Squaric acid as an internal standard for temperature measurements in C-13 MAS NMR

Squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione) is suggested as an internal standard for temperature measurements in C-13 MAS NMR studies in the temperature range from 373 K to 520 K. Compared to previously utilized standards, squaric acid has several important advantages, such as fast kinetics of the phase transition, higher temperature range for C-13 studies, and an almost constant sample volume through the transition temperature range. Thus, we found squaric acid to be suitable for the probe calibration with the reference temperature at 373.2 K. Moreover, upon partial deuteration the reference temperature can be gradually increased to 520 K. Additionally, the described procedure makes it possible to quantitatively estimate the temperature gradient across the sample. We also discuss the effect of spinning related stress on the temperature measurement technique and how this effect could be minimized.