Solute-solvent interactions. IV. Infrared studies of solvent effects on rotational isomers of 1,2-dichloroethane and 1,1,2,2-tetrachloroethane

Solvent Effects on trans/gauche Conformational Equilibria of Substituted Chloroethanes: a Polarizable Continuum Model Study

“…We report in Table calculated harmonic frequencies for the C−Cl stretching mode of DCE both in vacuo and in selected solvents. Experimental values taken from Oi and Coetzee are also reported. For the gauche conformer, both the symmetric and antisymmetric stretches are considered, while for the trans isomer, we focus on the antisymmetric mode only because the symmetric stretch is IR inactive.…”

“…In Table , the values of Δ G ° obtained through eq 1 from the experimental A T / A G ,,, and the calculated ε T /ε G (Tables and ) are collected. As it can be seen, all of the Δ G ° values are positive; this means that T is the most stable conformer for the solvents here considered.…”

“…b Experimental data at 296 K for the band around 1200 cm -1 were taken from ref . c Experimental data for the C−Cl stretching band were taken from ref . d Experimental data at 296 K for the band around 1400 cm -1 were taken from ref .…”

Solvent Effects on 1,2-Dihaloethane Gauche/Trans Ratios

“…30 The observed transitions are fairly well reproduced at the MP2 level of theory, but the Becke3LYP level gives a significantly better fit. Leaving out the CH stretching modes, which are considerably shifted due to anharmonicity, the root mean square (rms) deviation between the calculated and observed transitions is only 25 cm-1 without scaling the calculated frequencies. " These energies i were calculated at the t 47 optimized geometries.…”

Dipole moments and conformational energies of the chloroethanes

“…Whenever possible, we exclude from consideration measurements carried out in solvents known to interact strongly with polar solutes; aromatic solvents are apparently particularly suspect in this regard.5•6•38-41 It should be noted, however, that recent measurements of the influence of solvents on C-Cl stretching absorption bands in 1,2-dichloroethane and 1,1,2,2-tetrachloroethane suggest that even in the case of aromatic solvents, the primary effect of solvent on conformational energy is simply through modification of electrostatic interactions. 54 In the case of these molecules, the barrier heights quoted by Allen and coworkers are unsuitable, since they are calculated assuming symmetric threefold barriers for each rotational isomer.43 The most reliable estimates of such barrier heights are probably those obtained from ultrasonic relaxation measurements.55-56 Such measurements on 1,2-dichloroethane in diethyl ether solution, from -90 to 40°, yield the value 3.2 (±0.5) kcal/mol for the height of the barrier for the conversion of a gauche to a trans isomer. The calculated value for e = 1.0 is 2.0 kcal/mol (Figure 2); modification of e to 4.3, the dielectric constant of diethyl ether42-57 in the vicinity of room temperature, increases this value to 2.6 kcal/mol, in reasonable agreement with experiment.…”