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 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 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 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.     

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.     

Chemistry of 2-aryl-substituted 3-hydroxypyridines

Russian Chemical Bulletin -     

Intramolecular hydrogen bond energy in polyhydroxy systems: a critical comparison of molecular tailoring and isodesmic approaches

The intramolecular hydrogen bond (H-bond) energies in several polyhydroxy systems are estimated using an isodesmic/homodesmic reaction approach as well as a molecular tailoring approach (MTA) [Deshmukh, M. M.; Gadre, S. R.; Bartolotti, L. J. J. Phys. Chem. A 2006, 110, 12519]. It is shown that the isodesmic/homodesmic reaction approach as advocated in the literature does not give true H-bond energy but includes the effect of strain energy due to the formation of a ring structure. Such a ring strain is duly accounted for in the MTA method. The isodesmic H-bond energies are found to be smaller than their MTA energy counterparts typically by the strain energy. The MTA is applied to decitol, a system with more than five different H-bonds for which an application of an appropriate isodesmic reaction is extremely difficult. It has been shown that the MTA method is able to predict not only the H-bond energies but also the trends in conformational energies for three different conformers of decitol studied in the present work.     

Intramolecular hydrogen bond, molecular structure and vibrational assignment of tetra-acetylethane. A density functional study

The intramolecular hydrogen bond, molecular structure and vibrational frequencies of tetra-acetylethane have been investigated by means of high-level density functional theory (DFT) methods with most popular basis sets. Fourier transform infrared and Fourier transform Raman spectra of this compound and its deuterated analogue were recorded in the regions 400-4000 cm(-1) and 40-4000 cm(-1), respectively. The calculated geometrical parameters of tetra-acetylethane were compared to the experimental results of this compound and its parent molecule (acetylacetone), obtained from X-ray diffraction. The O...O distance in tetra-acetylethane, about 2.424A, suggests that the hydrogen bond in this compound is stronger than acetylacetone. This conclusion is well supported by the NMR proton chemical shifts and O-H stretching mode at 2626 cm(-1). Furthermore, the calculated hydrogen bond energy in the title compound is 17.22 kcal/mol, which is greater than the acetylacetone value. On the other hand, the results of theoretical calculations show that the bulky substitution in alpha-position of acetylacetone results in an increase of the conjugation of pi electrons in the chelate ring. Finally, we applied the atoms in molecules (AIM) theory and natural bond orbital method (NBO) for detail analyzing the hydrogen bond in tetra-acetylethane and acetylacetone. These results are in agreement with the vibrational spectra interpretation and quantum chemical calculation results. Also, the conformations of methyl groups with respect to the plane of the molecule and with respect to each other were investigated.     

Intramolecular catalytic cyclization of substituted 2-alkenylanilines

A study was carried out on the effect of the nature and structure of the substituent in the aromatic ring of 2-(1-methyl-2-butenyl)anilines on the direction and structural selectivity of the their intramolecular cyclization to derivatives of quinoline and indole by the action of catalytic amounts of PdCl₂ or PdCl₂-(DMSO)_n complexes.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 505–507, April, 1987.     

Hydrogen bonding and molecular association in methylated 3-hydroxypyridines

Association in derivatives of 3-hydroxypyridine has been examined via the chemical shifts for OH in PMR for equimolar mixtures of the derivatives of pyridine and phenol in CCl₄ and CH₂Cl₂. It is found that spatial screening of the pyridine N and phenol OH by methyl groups has an anomalous effect on the strength of the complex. A possible explanation is that the ionic form NH . . . O is stabilized on account of increased basicity of the N and acidity of the H in the methyl derivatives of pyridine and phenol. Domination by the intermolecular hydrogen bond between OH and N is the distinctive feature of association in 3-hydroxypyridines relative to association in phenol. The temperature dependence of _OH gives the enthalpy and entropy of association for 2, 4, 6-trimethyl-3-hydroxypyridine and of complex formation for various model systems. IR measurements confirm the conclusions.See [1] for the previous communication.We are indebted to G. G. Dvoryantseva for making the IR measurements.