Recent Research Developments

An Unusual Effect of Near-Neighbor Interactions on the Structure of a Molecule: The Hypersensitivity of HCN-BF₃

An accurate knowledge of molecular structure lies at the foundations of our understanding of virtually all chemical phenomena, and it is truly fortunate that with a remarkably simple set of principles, the structures of a wide variety of molecules can readily be estimated. With the simple concept of orbitals, for example, bond angles can often be approximated with essentially no calculation at all, while rough inter-atomic distances are easily obtained by simple addition once sets of covalent, ionic, or van der Waals radii have been established. It is implicit in the use of such procedures that one regards a molecule as a microscopic building block whose internal geometry is insensitive to the environment which surrounds it. Indeed, one normally expects to be able to make reasonable predictions of molecular structure and to further have the results be consistent between the gas phase and the solid state.

For most common systems, this is a very reasonable idea. However, research in the group of Professor Kenneth R. Leopold has recently explored a class of systems where such simple ideas no longer apply. These systems, for which the group has coined the term "partially bound", are Lewis acid-base adducts in which the donor-acceptor bond lies in the intermediate regime between chemical bonding and weak intermolecular interactions. For these systems, gas phase molecular structure can be quite different from that in the solid state, with the process of crystallization driving the formation of the dative bond forward.¹

A portion of the microwave spectrum of HCN···HCN-BF₃ is shown in

Figure 1. This spectrum, together with those of numerous isotopically substituted forms, indicate that the addition the remote HCN to HCN-BF₃ drives the B-N dative bond forward, shortening it by 0.174(57) Å! Essential features of the structures of both HCN-BF₃ and HCN···HCN-BF₃ are shown in Figure 2. The contraction represents an enormous effect, induced by a quite rarefied "environment", and graphically the hypersensitivity of partially bound

illustrates systems to near-neighbor influence. In terms of structure, the extra HCN unit drives the dative bond about 20% toward completion. Moreover, ab initio calculations indicate that the bond energy for the B-N dative bond is also increased by about 20% relative to that in "bare" HCN-BF₃. For more information about this and a host of other unusual molecular species, see http://www.chem.umn.edu/groups/kleopold/.

1. Leopold, K.R.; Canagaratna, M.; Phillips, J.A. Acc. Chem. Res. 1997, 30, 57.
2. Fiacco, D.L.; Leopold, K.R., J. Phys. Chem. A 2003, in press
3. Reeve, S.W.; Burns, W.A.; Lovas, F.J.; Suenram, R.D.; Leopold, K.R. J. Phys. Chem. 1993, 97, 10630.
4. Burns, W.A.; Leopold, K.R. J. Am. Chem. Soc. 1993, 115, 11622.

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