and N; the bonding situations in 6 and 7 are rather similar.
As suggested by the formal structure 9, a planar form would be expected to benefit from the Hiickel271 character of the four-membered ring. Nevertheless, 9 is 14.6 kcal/ mol less stable than 7 with the larger 6-31G* basis set (13.2 kcal/mol; 3-21G). The inversion barrer for 6 is indicated to be even larger, 20.6 kcal/mol (6-31G*//3-21G; 18.4 kcal/mol, 3-21G). According to the properties of p(r), electron delocalization in puckered 6 and 7 is still large and is supported by the now favorable 1,3-interaction. Both 71 and o effects contribute to the stability of the puckered forms [']. Once again, our calculational observations emphasize that an aromatic 271 electron ensemble does not necessarily imply planarity of a four-membered ring like 1, 2, 5 , or 6 .
Our results on 7 are pertinent to a large number of squaric acid derivatives, e.g., the "squaraines" ("quadrains")[71. While examples with good a donor substituents are planar[91, we are attempting to find cases with non-planar structures. Groups with C=N or C=C double bonds['01 when substituted at C2 and C4 of bicyclobutane may also lead to partially opened structures like 7 with elongated Cl-C3 bonds["].