Knowledge Bank

The Raman (3200 to $20cm-1)$ and infrared (3200 to $35cm-1$) spectra of 3-bromo-2-methylpropene, $H2C=C(CH3)CH2Br$, have been recorded for the gas and solid. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values obtained. The fundamental asymmetric torsional mode for the {gauche} conformer is observed at $81.5cm-1$ with one excited state falling at a lower frequency but the asymmetric torsion is not observed for the higher energy {s-cis} conformer. Utilizing the {gauche} observed torsional frequencies, the {gauche} dihedral angle, the enthalpy difference between conformers, and the calculated torsional fundamental frequency for the {s-cis} conformer, the potential function governing the interconversion of the rolamers has been estimated. This potential function gives values of $170cm-1$ (486 cal/mol), $2659cm-1$ (7.60 kcal/mol), and $725cm-1$ (2.07 kcal/mol), for the {s-cis} to {gauche, gauche to gauche}, and {gauche} to {s-cis} barriers, respectively, and it is compared to that obtained with the $RHF/STO-3G\ast$ basis set. From the methyl torsional frequency of $172cm-1$ for the {gauche} conformer, the threefold barrier of $695cm-1$ (1.99 Kcal/mol) has been calculated for the methyl group. A complete vibrational assignment is proposed on Raman depolarization data, group frequencies, and relative infrared and Raman intensities. The conformational energy difference and optimized geometries of both conformers have also been obtained form {ab initio} calculations.