Isomerization of cyanoborane anion

A previous study of the potential energy surface of borazirene revealed prohibitively large barriers on the reaction pathway from cyanoborane to borazirene. An alternative mechanism for its synthesis, based on a base-catalyzed reaction is proposed. It includes the deprotonation of cyanoborane and subsequent isomerization of the HBCN^- chain to the HBCN^- ring. Calculations of the X^- + H₂BCN = HBCN^- + HX equilibria (X = OH, H) show that proton transfer to the hydroxyl anion is thermodynamically feasible. The geometries of the stationary points on the potential energy surface corresponding to ring closure or ring opening to HBCN^- and/or HBNC^- chain-ions are obtained at the MBPT(2) level. Single-point calculations of the activation barriers and reaction enthalpies are performed at the CCSD + T(CCSD) level. The barriers are significantly lower compared to the isomerization of the neutral species and are in the range 100-130 kJ/mol. The reaction enthalpies are substantially lower and slightly favor the ring formation from the isocyanoborne anion. Trends in correlation energy contributions for the individual steps of the mechanism are analyzed.