Raman and infrared spectra, conformational stability, barriers to internal rotation, vibrational assignment and ab initio calculations of 3‐bromo‐2‐methylpropene

Abstract

AbstractThe Raman (3200–20 cm−1) and infrared (3200–35 cm−1) spectra of 3‐bromo‐2‐methylpropene, H2CC(CH3)CH2Br were recorded for the gas and solid. Additionally, the Raman spectrum of the liquid was recorded and qualitative depolarization values were obtained. The fundamental asymmetric torsional mode for the gauche conformer is observed at 81.5 cm−1 with one excited state falling at a lower wavenumber but the asymmetric torsion is not observed for the higher energy s‐cis conformer. Utilizing the gauche observed torsional wave numbers, the gauche dihedral angle, the enthalpy difference between conformers and the calculated torsional fundamental wavenumber for the s‐cis conformer, the potential function governing the interconversion of the rotamers was estimated. This potential function gives values of 170 cm−1 (486 cal mol−1) (1 cal = 4.184 J), 2659 cm−1 (7.60 kcal mol−1) and 725 cm−1 (2.07 kcal mol−1) for the s‐cis to gauche, gauche to gauche and gauche to s‐cis barriers, respectively, and it is compared with tht obtained with the RHF/STO‐3G* basis set. From the methyl torsional wavenumber of 172 cm−1 for the gauche conformer, the threefold barrier of 695 cm−1 (1.99 kcal mol−1) was calculated for the methyl group. A complete vibrational assignment is proposed based on Raman : depolarization data, group wavenumbers and relative infrared and Raman intensities. The conformational energy difference and optimized geometries of both conformers were also obtained from ab initio calculations with the RHF/STO‐3G*, RHF/LANL1DZ and MP2/LANL1DZ basis sets. Normal‐coordinate analyses were also performed with force fields determined from both the RHF/STO‐3G* and RHF/LANL1DZ basis sets. The calculated wavenumbers support the proposed vibrational assignment. These data are compared with the corresponding data for some similar molecules.