Grafton, thank you for your question!
In the approved-but-not-published GSTG we have the following that applies specifically to chemical expressions in TGs. The third paragraph in the following is most important in this case.
6.17.1.5 Whether transcribing according to UEB or Nemeth within UEB Contexts, it is preferred to draw the bonds and arrows as tactile (raised lines) rather than using braille symbols.
Electron dots must not be shown as tactile dots, but must be shown using the appropriate braille symbols.
If braille symbols are used to represent the chemical structure, the appropriate rules for UEB or Nemeth within UEB Contexts must be followed (e.g., grade 1 passage indicators, switch code indicators). These symbols must be listed on the Special Symbols page or explained in a transcriber's note prior to the tactile graphic.
The preferred means for achieving this is illustrated with the following graphic (attached) in those approved but not yet released updates to GSTG. In this case, we see grade 1 indicators are placed with the "dot locator for use" (RUEB 3.14) on a line of their own above and below the graphic.
Please note that the updates to GSTG <u>also indicate</u> it is allowable to use grade 1 indicators on individual symbols instead of using the method shown above. While the guidelines do not say so, this second method might be preferred if there are very few instances where grade 1 indicators would be required in a TG and where saving space that would be required from the "in use" indicators is a primary consideration.
However, because clarity of the technical material is critical, and the use of grade 1 indicators within an expression may introduce complexity for a reader, our advice is to wrap the braille and TGs in grade 1 indicators and the "dot locator for use" symbols as shown in the graphic above.
Finally, while you don't ask specifically about the following, braille contractions should not be used when the letters stand for individual chemical elements. With this in mind, even if the chemical symbols for C (carbon) and H (hydrogen) come together in a chemical expression, the "ch" contraction should not be used.
Please let us know if this answers your question.
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