INFRARED BAND INTENSITIES OF 1,2-DIBROMOETHANE IN SOLUTIONS - ELECTROSTATIC EFFECT AND INFLUENCE OF HYDROGEN-BONDING ON THE CONFORMATIONAL EQUILIBRIUM

The conformational equilibrium of 1,2-dibromoethane (DBE) in various m edia (vapor phase, liquid, and solutions in n-hexane, carbon tetrachlo ride, toluene, carbon disulfide, bromoform, acetone, nitromethane, deu terated acetonitrile and deuterated dimethylsulfoxide) has been studie d by IR absorption spectra. The enthalpy differences between trans (t) and gauche (g) conformers (Delta H-0) were determined from the depend encies of In(I-t/I-g) upon T-1, where I-t and I-g are the integrated i ntensities of the bands, belonging to different conformers. The values RT . ln(I-t/I-g) and (R . ln(I-t/I-g) + Delta H-0/T) obtained at 296 K were used as measures of the free enthalpy (Delta G(0)) and entropy differences of the conformers (Delta S-0) respectively, when consideri ng their changes with solvent. To minimize the errors due to solvent a nd temperature effects on the IR-band absorption coefficients, four di fferent band pairs of trans and gauche conformers were investigated. G ood correlations between Delta G(0), Delta H-0 and the function of die lectric permitivity of the medium (0.5 - (epsilon - 1)/(2 epsilon + 1) )(1/2) were observed for all solvents except toluene. Significant chan ges of Delta S-0 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 Delta S-0 values correlate with Delta H-0 ones (compensation effe ct); the slope of the dependence (Delta Delta S-0/Delta Delta H-0) = ( 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 (alpha(A/B)(str), 3200-2700, cm(-1)) and deformational (alpha(A/B)(def ), 1550-1300, cm(-1)) regions were measured for neat DBE and its solut ions in CCl4, CD3CN and (CD3)(2)SO. The alpha(A/B)(str) value noticeab ly increases when going from CCl4 to proton acceptor solvents, while t he alpha(A/B)(def) does not depend on the solvent. These results are i nterpreted within the framework of the formation of weak hydrogen bond s between CH2-groups of DBE and proton acceptor groups of the solvents . The enthalpies of specific interaction of DBE with CD3CN and (CD3)(2 )SO were estimated using the 'intensity rule'. These values are about 1 kcal mol(-1). Nevertheless, poor correlation between Delta H-0, Delt a G(0) and basicity parameters of the solvents indicate the minor effe ct of the H-bond formation on the conformational equilibrium of DBE.