Raman and infrared spectra, conformational stability, barriers to internal rotation, vibrational assignment and ab initio calculations of trans‐1‐bromobut‐2‐ene

Abstract

AbstractRaman (3200–10 cm−1) and infrared (3200–20 cm−1) spectra were recorded for the fluid and solid phases of trans‐1‐bromobut‐2‐ene (crotyl bromide), trans‐CH3CHCHCH2BR, and a complete vibrational assignment is proposed. The fundamental asymmetric torsion for the more stable gauche conformer was observed in the far‐infrared spectrum of the gas as a hybrid band centered at 75.0 cm−1. The enthalpy difference between the gauche and syn conformers is estimated to be 434 ± 111 cm−1 (1.24 ± 0.32 kcal mol−1) in the gas phase from the relative intensities of the Raman lines utilizing the differences in the Raman activities from the ab initio calculations with the RHF/STO‐3G* basis set. The enthalpy difference was determined experimentally for the liquid from the relative intensities of the C–Br stretches as a function of temperature. The determined value is 370 ± 77 cm−1 (1.06 ± 0.22 kcal mol−1) with the gauche conformer the more stable form. A reasonable range of values was obtained for the coefficients of the potential function governing internal rotation about the CC bond. The three‐fold barrier governing internal rotation of the CH3 group was determined from the far‐infrared spectrum of the gas. All of these data are compared to the corresponding quantities obtained from ab initio Hartree–Fock gradient calculations employing the RHF/STO‐3G* and RHF/LANL1DZ basis sets. Additionally, complete equilibrium geometries were determined for both rotamers. The results are discussed and compared with the corresponding quantities for some similar molecules.