The set of 1,3,4,6-tetraphenylhexa-1,5-dienes (1) represents a perturbation of Cope's rearrangement by four radical-stabilizing phenyl groups all positioned to drive the transition region toward the homolytic−colligative end of the mechanistic spectrum. The appearance of (Z)-isomers being suppressed thermodynamically by a steric interaction of +2.6 kcal mol-1 per cis double bond, an equilibration that is stereochemically not of any Cope type, emerges as the predominant reaction. It is an interconversion of rac -( E , E )-1 and meso -( E , E )-1 (48:52; 77.3−115.3 °C) with the following values of the enthalpy, entropy, and volume of activation:  ΔH ⧧ = 30.7 ± 0.2 kcal mol-1, ΔS ⧧ = +2.1 ± 0.4 cal mol-1 K-1, and ΔV ⧧ = +13.5 ± 0.1 cm-3 mol-1, respectively. Structures have been established by X-ray crystallographic analysis; a possible relationship between dihedral angle and bond lengths in the styrene portions is proposed. The entropy of activation is incompatible with a chair or boat Cope rearrangement; the volume of activation is neither low enough for a pericyclic Cope (“concerted”) mechanism nor high enough for a homolytic−colligative mechanism involving full dissociation as the rate-determining step. Trapping and a crossover experiment give some but only partial support to the intermediacy of free radicals. At higher temperatures, however, electron spin resonance experiments demonstrate an equilibrium with kinetically free (E,E)-1,3-diphenylallyl radicals. These observations are rationalized in terms of geometric reorganization within the confines of a ‘cage'. Resolution by chiral chromatography of rac -( E , E )-1 allows recognition of a fast racemization (40−65 °C), of which ΔH ⧧ (21.3 ± 0.1 kcal mol-1), ΔS ⧧ (−13.2 ± 0.3 cal mol-1 K-1), and ΔV ⧧ (−7.4 ± 0.4 cm-3 mol-1) are consistent with a pericyclic Cope rearrangement. Enriched (Z)-isomers undergo Cope rearrangements in accord with the known influence of axiality and the chair/boat alternative on the energy of the transition region.

Fluoroformyl trifluoroacetyl disulfide, FC(O)SSC(O)CF3, is prepared by quantitative reaction between FC(O)SCl and CF(3)C(O)SH. The conformational properties and geometric structure of the gaseous molecule have been studied by vibrational spectroscopy (IR(gas), Raman(liquid), IR(matrix)), gas electron diffraction (GED), and quantum chemical calculations (B3LYP and MP2 methods). The disulfide bond length derived from the GED analysis amounts 2.023(3) Angstroms, and the dihedral angle around this bond, phi(CS-SC), is 77.7(21) degrees, being the smallest dihedral angle measured for noncyclic disulfides in the gas phase. The compound exhibits a conformational equilibrium at room temperature having the most stable form C(1) symmetry with a synperiplanar (sp-sp) orientation of both carbonyl groups with respect to the disulfide bond. A second form was observed in IR spectra of the Ar matrix isolated compound at cryogenic temperatures, corresponding to a conformer that possess the carbonyl bond of the FC(O) moiety in antiperiplanar position with respect to the S-S single bond (ap-sp). A DeltaH degrees = - = 1.34(11) kcal/mol has been determined by IR(matrix) spectroscopy. The structure of single crystal of FC(O)SSC(O)CF3 was determinate by X-ray diffraction analysis at low temperature using a miniature zone melting procedure. The crystalline solid (monoclinic, P2(1)/n, a = 5.240(4)Angstroms, b = 23.319(17)Angstroms, c = 6.196(4)Angstroms, beta = 113.14(3) degrees) consists exclusively of the (sp-sp) conformation. The geometrical parameters agree with those obtained for the molecule in the gas phase.

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Et), which have been characterized by IR, IH-, I3C-and 31P-NMR spectroscopy. In addition, the molecular structure of 5b has been elucidated by a single-crystal X-ray structure determination.