The molecular complex of imidazole (Im) with 5,5-dimethyl-1,3,2-dioxaphosphorinane-2-thioxo-2-hydroxy acid (DPTHA) involves strong (O-H‚‚‚H and N .. H + ‚‚‚N) and weak (C-H‚‚‚O and C-H‚‚‚S) hydrogen bonds and has an unusual molecular packing. The crystals are monoclinic, space group I2/m, with a ) 10.609 (5) (000) ) 672, and µ ) 2.760 mm -1 . A comparison of 13 C CP/MAS NMR data for imidazole and the Im‚‚‚H + ‚‚‚Im cation shows that the 13 C isotropic chemical shifts of the C4 and C5 carbons, and of the δ 22 and κ parameters for C2 carbons for each compound are different. GIAO calculations at the HF/6-311 + G(2d,p)//B3LYP/6-31G* level reveal correlations between the differences in the 13 C chemical shift between the C4 and C5 atoms, the values of δ 22 and κ for the C2 carbon, and the N‚‚‚H + distance. An analysis of 13 C chemical shift parameters provides information about the strength of the hydrogen bonds and the localization of the proton in the N‚‚‚H + ‚‚‚N linkage.