Determination of the Structure of the Argon Cyclobutanone van der Waals Complex

The microwave spectra of the argon cyclobutanone van der Waals complex and its three ¹³C isotopomers have been assigned in the ground state of the ring puckering vibration. The rotational constants and centrifugal distortion constants for the ¹²C species are determined to be A = 3598.5200(4) MHz, B = 1388.4449(1) MHz, C = 1195.5254(2)MHz, Δ<INF>J</INF><INF></INF> = 4.846(2) kHz, Δ<INF>JK</INF><INF></INF> = 36.11(2) kHz, Δ<INF>K</INF><INF></INF> = −38.10(8) kHz, δ<INF>J</INF><INF></INF> = 0.6415(6) kHz, δ<INF>K</INF><INF></INF> = 20.62(4) kHz, H<INF>k</INF><INF></INF> = 0.067(4) kHz. The dipole moments have also been measured:  μ<INF>a</INF><INF></INF> = 0.29 D, μ<INF>b</INF><INF></INF> = 2.83 D. The cyclobutanone ring was found to remain planar after complexation with argon. The coordinates of the argon in the monomer's principal axis system are a = 0.23, b = 0.55 (across the ring due to large amplitude bending), and c = 3.48 Å (above the ring). The equilibrium position of argon is in the C<INF>s</INF><INF></INF> plane of the complex, 3.48 Å above the ring and shifted 0.23 Å from the C<INF>2</INF>−C<INF>2</INF><INF>‘</INF> ring diagonal toward the carbonyl.