Date: Wed, 29 Sep 1999 21:36:24 -0500 Reply-To: CH412F99-L@po.missouri.edu Sender: owner-CH412F99-L@po.missouri.edu Precedence: bulk From: "susan lopez"
To: Subject: POTW-Susan How Stable are Thiosulfoxides? MIME-Version: 1.0 X-Priority: 3 X-MSMail-Priority: Normal X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2314.1300 How Unstable are Thiosulfoxides? An ab Initio MO study of Various Disulfanes RSSR (R= H, Me, Pr, All), Their Branched Isomers R2SS and the Related Transition States Ralf Steudel*, Yana Drozdova, Karol Miaskiewicz, Roland H. Hertwig, & Wolfran Koch J. Am. Chem. Soc. 1997, 119, 1990. Goals of Paper: To study organic thiosulfoxides R2SS, which play an important role as reaction intermediates, yet cannot be isolated or directly observed. Using ab initio MO calculations, the structures and energies of these are obtained as well as the activation energies necessary to form them from disulfanes (RSSR). The structures and energies of the transition states for the interconversion (RSSR <-> R2SS) are also discussed. Methods: Geometries were optimized using Hartree-Fock and MP2 level (except Pr2S2 & All2S2) and 6-31G* and 6-311G** basis sets. Vibrational spectra info. was derived from force constant matricies and harmonic frequencies computed analytically for each stationary point. Additional calculations on H2S2 were done using much higher levels of theory to make sure that the structures and relative energies were accurate. Briefly this included: CASSCF level with contracted basis set ANO-S (atomic natural orbital); for energy calc. ANO-L basis set (larger) was used. Energies were obtained via CASPT2, MP4sdqt, and CCSD(T) [these are perturbation thory levels?]. Results: HSSH<->H2SS and TS: Using the higher level calc. the energy difference between the two isomers was found to be 113-117 kJ/mol, yet using 6-31G* or 6-311G** basis sets with MP2 or MP4 level the difference was found to be 141-145kJ/mol (in each of these the HSSH is the more stable species). This is an artifact of basis set choice, and the system is said to be "well-behaved"; enough that one can use the smaller basis sets at MP2. [30kJ/mol seems like a big enough E difference to me- I' m not sure why the 143kJ/mol estimate is ok in this case? Isn' t this a compelling argument to in fact use the higher level rather that dismiss it?] The SS bond lengths suggest double bond character in H2SS. The saddle point (TS) is 210/67 kJ/mol above these isomers respectively, this suggests that at low enough T it should be possible to detect H2SS using vibrational spectroscopy (Table 3). MeSSMe<->Me2SS and TS: The MeSSMe isomer is only 84kJ/mol more stable in this case. The reason is that SS bonds are "semipolar"and the (+) charge on S1 in H2SS weakens the adjacent SH bonds (enlongating it), yet in Me2SS the CS is shorter because C has (-0.4e) charge vs. H (+0.1e); making the isomerization less E costly. The TS in this case is 340kJ/mol higher than MeSSMe due to the SS and CS(C in transit) bond lengths being longer and therefore weaker. For this reason a different isomerization pathway is proposed, instead of intramolecular CH3 shift, ther is homolysis of CS bond, migration of CH3 radical to the other sulfur. This is supported by previous bond dissociation work (Benson). Pr2S2 and All2S2: The Pr2SS system was studied to see if the stabilty trend continues if you lengthen the alkyl chain, and it does 74kJ/mol less stable. All2SS is 78 kJ/mol less stable than disulfane. The later undergoes a sigmatropic rearrangement (eg.3) leading to a very different TS for which a five-membered C3S2 ring structure is proposed that is stabalized via solvent interactions. The E of the TS is 111/33 kJ/mol higher than the respective isomers. MeAllS2 and TS: The MeAllSS is 83kJ/mol less stable than MeSSAll. Two TS were found; TS1 forms a S-S-C triangle (like TS(Me)) with an activation E of 298kJ/mol compared to MeSSAll, and TS2 forms a five membered ring TS (like TS of All2S2) that is 110kJ/mol less stable than MeSSAll. Reactions: Me2SS -> Me2S +S2 is exothermic and spin forbidden fot triplet S2, yet allowed for singlet S2 but endothermic. Me2SS +Sx -> Me2S and Sx+1 (x>2) is exothermic. Bonding: SS bond in R2SS is considered as either double or "semipolar" depending on the electronegativity of the R. FSSF for example has very short SS bond length, due not only to the electronegativity of the F but also because hyperconjugation of the 3p sulfur lone pairs are partially deloccalized into the s* MO of the RS bonds in same plane (gauche effect), this way two pi bonds are created in planes perpendicular to one another. The more EN the R the lower the s* energy and the stronger the p-bond formed. I chose this paper because it was relatively straight forward, had quite a bit of theory and dealt with relatively simple molecular species. Best of all it contained transition states for all of these molecules!