Conformational stability from temperature dependent FT-IR spectra of liquid rare gas solutions, barriers to internal rotation, vibrational assignment, structural parameters and ab initio calculations for 3-bromopropene

Abstract

The infrared spectra of 3-bromopropene (allyl bromide), CH2CHCH2Br, dissolved in liquid krypton (−110 to −155°C) and liquid xenon (−55 to −110°C) at concentrations of about 1×10−2M are reported. The temperature dependence of the infrared spectra of three conformer pairs in the xenon solution has been used to obtain an enthalpy difference of 184±12cm−1(2.20±0.14kJ/mol) with the gauche conformer the more stable rotamer. Utilizing the asymmetric torsional fundamental of the cis conformer at 133cm−1 along with two accompanying hot bands, the gauche fundamental at 99cm−1, the enthalpy difference of 184cm−1, and the gauche dihedral angle (BrCCC) of 116.1°, the potential function governing the conformational interchange has been determined. The cis to gauche and gauche to gauche barriers have been determined to be 707 and 848cm−1, respectively. The adjusted r0 structural parameters have been obtained from the previously reported rotational constants and ab initio predicted values. The conformational stability and infrared intensities, along with the vibrational frequencies and structural parameters have been obtained from ab initio calculations. These data are compared to the corresponding experimental quantities when appropriate. A complete vibrational assignment is proposed for the gauche conformer and most of the fundamentals have been assigned for the cis conformer. The overall results are compared and contrasted to those for some similar molecules.