The microwave Stark and Fourier transform spectra, structure and quadrupole coupling constants of 1,2-dicyanocyclobutene

Abstract The microwave spectrum of 1,2-dicyanocyclobutene, C 4 H 4 (CN) 2 , has been investigated using a Stark microwave spectrometer in the 11.0–34.0 GHz spectral region, and a Fourier transform microwave spectrometer in the 6–18 GHz region. The ground and several vibrationally excited states of the parent species have been assigned, as have the ground state of several 13 C and 15 N isotopomers. The nuclear quadrupole coupling constants of the 14 N nucleus have been determined for several species. The effective ( r 0 ) structure of the molecule has been derived. The substitution ( r s ) structure of the heavy atoms has also been calculated. An anomalously short carbon–carbon double bond of 132.6(2) pm was found by the substitution method. This short bond length is shown to be unreliable. Advanced quantum chemical calculations have been carried out for the title compound as well as for the cyclobutene prototype. An accurate estimate of the equilibrium ( r e ) structures of cyclobutene as well as of 1,2-dicyanocyclobutene based on quantum chemical calculations has been made.