MOLECULAR-STRUCTURE AND CONFORMATIONS OF BICYCLOPENTYL, C5H9-C5K9, ASSTUDIED BY ELECTRON-DIFFRACTION, MOLECULAR MECHANICS AND AB-INITIO METHODS

The molecular structure of bicyclopentyl was established by gas phase electron diffraction, and it is compared with that calculated previous ly by molecular mechanics (MM3) and currently by ab initio methods (6- 31G). There is good agreement between theory and experiment for the s tructural parameters, and the conformational ratio, measured with a lo w precision, is in fair agreement with that calculated by both methods . Experiment and theory agree that the major conformer is equatorial, equatorial, anti (ee, anti). Comparisons are made with conformations o f similar bicyclic (bicyclopropyl, bicyclobutyl and bicyclohexyl) and open-chain (sym-tetramethylethane) compounds. The main structural para meters measured for the ee, anti conformer are as follows: C-C-ave, 1. 541(4) Angstrom; C-H, 1.121(12) Angstrom; bond angles: C-2-C-1-C-5, 10 3.7(3)degrees; C-1-C-2-C-3, 107.9(5)degrees; C-2-C-1-C-6, 113.3(3)degr ees; C-C-H, 112.7(10)degrees; H-C-H 106.3(8) Angstrom. Additionally, t he C-C bond length between the rings is assessed to be 1.545(10) Angst rom. Similar bond lengths in related bicyclic molecules are found to c orrelate with the ring C-C-C bond angles.