A. V. Chernova

Cooperative intramolecular hydrogen bond and conformations of thiocalix[4]arene molecules

The joint FTIR spectroscopic study and ab initio quantum-chemical calculations (HF/3-21G and PBE/TZ2P methods) showed that thiocalix[4] arene molecules adopt the cone conformation in CCl4 solutions. The weakening of the cooperative intramolecular H bond in thiocalix[4]arenes compared to the corresponding calix[4]arenes can be due to the larger thiocalixarene macrocycle, bifurcated hydrogen bond in it, and electron density transfer from the bridging S atom to the benzene ring.

Ring contraction in reactions of 3-benzoylquinoxalin-2-ones with 1,2-phenylenediamines. Quinoxaline-benzoimidazole rearrangement

The reactions of 3-benzoylquinoxalin-2-one and its N(1)-alkyl derivatives with 1,2-phenylenediamines were accompanied by ring contraction as a result of the quinoxaline-benzoimidazole rearrangement giving rise to 2-benzoimidazolyl-substituted quinoxalines. The possible pathways of these reaction are discussed.

Intramolecular interactions in acyclic and macrocyclic compounds containing nucleotide bases

Abstract Acyclic and cyclic counterparts containing thymine and two 3,6-dimethyluracil fragments bridged by methylene chains have been prepared and studied by UV and NMR spectroscopy; in water the uracil units of the acyclic counterpart form an intramolecular stack but arrange in a linear array in chloroform while the fragments of the uracilophane form an intramolecular stack both in chloroform and water; uracil units of bis(3,6-dimethyluracil-1-yl)butane and the macrocyclic counterpart form a stack in chloroform.

Energetics of intramolecular hydrogen bonds and conformations of ω-diphenylphosphoryl- and ω-diphenylthiophosphoryl-substituted aliphatic alcohol molecules

A series of conformationally heterogeneous and energetically multilevel ω-diphenylphosphoryl- and ω-diphenylthiophosphoryl-substituted aliphatic alcohols, whose molecules are capable of five-, six-, and seven-membered ring closure due to the OH...O=P or OH...S=P intramolecular hydrogen bond (IHB), was studied by IR spectroscopy and molecular mechanics. The data obtained confirm the previous assumption that the energy of IHB (EIHB) differs substantially from the difference of enthalpies of conformers with and without IHB (–ΔH ), which is experimentally determined by the temperature dependence of the intensity of IR ν(OH) absorption bands. The measured –ΔH values can be positive, zero, or negative at significant IHB effects, i.e., at high Δν(OH) and EIHB values.

Vibrational spectra and conformational isomerism of calixarene building blocks: 2-benzylphenol

The conformations and vibrational spectra of 2-benzylphenol have been analysed within the framework of scaled quantum mechanics. It is shown that the solid-state conformation of the 2-benzylphenol molecule corresponds to one of four potential energy minima predicted by non-empirical quantum chemical calculations for the isolated molecule. In the molten state and in diluted CCl4 solutions of the title compound this conformer coexists with another two spectroscopically detected conformations. The presence of several conformers gives rise to the observation of the corresponding number of vsCH2 and vasCH2 bands in the IR spectra, thus providing experimental evidence of the conformational sensitivity of the vCH2 frequencies. It is shown that vCH2 wavenumbers of different kinds of molecules, containing diphenylmethane fragments, are determined by the dihedral angles between the planes of the aromatic rings and the plane of the connecting methylene bridges, except for the case of short intramolecular contacts with the participation of the methylene protons.

Synthesis of pyrimidinophanes containing nitrogen atoms in polymethylene bridges

The reactions of 1,3-bis(ω-bromobutyl- or -pentyl)-6-methyluracil with 1,3-bis(ω-ethylaminobutyl- or -pentyl)-6-methyluracil afforded pyrimidinophanes containing N atoms in bridging polymethylene chains. Individual geometric isomers of pyrimidinophanes were isolated. The structure of one of these isomers was established by X-ray diffraction analysis. Quaternization of the bridging N atoms with o-nitrobenzyl bromide gave rise to water-soluble pyrimidinophanes.

Synthesis, IR Spectra, and Steric Structure of Macrocycles Derived from Pyrimidine Compounds

A new macrocycle including pyrimidine fragments, 12,23,36-trimethyl-24,40-dioxo-15,33-dithia-2,9,13,22,26,35,38,39-octaazatetracyclo[32.3.1.110,14.122,26]tetraconta-1(38),10(39),11,13,23,34,36-heptaene, was synthesized. According to the data of IR and UV spectroscopy and HF/6-31G** quantum-chemical calculations, macrocyclic compounds of this series in crystal exist in the amino form, one NH group of which is likely to be involved in intramolecular hydrogen bond, and the other, in intermolacular hydrogen bond. The strength of the latter depends on the macroring size. In solution, the above structures are supplemented by conformers containing both intramolecularly H-bonded and free amino groups, predominantly with trans structure of the HÄNÄCÍN fragment. The imino form of the aminopyrimidine moieties is hardly probable.

Copper(II) bromide complexes with acyclic and cyclic pyrimidine-containing phane ligands

Complexes of copper(II) bromide with cyclic and isostructural acyclic phane ligands containing derivatives of pyrimidine nucleobases (cytosine and uracil) were synthesized and characterized. In two cyclic pyrimidinophanes used, the macrocycles included two 6-methylthiocytosine and one 6-methyluracil units linked by polymethylene chains (L3) and two 6-methyluracil units linked by N-containing bridges (L5). Ligand L3 and its isostructural acyclic analogs are coordinated by the Cu2+ ion through the same donor sites (the ring N atoms of the thiocytosine units). The coordination polyhedra of the Cu atom in complexes with cyclic and acyclic ligands are different. Ligand L5 and its isostructural acyclic analog also form copper(II) complexes with different coordination polyhedra involving different donor sites. The acyclic ligand is coordinated by the Cu2+ ion via the bridging N atom, while cyclic ligand L5, via the uracil CO groups (the bridging N atoms become protonated). The resulting complexes are dielectrics.

Reactions of Pyrimidinophanes and Their Acyclic Analogs with Electron-Deficient Substrates

Acyclic and macrocyclic compounds having different numbers of pyrimidine fragments and nitrogen atoms in the polymethylene bridges or in substituents at the pyrimidine rings reacted with 2,3,5,6-tetrachloro-1,4-benzoquinone and 1,4-benzoquinone according to the charge transfer scheme to give products with an ionic structure. The reaction was accompanied by protonation of nitrogen atoms in the polymethylene bridges or in substiuents at the pyrimidine rings and reduction of 2,3,5,6-tetrachloro-1,4-benzoquinone or 1,4-benzoquinone to 2,3,5,6-tetrachlorobenzene-1,4-diol or benzene-1,4-diol, respectively. The isolated products are dielectrics.

Intramolecular hydrogen bonds and conformations of the 1,4-butanediol molecule

IR and Raman spectra of 1,4-butanediol (BD) versus variations in the medium (CCl4, CH3CN), concentration, temperature, and phase state were obtained. The observed changes attest to the conformational variety of BD molecules under the experimental conditions. On the basis of the analysis of the v(OH) region it is concluded that both in the gas phase and in CCl4 solution conformers of BD with free OH groups coexist with conformers with O-H...O intramolecular hydrogen bonds. The difference in enthalpies, ΔH, for the groups of conformers with and without intramolecular hydrogen bonds was found from the temperature dependence of the v(OH)free and v(OH)intra band intensities. The structures and energies for 70 possible spectrally and energetically distinguishable conformers of BD that do not take into account intramolecular hydrogen bonds were calculated by molecular mechanics with account of electron lone pairs of oxygen atoms. Using the experimental values of ΔH and the calculated relative conformer energies,E, the intramolecular hydrogen bond energyEintra= 3.7 kcal mol−1 was found.