Molecular structure and conformation of cyclopropylbenzene as determined by ab initio molecular orbital calculations, pulsed-jet Fourier transform microwave spectroscopic, and gas-phase electron diffraction investigations

Ab initio computational, microwave spectroscopic, and electron diffraction techniques have been used to study the gas-phase structure of cyclopropylbenzene. Theoretical calculations at the HF, B3LYP, and MP2 levels for basis sets 6-31G(d) and 6-311G(d) have been carried out. Both MP2 and B3LYP calculations showed the bisected form to be lower in energy (245/157 and 660/985 cal mol(-1), respectively, for basis sets 6-311G(d)/6-31G(d)). Rotational constants for the bisected form of the parent and eight singly substituted C-13 isotopic species were obtained. The selection rules of the observed rotational transitions and the facts that eight (rather than six) singly substituted 13C isotopers are observed and assigned and that seven of the compound's nine carbon atoms lie in the molecule's symmetry plane required the molecule to exist in the bisected conformation. No transition from the perpendicular form was observed in the pulsed-jet microwave experiment. Gas-phase electron diffraction data were collected at a nozzle-tip temperature of 265 K. Least squares analyses were carried out using ED data alone and with the inclusion of microwave rotational constants. The principal structural results (r(g) and angle alpha) obtained from the combined ED/MW least-squares analysis are r(C-H)(av) 1.093(6) Angstrom, r(C-7-C-8)(v) = 1.514(20) Angstrom, r(C-8-C-9)(d) = 1.507(26) Angstrom, r(C-7-C-1) = 1,520(25) Angstrom, r(C-C)(Ph) = 1.395(1) Angstrom, angleC(1)C(7)C(8) = 119.6(17)degrees, angleC(2)C(1)C(7) = 122.5(25)degrees, degrees angleC(1)C(2)C(3) = 120.9-(35)degrees, angle (HCC9)-C-8 = 116.7(20)degrees, angle HCCPh = 120.0 degrees (assumed).