Revise adamantane standard references

Just prior to the holidays I had a discussion with @jtrebosc regarding adamantane references.

Current situation

Currently ssNake has three build-in adamantane references:

1H: Solid Adamantane (centre)	1.85
13C: Solid Adamantane (high ppm)	38.520
13C: Solid Adamantane (low ppm)	29.472

Originally added in bdb0bb6d

13C, high ppm

Per the IUPAC recommendations 2008, section 9 the 13C chemical shift should be 37.77 ppm (methylene, "high ppm"). This value is w.r.t. 1% TMS in CDCl3. The value of (approximately) 38.5 ppm is relative to neat TMS instead.

13C, low ppm

There is a deviation between the 38.52 value in ssNake and the "neat TMS" value according to IUPAC, specifically: 38.52 vs. 38.48 ppm. I do not know the original source of the 38.520 ppm used here, but using the fixed difference of -0.71 ppm between neat TMS and 1% TMS in CDCl3, the "low ppm" peak should be: 28.76 ppm (vs. current ssNake reference), 28.73 ppm (vs. this, which matches my experiment at RT) or 28.75 ppm (vs. this, corrected by -0.71 ppm), w.r.t. 1% TMS in CDCl3.

1H

Most likely the 1H reference is thus also incorrect w.r.t. 1% TMS in CDCl3. A preliminary experiment of mine suggests that the 1H chemical shift (when referenced w.r.t. the methylene peak at 37.77 ppm and using the frequency ratios) should be at 1.74 ppm. However, my spectra were not background free nor did they have a good S/N ratio, required for such statements. Unfortunately I did not find any source for the 1.85 ppm in ssNake, so I cannot say for certain whether this was referenced w.r.t. TMS, 1% TMS in CDCl3, or something else. In literature, other values are also found. For example, this tutorial on referencing by the university of Iowa suggests adamantane has its average 1H resonance at 1.82 or 1.71 ppm vs. neat and 1% TMS/CDCl3 respectively.

My suggestions for a fix

I think that it is important to keep the old reference for a while, coexisting with the new references. Otherwise, people that just blindly apply the "adamantante-reference" will suddenly see a shift in their experimental data when they process new data. However, I would rename the old references.

Hence, I would recommend:

  1. Changing the existing 13C: Solid Adamantane (high ppm) at 38.520 ppm to 13C: Solid Adamantane (high ppm; neat TMS) for backward compatibility. Possibly updating the shift to 38.48 ppm instead, but again, I'm not sure about the source of this 38.52 ppm.
  2. Adding an extra reference: 13C: Solid Adamantane (high ppm; 1% TMS in CDCl3 (IUPAC)) at 37.77 ppm
  3. Removing the methine ("low ppm" 13C) reference - why would one use this reference? Alternatively, change the name similar to the first point
  4. Updating the 1H reference in line with the first two points (thus having two coexisting references for the time being) once someone can confirm the correct average 1H chemical shifts.

Note: I don't know if the other 13C references in ssNake (for glycine) are also affected, e.g. whether they're referenced w.r.t. TMS or 1% TMS in CDCl3