is in better agreement with the MO calculations [HF, 116.9° MP2(f.c.), 117.1° B3LYP, 116.9°]. 相似文献
2.
Kinetics and equilibria for the formation of a 1:1 complex between palladium(II) and chloroacetate were studied by spectrophotometric measurements in 1.00 mol HClO4 at 298.2 K. The equilibrium constant, K, of the reaction
was determined from multi-wavelength absorbance measurements of equilibrated solutions at variable temperatures as log 0.006 with and , and spectra of individual species were calculated. Variable-temperature kinetic measurements gave rate constants for the forward and backward reactions at 298.2 K and ionic strength 1.00 mol as and , with activation parameters and , respectively. From the kinetics of the forward and reverse processes, and were derived in good agreement with the results of the equilibrium measurements. Specific Ion Interaction Theory was employed for determination of thermodynamic equilibrium constants for the protonation of chloroacetate () and formation of the PdL+ complex (). Specific ion interaction coefficients were derived. 相似文献
3.
Annemarie MacKenzie Thomas M. Klapötke György Schultz Magdolna Hargittai 《Structural chemistry》1999,10(1):59-66
The molecular structure of tert-butylazide has been determined by gas-phase electron diffraction and quantum chemical calculations. The HF/6-31G* and B3LYP/6-31G** calculations yielded near C
s symmetry for the tert-butyl group, anti conformation of the (C)N—N bond with respect to one of the
bonds, and an essentially free rotation around the
bond with a 0.34 kcal/mol energy difference between syn and anti conformations of the CNNN moiety, the anti being the more stable form. The electron diffraction analysis was carried out by modeling a mixture of conformational isomers, generated by rotating the terminal nitrogen of the azide group, using a computed rotational potential. The data are consistent with C
s symmetry for the tert-butyl group. The
bond, however, was found to be rotated out of the anti position, with respect to one of the
bonds, by 12.5(12)°. The electron diffraction analysis yielded the following bond lengths (r
g), bond angles, and torsional angles:
,
. 相似文献
4.
Radosław Szmytkowski 《Journal of mathematical chemistry》2009,46(1):231-260
The derivative of the associated Legendre function of the first kind of integer degree with respect to its order, , is studied. After deriving and investigating general formulas for μ arbitrary complex, a detailed discussion of , where m is a non-negative integer, is carried out. The results are applied to obtain several explicit expressions for the associated
Legendre function of the second kind of integer degree and order, . In particular, we arrive at formulas which generalize to the case of (0 ≤ m ≤ n) the well-known Christoffel’s representation of the Legendre function of the second kind, Q
n
(z). The derivatives and , all with m > n, are also evaluated. 相似文献
5.
Previously developed additivity schemes for nonelectrolytes have been used to estimate
and
for tetraalkyl and tetraphenyl methanes in methanol and water. Corrections have been applied to the thermodynamic values of these model compounds to account for a variation in size of the central atom, and these were used to ascertain the effect of charge on
and
of alkyl and phenyl quaternary ions having N, P and B as central atoms. Investigations of R4NBr, (R=methyl to heptyl) salts show that the charge effect on
and
of R4N+ ions is large and relatively independent of ion size suggesting that the solvent molecules penetrate the ions. The ability to estimate
and
of the quaternary ions in the bromide salt solutions has made it possible to make ionic assignments with some confidence;
(Br–) has been evaluated as 19.7±2 and 30.2±7 cm3-mol–1 and
(Br–) as –83±7 and –68±30 J-K–1-mol–1 in methanol and water, respectively. The use of organic ions for making ionic assignments of
and
is critically examined and comparisons with other assignments are made. The scaled particle theory is employed to divide the heat capacities of electrolytes into cavity and interaction contributions. 相似文献
6.
Y. Marcus 《Journal of solution chemistry》1991,20(9):929-944
Taft and Kamlet's -scale of solvent hydrogen bond donation ability is reexamined with regard to its correlations with three widely used polarity scales: Dimroth and Reichardt's E
T
(30), Kosower's Z and Mayer's A
N
, as well as with the m values of the solvents when present as monomeric solutes. The correlation with E
T
serves to extend the solvent -scale according to the expression:
相似文献
7.
Using a hydrogen-electrode thermocell with a temperature difference of 8 K, the initial thermoelectric power in has been determined for aqueous NaOH solutions from 0.003 to 0.2 molal at mean temperatures of 25 and 15°C. Graphs of a function of in vs. a function of m1/2 yield infinitedilution limiting slopes as m1/2 0 which are shown to be in agreement with the five-term theory of this laboratory. Extrapolation of these curves to infinite dilution yields the value for the standard transported entropy of the hydroxide ion
. 相似文献
8.
Conclusions FromHerniaria glabra L. two triterpene glycosides, glabrosides B and C, have been isolated. We have shown that the first is the -D-glucopyranosyl-(16)--D-glucopyranoside, and the second is the
Khimiya Prirodnykh Soedinenii, Vol. 6, No. 3, pp. 307–311, 1970 相似文献
9.
10.
In the present work
as well as HRO. radicals were generated in the photochemical interaction of 1,2-benzanthracene with -ethyl phenyl hydroperoxide /HROOH/ in C6H6 and CCl4 at 304 K.
radicals were trapped by C6H6. The main reaction of HRO. radicals is hydrogen abstraction from the hydroperoxide group of HROOH. Although OH radicals are less selective, the hydrogen abstraction is the main process during their interaction with aromatics in contrast to reactions in aqueous solutions, where addition to the benzene ring is the rate-determining process in CCl4:
|
设为首页 | 免责声明 | 关于勤云 | 加入收藏 |
Copyright©北京勤云科技发展有限公司 京ICP备09084417号 |