Equilibrium and thermal average structure of 1,4-disilacyclohexa-2,5-diene (DSCHD) was determined by applying procedures that have been recently developed for joint treatment of gas phase electron diffraction and molecular spectroscopy data guided by ab initio calculations. Both large amplitude motion and anharmonic vibrational effects were taken into account. Similar structural results were obtained by considering DSCHD molecule as a unique equilibrium conformer and as a set of quasi-conformers describing large amplitude motion. The determined equilibrium structural parameters for DSCHD (planar configuration) are: re(Si–C)=1.861(2)Å, re(CC)=1.346(3)Å, re(Si–H)=1.498(10)Å, re(C–H)=1.074(10)Å, ∠(C–Si–C)=109.9(3)°, ∠(H–CC)=116.7(10)°, and ∠(H–Si–H)=107.9(10)°. Uncertainties given in parentheses include three times standard deviation and a systematic error.

As a continuation of our systematic study of small and medium cyclic systems we synthesized the novel compound cyclobutyltrifluorosilane (CBTFS), and investigated its structure and conformational composition by means of electron diffraction and ab initio calculations. This molecule exists in equatorial and axial forms with the equatorial conformer being significantly more stable (with 81(15)%), which is in good agreement with the ab initio results provided by MP2 (71.6%) and DFT/b3pw91 (70.0%) calculation. Some of the more important structural parameters (ra values) for the predominant equatorial conformer are: r(C1–C2)=1.555(7)Å, r(Si–C)=1.832(3)Å, r(Si–F)=1.579(1)Å, ∠(C2–C1–C4)=89.3(2.8)°, ∠(C1–Si–F6)=110.7(1.6)°, ∠(C1–Si–F7,8)=113.4(1.2)° and ∠(Si–C(C2–C1–C4)plane)=127.3(1.4)°. The ring-puckering angle is 24.4(5.0)° (eq) and 19.6(10.0)° (ax). Uncertainties in parantheses are 3σ. In addition to CBTFS, the geometric parameters of cyclobutylmonofluorosilane and cyclobutyldifluorosilane were optimized. The simultaneous shortening of both the Si–C and Si–F bonds with progressive fluorination is rationalized by invoking electrostatic arguments and increasing bond ionicity. The obtained structural and conformational results are compared with those of other cyclobutyl derivatives and discussed on the basis of the Jonvik and Boggs’ predictions which postulate a linear relationship between the electronegativity of the substituent and the structural parameters, and conformational stability in four-membered rings.

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 MP2/6-31G*. 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)degrees 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/6-31G* (and MP2/6-31G*) 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) (376.4cal 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(alpha) structure. The major bond distances (r(a)) obtained from least-squares analysis (uncertainties in parentheses are 3 sigma) are: r(C-H) = 1.098(3) Angstrom, r(C equivalent to C) = 1.197(3) Angstrom, r(C-C equivalent to) = 1.438(4) Angstrom, r(C-1-C-2) = 1.525(2) Angstrom, and r(C-2- C-3) = 1.511 Angstrom (assumed). The torsional angle around the C-C equivalent to C-C axis tau = 91.7(7.5)degrees. 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 pi-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 tau, were found to be 98.65 degrees, 102.11 degrees, and 107.08 degrees, respectively. The structural results of DCPA and its dihalogenated derivatives are compared and discussed.

The microwave spectrum of (cyclopropylmethyl)acetylene (a newly prepared compound) was investigated in the frequency range 28-40 GHz. Both gauche and cis conformers were observed, the latter being 64 +/- 30 cm(-1) lower in energy. The geometries of these conformers were obtained combining the ab initio and cm microwave spectroscopy results. A large-amplitude motion (the methylacetylene group torsion) was studied by assigning the rotational spectra of several torsionally excited states.