¹³C and ¹⁵N NMR shieldings of 1,2,4-diazaphospholes in the solid state and in solution

The solid state ¹³C and ¹⁵N CPMAS NMR spectra of 3,5-di-tert-butyl-1,2,4-diazaphosphole 4 and 3,5-diphenyl-1,2,4-diazaphosphole 5 have been recorded. The X-ray structure of the first compound was already known (it is a cyclic dimer with localized N–H protons) while the structure of the second cannot be determined due to the difficulty to grow suitable single crystals. NMR results pointed out that 4 is a “classical” compound while 5 is probably a tetramer showing Intermolecular Solid-State Proton Transfer (ISSPT). GIAO/ab initio calculations have been carried out to estimate the absolute ¹H, ¹³C and ¹⁵N shieldings. The agreement with the experimental chemical shifts is good enough to assign the signals of carbons C-3 and C-5.