Effect Of Intramolecular Hydrogen Bond Research Articles

Conformational equilibria in EE-2,6-di-[2-(furan-2-yl)vinyl]pyridine controlled by intramolecular hydrogen-type bonds

The conformational equilibria in solutions of the title compound were studied by a combined kinetic and statistical fluorimetric analysis in 9:1 methylcyclohexane/3-methylpentane at 293 K. The temperature and solvent effects on the equilibrium were also investigated. Two rotamers were evidenced: one short living, absorbing at shorter wavelengths, and a longer-lived one, more stable and with a bathochromically shifted absorption spectrum. The stabilising effect of intramolecular hydrogen bonds on the latter is discussed. The analysis allowed the spectral and radiative relaxation parameters of the two rotamers to be obtained.

Effect of intramolecular hydrogen-bond on the DNA-binding and photocleavage properties of polypyridyl cobalt(III) complexes

Two polypyridyl cobalt(III) complexes containing intramolecular hydrogen-bond [Co(phen)2HPIP]3+ (HPIP=2-(2-hydroxyphenyl)imidazo[4,5-f][1,10]phenanthroline) and [Co(phen)2HNAIP]3+ (HNAIP=2-(2-hydroxy-l-naphthyl)imidazo[4,5-f][1,10]phenanthroline) have been synthesized and characterized by UV–Vis, EA and MS spectra. Binding of both complexes with calf thymus DNA has been investigated by cyclic voltammetry, absorption spectroscopy, luminescence titration, viscosity measurements and DNA cleavage assay. The results suggested that the complex [Co(phen)2HPIP]3+ bound to CT DNA by intercalation via HPIP into the base pairs of DNA, while [Co(phen)2HNAIP]3+ bound with DNA by partial intercalating the ligand HNAIP. In addition, both complexes have been found to promote the single-stranded cleavage of plasmid pBR 322 DNA upon irradiation, and [Co(phen)2HPIP]3+ exhibited the higher cleaving efficiency.

Side-chain functionalized liquid crystalline polymers and blends, 4. Phase behavior of functionalized liquid crystalline copolymers containing maleic acid fragments

A number of new functionalized LC copolymers containing mesogenic oxycyanobiphenyl groups and 3–41 mol-% of maleic acid has been obtained by free-radical copolymerization in solution. A significant effect of intramolecular hydrogen bonds on the phase behavior of these copolymers is observed: for a maleic acid content of up to 8 mol-% LC copolymers form only the nematic and between 10–41 mol-% the SmA phase is formed. The process of formation of anhydrides in functionalized LC polymers at 130°C is investigated. It is shown that the formation of five-membered intramolecular anhydride rings does not influence the phase behavior of the LC polymers.

Side-chain functionalized liquid crystalline polymers and blends, 4. Phase behavior of functionalized liquid crystalline copolymers containing maleic acid fragments

A number of new functionalized LC copolymers contaning mesogenic oxycyanobiphenyl groups and 3-41 mol-% of maleic acid has been obtained by free radical copolymerization in solution. A significant effect of intramolecular hydrogen bonds on the phase behavior of these copolymers is observed, for a maleic acid content of up to 8 mol-% LC copolymers form only the nematic and between 10-41 mol-% the SmA phase is formed. The process of formation of anhydrides in functionalized LC polymers at 130°C is instigated. It is show a that the formation of five-membered intramolecular anhydride rings does not influence the phase behavior of the LC polymers.

Photophysics and photochemistry of 2,6-distyrylpyridine and some heteroanalogues

The photophysical and photochemical properties of some symmetric (E,E)-2,6-di(arylvinyl) derivatives of pyridine, where the side aryl groups were phenyl, pyridyl, furanyl, thienyl and thiazolyl were investigated. The conformational equilibria between the two more abundant rotamers and their role in the kinetic competition between the radiative and reactive (E,E→E,Z) relaxation channels of their lowest excited singlet state are described and compared with those of the parent hydrocarbon, 1,3-distyrylbenzene. The effect of intramolecular hydrogen bonds between the nitrogen atom and the ethenic hydrogens on the nature and properties of the S1 state is discussed.

Adiabatic Intramolecular Hydrogen Atom Transfer in 2-[2-(2-Pyrrolyl)ethenyl]pyridine

Abstract 2-[2-(2-Pyrrolyl)ethenyl]pyridine (3) forms an intramolecular hydrogen bond between the pyridine and pyrrole ring and undergoes one-way trans→cis isomerization on direct irradiation. Furthermore, cis isomer undergoes intramolecular hydrogen atom transfer to give a tautomer in the excited singlet state. Based on the experimental findings, effects of intramolecular hydrogen bond and the solvent on the photochemical behaviors of 3 were discussed.

Conformational Behavior of Methyl (3R)-3-{[(3′R)-3′ -Hydroxybutanoyl]oxy}butanoate in Solutions: Effect of Intramolecular Hydrogen Bond

Abstract Methyl (3R)-3-{[(3′R)-3′-hydroxybutanoyl]oxy}butanoate (MHBOB) has been prepared as a model compound of the hydroxy terminal part of poly[(R)-3-hydroxybutanoate] [P(3-HB)]; the NMR assignments and conformational analyses of the compound are reported. The NMR assignments of MHBOB were made by means of two-dimensional pulsed field gradient 1H–1H COSY and 1H–13C HMBC (heteronuclear multiple-bond correlation) spectroscopy. The conformational behavior of MHBOB generated by rotation about two CH2–CH bonds was investigated in chloroform and in aqueous solution at various temperatures by analysis of vicinal coupling in the 500-MHz 1H NMR spectra. In both chloroform and aqueous solution, the monomer unit adjacent to the methoxy group was found to adopt a similar conformational distribution to that of the P(3-HB) polymer backbone, in which the trans and gauche conformers were predominant, while the another gauche conformer was suppressed to almost zero. On the other hand, the monomer unit adjacent to the hydroxy group had a different conformational distribution relative to that of the other monomer unit, due to the formation of an intramolecular hydrogen bond between hydroxy and carbonyl groups. In chloroform, only the gauche conformer was predominant at temperatures between −60 and 57 °C, and even the fraction of another gauche conformer was higher than that of trans conformer at low temperatures below 0 °C. In aqueous solution, the fraction of gauche conformer was also higher than that of trans conformer, suggesting the formation of the intramolecular hydrogen bond. However, the formation of the intramolecular hydrogen bond in aqueous solution was not so strong as that in chloroform.

Effect of intramolecular hydrogen bonds on the thermodynamics stabilities of the iron (III) complexes with novel tripodal ligands

Effect of intramolecular hydrogen bonds on the thermodynamics stabilities of the iron (III) complexes with novel tripodal ligands

Tacticity-Dependent 13C NMR Chemical Shifts for Poly(vinyl alcohol) Models Studied by ab Initio Gauge-Included Atomic Orbital Calculations

Large differences in the experimental 13 C NMR chemical shifts for three resonance peaks assigned to the methine carbons in solid poly(vinyl alcohol) (PVA) are attributed not to the intramolecular hydrogen-bond effect but to the conformation effect due to tacticity on the basis of ab initio GIAO calculations for model compounds: 1,3-propanediol, 1,3,5-pentanetriol, pentane, and 4-methylheptane. The experimental chemical-shift differences of the methine carbons are well reproduced by the calculated shifts for not only the triad models of 1,3,5-pentanetriol but also those of 4-methylheptane. The strong intramolecular hydrogen bonds in the m tacticity maintain the all-anti conformation of the main chain of PVA. Both the inter- and intramolecular hydrogen-bond effects on 13 C chemical shifts are found to be small for carbons connected to hydroxyl groups forming a hydrogen-bond system, where the hydroxyl oxygens can serve as either donor or acceptor in the hydrogen bonding

Intramolecular Hydrogen Bond Effect in Meta-cycloaddition of Arene to Olefin

Abstract The intramolecular meta-photocycloaddition of bichromophores 1 a and 1 b is examined and the observed stereochemistry of cycloaddition is rationalized in terms of hydrogen bond effect introduced by the 3-hydroxy group in the 5-phenylpent-1-ene skeleton.

Cationic micellar effect on the kinetics of the protolysis of aromatic carboxylic acids studied by the ultrasonic absorption method

The protolysis of carboxylic acids has been kinetically studied by the ultrasonic absorption method in the presence of tetradecyltrimethylammonium bromide (TTAB) micelles in aqueous solution. The carboxylic acids studied were classified into two categories, one capable of formation of intramolecular hydrogen bond, namely the salicylic acid derivatives (SAD) and the other which cannot form the bond, namely the benzoic acid derivatives (BAD). The rate constant (γ2kf, kb), the apparent dissociation constant (Ka), and the volume change of the reaction (ΔV) were obtained. Different Ka dependences of the rate constants observed for SAD and BAD are discussed in relation to the effect of intramolecular hydrogen bond. pKa dependences were also observed for ΔV of SAD and BAD. These dependences are larger than those in aqueous solution. This result was attributed to the change of arrangement of water molecules around the solute in micellar solution and aqueous solution.

Chemometrical Analysis of Substituent Effects. V. ortho Effect

The dissociation constants of nineteen ortho substituted benzoic acids have been determined in eight organic solvents (methanol, ethanol, acetone, dimethyl sulfoxide, dimethylformamide, acetonitrile, pyridine, 1,2-dichloroethane). The correlation between the σI, σR, and υ constants were unsuccessful due to neglecting the description of intramolecular hydrogen bond effect. The method of conjugated deviations has been applied to the results obtained and to those given in literature for ortho substituted benzoic acids (the dissociation constants, the reaction with diphenyldiazomethane, 33 sets), and values of three types of substituent constants have been determined for 29 substituents. The first of these substituent constants, σoi, describes the electronic effects and was adjusted with the application of the isoparameter relation (σoi as a function of σmi) suggested in previous communications. This constant (after excluding the substituents NHCOCH3 and OCOCH3) correlates very well (R = 0.993) with the σI and σR constants. The second substituent constant, σHGi, describes the interaction of the reaction centre (the oxygen atom of carboxylate anion) with the substituent, and it has non-zero values for the substituents OH, SH, NH2, NHCH3, NHCOCH3, COOH, CONH2, and SO2NH2. The third substituent constant, σSi, describes the steric effects and is not significantly related to any of the known quantities of this type. The set given was tested together with the triad of σI, σR, and υ on the definition set and on a set extended by other 28 sets of processes with ortho substituted compounds. On the whole, the set of substituent constants suggested explains 94.6% of variability of data, whereas only 66.0% are explained with the use of σI, σR, and u constants. Moreover, the tests have shown that the σoi constant is not suitable for interpretations of processes involving direct conjugation between the reaction centre and substituent.

ChemInform Abstract: Trans→Cis One‐Way Photoisomerization of an Olefin Substituted with an Indole and a Pyridine Ring. Effect of Intramolecular Hydrogen Bond.

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Trans → cis One-way Photoisomerization of an Olefin Substituted with an Indole and a Pyridine Ring. Effect of Intramolecular Hydrogen Bond

Abstract On direct irradiation in benzene 2-[2-(2-pyridyl)ethenyl]indole undegoes trans-to-cis one-way isomerization in the excited singlet state.

Theoretical study of some nitriles: Intramolecular hydrogen bonds and anomeric effect

An ab initio study of 3-chloro-, 3-hydroxy-, 3-mercapto-, and 3-amino-propanenitrile and 4-chloro-bu- tanenitrile was carried out at several levels of theory. The calculated stabilities and geometrical trends are interpreted in terms of the effects of intramolecular hydrogen bonds and anomeric interactions, and compared with available experimental data.

Comparison of intramolecular and intermolecular reactions in protein folding

The intrinsic component of the standard free energy change for the formation of a disulfide bond in a protein molecule is compared to that for an analogous chemical reaction. The former reaction, which represents theintramolecular formation of a disulfide bond in a protein molecule from a cysteine group containing a mixed disulfide bond with glutathione, and a free cysteine residue, is a unimolecular reaction. In contrast, its chemical analogue is a bimolecular reaction, and corresponds to theintermolecular disulfide interchange between a mixed disulfide-bonded compound between a cysteine residue and glutathione, and a free cysteine molecule. The difference in the intrinsic free energy of the above two reactions is estimated by two different approaches. First, a theoretical estimate of the magnitude of the difference in free energy of the two reactions (for a standard state of 1 M) is obtained using a gas-phase statistical thermodynamic approach, which indicates that the intramolecular reaction is energetically favored over its intermolecular counterpart by as much as 15.6 kcal/mole. For comparison, an experimentally derived value is also obtained, using experimental data from a study by Konishi et al. of the regeneration of the protein ribonuclease A (RNase A) from its reduced form by reduced and oxidized glutathiones. The intrinsic component of the free energy change of the intramolecular reaction, as it occurs in the protein molecule, is obtained from such experimental data by accounting explicitly for the free energy change (assumed to be solely an entropy change) pertaining to the conformational changes (ring closure) that the protein molecule undergoes in the course of the reaction. On the basis of the value derived from such an experimental approach, the intramolecular reaction is also energetically more favorable as compared to its intermolecular analogue, but only by a difference of 2.3 kcal/mole (for a standard state of 1 M). The large apparent discrepancy between the two values estimated from the theoretical and experimental approaches is rationalized by the postulation of several additional factors not inherent in the gas-phase theoretical estimate, such as dehydration and intramolecular hydrogen-bonding effects, which can largely compensate for the otherwise favorable energetics of the intramolecular reaction.

Electron reorganization induced by intramolecular hydrogen bonding in phenol derivatives: relationship between proton chemical shifts and the hydroxylic electron populations

Intramolecular hydrogen-bonding effects in ortho-substituted phenols have been studied by 1H n.m.r. and i.r. spectroscopy and CNDO/2 calculations. Full geometry optimization using the CNDO/2 method indicates definite variations of the OH bond length in intramolecular hydrogen-bonded phenols whereas these lengths remain nearly constant in para- or meta-substituted derivatives. There is a good linear relationship between the OH bond length and the i.r. νOH frequency shift, considered as reflecting the intramolecular association strength. Comparison of ortho- and para-substituted phenols shows that intramolecular hydrogen-bond formation induces important δ electron transfer from the substituent X to the hydroxylic oxygen via the bonded hydrogen atom whereas the electron population of this hydrogen atom decreases. This transfer results in π-electron reorganization of the X–C(1)–C(2)–OH fragment, and thereby increases the electron-releasing character of the OH group and the electron-withdrawing power of acceptor X groups, but the remaining atoms of the ring are little affected. With such an unusual electron reorganization, the chemical shifts of the phenolic hydrogen in ortho-, meta-, and para-substituted phenols can be simultaneously correlated with the electron population only if we consider both the proton investigated and the oxygen atom bearing it. The correlation Δδ=–31.6ΔqHM+ 16.1ΔQσO+ 0.08 satisfactorily reflects the proton shifts for the 22 compounds investigated.

Effect of intramolecular hydrogen bond in nucleophilic azaheteroaromatic substitution reaction

1. An experimental and theoretical study was made of the accelerating effect of the intramolecular hydrogen bond on the nucleophilic substitution reaction of halopyrimidines. 2. In contrast to the existing literature data, the presence of specific solvation is not a necessary condition for the accelerating effect. 3. The theory was expressed that intrachelate tautomerism is possible in the σ complexes of azines that contain an intramolecular hydrogen bond.

Fluorophosphoranes containing the perfluoropinacolyl ring system. Part 5. Estimates of two-step exchange barriers in some amino-derivatives

Low-temperature n.m.r. spectra have confirmed the presence of a two-step intramolecular exchange process in aminodifluoro(perfluoropinacolyl)phosphoranes, PF2(NR1R2)(pfp)(R1= H, R2= H or But; R1= But or SiMe3, R2= SiMe3; R1= R2= Me or Et; R1R2= Me2C[CH2]3CMe2). Energy barriers for these processes have been estimated. Analysis of the spectra confirms the expected ground-state configurations. Rotational effects are observed in a highly hindered asymmetrically substituted amino-derivative. The compound PF2(NH2)(pfp) has been synthesised by cleavage of the bis(trimethylsilyl)amino-precursor with hydrogen chloride. Intramolecular hydrogen-bonding effects are suspected in this compound and in PF2(NButH)(pfp).