Formation Of H-bonded Complexes Research Articles

Spectral and proton acceptor properties of chloramphenicol

The absorption spectra of chloramphenicol aqueous solution were obtained experimentally. The absorption spectra of chloramphenicol isomers and the effect on the spectra of the formation of H-bonded complexes were calculated and interpreted using quantum chemistry methods. Calculation results showed that the absorption spectrum of chloramphenicol by position of bands and their nature is largely determined by the nitrobenzene fragment with little participation of propanol and dichloroacetamide fragments of chloramphenicol. The proton acceptor properties of individual chloramphenicol fragments and the effect of the formation of H-bonded complexes on them have been analyzed. Keywords: chloramphenicol, enantiomers, electronic absorption spectrum.

Спектральные и протоноакцепторные свойства хлорамфеникола

The absorption spectra of chloramphenicol aqueous solution were obtained experimentally. The absorption spectra of chloramphenicol isomers and the effect on the spectra of the formation of H-bonded complexes were calculated and interpreted using quantum chemistry methods. Calculation results showed that the absorption spectrum of chloramphenicol by position of bands and their nature is largely determined by the nitrobenzene fragment with little participation of propanol and dichloroacetamide fragments of chloramphenicol. The proton acceptor properties of individual chloramphenicol fragments and the effect of the formation of H-bonded complexes on them have been analyzed.

On the Structure and Chiral Aggregation of Liquid Crystalline Star-Shaped Triazines H-Bonded to Benzoic Acids.

The ability of a star-shaped tris(triazolyl)triazine derivative to hierarchically build supramolecular chiral columnar organizations through the formation of H-bonded complexes with benzoic acids was studied from a theoretical and experimental point of view. The combined study has been done at three different levels including the study of the structure of the triazine core, the association with benzoic acids in stoichiometry 1:3, and the assembly of 1:3 complexes in helical aggregates. Although the star-shaped triazine core crystallizes in a non-C3 conformation, the C3 -symmetric conformation is theoretically predicted to be more stable and gives rise to a favorable C3 supramolecular 1:3 complex upon the interaction with three benzoic acids in their voids. In addition, calculations at different levels (DFT, PM7, and MM3) for the 1:3 host-guest complex predict the formation of large stable columnar helical aggregates stabilized by the compact packing of the interstitial acids by π-π and CH⋅⋅⋅π interactions. The acids restrict the movement of the the star-shaped triazine cores along the stacking axis causing a template effect in the self-assembly of the complex. Theoretical predictions correlate with experimental results, since the interaction with achiral or chiral 3,4,5-(4-alkoxybenzyloxy)benzoic acids gives rise to supramolecular complexes that organize in bulk hexagonal columnar mesophases stable at room temperature with intracolumnar order. The existence of supramolecular chirality in the mesophase was determined for complexes formed by acids derived from (S)-2-octanol. Chiral aggregation was also evidenced for complexes formed in dodecane.

Structure and Regularities of Formation of H-Bonded Complexes in Aqueous and Nonaqueous Binary Solutions

Results of investigations carried out by the oscillation spectroscopy and quantum chemistry methods (B3LYP/6-31++G(d,p)) to study composition, structure, formation energy, and relative stability of hydrogen-bonded molecular complexes occurring in some aqueous and nonaqueous binary solutions are summarized and analyzed. Common complex formation regularities are established for these solutions.

H-Bond Acceptor Parameters for Anions.

UV/vis absorption titrations have been used to investigate the formation of H-bonded complexes between anionic H-bond acceptors (HBAs) and neutral H-bond donors (HBDs) in organic solvents. Complexes formed by three different HBDs with 15 different anions were studied in chloroform and in acetonitrile. The data were used to determine self-consistent HBA parameters (β) for chloride, bromide, iodide, phosphate diester, acetate, benzoate, perrhenate, nitrate, triflimide, perchlorate, hexafluorophosphate, hydrogen sulfate, methyl sulfonate, triflate, and perfluorobutyl sulfonate. The results demonstrate the transferability of H-bond parameters for anions between different solvents and different HBD partners, allowing reliable prediction of anion recognition properties in other scenarios. Carboxylates are the strongest HBAs studied, with β parameters (≈ 15) that are significantly higher than those of neutral organic HBAs, and the non-coordinating anion hexafluorophosphate is the weakest acceptor, with a β parameter comparable to that of pyridine. The effects of ion pairing with the counter-cation were found to be negligible, provided small polar cations were avoided in the less polar solvent (chloroform). There is no correlation between the H-bonding properties of the anions and the pKa values of the conjugate acids.

The effect of central and planar chirality on the electrochemical and chiral sensing properties of ferrocenyl urea H-bonding receptors.

A new series of chiral ureas containing one or two redox-active ferrocene units was synthesised and studied in order to investigate the effect of planar chirality and central chirality on electrochemical chiral sensing. Binding of chiral carboxylate anions in organic solvents through H-bond formation caused a negative shift in the potentials of the ferrocene/ferrocenium (Fc/Fc(+)) couples of the receptors, demonstrating their use as electrochemical sensors in solution. While the presence of two ferrocene units gave no marked improvement in the chiral sensing capabilities of these systems, the introduction of planar chirality, in addition to central chirality, switched the enantiomeric binding preference of the system and also caused an interesting change in the appearance of some voltammograms, with unusual two-wave behaviour observed upon binding a protected prolinate guest.

Dielectric properties and analysis of H-bonded interaction study in complex systems of binary and ternary mixtures of polyvinyl alcohol with water and DMSO

The dielectric relaxation properties and dipole ordering of binary mixtures of low molecular weight Poly Vinyl alcohol (PVA) with water (WAT) and ternary mixtures of PVA+WAT with Dimethylsulfoxide (DMSO) of different concentration have been measured at 298K using Time Domain Reflectometry (TDR) technique. The measured values of permittivity (ϵ′), dielectric loss (ϵ″), static dielectric constant (ϵ0) and high frequency limiting dielectric constant (ϵ∞) have been measured in frequencies 10MHz to 30GHz and used to determine the values of molecular relaxation time (τ), and excesses permittivity (ϵE) of the different composition of binary and ternary mixtures. The formation of H-bonded complexes between PVA and WAT molecules increases the number of dipoles of binary mixtures. The PVA+WAT mixture forms the strongest complexes corresponding to the various concentrations. The ternary mixtures indicate the existence of heterogeneous species of different characteristics and molecular interactions in dilute solutions. The ϵE values of the ternary mixtures confirm the increase in dipole number. This contributes the raise in dielectric constant values and formation of the ternary complexes. The ionic conductivity (σ) and activation energy (ΔF) justify that the electronic polarization is influenced by heterogeneous H-bond of complexes of both binary and ternary systems. The Kirkwood correlation factor (gf) and effective Kirkwood correlation factor (geff) indicates the dipole ordering of the binary and ternary mixtures. The comparative values of various binary and ternary mixtures of different composition suggest that the nature of heterogeneous interaction varies with the solvent. Variations of the constituent compositions in the binary and ternary mixture also identified.

The Role of Silanols in the Interactions between Methyl tert-Butyl Ether and High-Silica Faujasite Y: An Infrared Spectroscopy and Computational Model Study

It is generally believed that the retention of methyl tert-butyl ether (MTBE) by zeolites is positively correlated to the silica content of these materials. Nevertheless, highly dealuminated zeolites can contain relevant amounts of silanol groups. In this study, the effect of these point defects in a zeolite Y (SiO2/Al2O3 = 200) on the adsorption of MTBE was evaluated by means of infrared spectroscopy, supported by DFT calculations. The adsorption process is found to occur in two steps, involving isolated silanol sites and the siloxane network of the zeolite, respectively, with an average loading of 1.3 molecules per cage in the first and 1.3 molecules in the second stage. Both external and internal isolated silanol groups (stretching at 3746 and 3738 cm–1, respectively) are involved in the MTBE adsorption process with the formation of H-bonded complexes and associated shifts (516 and 358 cm–1, respectively), consistent with a H-bonding strength higher for external than for the internal ones. However, MTBE is more tightly adsorbed on the internal silanols as a result of the cage confinement effect. The band assigned to the methyl symmetric stretching of the CH3O– group can be used to discriminate between H-bond and van der Waals MTBE–zeolite interactions (2843 and 2828 cm–1, respectively). Ab initio models were used to compute the harmonic frequencies of different MTBE–zeolite models and to simulate the cage confinement of one and three ether molecules.

Effect of complementary small molecules on the properties of bicomponent hydrogel of riboflavin

Three new bicomponent hydrogels of riboflavin (R) with salicylic acid (S), dihydroxybenzoic acid (B) and acetoguanamine (D) in 1:1 molar ratio have been reported. FTIR and UV-vis spectra suggest formation of H-bonded complexes in 1:1 molar ratio of the components. The network consists of tape, bar and helical tubes for RB11, RS11 and RD11 systems, respectively. Reversible first order phase transition and invariant storage modulus (G') with angular frequency (ω) characterise the systems as forming thermoreversible hydrogels. The RD11 gel has the highest gel melting temperature and highest critical strain compared to other gels. WAXS study indicates different crystal structures for different gels. NMR spectra reveals higher shielding of protons in RD11 gel suggesting better π-stacking compared to RS11 and RB11 gels. RD11 gel shows two-fold enhancement of photoluminescence (PL) intensity with a substantial red shift of emission peak but RB11 and RS11 gels show PL-quenching. The gels exhibit a small decrease in lifetime and the PL property is very much temperature and pH dependent. So the complementary molecules have a pronounced effect on morphology, structure, stability and optical property of riboflavin gels.

Dielectric properties of binary and ternary mixtures of alcohols: Analysis of H-bonded interaction in complex systems

The heterogeneous H-bonded interactions in mixtures of glass-former liquids, i.e., binary mixtures of ethyl alcohol (EA) with ethylene glycol (EG) and ternary mixtures of equi-molar EA+EG system with glycerol (Gly) have been investigated at 21 concentrations by using precisely measured values of static and high frequency limiting dielectric constants at 25°C. Concentration dependent observed negative values of excess dielectric constant (εE) and effective Kirkwood correlation factor (geff) implies the formation of H-bonded complexes between EA and EG molecules which reduces the effective number of dipoles. The EA+EG system forms the strongest complexes corresponding to the molar ratio ∼1:1. The observed positive εE values of the (EA+EG)+Gly ternary mixtures confirm the increase in number of effective dipoles contributed to the dielectric constant values due to the formation of the ternary complexes. Critical analysis of the experimental results of the EA+EG and the (EA+EG)+Gly mixtures suggest the suitability of εE and geff study in order to explain the dipolar ordering mechanism formed during the complexes between the alcohol molecules in their ternary mixtures.

Effect of solvation on the reaction rate constants of the diphenylaminyl radical with phenols and hydroquinones

The kinetics of the reactions of the diphenylaminyl radical with unhindered phenols and hydroquinones in acetonitrile at 294 K was studied by laser flash photolysis. The rate constants k range from 104 to 106 L mol−1 s−1 and depend on the nature of substituents in phenols and hydroquinones. The k values also depend on the solvent nature and decrease by 6.5 and 50 times, on the average, when decane is replaced by toluene and acetonitrile, respectively. The logk values in decane, toluene, and acetonitrile depend linearly on the dissociation energies DOH of OH bonds in phenols and hydroquinones. Taking into account this dependence and using the experimental k value in acetonitrile, the estimate DOH = 346.6 kJ mol−1 was obtained for 2,5-dichlorohydroquinone. The thermodynamic parameters of formation of H-bonded complexes of phenols and hydroquinones with toluene and acetonitrile were calculated in the framework of the multiplicative approach to thermodynamic description of H-bonding. The rate constants k(free) of the reactions of the diphenylaminyl radical with free phenols and hydroquinones in toluene and acetonitrile were estimated. Comparison of the k(free) values with each other and experimental k values in decane suggests that the changes in k upon solvent replacement is mainly caused by the formation of H-complexes of phenols and hydroquinones with the solvent.

Tautomeric Transformations and Reactivity of Polyfunctional Hydroxypyrimidines: III. Tautomerism of 5-Acyl Derivatives of Hydroxypyrimidines and Formation of H-bonded Complexes with Solvents

Tautomerism of 5-formyl- and 5-acetyl derivatives of pyrimidine-2,4,6-trione, 2-thioxopyrimidine-4,6-dione, and 4,6-dihidroxy-2-methylthiopyrimidine was studied by NMR, UV, and IR spectroscopy, as well as by quantum chemistry. An equilibrium mixture of exo- and endo-enols in the neutral state and the presence of the same tautomers in monoanions were found. The energies of intramolecular hydrogen bonds in the gas phase and their changes in going to solutions in water, DMSO, and chloroform were calculated. The energies of intra- and intermolecular hydrogen bonds are close to each other. This fact suggests existence in solutions of two forms: tautomers with intramolecular H bond and H-bonded complexes with the solvents studied, whose formation involves cleavage of the intramolecular H bond. Characteristics of intermolecular hydrogen bonds are determined by the relative proton-donor and proton-acceptor powers of polyhydroxypyrimidines and solvents and are almost independent of the polarity of the medium.

Dimers of phenol in argon and neon matrices

The IR absorption spectra of phenol molecules in solid rare gas matrices of argon (10–12 K) and neon (4.5–5 K) are investigated at molar ratios of phenol:matrix of 1:1000 to 1:30 in the frequency range 400–4000 cm−1. Bands of dimers and larger complexes of phenol molecules are observed in the absorption spectrum of both matrices as the matrix ratio decreases. The first additional bands to appear in the spectral region of the stretching vibrations of the O–H group as the phenol concentration increases are two bands attributed to dimers with one and two hydrogen bonds. The absorption coefficients are determined for the bands of stretching vibrations of the O–H and C–O groups, O–H planar bending vibrations of monomers, and the stretching vibrations of the hydrogen-bonded O–H groups of the phenol molecules. The features of the formation of H-bonded complexes in low-temperature matrices are discussed. A model is proposed which permits calculation of the number of monomers, dimers, and larger complexes in argon and neon matrices for molecules which are close in size to the phenol molecule.

Quantum-chemical study of spectral and luminescent properties and photolysis of phenol and its complexes with water

The spectral and luminescent properties of phenol and the photoinduced O-H bond cleavage reaction, depending on complexation and excitation energy, were considered using the INDO method. It was shown that the formation of H-bonded complexes with water does not exert a noticeable effect on the spectral-luminescent properties of phenol. A decrease in the fluorescence quantum yield with increasing excitation energy is associated with the enhancement of the photoreaction rather than a change in the dynamics of intramolecular photophysical processes. The photolysis of phenol proceeds via the predissociation scheme. Complexation results in the enhancement of population of photodissociative singlet and triplet sates.

Spectral features of formation of H-bonded complexes in coumarins and psoralens

Spectral features of H-bonded complexes formed by molecules of coumarin 1 and psoralen with methanol (1∶1) have been studied by the methods of quantum chemistry [a semiempirical method with intermediate neglect of differential overlap (INDO) with special spectroscopic parametrization] and by the method of electrostatic potential (MEP). The MEP using INDO wave functions was used to find ways of approach to the methanol molecules and the structure of H-bonded complexes. The direction of spectral shifts of the different (with respect to the orbital nature) electronic excited states in the formation of the complexes and ways of approach to the solvent molecules in the formation of the first solvation shell are established.

IR Study of the Adsorption of Unsaturated Hydrocarbons on Highly Outgassed Silica: Spectroscopic and Thermodynamic Results

Fourier transform IR data are reported concerning the adsorption of ethyne, propyne, 1-butyne, 3-hexyne, benzene, and cyclohexene on Aerosil outgassed at 1073 K. With the three lightest acetylenic molecules, an ideal Langmuir-type interaction occurs. With the three six-carbon atom species, physisorption competes with the formation of H-bonded complexes, which are best studied at low coverage. From the intensity of the ν(OH) mode of the free hydroxyls as a function of pressure, the equilibrium constant for H-bond formation is calculated in different ways, according to the role of physisorption. Several correlations are drawn for Δν(OH) and ΔG°, based on the different basicities of the C−C triple bond as modulated by the alkyl substituents, and on the different C−C bond order in the other molecules. An estimate for ΔH° is made from Δν(OH) so that a fairly complete thermodynamic characterization of the H-bonding process is achieved. An explanation is proposed for the complex shape of the ν(OH) band in the case of 3-hexyne. The interplay between sheer H-bonding formation and dispersive contributions to the interaction is also discussed.

Ab initio calculation of isotope effects in IR and NMR spectroscopy of H-bonded systems

Abstract In this report recent computational developments are reviewed rather than novel H-bonded systems being presented. The well examined prototypes for neutral and ionic H-bonds, (H2O)2 and [H5O2]+, respectively, have been investigated anew by means of the MP2/6-311G(2df,p) level of ab initio theory, employing the program gaussian 92 . The complete sets of harmonic vibrational frequencies and the corresponding absolute IR intensities are presented. Most surprisingly, the main spectroscopic features of H-bonds, frequency decreases and IR intensity increases, can be understood quite well without consideration of anharmonicity at this level of theory. Frequency shifts and IR intensity decreases upon deuteration are also presented and discussed. Thermochemical relations are applied to obtain the enthalpies of formation of H-bonded complexes. NMR chemical shifts for 1H and 17O in the above H-bonded systems have been calculated ab initio by means of the Individual Gauche for Localized Orbitals (IGLO) method. The compositions of the chemical shift parameters are discussed in terms of dia- and paramagnetic contributions.

Excess molar heat capacities and excess molar volumes of binary mixtures of 2-bromo-2-chloro-1,1,1-trifluoroethane (halothane) with oxygenated and hydrocarbon solvents

Excess molar heat capacities C E p were determined at 298.15 K for the following thirteen mixtures: 2-bromo-2-chloro-1,1,1-trifluorethane (halothane)+acetone (ACT), +dipropyl either (DPE), +diisopropyl ether (DIPE), +methyl fert-butyl ether (MTBE), +tetrahydrofuran (THF), 1,4-dioxane (DIOX), +methyl acetate (MAC), +benzene (BENZ), +cyclohexane (cC6), +n-hexane (nC6), +2,2-dimethylbutane (22DMB), +n-heptane (nC7), and +2,4-dimethylpentane (24DMP). Excess molar volumes V E at 281.15 K were also measured for the following mixtures: halothane + DPE, +DIPE + DIOX, +BENZ, +cC6, +22DMB, +nC7, and +24DMP. V E values for the remaining five mixtures (ACT, MTBE, THF, MAC, and nC6) were previously reported. For all oxygenated solvents, C E p is positive and V E negative, while for the equal-structure hydrocarbon solvents (taken as homomorphs), they are of the opposite sign. This is interpreted in terms of the formation of H-bonded complexes between the oxygen atom in the solvent molecule and the highly acidic hydrogen atom in halothane. For benzene, C E p and V E are positive but small; this is consistent with the weak electron-donor character of benzene and indicates the presence of a weak halothane/BENZ complexation which most probably occurs through a charge transfer mechanism similar to that present in mixtures of fully halogenated frEons, such as Freon 113, with oxygenated solvents. The use of the homomorphs indicated that (i) for halothane + DPE, +DIPE, and +MTBE, the asymmetry of the C E p curves and their opposite-sign curvatures at low and high halothane concentrations are mainly due to the physical contributions to C E p , and (ii) for halothane + ACT, +MAC, and +DIOX, the solutions contain multisolvated species.