S—H Stretching Vibrations

The S—H group of thiol acids absorbs in the same region as mercaptans and thiophenols. 

2 C—S and C=S Stretching Vibrations Sulfides The stretching vibrations assigned to the C—S linkage occur in the region of 700-600 cm1. The weakness of absorption and variability of position make this band of little value in structural determination. 

Disulfides The S—S stretching vibration is very weak and falls between 500 and 400 cm-1. 

Thiocarbonyl Compounds Aliphatic thials or thi-ones exist as trimeric, cyclic sulfides. Aralkyl thiones 

Thioketo compounds that can undergo enolization exist as thioketo-thioenol tautomeric systems such systems show S—H stretching absorption. The thioenol tautomer of ethyl thiobenzoylacetate, 

Compounds that contain a thiocarbonyl group show absorption in the 1250-1020 cm1 region. Thiobenzo-phenone and its derivatives absorb moderately in the 1224-1207 cm-1 region. Since the absorption occurs in the same general region as C—O and C—N stretching, considerable interaction can occur between these vibrations within a single molecule. 

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There are four disulfide bonds in short-chain (Type I) neurotoxins. This means that there are eight half-cystines. However, all Hydrophiinae toxins have nine halfcystines with one cysteine residue. An extra cysteine residue can be readily detected from the Raman spectrum as the sulfhydryl group shows a distinct S-H stretching vibration at 2578 cm" Some Laticaudinae toxins do not have a free cysteine residue as in the cases of L. laticaudata and L. semifasciata toxins. In long toxins (Type II) there are five disulfide bonds (Table III). 

The X-H stretch region (4000-2500 cm-1), where strong contributions from O-H, N-H, C-H, and S-H stretch vibrations are observed. 

Location and appearance of bands similar to sulfides, except with the addition of the -S-H stretching vibration near 2575 cm-1. 

Characteristic group frequencies of thiols are shown in Figure 40. The S-H stretching vibration of aliphatic and aromatic thiols gives rise to a medium to. strong Raman line at 2.590-2530 cm this band is weak or very weak in the infrared. 

The S—H stretching vibration of a thiol occurs in the 2550—2600 cm region. This lower wave-number position compared to the O—H stretching vibration is the result of the weaker S—H bond. The absorption of the S—H bond is much less intense than that of the O—H bond because the S—H bond is less polar. The S—H absorption is not broad because S—H does not form hydrogen bonds. 

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