Ab initio studies of the conformations of ethynyl formate, cyano formate and related ethynyl and cyano compounds

The optimized geometries, relative free energies and related thermodynamic properties, harmonic frequencies, and dipole moments have been calculated at the HF and MP2 levels for ethynyl formate (1a), ethynyl acetate (1b), cyano formate, HCO2CN (1c), cyano acetate (1d), S-ethynyl thioformate (2a), S-ethynyl thioacetate (2b), S-cyano thioformate (2c), S-cyano thioacetate (2d), N-ethynylformamide (3a), N-ethynylacetamide (3b), N-cyanoformamide (3c), and N-cyanoacetamide (3d) with the gaussian 98 program. For ethynyl formate, the calculation for 25 °C at the MP2/6-311++G(df,pd) level predicts that the Z isomer is more stable by 1.23 kcal/mol. For S-ethynyl thioformate, calculations at the MP2/6-311++G(2d,2p) level predict that the E isomer is favored by 0.71 kcal/mol at 25 °C. The E isomers of N-ethynylformamide and N-ethynylacetamide were found at all levels to be more stable than the Z isomers at 25 °C. For cyano formate and cyano acetate, calculations at the MP2/6-311++G(df,pd) level predict that the Z isomers are more stable at 25 °C by 1.50 and 2.72 kcal/mol, respectively. At this level and temperature, the Z isomers of 2c, 2d, 3c, and 3d are predicted to have free energies of 0.46, −0.07, 1.22, and 2.28 kcal/mol, respectively, relative to the E conformations. Z to E free-energy barriers at 25 °C of 8.63, 10.64, 17.63, 7.39, and 14.03 kcal/mol were calculated for 1a, 2a, 3a, 1c, and 3c at the HF/6-311G(d,p) level, and at the HF/6-311+G(d,p) level, the free-energy barrier for 2c was 7.08 kcal/mol.