Infrared band intensities of 1,2-dibromoethane in solutions: Electrostatic effect and influence of hydrogen bonding on the conformational equilibrium

Abstract

The conformational equilibrium of 1,2-dibromoethane (DBE) in various media (vapor phase, liquid, and solutions in n-hexane, carbon tetrachloride, toluene, carbon disulfide, bromoform, acetone, nitromethane, deuterated acetonitrile and deuterated dimethylsulfoxide) has been studied by IR absorption spectra. The enthalpy differences between trans (t) and gauche (g) conformers (ΔHo) were determined from the dependencies of 1n(It/Ig) upon T-1, where It and Ig are the integrated intensities of the bands, belonging to different conformers. The values RT · 1n(It/Ig) and (R · 1n (It/Ig) + ΔHo/T) obtained at 296 K were used as measures of the free enthalpy (ΔGo) and entropy differences of the conformers (ΔSo) respectively, when considering their changes with solvent. To minimize the errors due to solvent and temperature effects on the IR-band absorption coefficients, four different band pairs of trans and gauche conformers were investigated. Good correlations between ΔGo, ΔHo and the function of dielectric permitivity of the medium (0.5 -( -1)/(2 + 1))1/2 were observed for all solvents except toluene. Significant changes of ΔSo with the media have been found: they cover more than 1.5 cal mol-1 K-1 when going from the vapor phase to polar solvents. The ΔSo values correlate with ΔHo ones (compensation effect); the slope of the dependence (ΔΔSo/ΔΔHo) = (1.0 ± 0.3) · 10-3 K-1 is close to those determined earlier for 1,2-dichloro- and bromofluoro-ethanes, trans-1,2-dichlorocyclohexane and o-iodophenol. The overall integrated intensities in the CH2-stretching (αA/B(str), 3200-2700, cm-1) and deformational (αA/B(def), 1550-1300, cm-1) regions were measured for neat DBE and its solutions in CCl4, CD3CN and (CD3)2SO. The αA/B(str) value noticeably increases when going from CCl4 to proton acceptor solvents, while the αA/B(def) does not depend on the solvent. These results are interpreted within the framework of the formation of weak hydrogen bonds between CH2-groups of DBE and proton acceptor groups of the solvents. The enthalpies of specific interaction of DBE with CD3CN and (CD3)2SO were estimated using the 'intensity rule'. These values are about 1 kcal mol-1. Nevertheless, poor correlation between ΔHo, ΔGo and basicity parameters of the solvents indicate the minor effect of the H-bond formation on the conformational equilibrium of DBE.