spin density at C3, and correspondingly divergent hfc's for H (CJ and H(C3), is foreshadowed in the preferred structure (7b) calculated for the monomethyl derivative ( p l = 0.51, hfc = -18.8 G; p3 = 0.74, hfc = -28.2 G). Clearly, 7b and 7c are unusual and unprecedented cyclopropane radical cations; their structures merit experimental verification. Appropriate experiments for the dimethyl derivative are under way. Concluding RemarksWe conclude that even simple alkyl substituents are capable of significantly altering the shapes of the potential energy surfaces of cyclopropane radical cations, always preferentially stabilizing the Y2Al-type" states possessing a long, one-electron C-C bond.** (28) A referee has questioned whether diffuse functions should have been added to the basis set to properly describe this long C C bond. We have used the 6-31G* basis set throughout our geometry optimizations. This basis set is of split-valence quality on all the atoms and contains d-type polarization functions on the carbon atoms as well. In 4a at the UHF/6-31G* level, we obtained C,-C, and Cz-C3 bond lengths of 1.477 and 1.875 A, respectively (Table I). Addition of diffuse valence functions to the basis set on the carbon atoms only (6-31G' -6-31+G*) and reoptimization leads to equilibrium C,-C, and C2-C3 bond lengths of 1.477 and 1.877 A, respectively. These values are unchanged upon addition of diffuse functions to the hydrogen atoms (6-31+G* -6-31++G*). Addition of p-type polarization functions on the hydrogen atoms (6-31++G* -6-31++G**) and reoptimization gives C I C land Cz-C, bond lengths of 1.476 and 1.876 A, respectively, for 4a. We conclude that the 6-31G* basis set is adequate for the description of the structural features of cyclopropane cations and that the role played by diffuse basis functions is minimal.Localization of the UHF/6-31G1 wave function using the natural bond orbital analysis procedures developed by Weinhold et al.29 progresses smoothly; in 4a, for example, this procedure predicts that a C2-C3 bond orbital containing 0.97 electron is formed almost exclusively (>95%) by C(2p) atomic orbitals. Despite the bonding characteristics exemplified by cyclopropane orbital 3, all Y2B2-type" states are low-energy transition states for interconversion of equivalent "2Al-type" minima. The predicted finetuning of the preferred geometry and resulting charge and spin distributions for substituted cyclopropane cations should be qualitatively verifiable by a combination of chemical (e.g., nucleophilic substitution) and spectroscopic (principally ESR and CIDNP) experiments. Such experiments are under way in our laboratories. AcknowRledgment. Supplementary Material Available:The optimized geometries of all stationary points for species 4 , 5 , 7 , and 8 in %matrix format (20 pages). Ordering information is given on any current masthead page.Abstract: The scope and limitations of the reaction between cyclopropylcarbene-chromium complexes and alkynes have been examined. A variety of cyclopropylcarbene complexes and alkynes...

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