Low temperature deuterium NMR studies on magnetism of TEMPO derivatives

We measured deuterium NMR of three TEMPO (2,2,6,6-tetramethylpiperidin-1-yloxyl) derivatives, 4-hydroxy-d1-TEMPO, 4-hydroxyimino-d1-TEMPO and Cl–C6D4–CDN-TEMPO down to 2 K to investigate the magnetic local structure around the selectively deuterated sites. Hyperfine coupling constants (HFCCs) determined from the spectra are in good agreement with the results of our previous experiments at higher temperatures. We found: (i) each deuterium, which forms a hydrogen bond with the nitroxide group in the crystalline phases of 4-hydroxy- and 4-hydroxyimino-TEMPO has a large negative HFCC; (ii) electron spin density on Cl–C6D4-group in Cl–C6D4–CDN-TEMPO crystal remains small in magnitude at 4.2 K.