Conjugated cyclopropyls: the molecular structures and conformations of 1,1-dicyclopropylethene and dicyclopropyl ketone as studied by gas phase electron diffraction and ab initio calculations

Abstract The molecular structure and conformations of the two structurally related molecules 1,1-dicyclopropylethene and dicyclopropyl ketone have been studied experimentally by the gas electron diffraction method. For the former molecule a conformational mixture of 59(8)% gg (as 47% g + g + , and 11% g + g − ) and 41% ga was observed, while the cyclopropyl rings of the ketone prefer a syn orientation (93(3)% syn syn ; 7% syn anti ). The syn oriented cyclopropyl groups of the ketone are substantially closer to the C O bond than the gauche oriented cyclopropyls of the hydrocarbon are to the C C bond (∠C 2 –C 3 –D: 121.3° vs. 127.2°). Results from ab initio MP2/6-31G* optimization calculations are in excellent agreement with the observed ones.