ISSN:0021-2148    

DOI:10.1002/ijch.197200012

出版商:WILEY‐VCH Verlag    

年代:1972

数据来源: WILEY

ISSN:0021-2148    

DOI:10.1002/ijch.197200013

出版商:WILEY‐VCH Verlag    

年代:1972

数据来源: WILEY

摘要:

AbstractThe basic operations of crystal‐structure determination are described in terms of optical imaging: the collection of the diffraction data corresponds to the process of scattering by the object, and Fourier synthesis is the production of the image from the scattered radiation. The “phase problem” expresses our inability to devise an apparatus for exploring the relative phases of diffracted X‐ra

ISSN:0021-2148    

DOI:10.1002/ijch.197200014

出版商:WILEY‐VCH Verlag    

年代:1972

数据来源: WILEY

摘要:

AbstractA comparison of minimum and expanded Slater basis set difference densities in H2, N2and B2H6indicates that minimum basis set results significantly underestimate the transfer and concentration of charge in molecular bonding regions.

ISSN:0021-2148    

DOI:10.1002/ijch.197200015

出版商:WILEY‐VCH Verlag    

年代:1972

数据来源: WILEY

摘要:

AbstractIt is shown that an experimental measurement of the absolute scale is highly desirable for the quantitative measurement of atomic form factors and, more generally, electron distributions in crystals. The experimental scale‐factor measurement on the mineral kernite confirms that atomic orbitals in molecules are contracted relative to isolated‐atom orbitals. Objectives of charge‐density measurements are divided into direct analysis of the charge density and the analysis of derived quantities, such as net atomic charges, dipole moments and the ionic character of

ISSN:0021-2148    

DOI:10.1002/ijch.197200016

出版商:WILEY‐VCH Verlag    

年代:1972

数据来源: WILEY

摘要:

Abstract1s binding energies are calculated for nitrogen atoms in thymine, pyrrole, adenine, and pyridine and for carbon atoms in thymine, adenine, pyrrole, and furan, and compared with experimental values. Calculations are performed in the framework of a perturbational model using CNDO molecular wave functions. Interactions of the 1s electron with its own atom are calculated exactly whereas interactions with neighbor atoms are calculated according to two levels of approximation: (1) Point charge approximation. (2) Rigorous calculations of integrals. Magnitudes of the different interactions and integrals are analysed. The questions of assignments of ESCA shifts and the errors inherent in the calculations are discussed.

ISSN:0021-2148    

DOI:10.1002/ijch.197200017

出版商:WILEY‐VCH Verlag    

年代:1972

数据来源: WILEY

摘要:

AbstractDi‐tert‐butyl trioxide has been prepared at −50° by photolysis of solid di‐tert‐butyl diperoxymonocarbonate and, more conveniently, by the reaction of ozone in Freon 12 (CF2Cl2) or methyl chloride with either dissolvedtert‐butyl hydroperoxide or suspended hydrated sodiumtert‐butyl peroxide. A low‐temperature procedure for freeing the product from accompanyingtert‐butyl alcohol and other impurities is described. The ozone ‐ sodium salt method has also been applied to the preparation of crystalline dicumyl trioxide. These trioxides have the properties previously observed in solution. They are unstable at temperatures above −30°. NMR spectra of the trioxides are reported and compared with those of related compounds. Approximate rate constants for the thermal decomposition of dicumy

ISSN:0021-2148    

DOI:10.1002/ijch.197200018

出版商:WILEY‐VCH Verlag    

年代:1972

数据来源: WILEY

摘要:

AbstractThe rate constants, the free energies (ΔF#), the energies (ΔH#) and entropies (ΔS#) of activation, and the steric parameter of substituent (Es), for both the acid and the alkaline hydrolyses of four highly branched ethylene carbonates (Ia ‐ d), and 11 trimethylene carbonates (IIa ‐ k) were analysed according to Taft's procedure for a quantitative separation of polar and steric effects of alkyl substituents in the total effect of structure upon reactivity in hydrolysis. Application of Eq. (II) with Σσ* to the most hindered substrates (“α‐axial methyl effect”) (IId ‐ f) and to the moderately hindered substrates (IIa ‐ c) gave well‐separated parallel straight lines for the two groups. Deviations from Eq. (II) were manifested by the rate‐enhanced group of compounds, geminally 2,2‐disubstituted trimethylene carbonates (IIg ‐ k) (“2,2‐gem‐dialkyl effect”), shown to exert a polar “field effect”; the assessment of its substituent constant σF* (= +0.17) is provided by Eq. (III). Analysis of 018exchange experiments and ΔΔFA#vs ΔΔFB#correlation (Eq. (IV)) show that the acid‐ and base‐catalysed reactions occur in parallel via similar transition states, the attainment of each designating the rate‐determining step in both systems. A value of 0.5 kcal/mole was found for the net steric interaction (ESB‐ ESA) exerted by anα‐axial methyl substituent on the attacking hydroxide ion in attaining the transition state. It was concluded that in alkaline hydrolysis, the transition states of compounds exhibiting an “α‐axial methyl effect” should closely resemble the tetrahedral intermediate (IX), while the transition states of the 2,2‐

ISSN:0021-2148    

DOI:10.1002/ijch.197200019

出版商:WILEY‐VCH Verlag    

年代:1972

数据来源: WILEY

摘要:

AbstractSeveral problems raised by initiation and propagation steps in solid‐state polymerizations are reviewed on the basis of data taken from the literature as well as from unpublished work. The nature of the initial active centers is briefly considered; their addition to double bonds within the crystal is discussed. Examination of initiation yields in radiation‐induced polymerizations shows that the efficiency of chain initiation rises with temperature. In most vinyl monomers, the propagation process, which is severely hindered in the crystal, presumably starts at lattice imperfections and thereafter proceeds on behalf of the additional imperfections introduced by the accumulated polymer. In a few specific cases the orientation of the double bonds in the crystal or their localization in certain regions of the crystal favours chain propagation and may result in a stereocontrolled reaction. The role of crystallite boundaries is emphasized particularly in the case of copolymerizations which take place in eutectic mixtures of two monom

ISSN:0021-2148    

DOI:10.1002/ijch.197200020

出版商:WILEY‐VCH Verlag    

年代:1972

数据来源: WILEY

摘要:

AbstractThe competitive through‐space and through‐bond interaction of two symmetry‐equivalent, semi‐localized orbitalsφa,φb, (=πor lone‐pair orbitals) in a molecule M of Csor higher symmetry can lead either to the natural order of the corresponding delocalized orbitals, i.e.,ψ−≈︁ (φa‐φb)/√2 aboveψ+≈︁ (φa‐φb)/√2 or to the inverted order, i.e.,ψ+aboveψ−.It is shown that one can discriminate between these two situations through an analysis of the PE spectrum of a molecule M′ which differs from M only by an additional semi‐localized orbitalφcof known symmetry behaviour. The method is exemplified by an investigation of the dependence of then‐orbital sequence in 3,6‐bridged 1,4‐cyclohexadienes on th

ISSN:0021-2148    

DOI:10.1002/ijch.197200021

出版商:WILEY‐VCH Verlag    

年代:1972

数据来源: WILEY