Tsonko Kolev

On the Origin of the Color in the Solid State. Crystal Structure and Optical and Magnetic Properties of 4-Cyanopyridinium Hydrogensquarate Monohydrate

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.

Vibrational spectra and structure of benzil and its 18O- and d10-labelled derivatives: a quantum chemical and experimental study

Geometry and vibrational spectroscopic data of benzil-d0 benzil-d10 and benzil-18O calculated at various levels of theory (RHF/6-31G*, B3LYP/6-31G*, BLYP/6-31G*) are reported. The theoretical results are discussed mainly in terms of the comparisons with infrared (4000-100 cm(-1)) and Raman (4000-50 cm(-1)) spectral data. The calculated isotopic frequency shifts, induced by the 18O- and d10-labeling, are in a good agreement with the measured values. A complete vibrational assignment was made with the help of ab initio force field calculations. The data thus obtained were used for reassigning some vibrational frequencies. The results of the optimized molecular structure obtained on the basis of RHF and the DFT calculations are presented and compared with the experimental X-ray diffraction for the benzil-d0 single crystal. It turns out that the best structural parameters are predicted by the B3LYP/6-31G* method.

Spectroscopic and structural elucidation of 4-dimethylaminopyridine and its hydrogensquarate.

A distortion of the aromatic character and stabilization of the imino-form as a result of the protonation of 4-dimethylaminopyridine was established by IR-, UV- and 1H NMR-spectral analysis of 4-diaminopyridinium hydrogensquarate. Quantum chemical calculations were carried out at MP2 and B3LYP levels of theory and a 6-311++G** basis set with a view to determining the changes in geometrical parameters and IR-spectroscopic characteristics resulting from Npy protonation. Linear-dichroic IR-spectroscopy coupled with the orientation techniques of solid samples as liquid crystal suspensions and melted solid polycrystalline films was applied for the identification of the IR-bands, characteristic for the structural fragments of the neutral and imino-form of the pyridine derivative. The spectral results were compared with the structure, obtained by a single crystal X-ray analysis. The salt contains dimmers of hydrogensquarate anions and Npy protonated cations of which the former are stabilized by strong intermolecular OH...O interactions (2.552 angstroms and 143.1(2) degrees). The 4-diaminopyridinium cation interacts with the anion through moderate NH...O bonds (2.729 angstroms and 165.0(0) degrees ). Individual cations are pi-pi stacked with their neighbors at a distance of 3.406 angstroms.

Vibrational spectra and structure of benzophenone and its 18O and d10 labelled derivatives: an ab initio and experimental study

Infrared (4000-100 cm(-1)) and Raman (4000-10 cm(-1)) spectra of benzophenone, benzophenone-d10 and benzophenone-(18)O have been studied in the solid state and in solution and their fundamental frequencies have been assigned using isotopic frequency shifts and differential infrared linear dichroic spectra of oriented polycrystalline layers (4000-400 cm(-1)). Ab initio MO calculations have been carried out for the three benzophenone isotopomers at the HF/3-21G, 6-31G and 6-31G** levels and the computed vibrational frequencies have been compared with the experimental ones. Best agreement is achieved with the 6-31G data, the mean deviation being 25.4 cm(-1). The calculated isotopic frequency shifts induced by the (18)O and d10 labelling, are also in a good accordance with the measured ones. All geometry parameters calculated for the isolated molecule are in good agreement with the X-ray data for the benzophenone single crystal.

Preparation and use of novel molybdenum-containing organic complexes as catalysts in the epoxidation of cyclohexene

AbstractNovel molybdenum-containing organic complexes based on 3,4-dihydroxy-3-cyclobutene-1,2-dione (“squaric acid”) andsome organic amines have been synthesized and characterized by infrared (IR) spectroscopy, thermogravimetric (TGA) andelemental analyses. The resulting molybdenum complexes were introduced as catalysts in the epoxidation of cyclohexenewith t-butylhydroperoxide (TBHP) and showed high activities and selectivities, comparable to those of the commerciallyavailable molydenyl acetylacetonate (MoO 2 (acac) 2 ).© 2002 Elsevier Science B.V. All rights reserved. Keywords: Squaric acid; Molybdenum-containing complexes; Catalysts; Epoxidation 1. IntroductionMolybdenum-containing complexes have long beenknown to be useful catalysts for the epoxidation ofalkenes by organic hydroperoxides. The industrial im-plementation of this reaction was first achieved in theHalcon process, which utilized a soluble Mo com-plex and in the Arco process, which made use of aheterogeneous Ti-SiO 2 catalyst to effect the epoxida-tion of propylene to the synthetically useful propyleneoxide precursor [1,2]. Since then, a number of solu-ble molybdenum-containing organic complexes withcarboxylic acids, ketones, alcohols, glycols, amines,etc. have been synthesized and employed as homoge-neous catalysts for the epoxidation of various alkenes[3–7]. The preparation of polymer-supported catalystsfor the epoxidation of alkenes has also received scien-

Spectroscopic and structural elucidation of amino acid derivatives and small peptides: experimental and theoretical tools

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 1H and 13C-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.

Spectroscopic, theoretical and structural characterization of hydrogensquarates of l-threonyl-l-serine and l-serine

Summary.Hydrogensquarates of dipeptide l-threonyl-l-serine (H-Thr-Ser-OH) and l-serine (HSq × Ser) have been synthesized, isolated and spectroscopic characterized by solid-state linear-polarized IR-spectroscopy, 1H- and 13C-NMR, ESI-MS and HPLC with tandem masspectrometry (MS-MS) methods. The structures of the salts and neutral dipeptide have been predicted theoretically by ab initio calculations. In the case of H-Thr-Ser-OH the theoretical data are supported by IR-LD ones. The hydrogensquarates consist in positive charged dipeptide or amino acid moiety and negative hydrogensquarate anion (HSq) stabilizing by strong intermolecular hydrogen bonds. The data about the l-serine hydrogensquarate are compared with known crystallographic data thus indicating a good correlation between the theoretical predicted structures and experimentally obtained by single crystal X-ray diffraction.

Solid-state IR-LD spectroscopy of codeine and N-norcodeine derivatives

Linear-dichroic infrared spectra (IR-LD) of 3-ethoxy-4-(N-norcodeino-)cyclobutene-1,2-dione (1) and codeine dihydrogenphosphate (Codeinum phosphoricum) (2), oriented as solid suspensions in nematic liquid crystals, have been measured. IR characteristic band assignments were made, and used in stereo-structural predictions. The results were compared with crystallographic structures of similar organic systems, and found to be in good agreement.

An Au(III) complex of glycyl- S -serine: a linear polarized IR and 1H- and 13C-NMR investigation

The structure of a mononuclear Au(III) complex of the dipeptide glycyl-S-serine (Gly-Ser) has been predicted using solid-state linear dichroic IR (IR-LD) spectroscopy, based on an orientation technique in a nematic liquid crystal suspension. Results are compared with data from 1H- and 13C-NMR, MS, elemental analysis, thermogravimetry and differential scanning calorimetry. The metal ion is coordinated as a tridentate through NH2, N (from deprotonated amide) and O (COO−) groups to form [Au(C5H9N2O4)Cl], with the fourth position of the square-planar coordination sphere being completed by a Cl− ion.

Linearly polarized IR spectroscopy : theory and applications for structural analysis

Linear-Dichroic Infrared (IR-LD) Spectroscopy: Background Theoretical Prerequisites Symmetry Analysis of Normal Vibrations and Dipole Moments of Transition Generated Therefrom Elements and Operations of Symmetry Symmetry Analysis of the Dipole Moments of Transition Orientation of the Samples Orientation in Liquid Crystal Solutions Orientation as Suspension in Liquid Crystals Photometrization and Processing of IR-LD Spectra: Differential Reducing Procedure Effects in the Infrared Spectra of Crystals Background of the Orientation Method of Colloid Suspensions in a Nematic Host Orientation Procedure Validation of the Orientation Procedure Accuracy and Repeatability The Quantitative Ratio of Liquid Crystals and Solid Samples Preliminary Rubbing Out of the KBr Plates Peak Function Type for the Curve-Fitting Procedure Number of Scans in the Measurements Experimental Design Mathematical Tools for the Interpretation of the IR Spectroscopic Patterns IR Spectra Subtraction The Smoothing Procedure Accuracy and Precision Deconvolution Curve-Fitting Procedure (CFP) Baseline Correction (BLC) Reducing-Difference Procedure (RDP) for IR-LD Spectra Interpretation Structural Elucidation of Inorganic Compounds and Glasses Inorganic Compounds R Spectroscopic Elucidation of Glasses Structural Elucidation of Organic Compounds Analysis of Heterocyclic Compounds Small Biologically Active Molecules Application in the Pharmaceutical Analysis Analysis of Morphine Alkaloids Application in the Chemistry of Dyes Stilbazolium Salts Dicyanoisophorone Derivatives Appendix: List of Acronyms References