Thermodynamic parameters of conformational equilibrium in 1,2-dichloroethane: influence of medium, benzene and compensation effects

The conformational equilibrium of 1,2-dichloroethane (DCE) in 19 different media (vapour phase, liquid, solutions in n-hexane, n-heptane, carbon tetrachloride, benzene, toluene, ethyl benzene (EB), carbon disulphide, diethyl ether, bromoform, chloroform, dichloromethane, pyridine, acetone, methanol-d3, acetonitrile, glassy polystyrene (PS) matrix and 1:1 mixture of PS and EB) has been studied by IR absorption spectra. The enthalpy differences (ΔH0) between trans (t) and gauche (g) conformers were determined from the dependencies of 1n(It/Ig) upon T-1, where I1 and Ig are the integrated intensities of the bands belonging to trans and gauche conformers, respectively. The values of RT ln(It/Ig) and (R ln(It/Ig) + Δ H0/T) obtained at 296 K were used as measures of the free enthalpy (ΔG0) and entropy (ΔSD) differences of the conformers respectively, when considering their changes with solvent. Good correlations between ΔG0, ΔH0 and the function of the dielectric permittivity of the medium (0.5 - (ε - 1)/(2ε + 1)) 1 2 were observed for all solvents except methanol-d3 (effect of self-association) and the aromatic solvents (benzene effect). Notwithstanding the similar structures and dielectric permittivities of EB and PS, the former medium behaves as a significantly more polar solvent. The analysis of the ΔH0 values obtained for DCE in PS and EB enabled confirmation of Eliel and Hofer's concept of the origin of the benzene effect; the effect is assigned to the relatively high anisotropy of the polarizability tensor of aromatic compounds. As a consequence, the electrostatic and dispersion interactions of a solute dipole with a neighbouring aromatic ring is crucial to their mutual orientation. Significantly non-linear (close to quadratic) dependencies ln(It/Ig) = f(T-1) were observed for DCE in toluene and EB in the range 180-370 K: the ΔH0 value increased from -170 cal mol-1 at 358 K to 820 cal mol-1 at 190 K for the toluene solution. These results are considered within the framework of the manifestations of the benzene effect. Significant changes in ΔS0 with the media have been found; they cover more than 2.0 cal mol-1 K-1 when going from the vapour phase to polar solvents. Satisfactory correlation between ΔH0 and ΔS0 (compensation effect) has been observed, the points corresponding to all the media, including methanol-d4 and the aromatic solvents, being scattered along a straight line. The tangent of the slope (ΔΔS0/ΔΔHD) = (1.4 ± 0.4) × 10-3 K-1 is close to those determined earlier for 1,2-bromofluoroethane, trans-1,2-dichlorocyclohexane and o-iodophenol. © 1995

Conformational mobility of small molecules in glass-forming solutions studied by FTIR spectroscopy

Small molecules with two or more stable conformations when embedded in a glass-forming liquid (matrix) serve the role of "conformational probes", i.e., their conformational transitions are used to follow local mobility in the matrix. In the present study, conformational probes were embedded in low-molecular-weight glass-forming liquids, and the molecular mobility was studied in a broad temperature range including the glass transition temperature (T g). Paraffin oil, dibutylphthalate, bis(2-ethylhexyl)phthalate and isopropylbenzene were used as glass-forming liquids while 1,2-dichloroethane, 1,2-diphenylethane, chlorocyclohexane and bromocyclohexane were used as conformational probes. For some of the matrix/probe systems, the conformational mobility was found to freeze-in at T g, while for the others it froze-in at certain temperatures T f < T g. In the latter case, it was possible to evaluate the activation energies related to mobility of free volume entities in the glassy matrices. In addition, it was possible to estimate the volumes of sub-molecular groups of the matrix that freeze-in at T f. © 2012 Elsevier B.V. All rights reserved

Compensation effect in the thermodynamics of conformational equilibria

The solvent dependence of thermodynamic parameters of conformational equilibria in trans-1,2-dichlorocyclohexane and trans-1,2-bromochlorocyclohexane was investigated by infrared absorption spectra. The results obtained show the existence of a compensation effect in the thermodynamics of conformational equilibria: the enthalpy (ΔH0) and entropy (ΔS0) differences change in the same direction when going from one solvent to another. A semi-quantitative estimation of the effect is given on the basis of the equations of statistical thermodynamics. It is shown that the temperature dependence of the ΔS0 value must be taken into account when determining the enthalpy difference of the conformers. This yields the equality of the true and observed ΔH0 values. © 1990

The determination of the parameters of conformational equilibria by infrared absorption spectra

New methods of determining the entropy difference of two conformations, the activation energy of the conformational transitions, of free enthalpy difference and the ratio of integral absorption coefficients of two conformations by i.r. absorption spectra are proposed. The basis of the method of ΔSo determination is the study of the i.r. spectra of compounds both in the temperature range where there are conformational transitions and at the temperatures were they do not occur. For the determination of E≠ ΔGo and αA/αB it is proposed to investigate the kinetics of the conformational transitions in liquid solutions. The methods have been applied to chloro-, bromo- and trans-dichlorocyclohexanes. © 1985

Infrared spectra and spinning diffusion of methyl bromide in solutions

Infrared absorption spectra of methyl bromide (CH3Br) dissolved in carbon tetrachloride (CCl4) and carbon disulfide have being studied in wide temperature ranges. IR absorption in the regions of degenerate (E-type) bands belonging to CH3-stretching (ν4) and deformational (ν5, ν6) vibrations were fitted by the sum of Cauchy-Gauss components. Each E-type band was reproduced by the two components: the narrower (n) and the broader (b) one. The narrower components of the bands belonging to deformational CH3-vibrations were interpreted within the framework of the orientational diffusion mechanism. The broader components of these bands were attributed to the unresolved gas-like vibration-rotational absorption of the molecules. The different temperature behaviour of the components has been found: the integrated intensities of the narrower components (In) decrease with temperature, while the intensities of the broader ones (Ib) increase. The enthalpy differences between the molecules absorbing via two different mechanisms ΔH) were determined from the dependencies of ln(In/Ib) upon T-1: 0.87 ±0.28 (ν5) and 0.65 ±0.10 kcal mol-1 ν6). These values are close to those determined previously for CH3I and CD3I. The narrower components' band widths were used for evaluating the spinning diffusion constants for CH3Br in solutions. © 1997 Elsevier Science B.V

Temperature and solvent effects on the infrared e-type bands of methyl iodide: orientational diffusion and free rotation

Infrared absorption spectra of liquid methyl iodide (CH3I) and its solutions in CCl4, CS2, heptane, benzene, chloroform and deuterated acetone-d6 have been studied. Infrared spectra in the regions 3400-2700, 1700-1100 and 1000-750 cm-1 were fitted by the sum of components, with the form of multiplication of the Lorentzian and Gaussian functions. E-Type bands under investigation (v4 = 3047, v5 = 1428, and v6 = 885 cm-1) were reproduced by the sums of two components: the narrower (n) and the broader (b) ones. A different temperature behaviour of the components has been found: the integrated intensity of the narrower component (In) decreases with the temperature, while the intensity of the broader one (Ib) increases. The narrower components of v5 and v6 were attributed to CH3l molecules moving according to the orientational diffusion mechanism; the broader ones were attributed to molecules, freely rotating about the C3v axis. Some additional mechanism (probably the interactions between CH stretching vibrations with single particle and collective motions of molecular dipoles) was proposed to play a part in forming the v4 bandshape. The enthalpy difference between freely rotating molecules and those moving via an orientational diffusion mechanism (ΔH) have been determined by the slopes of the dependencies of ln(In/Ib) upon T-1: ΔH = 0.8 ± 0.1 kcal mol-1. The temperature behaviour of δn has been studied in the 210-340 K temperature range, and Rakov's approach has been used to determine the activation enthalpy (ΔH*) and entropy (ΔS*) of parallel orientational diffusion in the pure liquid: ΔS* = -4.5 ± 0.2 cal mol-1 K-1, ΔH* = 0.1 ± 0.1 kcal mol-1. The CH3 stretching range was found to be strongly affected by a solvent. Total integral absorption coefficients of v1 and v4 bands increase two-fold when going from CCl4 to acetone-d6 solution, while δn values decrease by 3-9 cm-1. The observed effects were explained in terms of the existence of complexes with weak ICH3 · acetone hydrogen bonding. The strength of the hydrogen bonding was characterized by enthalpies of specific interaction ΔHint.CH3I/S (sp.). These values were estimated by the "intensity rule": ΔHint. CH3I/S (sp.) = 0.13 kcal mol-1 for self-association in pure CH3I and 0.4 kcal mol-1 for solution in acetone-d6. © 1995

Some applications of vibrational spectroscopy in conformational analysis

We report some vibrational spectroscopic approaches to studying conformational equilibria and kinetics, which were developed in recent years: (i) investigation of relative band dichroism in polarized IR spectra of crystals; (ii) kinetic investigations of conformational transitions in liquid solutions; and (iii) use of conformationally inhomogeneous compounds as probes for studying molecular mobility and free volume distribution in polymers

Universal laboratory complete for optical practice

A set of optical elements is presented. The set helps in the organization of practical classes for first and second year students. The set is based on the principle of optical constructing. It contains different optical elements: lenses, mirrors, prisms, crystal plates, polarizers, Fabry-Perot interferometers, glass absorption light filters, Fresnel biprism, Newton rings, transparent and reflection diffracting gratings and phase zone plates

Conformational mobility of 1,2-di-(p-bromophenyl)ethane dispersed in polymer matrices: Correlations with relaxation transition phenomena and free volume distribution in glassy polymers

Conformational mobility of 1,2-di-(p-bromophenyl)ethane (DPBPE) introduced into glassy poly(vinyl chloride) (PVC) and poly(methyl methacrylate) (PMMA) matrices has been studied by infrared absorption spectra. For both systems under investigation the freezing of the trans-gauche conformational transitions in DPBPE has been found. The temperatures of freezing of the conformational equilibrium (Tf) have been determined: 276±8 (PVC) and 326±18 K (PMMA). The Tf values are correlated with the secondary relaxation transition temperatures of the pure polymers. In the view of the obtained data, the processes responsible for the relaxation transitions in PVC and PMMA are analysed. The volume of the conformationally mobile group of DPBPE (108.2 Å3) was found to be close to the free volume entity sizes of the polymers, measured at Tf temperatures by positron annihilation lifetime spectroscopy