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

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 (angle C(2)-C(3)-D: 121.3 degrees vs. 127.2 degrees). Results from ab initio MP2/6-31G* optimization calculations are in excellent agreement with the observed ones. (C) 1999 Elsevier Science B.V. All rights reserved.