Molecular structure and puckering potential function of cyclobutane studied by gas electron diffraction and infrared spectroscopy

The electron diffraction intensity of cyclobutane was measured and analyzed conjointly with the rotational constant, B0=0.355 82(11) cm−1, determined by an analysis of FTIR spectra of the ν14 (CH2 scissoring, B1g) and ν16 (CH2 rocking, A2u) bands. The rz structure was determined to be rz (C–C)=1.552±0.001 Å, rz (C–H)=1.093±0.003 Å, αz (∠HCH)=106.4±1.3°, and the ring dihedral angle, θz =27.9±1.6°; the rg distances of the C–C and C–H bonds are 1.554±0.001 and 1.109±0.003 Å, respectively. The uncertainties represent estimated limits of error. The rocking angle βz between the bisectors of the adjacent H–C–H and C–C–C angles was found to be 6.2±1.2°, the axial C–H bonds in the 1,3 positions being tilted towards each other. The coefficient of coupling of the ring-puckering and CH2-rocking motions was estimated to be βz/θz =0.22±0.05. The combination and difference sideband structures appearing in the ν14 band due to the puckering mode ν6 were analyzed. The puckering energy levels thus obtained were consistent with the existing data. A double-minimum potential function for the puckering mode was determined by taking into account the coupling of the puckering motion with the CH2-rocking motion. The potential gave the zero-point vibrational average value of the ring dihedral angle, 27.5±1.1°, which agreed with that determined in the ED–IR joint analysis mentioned above. The puckering potential for C4D8 was found to be consistent with that for C4H8 when the coupling between the puckering and the rocking motions was considered.