Conformational Stability of 3-Fluoropropene: A Challenging Problem for Both Theory and Experiment

Abstract

Experiments have shown the cis conformer of 3-fluoropropene to lie lower in energy than the gauche conformer, with enthalpy differences ranging from 58 to 263 cm-1 depending upon the experimental method. Van der Veken, Herrebout, Durig, Zhao, and Durig have studied this system at the MP2 level. While their MP2/ 6-311G(d,p) results, predicting an enthalpy difference of 192 cm-1, agreed with the experimental evidence, their highest level of theory, MP2/6-311++G(d,p), gave a reversed energetic ordering, predicting the gauche conformer to be more stable by 117 cm-1. We have used much larger basis sets to resolve this discord between theory and experiment. Systematic basis set trends are observed in our results. In particular, the inclusion of diffuse functions with small basis sets significantly increases the stability of the gauche conformer relative to the cis conformer, sometimes resulting in an incorrect energetic ordering. This effect is greatly reduced with larger basis sets. Our highest level results agree qualitatively with experimental conclusions, predicting the cis conformer to lie lower by 40 and 21 cm-1 at the MP2/cc-pVQZ and MP2/aug-cc-pVQZ levels, respectively. Entropy considerations are seen to be very important.