A theoretical study has reported that CCBCC will rearrange to planar cyclo-C₄B if the excess energy of CCBCC is ≥ 67 kJ mol^–1 [calculations at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-31G(d) level of theory]. Cyclo-C₄B lies only 4.5 kJ mol^–1 above CCBCC. Although we were unable to prepare an ionic precursor (of CCBCC) in sufficient yield to allow the production of neutral CCBCC by neutralisation-reionisation, the analogue HCCBCCH can be prepared by one-electron vertical (Franck-Condon) reduction of cation [HCCBCCH]⁺. Some of the HCCBCCH neutrals formed in this way are stable for the microsecond duration of the ⁺NR⁺ experiment, whereas others rearrange. Calculations indicate that energised HCCBCCH may rearrange over a barrier of 203 kJ mol^–1 to cyclo-C₄H₂B, which lies 105 kJ mol^–1 above HCCBCCH. The cyclic isomer is not planar like cyclo-C₄B, but has trans geometry. The cyclic species is formed with an excess energy of 98 kJ mol^–1, and may undergo facile and degenerate formation of the alternative trans structure through a planar C₄H₂B transition state (+17 kJ mol^–1). If the cyclic isomer has an excess energy of ≥ 133 kJ mol^–1 it may ring open to give an open chain C₄H₂B isomer containing a terminal B atom

Keywords: HCCBCCH neutral, molecular modelling, rearrangement, trans cyclo C₄H₂B