An empirically corrected quantum mechanical potential energy curve of internal rotation of acryloyl fluoride, CH2=CH-CF=O

The geometrical parameters of acryloyl fluoride were optimized completely at the MP2/6-31G* computational level for 17 points on the internal rotation potential energy (IRPE) curve for rotation around the formal single carbon–carbon bond. The expansion coefficients of the reduced rotational constant function F(φ) and the four, five, and six-term expansions of the IRPE function,[Formula: see text]were obtained from these data. The theoretical IRPE functions were then refined using only the experimental torsional transition frequencies in both the s-trans and s-cis wells. The IRPE functions obtained are compared with those in the literature, calculated at lower levels of theory in both the rigid and nonrigid rotation approximations. The best representation of the refined IRPE function is given by the six-term expansion with V1 = 71.7, V2 = 1944.8, V3 = 113.0, V4 = −122.8, V5 = −8.7, and V6 = 12.5 cm−1, respectively. From this IRPE function, one correctly predicts the s-trans conformer to be more stable with ΔH0 = 168 cm−1. The barrier to rotation from the s-trans to the s-cis positions, ΔH#, is 2048 cm−1 at 88° from the s-trans well. The advantages of using the nonrigid rotation approximation, based on high-quality quantum mechanical calculations that include correlation effects, to construct the effective IRPE function for molecules are emphasized.