Abstract
Two hypotheses have been advanced to explain the conformational dependence of the disulfide stretching frequency of the CCSSCC moiety in unstrained disulfides. Such compounds can adopt conformations that exhibit disulfide stretching bands near 510, 525, and 540 cm-1. Sugeta et al. [Sugeta, H., Go, A. & Miyazawa, T. (1973) Bull. Chem. Soc. Jpn. 46, 3407-3411] have attributed these three bands to conformations having none, one, and two trans conformations, respectively, about the C-S bonds. In apparent contradiction to this correlation, Van Wart and Scheraga [Van Wart, H. E. & Scheraga, H. A. (1976) J. Phys. Chem. 80, 1812-1823] found that dithioglycolic acid crystals, believed at the time to exist only in the trans conformation about both C-S bonds, exhibited the S-S stretching band near 510 cm-1. On the basis of this observation, it was suggested instead that the 510, 525, and 540 cm-1 bands arose from CCSSCC moieties having none, one, and two A conformations (i.e., those with CC-SS dihedral angles close to 30 degrees), respectively, about the C-S bonds. It has recently been shown by Nash et al. [Nash, C. P., Olmstead, M. M., Weiss-Lopez, B., Musker, W. K., Ramasubbu, M. & Parthasarathy, R. (1985) J. Am. Chem. Soc. 107, 7194-7195] that dithioglycolic acid can crystallize in two different forms, one with trans and the other with gauche conformations about both C-S bonds, and that these have S-S stretching bands at 536 and 510 cm-1, respectively. These results are confirmed here and it is shown that our earlier data were collected on the all-gauche (rather than the all-trans) form. Thus, the correlation proposed by Sugeta et al. is correct.
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