Microwave spectrum, dipole moment, quadrupole coupling constants, and conformation of cyanocyclobutane

The microwave spectrum of cyanocyclobutane has been observed in the ground and first four excited vibrational states. The spectrum consisted of a- and c-type R- and Q-branch transitions. 14N quadrupole coupling constants were determined to be χaa= −3.663± 0.005 MHz, χbb= 2.086± 0.006 MHz and χcc= 1.577± 0.006 MHz. Electric dipole moment measurements led to μa= 4.00 ± 0.003 D and μc= 0.95± 0.02 D (μb = 0 by symmetry). All of the data are in accord in specifying the equatorial conformation for the puckered-ring system. Data from the excited states of the ring-puckering motion are in accord with a recently proposed potential function.