SummaryThe enthalpy of formation of tris (methy1idene)-cyclopropane (" [3]radialene", 1) has been determined as A H : = 396f 12 kJ mol-' from three fragmentation reactions of its molecular ion I+ formed from 1 by photoionisation using synchrotron radiation. Comparative electron impact measurements using conventional mass spectrometry were also performed. A treatment of the latter data is described which leads to satisfactory agreement with the photoionization data. The experimental value of d HF (1) is compared with various theoretical estimates. The strain energy of 1 is calculated to be 226.3 kJ mol-'. Linear extrapolation of this quantity from the increase of strain in passing from cyclopropane to methylidenecyclopropane yields 282.4 kJ mol-'. The discrepancy between these values, already predicted by Dewar and Baird ten years ago from theoretical calculations, is discussed on the basis of maximum overlap considerations. The enthalpy of formation of bis(methy1idene)cyclopropane is predicted to be d HF= 309 kJ mol-'.

The mass spectra of the cyanoaceQlenes X-C=C-C=N, X = H , F, C1, Br, 1, are reported, together with electron impact appearance energies as determined using a modiíied second derivative method. From these data the enthaipies of formation and the C-X as weU as the C=C bond energies are deduced. These bond energies are discussed in companson with vaiues of corresponding saturated and acetylenic compounds.