been neglected. For these compounds it has been shown that the latter "memory effects" may cancel the nearest neighbor control. Such processes have to be expected in compounds with "unstable" local electronic configurations in the absence of "memory effects". VII. Final OutlookThe many-particle nature of a electron bonding in alternant hydrocarbon systems has been investigated by the method of the local approach supplemented by rather simple model Hamiltonians. The present authors believe that it is most convenient for the reader to review the principal findings in the form of characteristic catchwords. (i) The A electrons in annulenes and linear polyenes are sizably localized. It seems to be necessary to substitute widely accepted traditional models for a compounds. (ii) Bond alternation leads throughout to an increase in the A electron delocalization. (iii) The respective enhancement of the charge fluctuations is probably the microscopic origin for the distortive nature of A networks. (iv) An enhancement of the charge fluctuations (decreasing electronic correlation-strength Ai) and increasing interatomic A correlation energies caused by bond dimerization are not mutually exclusive in larger hydrocarbons. The out-of-phase modulation is obviously the rule. (v) Cyclic 67 electron structures allow for the optimum possible a delocalization in alternant hydrocarbons; but even this takes place far from the "free-electron" limit. The latter process may lead to the situation that extended a systems prefer to form more or less decoupled spatially localized 6 r subunits. (vi) Extension of linear monocyclic or polycyclic A systems is frequently not coupled to an increasing delocalization of the A electrons (effects due to the end atoms neglected); their fluctuations saturate quite early. Exceptions have been discussed in the above sections. (vii) The interatomic A correlation energy is one driving force for the bond length alternation in linear and monocyclic hydrocarbons exceeding a certain threshold dimension. (viii) Strong electronic correlations tend to attenuate Jahn-Teller or Peierls instabilities. The attenuation is maximized with increasing correlation strength; see the series C4H4, CsHs, CI2Hl2. (ix) The latter effect may be of some influence in "metastable" solids, i.e. solids where the instability is scarcely suppressed. In these compounds superconductivity under strong coupling conditions may become possible.Exposure of phosphorus trichloride (C13P) and dimethyl sulfide (Me#) dissolved in halocarbons (CFC13, CF3CC13, CF2C1CFCl2, and CH2Clz) to X rays at 77 K results in the corresponding parent cations and several cationsubstrate adducts. The radicals are detected and identified by ESR spectroscopy. In dilute solution exclusive formation of the parent C13P.+ and Me2S'+ radical cations is observed. In CFC13, Me2S'+ exhibits superhyperfine interactions due to chlorine and fluorine nuclei of the matrix molecule(s). At increased concentration, or on warming the sample, the parent radical cations readily react with d...

Intermolecular effects on the radiogenic formation of electron-capture phosphorus-centered radicals. A single-crystal ESR study of diastereoisomeric precursors.

. (1990). Intermolecular-directed reactivity in solid media. Radiogenic formation of phosphorus-centered radicals in chiral diphosphine disulfides studied by ESR. Journal of the American Chemical Society, 112(14), 5432-5447. DOI: 10.1021/ja00170a006General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. A W c t Singlecrystal, powder, and frozen-matrix ESR experiments have been performed to study the radiogenic electron-capture properties of several diastereoisomeric and asymmetric diphosphine disulfides (R,RzP(S)P(S)R3R4). The principal values of the hyperfine couplings of several phosphorus-centered radical configurations are determined and related to the spin density distribution. Attention is focused on the strong differences in radiogenic properties, observed between the meso and racemic forms of phenyl-and tolyl-substituted diphosphine disulfides. The most striking result is that X irradiation of the crystalline meso compounds MePhP(S)P(S)MePh, Me@-Tol)P(S)P(S)Me@-Tol), and Ph(PhCHz)P(S)P(S)Ph(CH2Ph) does not lead to the formation of a three-electron bond PAP u* radical but invariably results in configurations in which the unpaired electron is primarily localized on one half of the molecule. X irradiation of the corresponding racemic forms, on the other hand, gives rise to P L P u* configurations. The observed discrimination between symmetric and asymmetric configurations is explained in terms of intermolecular steric interactions affecting the geometry relaxation of the precursor molecule after initial electron addition. For a quantitative assessment, the change in van der Waals energy resulting from elongation of the P-P bond of the molecules in their respective crystal lattices was calculated with X-ray crystallographic data. The calculations reveal significantly stronger steric interactions for the aromatic meso compounds than for their racemic forms, in agreement with the absence of PAP u* configurations in the first. X irradiation of diphosphine disulfides in a frozen THF matrix almost invariably results in a single radical product, being the PAP u* configuration, and differences between meso and racemic isomers disappear. This is a consequence of the fact that in a randomly oriented solid matrix the molecular packing is less tight than in a molecular crystal, making more sp...