The molecular structures and conformational compositions of 1-bromo-3-chloropropane and 1,3-diiodopropane, as determined by gas-phase electron diffraction and molecular mechanics calculations

Abstract

The molecular structures and conformational compositions of 1-bromo-3-chloropropane (BCP) and 1,3-diiodopropane (DIP) have been studied by gas-phase electron diffraction at 27 and 38°C, respectively. BCP was found to exist as a mixture of the following four conformers, with estimated mole fractions in brackets: AA (0.1), AG (0.3), GA (0.1) and GG (0.5). These estimated mole fractions correspond to the following relative conformational energies: ΔEAA = 0.5 kcal mol−1, ΔEAG = 0.3 kcal mol−1, ΔEGA = 0.9 kcal mol−1 and ΔEGG = 0.0 kcal mol−1. DIP was found to exist as a mixture of AA, AG and GG. The mole fractions and uncertainties estimated at 2σ were found to be 0.05(9), 0.40(21) and 0.55(24), respectively. This corresponds to the following relative conformational energies: ΔEAA ≈ 1.1 kcal mol−1, ΔEAG = 0.7(6) kcal mol−1 and ΔEGG = 0.0 kcal mol−1. In order to conduct a more detailed investigation of the conformational geometries, results from molecular mechanics (MM) calculations have been included in the analysis.