Enthalpy Of Hydrogen Bond Formation Research Articles

Liquid pharmaceuticals formulation by eutectic formation

The amphiphilic nature of many pharmaceutical active ingredients often makes them difficult to solubilise and leads to significant wastage through non-optimal dosage. In this study it is shown that highly concentrated liquid formulations can be produced from pharmaceutical active ingredients which either contain a strong hydrogen bonding functionality e.g. -OH or -COOH or a quaternary ammonium moiety. These mixtures can overcome solubility issues in water as the eutectics prevent recrystallization of the active ingredient when dispersed in water. The depression of freezing point for these eutectic mixtures is modelled using the enthalpy of hydrogen bond formation which was calculated using calorimetric data. The study also demonstrates that complex drug molecules which exhibit polymorphism such as Adiphenine and Ranitidine can be formulated into a homogeneous liquid and the hydrogen bond donor can also be a pharmaceutical active ingredient e.g. aspirin.

Materials produced from plant biomass: part III: degradation kinetics and hydrogen bonding in lignin

In this study Klason lignins from Eucalyptus grandis (LEUG) and Pinus taeda (LPIT) were characterized using Fourier transform infrared (FTIR) spectroscopy and thermogravimetry (TGA). The degradation kinetic parameters were determined by TGA using the Kissinger method. Thermogravimetric results showed that LPIT had higher thermal stability and also higher activation energy than LEUG. FTIR results showed that the average strength of intermolecular interactions and enthalpy of hydrogen bond formation among the phenolic groups are higher for LPIT than for LEUG. The results demonstrated that differences between thermal stability and degradation mechanisms might be associated with differences in hydrogen bonding in lignin.

Fundamental and overtone vibrational spectroscopy, enthalpy of hydrogen bond formation and equilibrium constant determination of the methanol–dimethylamine complex

We have measured gas phase vibrational spectra of the bimolecular complex formed between methanol (MeOH) and dimethylamine (DMA) up to about 9800 cm(-1). In addition to the strong fundamental OH-stretching transition we have also detected the weak second overtone NH-stretching transition. The spectra of the complex are obtained by spectral subtraction of the monomer spectra from spectra recorded for the mixture. For comparison, we also measured the fundamental OH-stretching transition in the bimolecular complex between MeOH and trimethylamine (TMA). The enthalpies of hydrogen bond formation (ΔH) for the MeOH-DMA and MeOH-TMA complexes have been determined by measurements of the fundamental OH-stretching transition in the temperature range from 298 to 358 K. The enthalpy of formation is found to be -35.8 ± 3.9 and -38.2 ± 3.3 kJ mol(-1) for MeOH-DMA and MeOH-TMA, respectively, in the 298 to 358 K region. The equilibrium constant (Kp) for the formation of the MeOH-DMA complex has been determined from the measured and calculated transition intensities of the OH-stretching fundamental transition and the NH-stretching second overtone transition. The transition intensities were calculated using an anharmonic oscillator local mode model with dipole moment and potential energy curves calculated using explicitly correlated coupled cluster methods. The equilibrium constant for formation of the MeOH-DMA complex was determined to be 0.2 ± 0.1 atm(-1), corresponding to a ΔG value of about 4.0 kJ mol(-1).

Spectroscopic and Thermodynamic Evidence of Dimer and Trimer Hydrogen Bonded Complex Formation between Chloroform and 2-Butanone. Excess Molar Enthalpy for the Chloroform + 2-Butanone Binary System at 303 K

FT-Raman and FT-infrared spectra of pure chloroform (A) and 2-butanone (B), as well as of the binary system chloroform + 2-butanone, were recorded to investigate the type and nature of the intermolecular complexes formed when both chemicals are mixed. The optimized structures and vibrational frequencies for 2-butanone, chloroform, and their 1:1 and 1:2 complexes were calculated by means of density functional theory (DFT) techniques using the B3LYP functional combined with the 6-31G(d,p) and 6-311++G(d,p) basis set. The recorded FTIR and Raman spectra confirm the existence of these types of hydrogen-bonded complexes, making it possible, furthermore, to calculate the heteroassociation constants. Heat of mixing at 303 K over the whole mole fraction range at atmospheric pressure was also measured. The excess molar enthalpy was fitted to a Redlich-Kister-type equation, using least-squares to obtain its dependence on concentration. The ideal associated solution model was also used to calculate these equilibrium constants among the chemical species in solution, which compare well with that calculated with the spectral determinations and the enthalpy of hydrogen bond formation. Furthermore, the McGlashan-Rastogi linearization test was also used to provide thermodynamic evidence about the stoichiometry of the formed complexes.

Enthalpy of solvation correlations for organic solutes and gases dissolved in dichloromethane and 1,4-dioxane

Enthalpies of solution at infinite dilution of 53 organic solutes in dichloromethane and 10 organic solutes in 1,4-dioxane were measured using semi-adiabatic solution calorimeter. Enthalpies of solvation for 103 organic vapors and gaseous solutes in dichloromethane and for 116 gaseous compounds in 1,4-dioxane were determined from the experimental and literature data. It is shown that an Abraham solvation equation with five descriptors can be used to correlate the experimental solvation enthalpies within standard deviations of 2.07 and 2.29 kJ mol−1 for dichloromethane and 1,4-dioxane, respectively. The derived correlations provide very accurate mathematical descriptions of the measured enthalpy of solvation data at 298 K, which in the case of 1,4-dioxane span a range of 121 kJ mol−1. Division of the experimental values into a training set and a test set shows that there is no bias in predictions, and that the predictive capability of the correlations is better than 3.5 kJ mol−1. Enthalpies of hydrogen bond formation of proton donor solutes (alcohols, amines, chlorinated hydrocarbons, etc.) with 1,4-dioxane were calculated based on the Abraham solvation equation. Obtained values are in good agreement with the available literature data.

Fourier Transform Infrared Spectroscopy and Theoretical Study of Dimethylamine Dimer in the Gas Phase

Dimethylamine (DMA) has been studied by gas-phase Fourier transform infrared (FTIR) spectroscopy. We have identified a spectral transition that is assigned to the DMA dimer. The IR spectra of the dimer in the gas phase are obtained by spectral subtraction of spectra recorded at different pressures. The enthalpy of hydrogen bond formation was obtained for the DMA dimer by temperature-dependence measurements. We complement the experimental results with ab initio and anharmonic local mode model calculations of monomer and dimer. Compared to the monomer, our calculations show that in the dimer the N-H bond is elongated, and the NH-stretching fundamental shifts to a lower wavenumber. More importantly, the weak NH-stretching fundamental transition has a pronounced intensity increase upon complexation. However, the first NH-stretching overtone transition is not favored by the same intensity enhancement, and we do not observe the first NH-stretching overtone of the dimer. On the basis of the measured and calculated intensity of the NH-stretching transition of the dimer, the equilibrium constant for dimerization at room temperature was determined.

Hydrogen-bond basicity of push–pull α,β-unsaturated enaminoketones

A method is proposed for evaluating the hydrogen-bond basicity of certain conformers of two push–pull enaminoketones with the general formula R C(O) CH CH N(CH 3) 2, where R=CH 3 (DMBN); R=CF 3 (DMTBN). It has been shown, for both enaminoketones, that the ( EE) conformer has a higher basicity than the ( EZ) conformer. Moreover, the (DMBN) has one of the highest general p K HB values in the hydrogen-bond basicity scale of ketones. Substitution of the CH 3-group in (DMBN) with the CF 3-group in (DMTBN) reduces general p K HB sharply as a consequence of electron withdrawal of CF 3-group. Hydrogen bond sites are also discussed; it is shown that, in both enaminoketones, carbonyl oxygen possesses the maximal basicity, whereas nitrogen of the dimethylamino group has less basicity, and the vinyl moiety is the least basic site. Enthalpies of hydrogen bond formation (−Δ H) in absolute values, as well as the respective p K HB values, are greater for the ( EE) conformer than for the ( EZ) as a consequence of greater contribution of resonance structure in the ( EE) form. ▪

Hydrogen bonding to divalent sulfur

A combination of vapor phase infrared spectroscopy and ab initio calculations has been used to show that sulfur is weaker than, but nearly equivalent to, oxygen as a hydrogen bond acceptor. Enthalpies of hydrogen bond formation were obtained for the hydrogen bonded complexes formed between methanol and either dimethyl sulfide or dimethyl ether by temperature dependence studies of the infrared spectra.

Kinetics of protonation of tungsten hydrides WH(CO)2(NO)L2 by weak OH-acids

The kinetics of protonation of tungsten hydrides WH(CO)2(NO)L2 (1, L = PMe3, PEt3, P(OPri)3, PPh3) by weak OH-acids (PhOH, (CF3)2CHOH, (CF3)3COH) in hexane was studied by IR spectroscopy. The study of the reactions of compounds 1 with OH-acids at 190–270 K revealed that the first step involves the formation of dihydrogen-bonded W(CO)2(NO)L2(H)...HOR complexes. When the temperature increases to ambient, the proton transfer and evolution of molecular hydrogen occur, affording the final products: organyloxy derivatives W(OR)(CO)2(NO)L2. The study of the kinetics at 298 K found that the proton transfer is the rate-determining step. The rate constants kapp are 2.2·10−5–6.3·10−4 s−1, and the free activation energies are ΔG≠298K = 22–23 kcal mol−1. The rate constants depend on the proton-accepting properties of the hydride and the acidic properties of the OH-proton donor and increase in the same order as the enthalpy of hydrogen bond formation.

Interpolymer‐specific interactions and miscibility in poly(styrene‐co‐acrylic acid)/poly(styrene‐co‐N,N‐dimethylacrylamide) blends

AbstractThe miscibility or complexation of poly(styrene‐co‐acrylic acid) containing 27 mol % of acrylic acid (SAA‐27) and poly(styrene‐co‐N,N‐dimethylacrylamide) containing 17 or 32 mol % of N,N‐dimethylacrylamide (SAD‐17, SAD‐32) or poly(N,N‐dimethylacrylamide) (PDMA) were investigated by different techniques. The differential scanning calorimetry (DSC) analysis showed that a single glass‐transition temperature was observed for all the mixtures prepared from tetrahydrofuran (THF) or butan‐2‐one. This is an evidence of their miscibility or complexation over the entire composition range. As the content of the basic constituent increases as within SAA‐27/SAD‐32 and SAA‐27/PDMA, higher number of specific interpolymer interactins occurred and led to the formation of interpolymer complexes in butan‐2‐one. The qualitative Fourier transform infrared (FTIR) spectroscopy study carried out for SAA‐27/SAD‐17 blends revealed that hydrogen bonding occurred between the hydroxyl groups of SAA‐27 and the carbonyl amide of SAD‐17. Quantitative analysis carried out in the 160–210°C temperature range for the SAA‐27 copolymer and its blends of different ratios using the Painter–Coleman association model led to the estimation of the equilibrium constants K2, KA and the enthalpies of hydrogen bond formation. These blends are miscible even at 180°C as confirmed from the negative values of the total free energy of mixing ΔGM over the entire blend composition. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1011–1024, 2007

Hydrogen-bonding between a dichroic dye and a liquid crystal-forming molecule, for application to LCDs

The performance of guest–host type liquid crystal displays (LCDs) is governed by the solubility and dissolution stability of dichroic dyes in the liquid crystal medium in guest–host kind of LCDs. The solubility and stability is influenced by hydrogen-bonding between the dye and the liquid crystal host. An earlier work was carried out to study the hydrogen-bonding behavior of 4-pentyl-4′-cyanobiphenyl (5CB) (liquid crystal), benzonitrile, and acetonitrile, with 1-hexanol in CCl4 using FTIR spectroscopy, ab initio and equation of state calculations [Ind. Eng. Chem. Res. 42 (2003) 1129]. To gain further thermodynamic understanding, hydrogen-bonding behavior of 5CB (liquid crystal), benzonitrile, and acetonitrile, with a dichroic dye Disperse Red 1 (DR1) {2-[ethyl[4-[(4-nitrophenyl)azo]phenyl]amino]ethanol} in CCl4 were studied in this work. In all three mixtures, -OH acts as a hydrogen-bond donor and -CN acts as hydrogen-bond acceptor. Based on FTIR analysis for DR1/acetonitrile, DR1/benzonitrile, and DR1/5CB, the enthalpies of hydrogen-bond formation are −24.2, −23.7, and −3.7kJ/mol, respectively; and the entropies of hydrogen-bond formation are −58.9, −58.1, and −0.4J/(molK), respectively. Though, for DR1/5CB, enthalpy of H-bonding is not too negative, the entropy of H-bonding is more favorable; hence the H-bonding appears to be entropically driven. Lattice fluid hydrogen-bonding (LFHB) parameters obtained from the spectroscopic data were observed to match well with the experimental trends. LFHB equation of state was then used to predict hydrogen-bonding for a wider range of conditions.

Hydrogen-Bonding Effects in Liquid Crystals for Application to LCDs

Hydrogen bonding plays an important role in the performance of liquid crystal displays (LCDs) by controlling the solubility and dissolution stability of dichroic dyes in the liquid crystal medium, for guest-host type LCDs. The hydrogen-bonding behaviors of 4-pentyl-4‘-cyanobiphenyl (5CB) (liquid crystal), benzonitrile, and acetonitrile, with 1-hexanol in CCl4 were studied using FTIR spectroscopy, equations of state, and ab initio calculations. In all three mixtures, the −OH group acts as a hydrogen-bond donor, and the −CN group acts as a hydrogen-bond acceptor. On the basis of FTIR analysis, the enthalpy of hydrogen-bond formation is observed to be −14.7 kJ/mol for 1-hexanol/acetonitrile, −5.4 kJ/mol for 1-hexanol/benzonitrile, and −4.6 kJ/mol for 1-hexanol/5CB. The entropy of hydrogen-bond formation is more favorable for 1-hexanol/5CB than for 1-hexanol/acetonitrile or 1-hexanol/benzonitrile. Trends in the spectroscopic results agree with ab initio calculations. Lattice fluid hydrogen-bonding parameters ...

Infrared Study of Hydrogen Bond Types in Asphaltenes

The structural characteristics of asphaltenes are important to refiners for determining the yields from the residual fraction of crude oils and the operating parameters of the process units for deep conversion processes. In this article, we studied the hydrogen bonding distribution of four Saudi Arabian crude-oil-derived asphaltenes against the phenol solutions of various concentrations in carbon tetrachloride. In an attempt to explain relative hydrogen bonding capabilities of asphaltenes, OH peak intensity of phenol and enthalpies of hydrogen bond formation were used in combination with the content of heteroatoms (N, O, and S) in asphaltenes. The OH peak intensity of phenol depends on the presence of acidic and basic moieties or functional groups on the surface of asphaltenes. The results obtained show that the asphaltenes with high oxygen and low nitrogen contents have poor interaction with phenol, which indicates that oxygen might be incorporated as acidic hydroxyl groups in asphaltenes. In a similar way, asphaltenes with low oxygen and high nitrogen contents give high phenol interaction values. Piperidine was also tried to evaluate the hydrogen bonding capabilities of NH group with the asphaltenes. The trace metal and GPC molecular weight measurements showed that the Arab heavy asphaltenes possess highest molecular weights and maximum V and Ni contents.

A counterintuitive observation concerning hydrogen bonding in polymer blends

It has been determined from an infrared analysis of miscible blends of n-butyl methacrylate-co-4-vinylphenol and styrene-co-2-vinyl pyridine copolymers that the fraction of hydrogen-bonded carbonyl groups actually increases with temperature. At first glance, this is counterintuitive, but it is entirely consistent behaviour for a multicomponent hydrogen-bonding system containing competing acceptor groups that have disparate enthalpies of hydrogen bond formation.

FTIR studies of hydrogen bonding between α,β-unsaturated esters and alcohols

The enthalpy (and entropy) of hydrogen bond formation has been measured between the ester carbonyl groups of the two α,β-unsaturated esters thienylacryloyl (TAOMe) and 5-methylthienylacryloyl (5MeTAOMe) methyl ester and the hydrogen bond donors ethanol, phenol and 3,5-dichlorophenol in CCl 4. For the esters, the hydrogen bonding strengths were measured by quantitating the amount of bound and unbound donor, using the OH stretching region, as a function of temperature and applying the van't Hoff equation. The decrease in ν CO of the ester carbonyl group upon hydrogen bond formation Δν CO has also been measured and correlated with the enthalpy of hydrogen bond formation. A linear correlation is observed between the enthalpy of hydrogen bond formation − ΔH and Δν CO, with − ΔH = 1.36 Δν CO − 16.1, where ΔH is measured in kJ mol −1 and Δν in cm −1. Comparison with data for other carbonyl acceptor compounds indicates that the carbonyl group of the above α,β-unsaturated esters is more readily polarized than the carbonyl group of saturated esters or ketones. The quantitative relationship between − ΔH and Δν CO derived here has been used to determine the change in the enthalpy of hydrogen bond formation between substrate and enzyme groups in a series of acylserine proteases.

Spectroscopic Characterization of Hydrogen Bonding in Poly(urethane−rotaxane)s

The hydrogen-bonding interactions in a series of poly(urethane−rotaxane)s were studied by infrared spectroscopy. Spectroscopic data along with a one-dimensional mean-field model was used to calculate the enthalpy of hydrogen bond formation involving a polyurethane N-H moiety and an ether oxygen atom located in a macrocycle threaded along the polyurethane backbone. The average enthalpy of hydrogen bonding was estimated at 3.0 kcal/mol.

Association Equilibria and Miscibility Prediction in Blends of Poly(vinylphenol) with Poly(hydroxybutyrate) and Related Homo- and Copolymers: An FTIR Study

Blends composed of poly(vinylphenol) (PVPh) and bacterial (tactic) poly(hydroxybutyrate) (PHB) and synthetic (atactic) poly(hydroxybutyrate) (PHBa) have been investigated by FTIR. From quantitative analyses of hydrogen bonded PHB carbonyl groups as a function of temperature and composition, the interassociation constant, KA, describing the formation of hydroxyl−carbonyl interaction as well as the related enthalpy of hydrogen bond formation have been determined. Using the association model of Painter and Coleman, phase diagrams have been predicted for blends of PVPh with the poly(hydroxyalkanoate) family.

Enthalpy of hydrogen bond formation in a protein-ligand binding reaction.

Parallel measurements of the thermodynamics (free-energy, enthalpy, entropy and heat-capacity changes) of ligand binding to FK506 binding protein (FKBP-12) in H2O and D2O have been performed in an effort to probe the energetic contributions of single protein-ligand hydrogen bonds formed in the binding reactions. Changing tyrosine-82 to phenylalanine in FKBP-12 abolishes protein-ligand hydrogen bond interactions in the FKBP-12 complexes with tacrolimus or rapamycin and leads to a large apparent enthalpic stabilization of binding in both H2O and D2O. High-resolution crystallographic analysis reveals that two water molecules bound to the tyrosine-82 hydroxyl group in unliganded FKBP-12 are displaced upon formation of the protein-ligand complexes. A thermodynamic analysis is presented that suggests that the removal of polar atoms from water contributes a highly unfavorable enthalpy change to the formation of C=O...HO hydrogen bonds as they occur in the processes of protein folding and ligand binding. Despite the less favorable enthalpy change, the entropic advantage of displacing two water molecules upon binding leads to a slightly more favorable free-energy change of binding in the reactions with wild-type FKBP-12.

Hydrogen bond formation to cyclic and acyclic polyethers

Equilibrium constants and enthalpies of hydrogen-bond formation of mcresol to various cyclic (crown) and acyclic polyethers have been determined in benzene solvent. Equilibrium constants indicated no evidence for an operative macrocyclic effect; the relationship between the increasing size of the equilibrium constant and the number of ether oxygens was rationalized with a simple statistical thermodynamic model. Enthalpies of interactions ranged between −19 and −23 kJ-mol−1. In agreement with PCILO calculations, enthalpies of interaction were essentially independent of the number of oxygen atoms in the ether; no significant difference in enthalpies of interaction between cyclic and acyclic ethers was found.

Studies of the phase behaviour of poly(vinyl phenol)-poly(n-alkyl methacrylate) blends

The results of theoretical and experimental studies of poly(4-vinyl phenol) (PVPh) blends with a series of poly(n-alkyl methacrylates) are presented. PVPh is miscible with poly(methyl methacrylate), poly(ethyl methacrylate) and poly(n-propyl methacrylate) over the entire composition range at temperatures between ambient and 200°C. In contrast, PVPh—poly(n-butyl methacrylate) blends phase separate at temperatures below 200°C. Quantitative analyses of the fraction of hydrogen bonded carbonyl groups were obtained by FTi.r. spectroscopy for all the PVPh blends—poly(n-alkyl methacrylates) as a function of composition and temperature. The data obtained from the miscible blend systems was employed to determine values of the equilibrium constant describing inter-association, K A, and the enthalpy of hydrogen bond formation, h A. Theoretical fractions of hydrogen bonded carbonyl groups and spinodal phase diagrams were calculated using a recently reported association model. The results compare very favourably to experiment.