The author replies to Jensen:

I was not aware of Dr. Jensen's monograph, and from his discussion it is clear that much of what appears in my paper also appears in his monograph. More importantly, the fact that neither I nor the original reviewers of the manuscript knew of these papers by Jensen surely reinforces one of the central theses of my paper--that unoccupied orbitals as the major arbiters of reactivity have been long ignored by the organic chemistry community when we teach introductory organic chemistry courses. I view his work as a welcome addition to the list of literature citations already present in the paper itself.

The discussion in this paper was largely restricted to organic reactions, where the interaction between HOMO and LUMO is large and where movement of electrons into an antibonding LUMO will be accompanied by bond rupture. However, it is recognized that less significant interactions between filled orbitals and antibonding orbitals does not necessarily lead to bond rupture. Ab initio calculations of the ethane molecule, for example, show that the C-C bond distance is shorter and the C-H bond distance is longer in the staggered conformation than in the eclipsed conformation, which would be expected if there were a small degree of p delocalization of electron density from the C-H s orbital to the vicinal C-H s^* orbital in the staggered conformation, thus strengthening (shortening) the C-C bond and weakening (lengthening) the C-H bonds. This argument was set forth in the paper itself during the discussion of s + s^* overlap.

Likewise, Jensen's point about extending this discussion to free radicals is well taken, although I chose not to address this subject in the paper. In similar vein, the point made in his final paragraph is correct: it is known that BF₃ has p backbonding between the boron and the halogens. However, it is not essential to invoke this set of orbitals to rationalize the reactivity of BF₃, whose Lewis acidity (but not detailed structure) is adequately rationalized on the basis of an empty 2p orbital on boron. (Fortunately, the paper also used the second step of the S_N1 reaction involving a tert-butyl cation as an example of this type of reagent, and in this case there is no such p backbonding possible except for hyperconjugation.)