Intramolecular hydrogen bond in enol form of 3-substituted-2,4-pentanedione

The 3-substituent effect on the enolic ring of 2,4-pentanedione has been investigated. A linear relationship between the chemical shifts of the enolic proton and the chelated carbonyl stretching vibrations has been found for the various highly enolized 3-substituted-2,4-pentanediones. The stronger electron-withdrawing resonance effect of the substituent at 3-position results in the lower magnetic field shift of the enolic proton and the lower frequency shift of the chelated carbonyl stretching. Semi-empirical Hückel calculations have been performed to enable discussion of the electronic effect of the substituents.     

Intramolecular hydrogen bond in the hydroxycyclohexadienyl peroxy radicals

The hydroxycyclohexadienyl peroxy radicals (HO? C₆H₆? O₂) produced from the reaction of OH‐benzene adduct with O₂ were studied with density functional theory (DFT) calculations to determine their characteristics. The optimized geometries, vibrational frequencies, and total energies of 2‐hydroxycyclohexadienyl peroxy radical II_s and 4‐hydroxycyclohexadienyl peroxy radical III_s were calculated at the following theoretical levels, B3LYP/6‐31G(d), B3LYP/6‐311G(d,p), and B3LYP/6‐311+G(d,p). Both were shown to contain a red‐shifted intramolecular hydrogen bond (O? H … O? H bond). According to atoms‐in‐molecules (AIM) analysis, the intramolecular hydrogen bond in the 2‐hydroxycyclohexadienyl peroxy radical II_s is stronger than that one in 4‐hydroxycyclohexadienyl peroxy radical III_s, and the former is the most stable conformation among its isomers. Generally speaking, hydrogen bonding in these radicals plays an important role to make them more stable. Based on natural bond orbital (NBO) analysis, the stabilization energy between orbitals is the main factor to produce red‐shifted intramolecular hydrogen bond within these peroxy radicals. The hyperconjugative interactions can promote the transfer of some electron density to the O? H antibonding orbital, while the increased electron density in the O? H antibonding orbital leads to the elongation of the O? H bond and the red shift of the O? H stretching frequency. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Intramolecular hydrogen bond and hyperfine splitting at the hydroxyl group

The ESR spectra of 1, 2-, 1, 4-, 1, 5-, and 1, 8-dihydroxyanthrasemiquinone were measured. A hyperfine splitting at the protons of the hydroxy groups participating in the formation of stable intramolecular hydrogen bonds with the carbonyl oxygen atoms is observed for all radicals. The spin density was calculated by the MO LCAO method in the Hückel approximation, taking into consideration the configurational interaction according to McLachlan. Two models were used: the Pullman two-center models, one with and one without consideration of hydrogen bonds. The existence of the splittings indicates the participation of H-bonds in the transfer of the conjugation. A transfer of the spin density to the proton of the H-bond takes place principally from a proton-acceptor atom. It has been shown that the formation of intramolecular hydrogen bonds has a significant effect on the distribution of the spin density in the radical. Reasons are given for the advantage of the qualitative usefulness of the Pullman model in the calculation of H-bonds.     

Intramolecular easily polarizable charged and non-charged hydrogen bonds: IR continua and hydrogen bond length

Substances with intramolecular, easily polarizable hydrogen bonds of different length are compared. Long hydrogen bonds cause IR continua which show band-like structures in the region 2800–1800 cm^?1, extending with weaker intensity toward smaller wave-numbers over the whole region studied. Medium length bonds cause continua beginning at 3200 cm^?1 and extending over the whole region studied. Very short, easily polarizable hydrogen bonds, such as the bond in the HAuCl₄ salt of NN'-tetramethyl-o-xylildiamine di-N-oxide, cause continua with very great intensity in the region 1500–850 cm^?1. Comparison of the experimental with calculated continua shows very good agreement with regard to wavenumber regions in which easily polarizable hydrogen bonds of different length cause IR continua.By comparing all these intramolecular, easily polarizable hydrogen bonds causing IR continua it is shown that the charge of the hydrogen bonds is not the decisive property for the occurrence of the continua but rather the shape of the proton potentials. Continua may occur only with hydrogen bonds with double minima or broad flat proton potential.     

Intramolecular hydrogen bond in 3‐imino‐propenylamine isomers: AIM and NBO studies

The molecular structure and intramolecular hydrogen bond energy of 18 conformers of 3‐imino‐propenyl‐amine were investigated at MP2 and B3LYP levels of theory using the standard 6‐311++G** basis set. The atom in molecules or AIM theory of Bader, which is based on the topological properties of the electron density (ρ), was used additionally and the natural bond orbital (NBO) analysis was also carried out. Furthermore calculations for all possible conformations of 3‐imino‐propenyl‐amin in water solution were also carried out at B3LYP/6‐311++G** and MP2/6‐311++G** levels of theory. The calculated geometrical parameters and conformational analyses in gas phase and water solution show that the imine–amine conformers of this compound are more stable than the other conformers. B3LYP method predicts the IMA‐1 as global minimum. This stability is mainly due to the formation of a strong N? H···N intramolecular hydrogen bond, which is assisted by π‐electrons resonance, and this π‐electrons are established by NH2 functional group. Hydrogen bond energies for all conformers of 3‐imino‐propenyl‐amine were obtained from the related rotamers methods. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Intramolecular hydrogen bond investigations in some Schiff bases using C-C and N-C coupling constants

Five Schiff bases, derived from substituted salicylaldehydes and methylamine, have been investigated by analysis of C-C and N-C coupling constants to check potential of those spectral parameters in intramolecular hydrogen bonds research. Two remaining imines, without OH substituent in position 2, were applied as model compounds for imine structure. The one-bond C-C couplings in aromatic ring provide valuable information about bond orders and correlate with bond lengths obtained by X-ray. The one-bond heteronuclear C-N couplings can be used very easily to distinguish between imine and enamine form of Schiff bases. Additionally the two-bond N-C couplings supply interesting information about geometry of the investigated molecules.     

Intramolecular hydrogen bonds in 3-diethylaminomethyl-5-R-salicylic aldehydes

Two 3-diethylaminomethyl-5-R-salicylic aldehydes were obtained and studied in chloroform solutions by FTIR and NMR spectroscopy. The existence of an equilibrium between the structures with OHO=C and NHO intramolecular hydrogen bonds was suggested. In the case of compound 1 (R=OCH₃) the OHO=C intramolecular hydrogen bond was more favorable whereas in the case of compound 2 (R=Br) the structure with the OHN intramolecular hydrogen bond was predominant.     

Intramolecular AR--O--AR bond formation in calixarenes

The formal dehydration of two vicinal phenol moieties of p-tert-butylcalix[6]arene was achieved in two steps by mild oxidation of the calixarene followed by treatment of the resulting monospirodienone derivative (9c) with an ionic hydrogenation mixture (Et(3)SiH/CF(3)COOH). Reaction of 9c yielded the unsubstituted xanthenocalix[6]arene 11d, while treatment of the monospirodienone derivative of a spherand-type calixarene (13) with Et(3)SiH/CF(3)COOH afforded the dibenzofuran derivative 15. The formation of the latter product indicates that, at least for 13, the rings forming the Ar--O--Ar bond in the product are not those connected by the spiro bond in the starting material. Methylation of the phenolic hydroxyl groups of 11d with methyl p-toluenesulfonate/K(2)CO(3) or dimethyl sulfate/base afforded its dimethyl and tetramethyl ether derivatives. The parent xanthone calix[6]arene derivative 17b was prepared by O-methylation of the phenol groups followed by CrO(3) oxidation of the xanthene methylene group and deprotection of the OH groups. McMurry coupling of calixanthone 17a afforded the dixanthylene 18. Calixarenes 11d and 15 (which possess a xanthene and dibenzofuran group, respectively) were structurally characterized by X-ray crystallography.     

Chemistry of 2-aryl-substituted 3-hydroxypyridines

Russian Chemical Bulletin -     

Intramolecular hydrogen bonding in calixarenes

The construction of a molecular cavity for recognition and catalysis requires either covalent synthesis or intermolecular self-assembly of complementary units. Intramolecular hydrogen bonding is another tool to control the cavity-forming process. When properly positioned within the same molecular structure, hydrogen bonding sites are responsible for the formation, preorganization, and binding ability of the host. The most typical examples from the supramolecular chemistry of calixarenes, the key cavity-containing building blocks, and derived from them receptor molecules are discussed.