Partially Bound Systems as Sensitive Probes of Microsolvation:  A Microwave and ab Initio Study of HCN···HCN−BF3

The complex HCN···HCN−BF3 has been investigated by rotational spectroscopy and ab initio methods. The experimental B−N bond distance is 2.299(28) Å, which represents a contraction of 0.174(57) Å relative to that previously determined in HCN−BF3. The observed N···H distance, on the other hand, is 2.185(25) Å, which is only 0.045(25) Å shorter than that in (HCN)2. A block-localized wave function energy decomposition analysis indicates that significant energetic differences between HCN−BF3 and HCN···HCN−BF3 arise from a combination of distortion, polarization, and charge-transfer energies, with the net result that the remote HCN unit increases the effective dative bond energy by about 20%. In addition, electron density difference maps reveal migration of charge from the inner nitrogen to the boron atom. The large magnitude of the bond contraction, the increase in bond strength, and the flow of charge from HCN to BF3 indicate that the additional HCN unit drives the dative bond formation forward within the HCN−BF3 complex. The effect is large, even at a very small degree of microsolvation, demonstrating an intrinsic hypersensitivity to near neighbor interactions which we attribute to the partially bonded character of HCN−BF3. We suggest that partially bound complexes, in general, are highly sensitive probes of their local environment and may represent an interesting class of systems in which to investigate microsolvation effects.