The molecular complex of imidazole (Im) with 5,5-dimethyl-1,3,2-dioxaphosphorinane-2-thioxy-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 12/m, with a = 10.609(5) Angstrom, b = 11.1739(2) Angstrom, c = 14.069(7) Angstrom, beta = 95.47(2)degrees, V= 1660.1(11) Angstrom(3), Z=8, D-c = 1.274 g/cm(3), F(000) = 672, and mu = 2.760 mm(-1). A comparison of C-13 CP/MAS NMR data for imidazole and the Im Hf Im cation shows that the C-13 isotropic chemical shifts of the C4 and C5 carbons, and of the delta(22) and kappa 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 C-13 chemical shift between the C4 and C5 atoms, the values of delta(22) and kappa for the C2 carbon, and the N ... H+ distance. An analysis of C-13 chemical shift parameters provides information about the strength of the hydrogen bonds and the localization of the proton in the N ... H+... N linkage.