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221.
The average long-range interaction energy between two molecules can be written as an inverse asymptotic series in the intermolecular separation distanceR. Using Frost-model wave functions, the dispersion coefficients of the first three (R
–6,R
–8,R
–10 terms in the series are obtained. Coefficients of three- and four-body non-additive interaction energies are also calculated and the form of the dispersion interaction when retardation effects are included is examined. 相似文献
222.
[structure: see text] A highly convergent, fourth-generation total synthesis of (+)-discodermolide (1), with a longest linear sequence of 17 steps and an overall yield of 9.0%, has been achieved. Highlighting the strategy is the efficient construction and sequential, bidirectional union of a linchpin comprising the C(9)-C(14) Wittig salt-vinyl iodide (-)-18. Importantly, Wittig salt generation proceeded in excellent yield under ambient pressure. 相似文献
223.
The problem of perturbations of excited states is discussed and three methods are developed. The first of these uses a zero order wave-function made up of a linear sum of singly excited SCF configurations, whereas the second uses just one of these configurations. The third method is restricted to small -systems, the zero order wave-function being a linear sum of all possible determinants formed from the basis set used. The perturbations considered here are one-electron operators. Example calculations are performed on the butadiene molecule within the context of the -electron approximation.
Zusammenfassung Für das Problem einer Störung von Einelektronen-Operatoren für angeregte Zustände werden drei Verfahren vorgeschlagen: Erstens die Verwendung einer Zustandsfunktion nullter Ordnung, die eine Linearkombination einfach angeregter SCF-Konfigurationen ist, zweitens die entsprechende Verwendung nur einer ausgewählten Konfiguration. Drittens läßt sich, wenn auch nur bei kleinen -Elektronensystemen, als nullte Näherung eine Linearkombination von allen möglichen angeregten Konfigurationen verwenden. Als Beispiel wird das -Elektronensystem des Butadiens gewählt.
Résumé Discussion du problème de perturbation pour les états excités et développement de trois méthodes. La première utilise une fonction d'ordre zéro combinaison linéaire de configurations SCF monoexcitées, alors que la seconde utilise seulement une de ces configurations. La troisième méthode est restreinte à de petits systèmes , la fonction d'onde d'ordre zéro étant une combinaison linéaire de tous les déterminants construits dans la base utilisée. Les perturbations envisagées ici sont constituées par des opérateurs monoélectroniques. La molécule de butadiène sert d'exemple dans le cadre de l'approximation à électrons .相似文献
224.
An accurate examination of features of the ground state surfaces of Si2H4 and Si2H
4
+
is reported; they are compared to C2H4 and C2H
4
+
. For the neutral species, accurate SCF calculations show disilene to be planar, but silylsilylene has the lower energy, whereas at the correlated (CI, MP2, MP3, MP4(SD)) levels disilene becomes trans bent and has the lower energy by 6 kcal/mol. In view of a recent theoretical suggestion that this value should be 23 kcal/mol, we have used large basis sets in these investigations. Our calculations cannot support this large value. Similar investigations are reported for the cation, where the planar disilene structure is predicted to be the most stable. It may be very slightly twisted at high accuracy CI, but it is much lower in energy than the silylsilylene structure. Vibrational frequencies and infra-red intensities are also reported. Theoretical photoelectron spectra of C and Si systems are presented and compared with experiment. 相似文献
225.
R.D. Amos 《Chemical physics letters》1984,108(2):185-190
The dipole and quadrupole derivatives of H2O and H2S are calculated analytically, using the coupled Hartree—Fock method first proposed by Gerratt and Mills. The greater efficiency, of this method allows SCF wave functions very, close to the Hartree—Fock limit to be used. Agreement, with experimental data is good. 相似文献
226.
A special burner is proposed for the atomic absorption spectrophotometric determination of aluminium. With a suitable mixture of nitrogen, oxygen and acetylene, a limit of detection of less than 2 p.p.m. was achieved in aqueous solution. The experimental variables and potential interferences were studied. Iron and chloride interfered in high concentrations, and must be either absent or allowed for in the preparation of standards. Several standard samples of various types were analysed for aluminium with satisfactory results. 相似文献
227.
[reaction: see text] In this paper, we report assembly of the novel dioxabicyclo[3.2.1]octane subtarget (-)-2, comprising the signature structural element of the potent antibiotic (+)-sorangicin A (1). The synthesis was achieved in 15 steps (1.5% overall yield) via a series of acid-catalyzed epoxide ring openings. The first, facilitated by the complex of alkyne (+)-3 with Co(2)(CO)(8), proceeded in a highly regio- and stereoselective fashion. 相似文献
228.
[reaction: see text] An efficient, stereocontrolled assembly of the indolizidine alkaloid, (-)-indolizidine 223AB, exploiting a three-component linchpin coupling employing an N-Ts aziridine as the second electrophile, followed by a one-pot sequential construction of the indolizidine ring system, has been achieved. The longest linear sequence was 10 steps, proceeding in 10% overall yield. 相似文献
229.
A very simple pathway for the preparation of amphiphilic analogues of natural bioactive peptidoamines such as carnosine (β-alanylhistidine) or carcinine (β-alanylhistamine), is presented. The strategy makes it possible to synthesise original bialkylchain or trialkylchain perfluorinated surfactants with or without perhydrogenated chains. 相似文献
230.
Moon G. Kim Larry W. Amos Edwin E. Barnes 《Journal of polymer science. Part A, Polymer chemistry》1993,31(7):1871-1877
Various molecular weight fractions and oligomers isolated from a novolac type resorcinol–formaldehyde (RF) wood adhesive resin were studied by 13C-NMR, gel permeation chromatography (GPC), and intrinsic viscosity measurements. The 13C-NMR results indicate that methylene groups occur mostly between C4–C′4 and C2–C′4 carbons and in minor amounts between C2–C′2 carbons of resorcinol rings. By suppressing the nuclear Overhauser effect (nOe) and using a long delay time for 13C-NMR measurements, reasonable methylene/aromatic carbon ratios were obtained from the aromatic C1 carbon patterns that have resulted from different numbers of methylene groups bonded on the aromatic ring. This analysis results indicate that the RF resin has an appreciable amount of branch structures and compare favorably with the gel permeation chromatography and intrinsic viscosity measurement results. © 1993 John Wiley & Sons, Inc. 相似文献