Êtes-vous un étudiant de l'EPFL à la recherche d'un projet de semestre?
Travaillez avec nous sur des projets en science des données et en visualisation, et déployez votre projet sous forme d'application sur GraphSearch.
The attempt to prep. hitherto unknown homopolyat. cations of sulfur by the reaction of elemental sulfur with blue S8(AsF6)2 in liq. SO2/SO2ClF, led to red (in transmitted light) crystals identified crystallog. as S8(AsF6)2. The X-ray structure of this salt was redetd. with improved resoln. and cor. for librational motion: monoclinic, space group P21/c (No. 14), Z = 8, a = 14.986(2) .ANG., b = 13.396(2) .ANG., c = 16.351(2) .ANG., b = 108.12(1) Deg. The gas phase structures of E82+ and neutral E8 (E = S, Se) were examd. by ab initio methods (B3PW91, MPW1PW91) leading to DfH0[S82+, g] = 2151 kJ/mol and DfH0[Se82+, g] = 2071 kJ/mol. The obsd. solid state structures of S82+ and Se82+ with the unusually long transannular bonds of 2.8-2.9 .ANG. were reproduced computationally for the first time, and the E82+ dications were shown to be unstable toward all stoichiometrically possible dissocn. products En+ and/or E42+ [n = 2-7, exothermic by 21-207 kJ/mol (E = S), 6-151 kJ/mol (E = Se)]. Lattice potential energies of the hexafluoroarsenate salts of the latter cations were estd. showing that S8(AsF6)2 [Se8(AsF6)2] is lattice stabilized in the solid state relative to the corresponding AsF6- salts of the stoichiometrically possible dissocn. products by at least 116 [204] kJ/mol. The fluoride ion affinity of AsF5(g) was calcd. to be 430.5 +- 5.5 kJ/mol [av. B3PW91 and MPW1PW91 with the 6-311+G(3df) basis set]. The exptl. and calcd. FT-Raman spectra of E8(AsF6)2 are in good agreement and show the presence of a cross ring vibration with an exptl. (calcd., scaled) stretching frequency of 282 (292) cm-1 for S82+ and 130 (133) cm-1 for Se82+. An atoms-in-mols. anal. (AIM) of E82+ (E = S, Se) gave eight bond crit. points between ring atoms and a ninth transannular (E3-E7) bond crit. point, as well as three ring and one cage crit. points. The cage bonding was supported by a natural bond orbital (NBO) anal. which showed, in addn. to the E8 s-bonded framework, weak p bonding around the ring as well as numerous other weak interactions, the strongest of which is the weak transannular E3-E7 [2.86 .ANG. (S82+), 2.91 .ANG. (Se82+)] bond. The pos. charge is delocalized over all atoms, decreasing the Coulombic repulsion between pos. charged atoms relative to that in the less stable S8-like exo-exo E82+ isomer. The overall geometry was accounted for by the Wade-Mingos rules, further supporting the case for cage bonding. The bonding in Te82+ is similar, but with a stronger transannular E3-E7 (E = Te) bonding. The bonding in E82+ (E = S, Se, Te) can also be understood in terms of a s-bonded E8 framework with addnl. bonding and charge delocalization occurring by a combination of transannular np*-np* (n = 3, 4, 5), and np2 -> ns* bonding. The classically bonded S82+ (Se82+) dication contg. a short transannular S+-S+ (Se+-Se+) bond of 2.20 (2.57) .ANG. is 29 (6) kJ/mol higher in energy than the obsd. structure in which the pos. charge is delocalized over all eight chalcogen atoms. [on SciFinder (R)]