Gas‐phase detection of the HBCC (X1Σ) molecule: a combined crossed beam and computational study of the B(2P)+C2H2(1Σg+) reaction

A novel supersonic beam of ground‐state boron atoms B(²P)] was employed to investigate the reaction of B(²P) with acetylene C₂H₂(¹Σ_g⁺)] at an average collision energy of 16.3±0.4 kJ mol^?1 at the most fundamental microscopic level. The crossed molecular beam technique was used to record time of flight spectra at mass to charge ratios of 36 (¹¹BC₂H⁺), 35 (¹⁰BC₂H⁺/¹¹BC₂⁺), and 34 (¹⁰BC₂⁺) at different laboratory angles. Forward‐convolution fitting of the laboratory data showed that only a product with the gross formula BC₂H was formed via a boron versus hydrogen exchange. By combining experimental results with electronic structure calculations, the conclusion was that the reaction proceeded via the initial addition of B(²P) to the two carbon atoms of acetylene, leading to the formation of a first intermediate, the borirene radical (c‐BC₂H₂). This intermediate underwent various isomerization processes on the BC₂H₂ potential energy surface before decomposing into the linear HBCC(X¹Σ) isomer via a hydrogen atom elimination. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1359–1365, 2001