Heavy bonding analysis

Diacetyl sulfide exists in the gas phase primarily in a conformation with a planar heavy-atom skeleton, and with one C = O bond syn and one anti with respect to the further S-C bond. Analysis of the electron diffraction data indicated that there must be less than 10% of the syn, syn conformer, and calculations using MP2 and B3LYP methods with the 6-31-I-G basis set placed the syn, syn form between 4 and 10 kJ mol above the syn, anti form, while the anti, anti conformation was less stable by about 30 kJ mol . Vibrational spectra showed that more than 99% of the gas was one conformer, and the crystal structure also revealed planar molecules with the same conformation as the gaseous molecules. Diacetyl sulfide thus resembles most related compounds in its conformational preferences, although apparently it differs from acetic anhydride, which has been reported to consist of a mixture of non-planar conformers. Distances in the ra structure of diacetyl sulfide include C = O 119.8(2) and 119.6 pm (the second parameter, without a quoted uncertainty, was set to have a fixed difference from the first parameter, as computed by the MP2... 

Dubillard S, Rota JB, Saue T, Faegti K (2006) Bonding analysis using localized relativistic orbitals water, the ultrarelativistic case and the heavy homologues H2X (X = Te, Po, eka-Po). J Chem Phys 124(15) 154307... 

The closely related dichloroethylenediamine platinum(II) was found to be slightly better than PtCll- as a heavy atom. A platinum-sulphur bond to methionine 29 is formed according to current analysis (now confirmed). The platinum cannot be removed by washing the crystal in sharp contrast to the reversible binding of other metal complexes. ... 

Final proof for the inferred structure (1) for cyclosporin A and a first insight in the shape of the molecule resulted from X-ray analysis and high-resolution NMR spectra. The preparation of a crystallized derivative containing a heavy atom was achieved by an addition reaction using iodine and thallium(I) acetate. Instead of the expected iodoacetoxy derivative, the cyclic product (11) was obtained. Obviously, the reaction proceeded by a selective addition of iodine to the double bond of the MeBmt unit followed by an internal cyclization with the participation of the OH group. Iodocyclosporin A (11) reverted easily with Zn powder in acetic acid into genuine cyclosporin A by rranr-elimination. X-ray analysis [6] revealed that iodocyclosporin A assumes a rather rigid backbone conformation. The amino acids 1-6 adopt an antiparallel, markedly twisted /i-pleated sheet conformation, whereas the residues 7-11 form a loop. 

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