Torsional potentials and conformational structures in chloro and bromo derivatives of propenal and propenoic acid obtained by molecular-mechanics calculations

Abstract

Gas-phase data on conformational energies and structures of the molecules H 2CCHCHO, H 2CCHCClO, H 2CCClCClO and H 2CBrCHO are used to establish parameter values for the nonbonding atom⋯atom interaction C( sp 2)⋯OC (carbonyl). The parameter values within the Morse potential formulation are R 0 = 3.00 Å, R m = 3.60 Å and ϵ = 2.50 kcal mol −1. Torsional potentials for the molecules (X = Cl, Br) H 2CCHCXO, H 2CCXCHO, cis-XHCCHCHO, H 2CCXCXO, cis-XHCCXCHO, cis-XHCCHCXO and X 2CCXCXO are calculated, using the new parameter values together with previously established potentials for the remaining interactions. For the aldehydes two planar conformations, syn and anti, are available. In all cases, except for H 2CCClCHO, anti is the low-energy form. However, the conformational situations for the acid chlorides and bromides are quite complicated. For the molecules H 2CCHCXO and H 2CCXCXO anti and syn conformers are stable, while for X 2CCXCXO and for cis-XHCCXCXO only gauche conformers should exist. Only a planar syn form is found for cis-BrHCCHCBrO, while syn and gauche forms are possible for cis-ClHCCHCClO. Torsional barrier heights in the wide range 1–7 kcal mol −1 are calculated. However, the barrier heights for the aldehydes fall in the narrow range 5.5–7.0 kcal mol −1. Torsional force constants are calculated.