Determination of the structure of methylene cyclobutane confirming a non-planar ethene and the structure of the argon-methylene cyclobutane van der Waals complex

Rotational spectra of the parent and four 13C isotopomers of methylene cyclobutane in the ground and first vibrational states of the ring-puckering vibration, and the parent and four 13C isotopomers of the argon methylene cyclobutane van der Waals complex in the ground vibrational state have been measured and assigned in the frequency range of 5-26 GHz. Ten c-type transitions between the ground inversion levels of methylene cyclobutane were observed for the first time by pulsed jet Fourier transform microwave spectroscopy. This improves the accuracy of the measured inversion frequency, ν01 = 33615.53(4) MHz. The substitution structure of each carbon in methylene cyclobutane was determined from the rotational constants of the normal and singly-13C substituted isotopomers in natural abundance. This structure experimentally confirms the non-planarity of the ethene group with respect to the plane defined by the cross ring carbons and the double bonded ring carbon in methylene cyclobutane. The double bond is tilted 6° toward an axial position in the puckered ground inversion states. This leads to a dramatic increase in the reduced mass of the inversion vibration. The rotational constants for the parent isotopomer of argon methylene cyclobutane, Ar-C5H8, are A = 3484.0659(3) MHz, B = 1308.0491(2) MHz, and C = 1127.9166(1) MHz. The position of the argon has been determined to be endo to the puckered ring, 3.624(1) Å above the ring center of mass, 0.110(7) Å forward toward the ethene group and 0.508(3) Å to the side of the ring. Large amplitude cross ring motion by the argon causes the cross ring 13C isotopomers to be equivalent and in 2% natural abundance.