J. Am. Chem. zyxwvut SOC. 1991, 113, zyxwvu 4505-4517 4505 the hyperfine coupling zyxwvutsr constants measured in derivatives of C O T - may be providing less information about the degree of bond al- ternation at the equilibrium geometries of these radical ions than about the ease with which geometries with more nearly equal bond lengths are accessed. On the basis of INDO calculations, Hammons, Bernstein, and MyersI3 have advanced explanations of the effects of substituents on the EPR spectra of COT'- that are similar to those presented here. Other re~earchers~~.~~ have proposed an alternative model, which assumes bond alternation does not occur, so that the NBMOs in eq 1 are not mixed. Instead, this latter model pos- tulates that there is a Boltzmann poDulation of the lowest excited methyl, and cyano derivatives. zyxw Our calculations suggest that these substituents have a relatively small effect on the extent of bond length alternation at the equilibrium geometry. However, both F, a zyxwvu A donor, and CN, a A acceptor, are found to reduce sub- stantially the barrier to bond equalization. Acknowledgment. We thank the National Science Foundation for its support of this research and for providing funds that enabled the purchase of the Convex C-2 computer, on which some of the calculations reported here were performed. We also thank the San Diego Supercomputer Center for a generous allocation of time on the Cray YMP-8/864 computer at SDSC. electronic state in which one eledtron is thermally excited from Registry No. COT, 34510-85-5; F-COT, 70741-95-6; CH3-COT, the lower energy of the two NBMOs to the upper. 3451 9-36-3; CN-COT, 70741-98-9. Our CI calculations indicate that the basic assumption of the latter model is incorrect, since we find that bond alternation is energetically favorable, not only in COT*-, but also in its fluoro, Supplementary Material Available: ROHF/3-21G-optimized geometries and ROHF and CI energies for bond-alternated (Ch angle-alternated (C,,,), and midpoint geometries of fluorocvclo- . -,- ocktetraene radical anion (2 pap&). OTdering information is given on any current masthead page. (31) Concepcion, J. G.; Vincow, G. J. Phys. Chem. 1975,79,2042-2048. Molecular Mechanics (MM3) Calculations on Aldehydes and Ketones Norman L. Allinger,* Kuohsiang Chen, Mita Rahman,' and Ahammadunny Pathiaseri12 Contribution from the School of Chemical Sciences, Department of Chemistry, University of Georgia, Athens, Georgia 30602. Received July 26, 1990 Abst~act: Aldehydes and ketones have been studied in some detail by using the MM3 molecular mechanics method. Approximately 50 structures have been calculated and compared with experimental data where available. Comparisons are also made of conformational equilibria, torsional potentials, moments of inertia, vibrational spectra, heats of formation, and other data. On the whole, the calculations yield information of experimental accuracy. The exception is in the case of vibrational spectra, where the rms error over four simple compounds amounts to 42 cm-I. Heats of formation for 35 compounds are calculated to within 0.41 kcal/mol. Introduction Earlier papers have described the MM3 force field,3 which has previously been used for calculations on hydrocarbons$ alcohols and ethers? amines: alkenes,' and conjugated hydrocarbons.8 The present work is concerned with the extension of these calculations to the important class of carbonyl compounds. A comprehensive study of carbonyl compounds with an early force field was reported some years ag0.~9'O It was shown that ~~ (1) Current addrm: Molecular Design Limited, 2132 Farallon Drive, San Leandro, CA 94577. (2) This paper is taken in part from the Ph.D. Dissertation of A.P., submitted to the University of Georgia, March 1987. Current address: School of Pharmacy, Division of Medicinal Chemistry, 425 North Charter Street, University of Wisconsin, Madison, WI 53706. (3) The MM3 program is available from the Technical Utilization Cor- poration, Incorporated,235 Glen Village Court, Powell, OH 43065, and from Molecular Design Limited, 2132 Farallon Drive, San Leandro, CA 94577. The current version is available to run on VAX computers. Modifications for other machines are being made, and interested parties should contact one of the distributors directlv. ~~~~ ~...~ (4) Allinger, N. L.;'Yuh, Y. H.; Lii, J.-H. J. Am. Chem. Soc. 1989, Ill, 8551. 8566. 8576. . . - . , . . - -. - - -. (5) Allinger, N . L.; Rahman, M.; Lii, J.-H. J. Am. Chem. Soc. 1990,112, (6) Schmitz, L. R.; Allinger, N . L. J. Am. Chem. Soc. 1990, 112, 8307. (7) Allinger, N. L.; Li, F.; Yan, L. J. Compur. Chem. 1990, 11, 848. (8) Allinger, N . L.; Li, F.; Yan, L.; Tai, J. C. J. Compur. Chem. 1990,II, 8293. 868. in general, a great many structural features for these compounds could be well reproduced. At the time that work was carried out, the rotational profile about the central bond in 2-butanone was not known, and it was assumed to be quite similar to the similar profile in propanal. It was subsequently shown by ab initio calculations reported by Wiberg and Martin" that the gauche conformation is not really a stable conformation, separated by a significant barrier from the anti, but rather the gauche con- formation is just a shoulder on the side of the anti potential well. This was corrected in MM2(77).'2313 Over the years it was showni4that there are many errors, mostly small, built into the MM2 force field. Rather than try to continue to patch these, it was decided to start again from the beginning and generate a new force field, which is called MM3. In addition to fitting the information which was previously fit for carbonyl compounds with MM2, including the corrected 2-butanone ro- (9) Allinger, N. L.; Tribble, M. T., Miller, M. A. Tetrahedron 1972, 28, tin-ii~n .. .- (10) Profeta, S., Jr.; Allinger, N. L., this work was never published in the ordinary way, but the results of it were included in the MM2 parameter set (ref 15). An updated modification of this work was published in ref 12. (11) Wiberg, K. B.; Martin, E. J. Am. Chem. Soc. 1985, 107, 5035. (12) Bowen, P.; Pathiaseril, A.; Profeta, S., Jr.; Allinger, N . L. J. Org. Chem. 1987, 52, 5162. (13) Goldsmith, D. J.; Bowen, J. P.; Qamhiyeh, E.; Still, W. C. J. Org. Chem. 1987, 52,951. (14) Lipkowitz, K. B.; Allinger, N. L. zyxw QCPE Bulletin 1987, 7, 19. 0002-7863/91/1513-4505$02.50/0 0 1991 American Chemical Society