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J. Phys. Chem. 1992, 96, 3712-3116
group become more positive in (Z)-ld compared to toluene, while the N of the cyano group in (2)-lc behaves in a similar manner when compared to that of acetonitrile. The calculations are in reasonable agreement with the N M R studies’ that indicate benzohydroxamic acid to exist predominantly in the (Z)-1 form, while monoalkylhydroxamic acids exist in several forms. Hopefully, the data presented here will encourage the further work necessary to provide a more appropriate compar ison.
MP2/6-31G(d,p) @)-la is predicted to be most stable, but two other conformations ((Z)-la, and (Z)-h) are so similar in energy that careful experimental work might be needed to accurately determine the structural preference of this molecule in the gas phase. The presence of substituents on the carbon of formohydroxamic acid is predicted to strongly affect the structural preferences of the acids. Therefore, experiments performed on the substituted acids may not be relevant to the parent. Acknowledgment. This work was supported in part by grants from the National Science Foundation, PSC-BHE, and IBM Corp.
Conclusion The ab initio resultsclearly show that Hartree-Fock calculations are not sufficient for describing the conformational preferences of formohydroxamic acid. At the highest levels optimized with
Rdstry NO. 1, 4312-87-2; (E)-2, 77269-31-9; (Z)-2, 77269-30-8; H$CONHOH, 546-88-3; NCCONHOH, 140177-1 1-3; PhCONHOH, 495-18-1.
Closs’s Diradicai: Some Surprlses on the Potential Energy Hypersurface C. David Sherrill, Edward T. Seidl, and Henry F. Schaefer III* Center for Computational Quantum Chemistry, University of Georgia, Athens, Georgia 30602 (Received: December 5, 1991)
The singlet and triplet potential energy surfaces for 1,3-~yclopentanediyl(Closs’s diradical) have been investigated using ab initio electronic structure theory. The triplet C, structure previously postulated to be an intermediate in the ring inversion of bicyclo[2.1.0]pentane(BCP) is found to correspond to a saddle point, rather than a minimum, on a potential energy surface more complex than that originally proposed by Closs. The singlet and triplet surfaces share several qualitative features, but the triplet stationary points lie 1 kcal/mol below the corresponding singlets. The BCP ground state and the singlet and triplet stationary points for Closs’s diradical have been fully optimized using a DZ + d basis set at the SCF and CISD levels of theory.
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introduction Because they have so often been proposed as reaction intermediates,’ diradicals have become a principal subject of investigation in physical organic chemistry. In 1975, Buchwalter and Closs became the first to characterize a “localized” diradical, 1,3-~ycIopentanediyl.~ Their ESR and CIDNP results suggested that 1,3-cyclopentanediylhas a triplet ground state which is planar or near-planar, and the triplet decay kinetics led them to propose the schematic potential energy surface in Figure 1. The shaded region represents the barrier to quantum mechanical tunneling, which Buchwalter and Closs believed responsible for the temperature independence of the triplet decay rate a t low temperatures. The barrier height was estimated to be 2.3 0.2 kcal/mol. Despite Benson-type thermochemical calculations that suggest a minimum on the singlet surface with a well depth of 5 k~al/mol,~ Buchwalter and Class depicted the singlet diradical as a transition state on its potential energy surface because they found such a deep well inconsistent with the observed instability of the triplet diradical. A more recent study by Goodman and Herman using time-resolved photoacoustic calorimetry also suggests that the
*
(1) (a) Borden, W. T., Ed. Diradicals; Wiley-Interscience: New York, 1982. (b) Berson, J. A. In Rearrangements in Ground and Excited Stares; de Mayo, P., Ed.;Academic Press: New York, 1980;Vol. I, pp 31 1-390. (c) Gajewski, J. J. Hydrocarbon Thermal Isomerizations; Academic Press: New York, 1981. (d) Wagner, P.J. In Rearrangements in Ground and Excited States; de Mayo, P., Ed,;Academic Press: New York, 1980; Vol. 111, pp 381-444. (2) (a) Buchwalter, S.L.;Closs, G. L.J . Am. Chem. Soc. 1975,97,3857. (b) Buchwalter, S. L.;Closs,G. L. J . Am. Chem. Soc. 1979, 101, 4688. (3)(a) Beadle, P.C.; Golden, D. M.; King, K. D.; Benson, S. W. J . Am. Chem. Soc. 1972,94,2943. (b) ONeal, H.E.; Benson, S. W. Inr. J . Chem. Kiner. 1970,2,423.(c) Luo,Y.-R.; Benson, S. W. J . Phys. Chem. 1989,93, 3304.
singlet diradical is a transition state or a shallow local minimum! Conrad et al. have previously studied 1,3-~yclopentanediylwith a b initio electronic structure theory,s using the spin-restricted self-consistent field (SCF) method with a double-l (DZ) basis set. They performed a partial geometry optimization of the triplet diradical, subject to the constraints of C , symmetry, 1.09-A C-H bond lengths, and 109’ methylene bond angles. At the constrained triplet geometry, the lowest lying singlet state, treated with tweconfiguration self-consistent field (TCSCF) theory, was found to lie 0.9 kcal/mol above the triplet ground state, in good agreement with the work of Buchwalter and Closs. In this work we seek to better characterize the singlet and triplet potential energy surfaces, and to answer the long-debated question of whether the singlet surface contains a local minimum. Using a DZ + d basis set, we optimize the geometries of the ring-closed ground state of bicyclo[2.1.O]pentane (BCP) and the diradical stationary points on the singlet and triplet surfaces at the SCF, TCSCF, CISD, and TC-CISD levels of theory. We perform harmonic vibrational frequency analyses at the SCF level to help determine the relationships among the stationary points on the schematic potential energy surface. Theoretical Methods The basis set used in this study includes Dunning’s6double-l contraction of Huzinaga’s’ 9s5p primitive set for carbon and 4s primitive set for hydrogen. The basis set for carbon was aug(4)Herman, M.S.; Goodman, J . L.J . Am. Chem. SOC.1988,110,2681. ( 5 ) Conrad, M. P.; Pitzer, R. M.; Schaefer, H. F. J . Am. Chem. Soc. 1979, 101, 2245.
(6)Dunning, T.H. J. Chem. Phys. 1970,53, 2823. (7) Huzinaga, S. J . Chem. Phys. 1965,42, 1293.
0022-365419212096-3712$03.00/00 1992 American Chemical Society
Closs's Diradical
The Journal of Physical Chemistry, Vol. 96, No. 9, 1992 3713 TABLE I: Geometrical Parameters (angstroms and degrees) for the Triplet Diradical Stationary Points Optimized at the DZ + d SCF and CISD Levels of Theory"
SCF re(C1-C2)
1.505 1.508 2.357 re(C4-G) 1.551 1.092 re(c 1-H7) 1.092 re(C1-H6) 1.075 re(C2-H8) 1.088 re(C4-H12) re(C4-H10) 1.088 Be(H7-C1-H6) 105.9 Be(C2-C1-H7) 112.0 112.0 Be(C2-C1-H6) 112.0 Be(C3-C1-H7) Be(C3-C1-H6) 112.0 123.7 Be(H&-C1) 123.4 Be(H&-C4) 111.3 Be(C2-C4-H12) Be(C2-C4-H1o) 111.3 Be(C5-C4-H12) 111.3 1 1 1.3 Be(CS-C4-Hlo) 0.0 7,(Cj-Cs
