Structure and vibrational spectra of 1,4-dimethylpiperazine di-betaine dihydrate

1,4-Dimethylpiperazine di-betaine, DBPZ (1,4-dicarboxymethyl-1,4-dimethylpiperazinium inner salt) crystallizes with two molecules of water. The crystals are triclinic, space group P1¯, a = 6.181(2) Å, b = 6.2600(18) Å, c = 8.869(2) Å, α = 97.50(2)°, β = 94.40(2)°, γ = 112.48(3)°, Z = 1, R = 0.0418. Double water-molecules bridges link the DBPZ molecules into infinite chains by two sequences of O-H⋯O hydrogen bonds of 2.771(2) and 2.810(2) Å. The piperazine ring has a chair conformation with the methyl groups in the axial positions and the CH2COO− groups in the equatorial ones. The FTIR spectra of DBPZ(H2O)2 and DBPZ(D2O)2 are interpreted and the assignment of bands to the corresponding vibrations is proposed on the grounds of the spectra calculated by the B3LYP/6-31G(d,p) level of theory. The DBPZ(H2O)2 hydrogen-bonded complex is more stable by 18.5 kcal/mol than anhydrous DBPZ.