Microwave spectrum of tetracyclo[3.2.0.0 2,7.0 4,6]heptan-3-one and the effect of cyclopropyl-carbonyl group interactions on the dipole moment

The microwave spectrum of tetracyclanone has been analyzed, resulting in a dipole moment of 3.65 ± 0.02 D. This rather large dipole moment is explained as another manifestation of interactions between the Walsh orbitals of cyclopropyl groups and π-orbitals on substituent groups. We have developed an empirical model, based on the ideas of Hoffmann, for the effect of such an interaction on the dipole moment, and show that the model gives good quantitative agreement with the dipole moments of many related compounds. The ground state rotational constants, obtained from an iterative least-squares fit to a rigid rotor, are A = 4361.92 ± 0.07, B = 2469.960 ± 0.003, and C = 2028.872 ± 0.003 MHz. The uncertainties are standard deviations determined solely from the goodness of fit to 10 transition frequencies.