The CH3CH2⊖ internal rotation in thiopropionic acid as studied by ab initio SCF-MO method

Abstract

The CH3CH2⊖internal rotation in thiopropionic acid was studied by ab initio SCF-MO calculations using 3–21G and 3–21G+d(0.65S) basis sets. Energies and structures of several conformations of the two tautomeric forms of this molecule (the thiol and thion forms) determined by gradient geometry refinement are reported, and some conformationally dependent local geometry trends are discussed. The results are compared with available data on propionic and dithiopropionic acids to assess the effects of substitution of oxygen by sulphur on molecular properties. While the CH3-CH2-CO axis adopts preferentially the syn conformation (C-C-CO dihedral angle equal to 0°), the lowest-energy conformation of the CH3-CH2-CS axis occurs for a C-C-CS dihedral angle in the 100–120° region. However, both axes possess energy maxima near 60° and at 180°. The calculated molecular atomic charges, dipole moments and ionization potentials are reported and compared with available experimental data.