Structural and Electronic Effects on One-Bond Spin?Spin Coupling Constants 1J(B-N), 1J(B-H), and 1J(B-F) for Complexes of Nitrogen Bases with BH3 and Its Fluoro-Substituted Derivatives.
Ab initio equation-of-motion coupled cluster (EOM-CCSD) one-bond spin−spin coupling constants 1J(B−N), 1J(B−H), and 1J(B−F) have been evaluated for complexes X:BHnF3−n with X = N2, NCH, NCLi, H2CNH, NF3, and NH3, for n = 0−3. These complexes can be classified as either covalent or van der Waals complexes, on the basis of their binding energies and B−N distances. 1J(B−N) for covalent complexes varies significantly from −19 to +9 Hz, whereas 1J(B−N) is less than 2 Hz for van der Waals complexes. An absolute value of 1J(B−N) of 3 Hz or greater indicates that the complex is covalently bonded, but a small value of this coupling constant does not necessarily mean that it is a van der Waals complex, in view of the variation among these complexes found for 1J(B−N) as a function of the B−N distance. Deformation of the boron acid upon complex formation and electron donation by the nitrogen base has opposing effects on both 1J(B−H) and 1J(B−F). These effects are relatively small in van der Waals complexes. In covalent complexes, electron donation has the dominant effect on 1J(B−H), and on 1J(B−F) in complexes with BH2F and BHF2, but acid deformation has the dominant effect on 1J(B−F) in complexes with BF3. Values of both 1J(B−H) and 1J(B−F) reflect the van der Waals or covalent nature of the B−N bond.