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The 1:2 complex of 1,4-dimethylpiperazine mono-betaine (MBPZ) with squaric acid (H2SQ) has been characterised by single-crystal X-ray analysis, FTIR and NMR spectroscopies, and by DFT calculations. The crystals are monoclinic, space groupP21/c. Two MBPZ cations and four hydrogen squarate anions (HSQ− ) are linked by strong O(1)–H…O(13) (2.525(4) Å), O(14)–H…O(21) (2.511(4) Å) and N(4)–H…O(23) (2.607(3) Å) hydrogen bonds into a cyclamer. In turn, the cyclamers are linked into a helixthrough two O(24)–H…O(11) hydrogen bonds of 2.516(4) Å. The piperazinium ring has a chair conformation with N(4)–CH3and N(1)–CH2COOH substituents in the equatorial positions, and N(1)–CH3in the axial position. The FTIR spectrum is consistent with the crystal data. Two models of the 1:2 complex of MBPZ with H2SQ have been optimised at the B3LYP/6-311++G(d,p) level of theory and have been used to calculate harmonic IR frequencies. One of the models (2) is dominated by electrostatic attraction between NH(4)+and HSQ− , whereas in the other (3) squaric acid interacts with a zwitterionic MBPZ through the O–H…O and O–H…N hydrogen bonds. [ABSTRACT FROM PUBLISHER]

• Structure of 8 - hydroxyquinoline betaine monohydrate was solved by X-ray. • The one proton of disordered water molecules is shared between two half-occupied sides. • Three broad bands classify the FTIR spectrum as the ABC type. • PED are used to assign the bands in the vibrational spectrum. • Hirshfeld surface, Mulliken atomic charges, MEP, HOMO-LUMO were computed. • The assignment of 1H and 13C resonance signals are based on the 2D measurements. Molecular structure and properties of 8-hydroxyquinoline betaine monohydrate (1-carboxymethyl-8-hydroxyquinolinium inert salt monohydrate) were studied by X-ray diffraction, DFT calculations, FTIR and NMR spectra. In the crystals, 8-hydroxyquinoline betaine and water are bonded by an intermolecular and asymmetric COO···H 2 O hydrogen bond of 2.782(3) Å. The water molecules are disordered so that one of the proton is shared between two half occupied sites. The hydroxyl group of quinoline is bonded to the neighboring molecule by the O−H···OOC hydrogen bond of 2.571(3) Å. The molecular geometry, Hirshfeld surface, Mulliken atomic charge distribution, molecular electrostatic potential, frontier molecular orbitals, vibrational wavenumbers, magnetic isotropic shielding constants were determined using the density functional theory (DFT/B3LYP) with 6–311++G(d,p) basis set. The FTIR spectrum shows a characteristic ABC structure, typical of medium-short hydrogen bonds. The potential energy distribution (PED) was used to assign the bands in the vibrational spectrum. 1H and 13C NMR spectra were analyzed. [Display omitted] [ABSTRACT FROM AUTHOR]

N- Methylpyrrolidine betaine (N- carboxymethyl- N- methyl-pyrrolidinium, inner salt, MPRB) cocrystallizes with p -hydroxybenzoic acid (HBA) at the 1:1 ratio. The complex is characterized by single-crystal X-ray diffraction, FTIR and NMR spectra. The pyrrolidinium ring adopts an envelope conformation with the methyl group in axial and the carboxymethyl substituent in the equatorial positions. In the cocrystal the COO⋯HO and COO⋯HOOC hydrogen bonds are of 2.574 (4) and 2.586 (4) Å, respectively. The molecular geometry, Hirshfeld surface, natural bond orbitals (NBO), molecular electrostatic potential (MEP), vibrational wavenumbers, magnetic isotropic shielding constants were performed using the density functional theory (DFT/B3LYP) with 6–311++G (d,p) basis set. The molecular structures of isolated 1:1 and 1:2 complexes were optimized. The potential energy distribution (PED) of the IR vibrational modes were used to assign the bands in the IR spectrum. Diagrams of the HOMO and LUMO of components and complex are presented. Theoretical and experimental results are in a good agreement. Image 1 • The 1:1 complex of N-methylpyrrolidine betaine with p -hydroxybenzoic acid was prepared. • Structure of complexes were characterized by X-ray diffraction, FTIR and NMR spectra. • The structures of 1:1 and 1:2 complexes were optimized by the B3LYP/6-311++G(d,p) approach. • The molecules are linked through the OH∙∙∙OOC and COOH∙∙∙OOC hydrogen-bonds. • The IR and NMR experimental and computed spectra were discussed. [ABSTRACT FROM AUTHOR]