Hydrogen-bonded Hydroxy Group Research Articles

Hydration/Dehydration Behavior of Polyalcoholic Compounds Governed by Development of Intramolecular Hydrogen Bonds

Hydration/dehydration behavior of polyalcoholic compounds with a rigid cyclohexane-type aliphatic molecular frame, such as 1,4-cyclohexanediol (ch(OH)2), 1α,3α,5α-cyclohexanetriol (cis-phloroglucitol, ch(OH)3), and cis-1,2,3,5-trans-4,6-cyclohexanehexaol (myo-inositol, ch(OH)6) was investigated using extremely high-frequency dielectric relaxation measurements up to 50 GHz over a temperature range from 10 to 70 °C. The temperature dependencies of hydration numbers per hydroxy group (mOH) in the polyalcoholic compounds were determined. Although the obtained mOH value was ca. 5 in a temperature range lower than 30 °C, it decreased and approached to ca. 3 with increasing temperature for ch(OH)2 and ch(OH)3, in which hydroxy groups hardly form intramolecular hydrogen bonds due to separations between them. This temperature-dependent hydration/dehydration behavior is characteristic of isolated hydroxy groups without intramolecular hydrogen bond formation. On the other hand, a constant mOH value of ca. 1 was observed irrespective of temperature for ch(OH)6. Because all the separations between adjacent hydroxy groups in ch(OH)6 are proper for (circular type) intramolecular hydrogen bond formation, the obtained temperature-independent small mOH value resulted from an inherent feature of intramolecularly hydrogen bonded hydroxy groups.

Dissociative Electron Capture of Hydrogen-Bonded Hydroxy Groups: Molecular Dynamics and Matrix Isolation ESR Study

Abstract While examining PFPE (perfluoropolyether)-based disk lubricants, extra lability was suspected for a hydrogen-bonded pair of hydroxy end-groups in the presence of a nucleophile (electron donor). Plausible reaction sequences upon electron capture of hydrogen-bonded methanol dimer, hydrogen-bonded ethanol dimer, and 1,2-propanediol were first examined by a molecular dynamics method based on the potential given by a semiempirical SCF molecular orbital calculation, and subsequently by matrix isolation ESR spectroscopy. In the latter experimental study, alcohol dimer anions were generated by co-trapping Na atoms and alcohol molecules in argon matrices and inducing electron transfer by photo-irradiation. In the cases of methanol and ethanol dimers, the molecular dynamics study predicted that on capture of an electron, the hydroxy hydrogen of the acceptor side would cleave as an atom, and then abstract a hydrogen atom from the nearest carbon of the proton donating alcohol. For 1,2-propanediol anion, the molecular dynamics study predicted C1–C2 bond scission followed by abstraction of the C2 hydroxy hydrogen by the C1 radical fragment. The matrix isolation ESR study revealed generation of radicals upon photo-irradiation when the alcohol concentration was sufficiently high to warrant the presence of dimers. The spectral patterns thus observed were totally consistent with the radical products predicted by the molecular dynamics calculations.

Experimental and theoretical studies of a naturally occurring non-oligomeric steroidal supramolecular zipper

Methanol extraction of the bulbs of Ornithogalum tenuifolium afforded a novel crystalline steroidal sapogenin (25R)-5β-spirostane-1β,3α-diol (compound 1). The structure and stereochemistry, but not the enantiomer, of this compound was unambiguously assigned using X-ray diffraction and multidimensional 1H and 13C NMR data. The crystal structure of 1 (monoclinic space group P21) is both novel and remarkable in that the extended lattice or “supramolecular” structure comprises an unusual one-dimensional hydrogen-bonded polymer with 21 symmetry (the polymer axis is co-linear with the b-axis of the unit cell). Each ladder-like hydrogen-bonded polymer stacks in an interlocking fashion with a neighbouring polymer along the c-axis of the unit cell. The self-recognition by hydrogen-bonding that leads to the formation of the polymer is characterized by discrete 10-membered supramolecular rings. Moreover, the steroid units of the one-dimensional polymer interlock perfectly to form a supramolecular zipper. The newly-discovered supramolecular synthon of the zipper has been elaborated on due to its possible use in crystal engineering applications. The supramolecular zipper system has also been simulated in vacuo using semi-empirical (AM1) quantum mechanics techniques, which correctly reproduce not only the experimental conformation but also the hydrogen-bonded extended structure of the compound. The simulations showed that formation of the supramolecular zipper structure of 1 most likely reflects charge-complementarity of its hydrogen-bonded hydroxy groups and that the electrostatically driven process is characterized by a favourable and cooperative in vacuo enthalpy of association. A further hallmark of the self-assembly of 1 was the lowering of the mean vibrational frequency of the hydrogen-bonded hydroxy groups upon formation of the zipper.

Forming Composites Based on Mineral Fibers

The effect of aluminum sulfate additives on the structure of a composite material based on basalt fibers has been studied using electron microscopy and IR spectroscopy. The introduction of aluminum sulfate leads to the appearance of outgrowths fastening mineral fibers to each other at their intersection contacts. This is accompanied by the appearance of an absorption band in the IR spectra, which is attributed to hydrogen-bonded hydroxy groups. Estimates of the hydrogen bond lengths are obtained from an analysis of the IR absorption band shape.

Hydrogen bonding and steric effects on rotamerization in 3,4-alkylenedioxy-, 3-alkoxy- and 3,4-dialkoxy-2-thienyldi(tert-butyl)methanols: an NMR, IR and X-ray crystallographic studyElectronic supplementary information (ESI) available: NMR data; activation parameters for rotation; MMFF94 steric energies and alkoxy group geometries; thermodynamic data; quantum mechanical calculations of geometries; bond lengths, bond angles and torsion angles of 8A-Et; NMR and IR data on new compounds. See

The equilibrium constant for the anti ↔ syn rotamerization (anti: intramolecularly hydrogen-bonded hydroxy group; syn: “free” hydroxy group) of 3,4-alkylenedioxy-, 3-alkoxy- and 3,4-dialkoxy-2-thienyldi(tert-butyl)methanols depends on the 3,4-alkylenedioxy or alkoxy group(s) and the solvent, hydrogen-bonding solvents such as DMSO and pyridine favouring the syn isomer. Equilibrium constants ([syn]/[anti]) in chloroform and benzene decrease in the order: 3,4-OCH2O-, 3,4-O(CH2)2O-, 3-OMe, 3-OEt, 3,4-(OMe)2 ≈ 3-Oi-Pr, 3,4-(OEt)2, ranging over about 2.5 orders of magnitude. Variations in the IR OH stretching frequencies and the NMR OH proton shifts for the anti isomer indicate that intramolecular hydrogen bonding increases in roughly the same order. The syn → anti rotation barrier in DMSO increases with substituent size and number. The 3,4-methylenedioxythienyl derivative has a rather lower barrier (17.5 kcal mol−1) than all the others (21.0–22.3 kcal mol−1). The syn → anti rotation barrier is largely determined by steric effects but intramolecular hydrogen bonding in the anti isomer contributes to the variation of the anti → syn rotation barrier. A single crystal X-ray diffraction study of the anti-3,4-diethoxy derivative shows that the orientation of the 3-alkoxy group is very different from that in anti-3-methoxy-2-thienyldi(1-adamantyl)methanol. Molecular mechanics and quantum mechanical calculations are used in an attempt to rationalize the equilibrium data.

Role of the (O‒H)6 Synthon in the Construction of Organic Inclusion Compounds

The racemic diol 5 crystallises from ethanol in the orthorhombic space group Cmca (a 19.05, b 14.93, c 10.12 Å) as the cocrystalline solid 5°C2H5OH. In contrast, the racemic diol 6 crystallises from benzene in the cubic space group Ia3 (a 19.85 Å) as a clathrate compound. The principal supramolecular synthon involved in both cases is a cyclic hexamer of hydrogen bonded hydroxy groups, (O-H)6. While the former cycle is almost planar, rather uncommonly the latter is cyclohexane-like in geometry. A major function of the (O-H)6 cycle is to allow efficient packing between opposite enantiomers within the crystal lattice.

Chromium-aluminium orthophosphates, II. Effect of AlPO4 loading on structure and texture of CrPO4−AlPO4 (20–50 wt.% AlPO4 catalysts obtained in aqueous ammonia

The AlPO4 content in CrPO4−AlPO4 (CrAlP-A) systems has a marked influence on thermal behavior, speeding up the transformation of amorphous CrPO4 to crystalline β-CrPO4. Thus, XRD analysis showed that amorphous systems are formed at AlPO4 loadings less than 50 wt.% and calcination temperatures below 1273 K. Moreover, on 50 wt.% AlPO4 developed crystalline β-CrPO4 in CrAlP-A systems for every calcination temperature. Besides, at 1273 K crystalline systems (β-CrPO4 and tridimite-AlPO4) are formed for every composition. Moreover, surface area at any calcination temperature increases with the rise in the AlPO4 content. Also, a surface area decrease on increasing calcination temperature is shown. Furthermore, CrAlP-A catalysts thermally treated below 1273 K only exhibited hydrogen bonded hydroxy groups (υO-H≃3450 cm-1).

Effect of precipitation medium and thermal treatment on structure and textural properties of chromium orthophosphates

The effect of the precipitation agent on the structure and textural properties of CrPO4 (Cr/P=1) catalysts has been studied. Catalysts obtained in propylene oxide-aqueous ammonia exhibited higher thermal stability and, hence, greater surface areas and pore volumes than catalysts obtained in propylene oxide or aqueous ammonia exclusively. The former also showed a small surface area decrease on increasing calcination temperature up to 1073 K. Nevertheless, calcination at 1273 K develops in all cases crystalline CrPO4, showing the β-structure. Moreover, CrPO4 catalysts thermally treated below 1273 K only exhibited hydrogen bonded hydroxy groups $$(\nu _{{\rm O} - {\rm H}} \simeq 3450 cm^{ - 1} )$$ .

Synthesis and thermal and photo-oxidative behaviour of novel 4-substituted alicyclic amino-2-hydroxybenzophenone stabilizers in polypropylene films

Four light stabilizers have been synthesized and characterized, based on the substitution of an alicyclic amine group into the 4-position of 2-hydroxybenzophenone. The compounds prepared were bis- N, N′-alkylpiperazines, N-propoxymorpholinyl and 4-amino-2,2,6,6-tetramethylpiperidinyl derivatives. The thermal and photo-oxidative stabilization activities of the three compounds have been examined and compared with those of 2-hydroxy-4-n-octoxybenzophenone (HOBZ) in polypropylene film at 0.1% w/w concentration using i.r. absorption spectroscopy, hydroperoxide analysis and yellowness index. Through both carbonyl index and embrittlement times, both alkylpiperazine derivatives were found to be ineffective on exposure to QUV irradiation but both the propoxymorpholinyl and the hindered piperidine compounds were found to be more effective than HOBZ. These stabilizing effects followed closely their ability to inhibit hydroperoxide formation before and during oven ageing. With the commercial hindered phenolic antioxidant, Irganox 1010, all the stabilizers were synergistic, especially HOBZ. On thermal ageing, all the light stabilizers including HOBZ behaved as antioxidants. The hindered piperidine derivative was the most effective, exhibiting a stabilizing effect close to that of the Irganox 1010. In the presence of Irganox 1010, weak antagonism was observed with HOBZ exhibiting the most effective stabilization. Both piperazine derivatives discoloured markedly on irradiation while the alkylmorpholinyl exhibited an initial photobleaching effect. The steric hindrance, molecular size and oxidizability of the cyclic amine are all concluded to be important factors in controlling the antioxidant behaviour of the stabilizer, with hydroperoxides playing a key role in the latter case. The results also demonstrate the importance of the ortho hydrogen bonded hydroxy group in 2-hydroxybenzophenones as an active chain-breaking donor and not the extinction coefficients. It is further concluded that the results show the potential for the development of a single stabilizer entity which will behave as an effective photothermal antioxidant.

ChemInform Abstract: Intramolecular Catalysis of the Hydrolysis of an Acetal by an Internally Hydrogen‐Bonded Hydroxy Group.

The hydrolysis of 1-methoxymethoxy-8-hydroxynaphthalene in aqueous solution at pH 2–4 is catalysed by the hydronium ion and the value of the catalytic coefficient is similar to that for 1-methoxymethoxynaphthalene. In the range pH 5–10 a pH-independent reaction with solvent is dominant and the rate coefficient for the neutral hydrolysis is estimated to be ca. 40-fold higher for 1-methoxymethoxy-8-hydroxynaphthalene than for 1-methoxymethoxynaphthalene. This is attributed to intramolecular catalysis by the hydrogen-bonded hydroxy group. As bond breakage and charge development occur in the transition state the internal hydrogen bond in the leaving group is strengthened.

Intramolecular catalysis of the hydrolysis of an acetal by an internally hydrogen-bonded hydroxy group

The hydrolysis of 1-methoxymethoxy-8-hydroxynaphthalene in aqueous solution at pH 2–4 is catalysed by the hydronium ion and the value of the catalytic coefficient is similar to that for 1-methoxymethoxynaphthalene. In the range pH 5–10 a pH-independent reaction with solvent is dominant and the rate coefficient for the neutral hydrolysis is estimated to be ca. 40-fold higher for 1-methoxymethoxy-8-hydroxynaphthalene than for 1-methoxymethoxynaphthalene. This is attributed to intramolecular catalysis by the hydrogen-bonded hydroxy group. As bond breakage and charge development occur in the transition state the internal hydrogen bond in the leaving group is strengthened.

Pattern recognition applied to vapour-phase infrared spectra: characteristics of νOH bands

With the commercial availability of infrared spectrum libraries in computer accessible form, a new application of these libraries, the investigation of structure-spectrum correlations, has become possible in addition to the conventional application, the identification of unknown substances on the basis of spectral search in the library. This paper deals with the νOH bands of alcohols, also including diols and diol monoether type compounds, investigating correlations between chemical structure and the features of spectra. For monoalcohols, the frequencies measured in dilute solution and vapour phase, respectively, are correlated; the former were taken from literature, the latter from the vapour phase spectrum library of EPA. For the other types of compound, vapour phase νOH frequencies of free and hydrogen bonded hydroxy groups were studied, and the differences correlated with the hydrogen bonded structures. Peak maxima were determined by a peak picking program; in the case of overlapping bands Fourier self-deconvolution was applied prior to peak picking. For more seriously overlapping bands least-squares curve fitting was applied. In studying correlations, a cluster analysis method (KMEANS) was also used.

Barriers to rotation in intramolecularly hydrogen-bonded 2-phenylazoresorcinols

Separate peaks at δ 13.0 and 7.5 are observed for the intramolecularly hydrogen-bonded and nonhydrogen-bonded hydroxy groups in the 1H n.m.r. spectrum of 2-phenylazoresorcinol in [2H8]toluene at temperatures below ca. 300 K. At 357 K, equilibration of the two conformers in which the phenylazo group is hydrogen-bonded to each hydroxy group is rapid (k 5.4 × 104 s–1) and an averaged hydroxy peak at δca. 10 is observed. Energy barriers for substituted 2-phenylazoresorcinols in [2H8]toluene and CD2Cl2 have been obtained from linewidth measurements of the hydroxy peaks and from observation of coalescence temperatures of aromatic 1H and 13C signals.

Infrared studies and thermodynamics of hydrogen bonding in 2-halogenoethanols and 3-halogenopropanols

From a non-linear least-squares curve analysis of the spectra of 2-halogenoethanols and 3-halogenopropanols in the hydroxy stretching region in dilute carbon tetrachloride solution at 298–338 K, the thermodynamics of the equilibrium between free and intramolecularly hydrogen-bonded species have been determined. The amount of intramolecularly hydrogen-bonded species decreased with increasing size of halogen and increasing steric repulsion. The similar frequency shifts of the intramolecularly hydrogen-bonded hydroxy group in both the 2-halogenoethanols (17–50 cm–1) and 3-halogenopropanols (15–52 cm–1) established the poor proton-acceptor properties of the halogens, including the highly electronegative fluorine. The absence of intramolecularly hydrogen-bonded species in 4-chlorobutanol is a case where the entropy change over-rides the enthalpy change due to hydrogen bonding. Hydroxy frequency shifts due to intramolecular hydrogen-bonding ran opposite to the enthalpy changes disobeying Badger–Bauer rule. These alcohols associate forming dimers (ca. 3500 cm–1) in which the hydroxy groups alone participate in intermolecular hydrogen bonding.

Intramolecular hydrogen bonding and conformational behaviour of N-methyldiethanolamine and triethanolamine

In N-methyldiethanolamine and triethanolamine, bicyclic five- and eight-membered ring intramolecular hydrogen bonds are formed such that the lone pairs of an oxygen whose hydroxy proton is already involved in an intramolecular hydrogen bond with the amino nitrogen participates in further hydrogen bonding with the second and third hydroxy groups, respectively. In triethanolamine, the participation of all three hydroxy groups in intramolecular hydrogen bonding was brought about by the formation of a cis intramolecularly hydrogen-bonded O—H⋯N five-membered ring and two intramolecularly hydrogen-bonded O—H⋯O eight-membered rings located symmetrically on either side of the five-membered ring. In both ethanolamines, nearly all the molecules had one intramolecularly hydrogen-bonded hydroxy group forming a five-membered ring. In half of these species there were one or two additional hydroxy groups involved in eight-membered ring intramolecular hydrogen bonds.

Kinetics and mechanism of tertiary amine-catalysed aqueous cleavage of maleimide

The reaction rates of the aqueous cleavage of maleimide in the presence of buffer solutions of trimethylamine, 1,4-diazabicyclo[2.2.2]octane, triethylamine, and triethanolamine have been studied at 30 °C. Both the ionized (S) and non-ionized (SH) forms of maleimide are reactive towards the free base component of buffer solutions of trimethylamine and 1,4-diazabicyclo[2.2.2]octane. These amines have exhibited general acid-catalysed and uncatalysed nucleophilic attack at carbonyl carbon of both SH and S–. Triethylamine has been found to be essentially unreactive towards SH and S– while triethanolamine has revealed significant nucleophilic reactivity towards SH and S–. The observed nucleophilic reactivity of triethanolamine and absence of such reactivity in the case of triethylamine have led to the proposal that the nucleophilic site in triethanolamine is the internally hydrogen-bonded hydroxy group rather than the free amine group. The absence of intermolecular general acid catalysis in the reactions of SH and S– with triethanolamine is attributed to both the steric effect as well as probable intramolecular general acid catalysis. A pre-association stepwise mechanism is proposed for the reactions of tertiary amines with S–while a stepwise mechanism has been favoured for the reactions of amines with SH.