Synthesis, Crystal Structures, and Density Functional Theory Studies of Two Salt Cocrystals Containing Meldrum’s Acid Group

[Image: see text] Two salt cocrystals, C(31)H(34)N(4)O(8) (DDD) and C(17)H(20)N(2)O(8) (MDD), were synthesized and their structures were determined by single-crystal X-ray diffraction. DDD is made up of one (C(13)H(13)O(8))(−) anion, one (C(9)H(11)N(2))(+) cation, and one 5,6-dimethyl-1H-benzo[d]imidazole molecule. MDD consists of one (C(4)H(7)N(2))(+) cation and one (C(13)H(13)O(8))(−) anion. DDD and MDD belong to the monoclinic, P21/c space group and triclinic, P-1 space group, respectively. A 1D-chained structure of DDD was constituted by N–H···N and N–H···O hydrogen bonds. However, a 1D-chained structure of MDD was bridged by N–H···O hydrogen bonds. Their density functional theory-optimized geometric structures with a B3LYP/6-311G(d,p) basis set fit well with those of crystallographic studies. By calculating their thermodynamic properties, the correlation equations of C(0)(p,m), S(0)(m), H(0)(m), and temperature T were obtained. By comparing the experimental electronic spectra with the calculated electronic spectra, it is found that the PBEPBE/6-311G(d,p) method can simulate the UV–Vis spectra of DDD and MDD. In addition, the fluorescence spectra in the EtOH solution analysis show that the yellowish-green emission occurs at 570 nm (λ(ex) = 310 nm) for DDD and the purplish-blue emission occurs at 454 nm (λ(ex) = 316 nm) for MDD.