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31.
The title compound has been synthesized and its aromatisation to the Kekulé isomer has been measured. From the kinetic data a diradical process is deduced. 相似文献
32.
W. Uhlig 《无机化学与普通化学杂志》1992,618(12):144-147
Synthesis of Siloxanes on the Basis of Silyltriflates The reaction of silyltriflates with silanols leads to the formation of siloxanes. This method is of importance if the synthesis of silyltriflates is more simply and selectively than the preparation of the corresponding chlorosilanes. This is observed in the case of functional substituted silyltriflates or triflate derivatives of oligosilanes. The synthetic potential of these compounds for the preparation of siloxanes is shown on selected examples. 相似文献
33.
34.
Bracher F Eisenreich WJ Mühlbacher J Dreyer M Bringmann G 《The Journal of organic chemistry》2004,69(25):8602-8608
The first biarylic bis-morphinanedienone alkaloids, saludimerines A (3a) and B (3b), isolated from a tree of Croton flavens (Euphorbiaceae) are described. These naturally occurring dimers of the known alkaloid salutaridine are joined together via a rotationally hindered biaryl axis, giving rise to atropo-diastereomers that are configurationally stable at room temperature but slowly interconvert in methanolic solution within several days. Their structures were established by spectroscopic methods and by partial synthesis, which was achieved by a highly atropo-diastereoselective biomimetic oxidative coupling of the monomeric precursor, salutaridine. Their axial configurations were elucidated by circular dichroism (CD) investigations, which succeeded despite the fact that the two atropo-diastereomers exhibit near-identical CD spectra. This remarkable phenomenon was rationalized by quantum chemical CD calculations. The configurational assignment of saludimerines A (3a) as P-axial and B (3b) as M was corroborated by atropisomer-specific NOE interactions between protons of the one molecular half with nuclei in the other. 相似文献
35.
The equilibrium bond distances, harmonic frequencies, and bond dissociation energies of the 21 homonuclear diatomics Li2—F2, Na2—Cl2, and K2—Br2 have been determined using approximate density functional theory (DFT) employing various widely used functionals and basis sets ranging from single zeta to triple zeta plus polarization quality. The results are in general much less sensitive to the size of the basis set as in conventional ab initio molecular orbital (MO) theory, while the choice of the functional is of much more significance. For one basis set (6-311G*), the performance of the DFT-based calculations has been compared and found to be superior to Hartree-Fock (HF) Møller Plesset second order perturbation theory (MP2), or configuration interaction with single and double excitations (CISD) calculations. Particularly, no pathological cases, such as the group 2 dimers (Be2, Mg2, Ca2), are observed. © 1995 by John Wiley & Sons, Inc. 相似文献
36.
37.
Synthesis and Reactivity of Triflate Substituted Siloxane Derivatives The reaction of amino substituted siloxane derivatives with trifluoromethanesulfonic acid leads under elimination of ammonium trifluoromethanesulfonate to the formation of siloxanyl triflates. The compounds are characterised by NMR-spectroscopy (29Si, 13C, 1H). The synthetic potential of these siloxanes is shown on selected examples. One interesting point of view is the synthesis of silicon containing oligo- and polymeres, in which siloxane and silylenealkine units are combined. 相似文献
38.
Zinc(II) Hydration in Aqueous Solution: A Raman Spectroscopic Investigation and An ab initio Molecular Orbital Study of Zinc(II) Water Clusters 总被引:1,自引:0,他引:1
Raman spectra of aqueous Zn(II)–perchlorate solutions were measured over broad concentration (0.50–3.54 mol-L–1) and temperature (25–120°C) ranges. The weak polarized band at 390 cm–1 and two depolarized modes at 270 and 214 cm–1 have been assigned to 1(a
1g), 2(e
g), and 5(f
2g) of the zinc–hexaaqua ion. The infrared-active mode at 365 cm–1 has been assigned to 3(f
1u). The vibrational analysis of the species [Zn(OH2)
2
+
] was done on the basis of O
h symmetry (OH2 as point mass). The polarized mode 1(a
1g)-ZnO6 has been followed over the full temperature range and band parameters (band maximum, full width at half height, and intensity) have been examined. The position of the 1(a
1g)-ZnO6 mode shifts only about 4 cm–1 to lower frequencies and broadens by about 32 cm–1 for a 95°C temperature increase. The Raman spectroscopic data suggest that the hexaaqua–Zn(II) ion is thermodynamically stable in perchlorate solution over the temperature and concentration range measured. These findings are in contrast to ZnSO4 solutions, recently measured by one of us, where sulfate replaces a water molecule of the first hydration sphere. Ab initio geometry optimizations and frequency calculations of [Zn(OH2)
2
+
] were carried out at the Hartree–Fock and second-order Møller–Plesset levels of theory, using various basis sets up to 6-31 + G*. The global minimum structure of the hexaaqua–Zn(II) species corresponds with symmetry T
h. The unscaled vibrational frequencies of the [Zn(OH2)
2
+
] are reported. The unscaled vibrational frequencies of the ZnO6, unit are lower than the experimental frequencies (ca. 15%), but scaling the frequencies reproduces the measured frequencies. The theoretical binding enthalpy for [Zn(OH2)
2
+
] was calculated and accounts for ca. 66% of the experimental single-ion hydration enthalpy for Zn(II).Ab initio geometry optimizations and frequency calculations are also reported for a [Zn(OH2)
2
18
] (Zn[6 + 12]) cluster with 6 water molecules in the first sphere and 12 in the second sphere. The global minimum corresponds with T symmetry. Calculated frequencies of the zinc [6 + 12] cluster correspond well with the observed frequencies in solution. The 1-ZnO6 (unscaled) mode occurs at 388 cm–1 almost in perfect correspondence to the experimental value. The theoretical binding enthalpy for [Zn(OH2)
2
18
] was calculated and is very close to the experimental single ion-hydration enthalpy for Zn(II). The water molecules of the first sphere form strong hydrogen bonds with water molecules in the second hydration shell because of the strong polarizing effect of the Zn(II) ion. The importance of the second hydration sphere is discussed. 相似文献
39.
40.
A new method for refinement of 3D molecular structures by geometry optimization is presented. Prerequisites are a force field and a very fast procedure for the calculation of chemical shifts in every step of optimization. To the energy, provided by the force field (COSMOS force field), a pseudoenergy, depending on the difference between experimental and calculated chemical shifts, is added. In addition to the energy gradients, pseudoforces are computed. This requires the derivatives of the chemical shifts with respect to the coordinates. The pseudoforces are analytically derived from the integral expressions of the bond polarization theory. Single chemical shift values attributed to corresponding atoms are considered for structural correction. As a first example, this method is applied for proton position refinement of the D-mannitol X-ray structure. A crystal structure refinement with 13C chemical shift pseudoforces is carried out. 相似文献