A critical comparison of the ab initio geometry and zero-point-average structure of bicyclo(3.1.0)hexane (original) (raw)

The molecular structure of the boat conformation of bicyclo(3.l.O)hexane (BCH) was determined by ab initio gradient geometry refinement using the 4-21G basis set both with and without polarization functions on the carbon atoms, in order to obtain a direct estimate for the basis set dependence of the C-C bond distances in this compound. This information is useful in a controversy involving the transannular C-C bond distance of this system which was found at 1.454(9) a in a joint gas electron diffraction-microwave (GED-MW) study, but at 1.509 a in a 4-31G ab initio geometry optimization. It is found that the addition of polarization functions on the 4-21G basis lowers the C-C bond in question from 1.518 to 1.502 a. Thus, it is difficult to expect that additional basis set improvements can further reduce this value significantly below 1.50 a. When previously established empirical corrections are applied to the 4-21G structure, an average rs C-C distance of 1.524 a results in agreement with the experimental ra average of 1.524(3) a. In addition, it is found that small adjustments of the 4-21G parameters (maximum changes of -0.008 a and +0.018 _& in C-C and C-H bond distances, respectively, and of +0.6' in bond angles) engender a structure of boat-BCH whose rotational constants are practically identical with published microwave constants without containing a C-C bond less than 1.51 a. Since the C-C bond distances of BCH are totally unresolved in the GED data, it appears that the unusually short 1.454(9) a C-C bond of the GED-MW investigation is not a necessary result of data analysis and not established with certainty. Apart from the structure of BCH, the conclusion is of general significance since the experimental technique employed is usually expected to protect against false minima in data analyses. The 4-21G structure of the unobserved chair conformation of BCH was also determined and found to be approximately 3 kcal mol" less stable than the boat form.