Cyclic Multimers Research Articles

Time domain reflectometry studies on benzaldehyde–ethanol binary solutions

Time domain reflectometry (TDR) study has been carried out on benzaldehyde (PhCHO) and ethanol (EtOH) binary solutions in the frequency range from 10 MHz to 25 GHz at different temperatures viz., 288.15, 293.15 and 298.15 K. The complex permittivity spectra have been fitted to the Debye function. Dielectric constant (ε0) vs mole fraction (X2, of EtOH ) graph shows that PhCHO-EtOH H-bond interactions are weaker in the solutions for which 0.0≤X2≤0.50 than in the solutions with 0.50<X2≤1. The relaxation time τ values suggest that lower order EtOH multimer involves in heteromolecular H-bond in PhCHO rich solutions. Antiparallel orientation of dipoles in pure PhCHO continues to exist upto X2 = 0.69. The smaller deviation of gf,εE/fB/ΔFE from the ideal behavior specify that the PhCHO interactions is weaker than EtOH-EtOH interactions. The well-known cyclic and linear dimers of PhCHO and cyclic multimers of EtOH is confirmed by geff and enthalpy values. The average number of hydrogen bonds have been analyzed using Luzar model calculations. EtOH multimers are found to be vulnerable on dilution by PhCHO. It appears that EtOH exhibits the same behavior in the solutions with aromatic carbonyl compounds.

Structure and hydrogen-bond properties of N-alkyl-N-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide and ethanol: A combination of FTIR and theoretical studies

Pyrrolidinium-based ionic liquids (ILs) demonstrate multiple irreplaceable features. One key to a better understanding of their unique properties is spectroscopic tools combined with density functional theory (DFT) calculations to study their structure features. In this regard, we investigated the structure features of four N-alkyl-N-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide ([CnMPyr] [Tf2N], n=3, 4, 6 and 8) ILs and ethanol mixtures using infrared spectroscopy (IR) and DFT calculations. Excess and two-dimensional correlation spectroscopy were employed to progressively enhance the infrared resolution and extract the structural information of the binary liquid systems from the O−D stretching vibrational region (v(O−D)) of the CD3CD2OD hydroxyl group. The mixing process is non-ideal, and different interaction complexes (the alcohol cyclic trimer/tetramer/larger multimers, anion−ethanol, ion pair−ethanol, and ion cluster−ethanol complexes) were identified. With increasing [CnMPyr] [Tf2N], the cyclic ethanol multimers were gradually broken apart, with the larger cyclic multimers vanishing first. For the hydrogen-bonded complex between IL and ethanol, the anion−ethanol complex appeared first, followed by the ion pair−ethanol complex. The ion cluster−ethanol appeared lastly in the mixture. The hydrogen-bonds with weak strength, closed shells and electrostatically dominant characteristics were found in different complexes.

The aggregation structure of a methanol/CHCl3 binary mixture investigated by polarized Raman spectroscopy and HNMR

The structural evolution of methanol in the protic solvent chloroform was studied by polarized Raman spectroscopy. Negative noncoincidence effects were detected for the full range of concentrations of the methanol/CHCl3 binary mixture, that is, ν⊥<ν∥ for the C-O stretching vibration. With the dilution of methanol/CHCl3 by CHCl3, the vibration frequency of the C-O bond moves to a lower wavenumber for both parallel ν∥ and vertical ν⊥ Raman spectra, contrary to the phenomenon observed for the methanol/CCl4 mixture. The absolute value of the frequency difference |ν⊥-ν∥| increased with volume fraction dilution from 1 to 0.3 and then sharply decreased from 0.3 to 0.03. Aggregation-induced split (AIS) theory was applied to explain these phenomena. PCM models of the most likely different structures of H-bond combined cyclic multimers, CHCl3 H-bonded multimers, and CHCl3 H-bonded monomers were established for different concentration stages. The DFT calculations based on these models of different concentrations can aptly reproduce the noncoincidence effect and concentration-dependent phenomenon.

Thermodynamics of mixtures with strongly negative deviations from Raoult's law. XVI. Permittivities and refractive indices for 1-alkanol + di-n-propylamine systems at (293.15–303.15) K. Application of the Kirkwood-Fröhlich model

Relative permittivities at 1 MHz, εr, and refractive indices at the sodium D-line, nD, are reported at 0.1 MPa and at (293.15–303.15) K for the binary systems 1-alkanol + di-n-propylamine (DPA). Their corresponding excess functions are calculated and correlated. For the methanol mixture, positive values of the excess permittivities, εrE, are found. Except at high concentrations of the alcohol in the 1-propanol mixture, the remaining systems show negative values of this property. This fact reveals that the creation of (1-alkanol)-DPA interactions contributes positively to εrE, being this contribution dominant in the methanol mixture. The negative contributions arising from the disruption of interactions between like molecules are prevalent in the other mixtures. At ϕ1 (volume fraction) = 0.5, εrE changes in the sequence: methanol > 1-propanol > 1-butanol > 1-pentanol < 1-heptanol. An analogous variation with the chain length of the 1-alkanol is observed in mixtures such as 1-alkanol + heptane, +cyclohexylamine or +n-hexylamine (HxA). Moreover, for a given 1-alkanol, εrE is larger for DPA than for HxA mixtures, suggesting that in DPA solutions multimers with parallel alignment of the molecular dipoles are favoured and cyclic multimers are disfavoured when compared to HxA mixtures. The (∂εr/∂T)p values are higher for the mixtures than for pure 1-alkanols, because (1-alkanol)-DPA interactions are stronger than those between 1-alkanol molecules. Calculations on molar refractions indicate that dispersive interactions in the systems under study increase with the chain length of the 1-alkanol and are practically identical to those in HxA solutions. The considered mixtures are treated by means of the Kirkwood-Fröhlich model, reporting the Kirkwood correlation factors and their excess values.

On similarity of hydrogen-bonded networks in liquid formamide and water as revealed in the static dielectric studies

The paper presents the experimental verification of the result obtained with the molecular dynamics simulation which revealed the differences in the topology of the hydrogen-bonded networks in liquid formamide and water, namely, the differences in their intermolecular cyclization process (I. Bakó, et al. J. Chem. Phys. 2010, 132, 014506). It is shown in our paper that the difference in the (simulated) size distribution of the hydrogen-bonded molecular rings in water (a relatively sharp maximum at about 6 molecules) and formamide (a broad maximum at about 11 molecules) strongly manifests itself in the experimental values of the Kirkwood correlation factor of the compounds. A much larger number of molecules included in the cyclic species (of more or less compensated dipole moment) leads to significant decrease of the Kirkwood correlation factor of formamide in comparison to that of water. Besides, as a consequence of an enhancement in formation of the cyclic multimers of formamide, one observes an essential reduction of the orientational entropy increment of that liquid, in comparison to the entropy effect related to liquid amides where the chain multimers are formed.

Depicting combinatorial complexity with the molecular interaction map notation

To help us understand how bioregulatory networks operate, we need a standard notation for diagrams analogous to electronic circuit diagrams. Such diagrams must surmount the difficulties posed by complex patterns of protein modifications and multiprotein complexes. To meet that challenge, we have designed the molecular interaction map (MIM) notation (http://discover.nci.nih.gov/mim/). Here we show the advantages of the MIM notation for three important types of diagrams: (1) explicit diagrams that define specific pathway models for computer simulation; (2) heuristic maps that organize the available information about molecular interactions and encompass the possible processes or pathways; and (3) diagrams of combinatorially complex models. We focus on signaling from the epidermal growth factor receptor family (EGFR, ErbB), a network that reflects the major challenges of representing in a compact manner the combinatorial complexity of multimolecular complexes. By comparing MIMs with other diagrams of this network that have recently been published, we show the utility of the MIM notation. These comparisons may help cell and systems biologists adopt a graphical language that is unambiguous and generally understood.

Orientational phasetransition between hexagonal solids in planar systems of hardcyclic pentamers and heptamers

Two-dimensional, hard-body systems of cyclic multimers(pentamers and heptamers) are studied by Monte Carlosimulations. Elastic properties (the elastic constants and thePoisson ratio) are computed for hexagonal solid phases withmolecular rotation by using an algorithm based on thestrain-fluctuation method. In both systems a minimum of thePoisson ratio is observed at the same density at which aqualitative change of the orientational singlet distributionfunction occurs. This result confirms that, in each of thestudied systems, a phase transition occurs between twoelastically isotropic solid phases exhibiting orientationaldisorder: a higher-density phase with (strongly) hinderedrotation and a lower-density phase with (almost) free rotation.The study also suggests that it is worth paying moreattention to the dependence of the Poisson ratio on thethermodynamic variables (volume, pressure or temperature) insolids because this quantity can play the role of a sensitiveindicator of (at least some) structural phase transitions.

Studies of Self-Association in Alcohols (ROH) as Functions of Concentrations and Temperature. Part II: Curve Resolution of Concentration Series

Self-association of six mono-hydroxyl aliphatic alcohols in CCl4 has been investigated with the use of infrared (IR) spectroscopy and methods for curve resolution. The alcohols were 1-octanol, 1-propanol, 2,4-dimethyl-3-pentanol, 2-methyl-2-propanol, 3-ethyl-3-pentanol, and 3-methyl-2-pentanol. Curve resolution of the OH stretching region assumes that distinct associates due to hydrogen bonding can be regarded as chemical species. The resolution aimed at determining the number, the structure, and the sizes of the associates in different concentration ranges. Analyses of the spectra, in limited concentration ranges, resulted in models containing up to three different species: monomers, open-chain multimers, and cyclic multimers. The size of the multimers was found to increase with increasing alcohol concentration. The resolved concentration profiles enable prediction of molar concentrations of the different alcohol species at all solute concentrations and estimation of the equilibrium constants involved. The molar extinction coefficients for all species at all wavenumbers can also be estimated.

Molecular association of pentanols in n-heptane IV: a photochromic reaction probe.

A photochromic reaction of a fulgide, Aberchrome 540 dye has been used to probe the molecular association of 1-,2-,and t-, pentanol in n-heptane. The kinetics of the photobleaching of the red dye (C-form) to the amber product (E-form) has been investigated spectrophotometrically over a low concentration range of < 0.3 mole fraction (F), at the temperature span of 303 to 323 K. Analyses of the results show that the photobleaching of the red dye follows a pseudo-first-order reaction, allowing for the extraction of the apparent rate constants (k) and the apparent activation energies (E a). Factors affecting the reaction are discussed. These factors are the solvent polarity effect, the solvent viscosity effect and the hydrogen bonding effect, deriving from the intermolecular hydrogen bond formation between the dye and the solvent molecules. The latter effect has been partially supported by the IR study of the dye in a number of solvents. Plots of E a against concentration (F) show extreme value for each pentanol. This result has been used to show the plausible presence of dominant multimers in the solutions. It appears that linear multimers are important for the 1-pentanol and 2-pentanol solutions. For the case of t-pentanol, E a of the reaction reaches a maximum at F=0.05. This is the same concentration at which the activation energy of viscous flow, E v was found to be maximum in the low concentration range. An explanation of the observed maximum E a is offered and it has led to the suggestion that the cyclic multimers may be the dominant species. Such a view is consistent with previous results from this laboratory i.e., the Kerr effect, viscometric and the NMR studies.

Growth Processes and Associative Properties in Alcohols by Dielectric and FTIR Spectroscopy

Abstract Dielectric permittivity and FTIR data of two isomeric alcohols, normal Pentanol (n-PeOH) and 2-Methyl-2-Butanol (2M-2BuOH) dissolved in an inert and apolar solvent, carbon tetrachloride, are presented. The extracted values of the Kirkwood correlation factor gk , from the two techniques, allow to explain the growth mechanism in these associated liquids starting from the monomeric state up to oligomers for increasing values of concentration. In particular, the Kirkwood factor plotted versus concentration exhibits, in the case of n-PeOH, a minimum lower than unity followed by a rise up to values larger than unity in the neat alcohol indicating that H-bond favours cyclic multimers at low concentrations and open multimers at high concentrations. In the case of 2M-2BuOH, instead, the gk behaviour suggests the persistence of both of cyclic and open dimers in all the concentration region.

Equation of state of planar hard molecular fluids; hard cyclic multimer system

A system of planar hard non-convex molecules consisting of hard discs, hard cyclic multimers (HCM), is studied in order to examine various semi-empirical approaches to the equation of state of a two-dimensional hard molecular fluid. Compressibility factors of the HCM fluid are obtained at different densities and molecular anisotropies by the Monte Carlo simulations. Various geometrical characteristics (molecular volumes and surfaces, interaction surfaces, and the second virial coefficients) are calculated. A one-parameter equation of state based on the modified scaled particle theory is proposed. It represents very well the simulation results over the entire density range even for fairly anisotropic molecules.

Excess heat capacities of alkan-1-ol–n-alkane systems at low alcohol concentrations. Description in terms of association

The association model proposed formerly has been used together with the other models to predict to excess heat capacities of alkan-1-ol–n-alkane systems for concentrations up to 0.02 mole fraction of the alcohol. All of the model parameters (i.e. standard entropies and standard enthalpies of association) have been estimated from the spectroscopic data of pure ethanol. The prediction accuracy of the proposed model of successive association taking into account both linear and cyclic multimers is fairly good, slightly but distinctly higher than that of the monomer–tetramer model. It disagrees with the conception of predominance of tetramers in the solutions studied as was suggested by Costas and Patterson. The other models give quite unacceptable results.

Open and cyclic multimers in alcohol solutions. A quantitative model for the association and the permittivity

When plotted versus concentration in an inert solvent, the dielectric Kirkwood g factor of most normal alcohols exhibits a minimum lower than unity followed by a rise up to values larger than unity in the neat alcohol. This behaviour is interpreted by the fact that H-bond association favours cyclic multimers at low concentrations and open multimers at high concentrations. A quantitative model using chain statistics and predicting this behaviour is summarized. A comparison is made between the results of this model and literature data on n-hexan-1-ol diluted in cyclohexane at 25 and 55°C. From the fitted values of the dissociation constants at these two temperatures it is deduced that the enhalpy for the destruction-formation of a hydrogen bond in this alcohol is about 20 kJ moL −1.

Association of ethanol in inert solvents

The previously derived association model is extended to account for cyclic multimers. If applied for ethanol + aliphatic hydrocarbon systems at low alcohol concentrations it predicts spectroscopic and calorimetric properties without any adjustable parameters better than other approaches do. It also provides more reliable values of the chemical contribution to the heats of mixing for the whole composition range.

HmE of (an n-alkane + a butanol isomer)

Excess molar enthalpies of (an n-alkane + a butanol isomer) were measured at 298.15 and 318.15 K. The peculiar shapes of the curves, especially for mixtures containing 2-methylpropan-2-ol, might be put down to the formation of cyclic multimers. The temperature coefficients of H m E are always positive and they increase as the chain-length of the n-alkane increases.

Molecular association of pentanols in n-heptane II: Viscosities as a function of temperature covering low concentration range

Abstract Viscosities of binary mixtures of isomeric pentanols in n-heptane at various mole fractions, F (F = 0.025 to 0.600) have been measured in the low temperature range (T = 233 to 293 K) with an automatic viscometer operating in a closed system. The temperature dependence of viscosity has been found to follow a simple relation, thereby allowing for the calculation of Ev, the activation energy for viscous flow. The latter is very sensitive to changes in molecular associates, especially at low concentrations. The results show that at low concentrations small multimers dominate for both 1-pentanol and 2-pentanol. At high concentrations, while the former may associate to give large linear multimers; the latter may result in the formation of large cyclic multimers with a low dipole moment. For 3-pentanol and tert-pentanol, small multimers are likely to dominate at low concentrations and there is a change in the dominating multimers from a linear type to a cyclic one, with increasing concentration in the limit F

Molecular association of pentanols in n-heptane I: Temperature dependence of Kerr effect

Electro-optic Kerr effect of various isomers of pentanols in n-heptane over a wide concentration range (0.3 to 1.0 mole fraction) has been measured from room temperature down to 202 K at a wavelength of 632.8 nm. The results show that for 1-pentanol, linear multimers consisting of small number of monomers are formed at low concentrations, and the chain lengths increase rapidly with increasing concentration. Tertiary-pentanol, on the other hand, tends to be dominated by cyclic multimers of a low dipole moment at low concentrations, as the concentration increases cyclic multimers still dominate but the number of monomers per multimers has increased. For secondary alcohols such as 2-pentanol and 3-pentanol, the formation of multimers appears to fall in between those of primary and tertiary alcohols. These findings are in general agreement with the results of the Kirkwood g-factor and the recent 13C NMR relaxation of related alcohols. Kerr effect has been demonstrated to be particularly sensitive to and useful for investigating molecular association in alcohols.

HmE and VmE of some (1-chlorobutane + alkanol or cyclohexanol) mixtures

Excess molar enthalpies and excess molar volumes of (1-chlorobutane + an alkanol or cyclohexanol) at 298.15 K (also 308.15 K in some case), were measured. The results were “explained” in terms of interactions involving the chlorine atom. Volumetric behaviour of mixtures containing 2-methylpropan-2-ol is analysed by taking into account the high proportion of cyclic structures in this alcohol and the conversion of linear into cyclic multimers when the mixture is formed.

Study of self-association of t-butyl alcohol by linear and non-linear dielectric effects

A method for the simultaneous determination of three independent values of the free energy of self-association from experimental dielectric data is proposed. This involves the determination of ΔG, ΔH and ΔS for linear bond formation in open multimers, for bond formation in cyclic trimers and in higher cyclic multimers of t-butanol. The proposed method allows one to evaluate the relative concentrations of open and cyclic structures with great accuracy. Moreover, it can determine which of the possible models of association describes the experimental system being tested.

Dilute solutions of nylon 12—III. Intermolecular and intramolecular association of end-groups

An equation has been derived relating the Huggins viscometric coefficient, k H , and molecular weight of polymers which in a given solvent associate via end-group interactions forming linear and cyclic multimers. The equation was used to calculate the association constant ( K a = 2.2 × 10 2 mol/l) for nylon 12 from experimental data. Values of the constant of intramolecular cyclization of unimers were estimated; their dependence on polymer molecular weight was less steep than suggested by the theory of macrocyclic equilibria.