The self-assembly of the hydrogensquarates is elucidated by means of linear-polarized infrared (IR-LD) spectroscopy of oriented colloids in nematic host and the so-called reducing-difference procedure for polarized IR-LD spectra interpretation. The scopes and limitation are discussed on five novel derivatives of squaric acid and its anions, that is, 2-chloro-3-aminopyridinium hydrogensquarate (1), bis (1,2,3,4-tetrahydroquinolinium) squarate (2), bis hydrogensquarate dihydrate salt of 4-(aminomethyl)pyridine (3), N-(2-ammoniumethyl)-piperazinium monohydrate hydrogensquarate squarate (4), and 3-nitropyridinium hydrogensquarate monohydrate (5), respectively. The structures of these compounds 1-5 were solved by means of single-crystal X-ray diffraction, and the crystallographic data were used for the experimental elucidation of the corresponding IR spectra of crystals with respect to studying Fermi-resonance (FR), Davydov splitting (DS), and Fermi-Davydov (FD) as well as Evans' hole effects. The various motifs for self-assembly of squaric acid and its anions in the organic crystals are discussed together with their IR-spectroscopic properties.

The novel hydrogensquarate salt of 4-cyanopyridine was synthesized, and its structure and properties were elucidated in detail spectroscopically, thermally, and structurally, using single-crystal X-ray diffraction, linear-polarized solid-state IR spectroscopy, UV spectroscopy, TGA, DSC, DTA, and positive and negative ESI MS as well as 1H and 13C NMR methods. Quantum chemical methods were used to calculate the electronic structure, vibrational data, and electronic spectra. 4-Cyanopyridinium hydrogensquarate monohydrate crystallizes in the space group P and exhibits a layered structure with molecules linked by intermolecular NH...O(HSq-) (2.651 A) and HOH...O(HSq-) (2.792 and 2.563 A) hydrogen bonds with participation of cations, anions, and the solvent molecule. The formation of stable layers of the type (2HSq-.2H2O)n and the observation of a red color in the solid state is discussed. The optical and magnetic properties were elucidated in comparison to the data for neutral 4-cyanopyridine as well as its four known salts.

This mini review deals with the modern aspects of the spectroscopy and structural elucidation of amino acid derivatives and small biologically active compounds. Free peptide bond rotation in these systems yields various conformers, which possess differing biological activities. Another phenomenon is the intermolecular or intramolecular stacking observed in aromatic small peptides. Specifically, the main aim is to illustrate the successful application of the "complex tool", consisting of a combination of the theoretical approximation methods with experimental linear polarized infrared (IR-LD) and/or Raman spectroscopy of oriented colloid suspensions in a nematic host. The possibilities and limitations of the approach for detailed vibrational assignment and structural elucidation of small peptides are discussed. Having in mind that physical and chemical properties of these systems can be precisely calculated by means of ab initio and DFT methods at Hartee-Fock, MP2 and B3LYP level of theory, varying basis sets, the results obtained allow a precise assignment of many vibrational bands to the corresponding normal modes, electronic structures and conformational state. The validity of the conclusions about the structure or vibrational properties of these systems have been supported, compared and/or additionally proved by the results from independent physical methods. In this respect (1)H and (13)C-NMR, single crystal X-ray diffraction, HPLC tandem mass spectrometry as well as thermal methods are all employed. A well ordered crystal must first be grown in order to determine the molecular structure by the absolute method of single crystal X-ray diffraction. Although the 3D structures of peptides have been determined over the past decades, peptide crystallization is still a major obstacle to X-ray diffraction work, the presence of chiral centre/s makes for this difficulty. For this reason the "complex tool" presented can be regarded as an alternative method for obtaining of structural information in the solid-state. It is obviously that only absolute crystallographic method can yield geometric parameters, bond lengths and angles, but the spectroscopic method presented can provide information about the dihedral angles for cis- and trans-configurated amide groups, mutual disposition of the aromatic fragments in peptides. Its validity is illustrated by comparing the theoretical and spectroscopic results obtained with available crystallographic data. The mini review can serve as a useful source of information not only for specialists in IR spectroscopy but, also, for other scientists, working in the field of structural analysis of amino acid derivatives and other small biologically active systems.

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3,4-diaminopyridinium bis(perchlorate) has been synthesized and its crystal structure has been solved by single crystal X-ray diffraction. The optical and magnetic properties of the N1, N4 protonated 3,4-diaminopyridinium dication have been elucidated in solution and in the solid-state by means of linear-polarized solid state IR-spectroscopy (IR-LD), UV-spectroscopy, TGA, DSC, and positive and negative ESI MS. Quantum chemical calculations were used to obtain the electronic structure, vibrational data, and electronic spectra of the dication. The studied compound crystallizes in the noncentrosymmetric space group Cc and exhibits infinite molecular chains formed by 3,4-diaminopyridinium dications and ClO 4 -anions along the c-axis by moderate intermolecular NH 3 + ÁÁÁOClO 3 -interactions with bond lengths of 3.031, 3.024, 2.825, and 2.875 Å . The NH group participates in intermolecular NHÁÁÁOClO 3 -contacts with bond lengths of 3.220 and 3.172 Å , respectively. The effect of N1, N4 diprotonation on the optical and magnetic properties of the 3,4-diaminopyridinium dication is discussed.

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