Bond Order: Bond Length Relationships in Conjugated Hydrocarbons: The Microwave Spectrum and Structure of 3,4-Dimethylene-cyclobutene. R. D. Brown, P. D. Godfrey, B. T. Hart, A. L. Ottrey, M. Onda and M. Woodruff. Aust. J. Chem., 36, 639-648, 1983