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1.
I.J. Galpin F.E. Hancock B.K. Handa A.G. Jackson G.W. Kenner P. McDowell P. Noble R. Ramage 《Tetrahedron》1981,37(17):3043-3050
The previously synthesised (1–37), (38–75), (76–93), (94–104), (105–117) and (118–129) fragments of the analogue were combined making extensive use of the DCCI/HONSu method. The final coupling involved the (1–75) and (76–129) sub-fragments. Aggregation of the latter fragment caused problems in purification by routine gel filtration methods employing Enzacryl K2 or Sephadex LH60. The fully protected (1–129) product was partially purified by washing, then deprotectcd and purified by gel filtration and ion exchange chromatography. Satisfactory removal of the acetamidomethyl group used for cysteine protection could not be achieved. 相似文献
2.
QSAR models have been under development for decades but acceptance and utilization of model results have been slow, in part, because there is no widely accepted metric for assessing their reliability. We reapply a method commonly used in quantitative epidemiology and medical decision-making for evaluating the results of screening tests to assess reliability of a QSAR model. It quantifies the accuracy (expressed as sensitivity and specificity) of QSAR models as conditional probabilities of correct and incorrect classification of chemical characteristic, given a true characteristic. Using Bayes formula, these conditional probabilities are combined with prior information to generate a posterior distribution to determine the probability a specific chemical has a particular characteristic, given a model prediction. As an example, we apply this approach to evaluate the predictive reliability of a CATABOL model and base on it a "ready" and "not ready" biodegradability classification. Finally, we show how predictive capability of the model can be improved by sequential use of two models, the first one with high sensitivity and the second with high specificity. 相似文献
3.
Frank R. Fronczek Richard D. Gandour Laura M. B. Gehrig Lyman R. Caswell Karen A. McDowell I. Alam 《Journal of inclusion phenomena and macrocyclic chemistry》1987,5(3):379-383
The first crystal structure of a potassium cation complex with dicyclohexano-18-crown-6 is reported. The potassium 2-nitrophenoxide complex ofsyn-cis-syn dicyclohexano-18-crown-6 crystallizes in the triclinic space group P
with cell constantsa=8.604(2),b=10.772(4),C=16.123(5)Å, =73.86(3)°,=77.61(3)°, =82.68(3)° andZ=2 forD
c
=1.31 g cm–3. Least-squares refinement based on 2742 observed reflections led to a final conventionalR value of 0.040. Dicyclohexano-18-crown-6 has the shape of a saddle with the potassium cation sitting at the saddlepoint. The structure of the 2-nitrophenoxide anion is dominanted by the quinoid resonance contributor. Because the complex is devoid of significant intercomplex interactions, it is a prototypical 1:1:1 complex.
Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82043 (26 pages).Now Mrs. K. M. Balo. 相似文献
4.
McDowell SA 《Journal of computational chemistry》2003,24(10):1201-1207
A theoretical study of the properties of the linear LiH dimer was undertaken. In this dimer, an unusual type of hydrogen bonding (termed "inverse" hydrogen bonding by some authors), which involves the hydrogen bonded molecule acting as an electron donor (rather than as a proton donor), is exhibited. The optimized geometry, dipole moment, interaction energy, atomic charges, harmonic vibrational frequencies, and frequency shifts for the dimer are computed at the SCF, MP2, and QCISD levels of theory using mainly a 6-31++G(d,p) basis set. We also examined the relative stability of the mono-deuterated isotopomers of linear (LiH)(2), i.e., Li-H...Li-D and Li-D...Li-H. Analysis of the normal vibrational modes, changes in the partial atomic charges, and changes in the vibrational frequencies of LiH on complexation were used to gain insight into the bonding and properties of the linear LiH dimer and its isotopomers. 相似文献
5.
The photoelectron spectrum of HCP has been obtained, Ionization potentials are found at 10.79 ± 0.01 eV (X2 Π) and 12.86 ± 0.01 eV (A 2 Σ). By comparison with HCP, a new interpretation of the first photoelectron band of HCN has been proposed which involves a Renner-Teller interaction in the Σ+ vibronic states of the molecular ion. 相似文献
6.
Five routes to stable chlorins bearing 0 or 1 meso substituents have been investigated, among which reaction of a 9-bromo-1-formyldipyrromethane and 2,3,4,5-tetrahydro-1,3,3-trimethyldipyrrin proved most effective. Application of this route afforded metallochlorins [Cu(II), Zn(II), and Pd(II)] including the chlorin lacking any β-pyrrole and meso substituents. 相似文献
7.
Sean A.C. McDowell C. Ruth Le Sueur A. David Buckingham Anthony J. Stone 《Molecular physics》2013,111(5):823-835
The integrated intensities of vibrational transitions depend on the magnitude of the derivatives of the dipole with respect to nuclear motion. Normally, the only reliable way to compute such derivatives is by tedious and expensive ab initio calculations. In this paper, we present a simplification for weakly bound complexes based on distributed schemes for describing the charge densities and polarizabilities of the monomers. Formulations based on both Cartesian and spherical harmonics are presented. The results for both these schemes agree exactly with each other, and qualitatively with full ab initio calculations for the hydrogen fluoride dimer, (HF)2. 相似文献
8.
We prove part of a conjecture by Johansson, Kahn, and Vu (Factors in random graphs, Random Struct. Algorithms 33 (2008), 1, 1–28.) regarding threshold functions for the existence of an H‐factor in a random graph . We prove that the conjectured threshold function is correct for any graph H which is not covered by its densest subgraphs. We also demonstrate that the main result of Johansson, Kahn, and Vu (Factors in random graphs, Random Struct. Algorithms 33 (2008), 1, 1–28) generalizes to multigraphs, digraphs, and a multipartite model. 相似文献
9.
The halogen-bonded complexes between H2Te/Me2Te and the dihalogen molecules XY (XY = F2, Cl2, Br2, I2, ClF, ClBr, BrF, BrCl, BrI, IF, ICl, IBr) have been studied to investigate the dependence of its strength and nature on the halogen donor X and its adjoining atom Y, as well as the methyl groups in the electron donor. The interaction energy varies between −1.7 and − 43.5 kcal/mol, indicating that the Te atom in H2Te/Me2Te has a strong affinity for the dihalogen molecules. For the H2Te-XY complex, the halogen bond is stronger for the heavier halogen donor X atom and the strong electron-withdrawing group Y. However, for Me2Te-XY, the halogen bond is stronger for the lighter halogen donor X atom. The H2Te/Me2Te-F2 complex has the largest interaction energy, although the σ-hole on F2 is the smallest in magnitude. In most of the complexes, the electrostatic and polarization contributions to the binding strength are similar in magnitude. However, for H2Te/Me2Te-F2, the polarization contribution is much larger than the electrostatic contribution, with a significant contribution from charge transfer. 相似文献
10.
A systematic method of obtaining relative signs of hyperfine coupling constants is described. It applies to systems consisting of (a) a set of one or more nuclei coupled fairly strongly to the electron spin, and possessing a two-fold (or higher) axis of symmetry, together with (b) a set of weakly coupled nuclei defining superhyperfine transitions. ENDOR measurements for several E.P.R. hyperfine transitions, with the field oriented along the symmetry axis, give relative signs of hyperfine components for this direction. Signs for the other directions can then be obtained through ENDOR measurements on a single hyperfine transition at various field orientations. Additional double ENDOR measurements may be necessary for very weakly coupled nuclei. This method can complement double ENDOR studies in favourable cases. It is illustrated by the determination of signs of coupling constants of protons and of 75As in the AsO4 4- radical in KH2AsO4. 相似文献