Electrophilic Attack on Sulfur−Sulfur Bonds. 1. Protonation of Various Isomers of the Homoatomic Sulfur Molecules S_n (n = 2−8)

Abstract

The structures and thermodynamic properties of 32 cations of composition [HS n ] + with n ) 2-8 have been studied by ab initio methods at the G3X(MP2) level of theory. In the global minimum structures, the proton binds to a single sulfur atom and, except for n ) 2, this also holds for the less stable isomers. The binding energies at 0 K range from 619.4 kJ mol -1 in [HS 2 ] + to 835.6 kJ mol -1 in [S 5 -SH] + , which represents the global minimum structure of composition [HS 6 ] + . The protonated branched rings are also the global minimum structures in the case of S 7 and S 8 , whereas [HS 5 ] + is most stable with a five-membered homocycle. The smaller cations [HS n ] + (n ) 2-4) are all chainlike with the hydrogen atom at a chain end. Unexpectedly, singlet chainlike structures of [HS n ] + (n ) 2-8) with the hydrogen terminating the chain at one end are more stable than the corresponding triplet chains. The protonation of neutral sulfur molecules always takes place at the atom of highest negative charge and the charge transfer to the proton is between 0.77 and 0.86 electrons. The S-H bond lengths are calculated as in the range 135-139 pm. The effect of protonation on the sulfursulfur bonds is dramatic, as can be seen in the strong bond length alternation within the cyclic or chainlike structures with the weakest SS bonds originating from the three-coordinate atoms. Hence, there is a pronounced bond activation by protonation. The calculated gas-phase basicities of S 6 and S 8 are in good accord with the experimental values.

Electrophilic Attack on Sulfur–Sulfur Bonds: Coordination of Lithium Cations to Sulfur‐Rich Molecules Studied by Ab Initio MO Methods

“…Compared with the high binding energies of protons to the various S 6 isomers (735-836 kJ mol À1 [40] ), the corresponding interaction of the Li + ion is much weaker.…”

“…[40] For this reason, the S 6 molecule had to be calculated in the various symmetries observed in the cationic lithium adducts reported below; these calculations have been published separately. [29] Isomers of cyclo-S 6 : Beside the experimentally observed chair conformation of D 3d symmetry (5 a), [8] the S 6 ring can also exist as a boat-shaped molecule of C 2v symmetry (5 b).…”

“…This is in sharp contrast to the interaction of protons with related sulfur-rich molecules in which case covalent bonds with mono-coordinate hydrogen are always formed with a transfer of % 0.8 electrons to the proton. [34,40] The coordination to the Li + ion stabilizes certain unstable S 6 isomers (see Table 5 in the Supporting Information). For example, the boat form of S 6 (5 b) has a relative energy of 52.4 kJ mol À1 compared to the chair form 5 a.…”

Isomers of cyclo-Heptasulfur and Their Coordination to Li+: an Ab Initio Molecular Orbital Study

“…In contrast, the asymmetrical isomer 1f exhibits an unusual rooflike conformation for the six-membered ring, which is not a local minimum structure on the PES of S 6 . 14 This structure is characterized by two relatively small torsion angles of -17.7°a nd -21.2°. Consequently, the bond lengths in this species vary between 193.0 pm for the exocyclic bond and 234.6 pm for the neighboring bond with the smallest torsion angle of the ring.…”

“…While the impact of nucleophiles (especially organic amines and phosphines) has been studied many times, 13 the interaction with electrophiles is less well understood. In previous publications, we have investigated the protonation of the sulfur molecules S 2 -S 8 14 as well as the addition of lithium cations to the homocycles S 6 15 and S 8 . 16 In the present work, we report on the corresponding coordination compounds between various S 7 isomers and Li + .…”