The FCHCN-BCl and ClCHCN-BCl complexes were investigated by quantum-chemical computations and low-temperature, matrix-isolation-IR spectroscopy. Theory predicts two stable equilibrium structures, with distinctly different B-N distances, for both complexes. One set of structures, which correspond to the global energy minima, exhibit B-N distances of 1.610 and 1.604 Å for FCHCN-BCl and ClCHCN-BCl, respectively (via M06-2X/aug-cc-pVTZ). The corresponding binding energies are 5.3 and 6.3 kcal/mol. For the metastable structures, the B-N distances are 2.870 and 2.865 Å for FCHCN-BCl and ClCHCN-BCl, respectively, and the corresponding binding energies are 3.2 and 3.3 kcal/mol. Also, the barriers between these structures on the B-N distance potentials are 2.5 and 2.8 kcal/mol, respectively, relative to the secondary, long-bond minima. In addition, several IR bands of both FCHCN-BCl and ClCHCN-BCl were observed in nitrogen matrices, but the assigned bands are consistent with M06-2X predictions for the short-bond, minimum-energy structures. None of the observed IR bands could be assigned to the metastable, long-bond structures.

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