Cavity Formation and Dipolar Contribution to the Gauche−Trans Isomerization of 1-Chloropropane and 1,2-Dichloroethane

A conformational study of 1-bromo-2-chloroethane in zeolites

The conformational properties of 1-bromo-2-chloroethane (BCE) in several representative zeolites including silicalite-1, siliceous Y (Si-Y), Na-Y, and zeolite L have been investigated by FT-Raman spectroscopy in combination with thermogravimetric analysis. The results indicate that the conformational and dynamic behavior of BCE inside a zeoltic host is strongly influenced by several factors such as the framework topology, the Si/Al ratio, and the nature and the locations of charge-balancing cations. For siliceous zeolites (silicalite-1 and Si-Y), the anti population of BCE increases significantly relative to pure liquid upon initial loading at room temperature. Interestingly, when lowering temperature, the anti population of BCE in silicalite-1 continues to increase, but the conformational equilibrium of BCE inside Si-Y shifts to the gauche conformer. Zeolite L and Na-Y with extra-framework cations show an almost exclusive preference for the gauche mode of the BCE molecule at room temperature, and the conformational population does not change significantly at low temperatures.

Conformational Equilibrium of 1,2-Dichloroethane in Water: Comparison of PCM and RISM-SCF Methods

The RISM-SCF and polarizable continuum model (PCM) approaches have been applied to study the conformational equilibrium of 1,2-dichloroethane (DCE) in water. Both the electron correlation effect and basis sets play an important role in the relative energies of the gauche and trans conformers in gas and solution phases. Both PCM and RISM-MP2 methods resulted in a consistent trend with the previous experimental and theoretical studies that the population of the gauche conformer increases in going from the gas phase to the aqueous solution. However, the PCM treatment could not describe the solvent effect completely in that the sign of the relative free energy of the gauche and trans forms is opposite to the most recent experimental and theoretical data, while the RISM-MP2 gives the right sign in the free energy difference. We found that the larger excess chemical potential gain (by ca. -4.1 kcal/mol) for the gauche conformer is large enough to result in the gauche preference of DCE in water, though it has to compensate for more solute reorganization energy (approximately 1.6 kcal/mol) and overcome the energy difference (approximately 1.6 kcal/mol) in the gas phase. The radial distribution functions between DCE and the nearest water shows that the electrostatic repulsion between chlorine and oxygen atoms is higher in the trans conformer than in the gauche one, while the attractive interaction between chlorine and hydrogen of water is higher in the gauche conformer.

Conformational Free Energies of 1,2-Dichloroethane in Nanoconfined Methanol

Monte Carlo simulations have been used to construct free energy surfaces of 1,2-dichloroethane dissolved in methanol confined in hydrophobic spherical cavities of varying size (10-15 A) and solution density (0.6-0.79 g/cm3). The free energy surfaces are functions of two variables: the (center-of-mass) distance from the cavity wall of 1,2-dichloroethane and the Cl-C-C-Cl dihedral angle. Umbrella sampling and the weighted histogram analysis method were used to obtain accurate results for the free energy in these two degrees of freedom. Our results indicate that the conformational equilibrium and the barrier to internal rotation of the 1,2-dichloroethane depend on the position in the cavity. The results are discussed in the context of the solvent density, orientational distributions, and packing effects.

Internal rotation in 1,2-di-(p-XC6H4)ethanes (X=H, Br, NO2): infrared spectra and compensation effect

Infrared absorption spectra and internal rotation of 1,2-di-(p-XC(6)H(4))ethanes (X=H, Br, NO(2)) in crystalline phase, liquid and solutions at various temperatures have been investigated. Band fitting was applied to conformationally sensitive regions of the spectra, and assignment of the peaks to trans and gauche conformations was performed. Enthalpy and entropy differences of the conformers (deltaH(0) and deltaS(0)) were found to be solvent-dependent, and it is interpreted in terms of previously discovered compensation effect. The values deltaH(0) and deltaS(0) for 1,2-di-(p-NO(2)C(6)H(4))ethane obtained are unusually large.