14 N nuclear quadrupole resonance study of the protonation of thiourea

14 N NQR frequencies of thiourea complexes with HCIO 4 , HCI and H 2 SO 4 obtained at 77 K are reported. Comparison of NQR data previously obtained in hexamethylenetetramine‐2thiourea showed that the perchloric acid complex was a hydrogen‐bonded complex. A similar comparison with uronium salts showed that the other complexes were thiouronium salts. The protonation site in thiouronium salts was shown to be the S atom. S protonation produced decreases of the N non‐bonding orbital occupation number and increases in those of the NN and NC sigma bonds. Changes in the N non‐bonding orbital produced by protonation were larger in thiourea than in urea.