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

Abstract

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 (Δ H o ) were determined from the dependencies of ln( I t / I g ) upon T −1 , where I t and I g are the integrated intensities of the bands, belonging to different conformers. The values R T ·ln( I t / I g ) and ( R ·ln( I t / I g )+Δ H o / T ) obtained at 296 K were used as measures of the free enthalpy (Δ G o ) and entropy differences of the conformers (Δ S o ) 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 Δ G o , Δ H o and the function of dielectric permitivity of the medium (0.5−( e −1)/(2 e +1)) 1/2 were observed for all solvents except toluene. Significant changes of Δ S o 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 Δ S o values correlate with Δ H o ones (compensation effect); the slope of the dependence (ΔΔ S o /ΔΔ H o )=(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 CH 2 -stretching ( α A/B (str), 3200–2700, cm −1 ) and deformational ( α A/B (def), 1550–1300, cm −1 ) regions were measured for neat DBE and its solutions in CCl 4 , CD 3 CN and (CD 3 ) 2 SO. The α A/B (str) value noticeably increases when going from CCl 4 to proton acceptor solvents, while the α A/B (def) does not depend on the solvent. These results are interpreted within the framework of the formation of weak hydrogen bonds between CH 2 -groups of DBE and proton acceptor groups of the solvents. The enthalpies of specific interaction of DBE with CD 3 CN and (CD 3 ) 2 SO were estimated using the `intensity rule'. These values are about 1 kcal mol −1 . Nevertheless, poor correlation between Δ H o , Δ G o and basicity parameters of the solvents indicate the minor effect of the H-bond formation on the conformational equilibrium of DBE.