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The molecular structure of dicylopropylacetylene (DCPA) has been studied by gas-phase electron diffraction and ab initio calculations using the basis sets 3–21G∗, 4–21G∗, 6–31G∗, 631-G∗∗, and MP 2 6–31 G ∗ . The diffraction data are consistent with the gauche rotamer ( C 2 symmetry) which has the bisectors of the cyclopropyl groups rotated 91.7(7.5)° away from one another. However, a model with a low torsional barrier in which the cyclopropyl rings are considered to be essentially freely rotating cannot be ruled out. Ab initio calculations utilizing the RHF 6–31 G ∗ (and MP 2 6–31 G ∗ ) basis sets are consistent with the gauche rotamer lying 383.8 cal mol −1 (362.8 cal mol −1 ) and 418.5 cal mol −1 (376.4 cal mol −1 ) lower in energy than the trans and cis rotamers, respectively. Large amplitude treatment has been undertaken applying a dynamical model consisting of a mixture of numerous pseudo-conformers. The force field provided by the ab initio calculations was utilized to calculate the vibrational mean amplitudes and to determine the r α structure. The major bond distances ( r a ) obtained from least-squares analysis (uncertainties in parentheses are 3σ) are: r( CH) = 1.091(3) A , r( CC>) = 1.197(3) A , r( CC) = 1.438(4) A , r( C 1 C 2 ) = 1.525(2) A , and r( C 2 C 3 ) = 1.511 A (assumed). The torsional angle around the CCCC axis τ = 91.7(7.5)°. The noticeable shortening of the carbon-carbon single bonds between the electron-rich systems, the triple bond and the cyclopropyl rings is discussed invoking the concept of π-electronic charge distribution of the carbon-carbon triple bond and other approaches. In order to obtain more pertinent details about the origin of the potential barrier in DCPA, the structures and the potential barriers for internal rotation of the dihalogenated derivatives of DCPA, bis-(1-fluorocyclopropyl)acetylene, (1-fluorocyclopropyl)(1-chlorocyclopropyl)acetylene, and bis-(1-chlorocyclopropyl)acetylene have been determined by ab initio calculations using the basis sets 4–21G∗ and 6–31G∗. The values of the most interesting parameter in this context, the dihedral angle τ, were found to be 98.65°, 102.11°, and 107.08°, respectively. The structural results of DCPA and its dihalogenated derivatives are compared and discussed. |
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