42 J. zyxwvutsrqp Am. Chem. zyxwvu SOC. 1995,117, 42-48 Ab Initio Molecular Dynamics Investigation of Singlet C2H2Li2: Determination of the Ground State Structure and Observation of LiH Intermediates Ursula Rdthlisberger and Michael L. Klein* Contribution ffom the Department of Chemistry, Universiry of Pennsylvania, Philadelphia, Pennsylvania 19104-6323 Received March 15, 1994@ Abstract: The structural, electronic, and dynamic properties of the organolithium compound C2H2Li2 have been investigated via ab initio molecular dynamics simulations based on the Car-Parrinello method. Dynamical simulated annealing techniques applied to search for the low-energy configurations resulted in a structure that is not a dilithioethylene isomer as suggested by the stochiometric formula but an acetylenic derivative that can be visualized geometrically as a complex of lithioacetylene with lithium hydride HCZLi-HLi. However, the ground state electronic structure is more suggestive of an ionic complex (H-C=C)-*(Li+H-Li+) in which the linear anion HCC- binds to the two lithium cations in the triangular complex Li+H-Li+. Several ethylene-like isomers were identified via high- temperature quenches, but these invariably turned out to lie at high energies (~'30 kcaymol). Analysis of the high-temperature dynamics indicated that ethylene-like isomers are always unstable toward an intramolecular hydrogen migration mediated via a lithium hydride unit. The direct observation of these intramolecular rearrangement reactions revealed the role of the lithium atoms as hydrogen transfer reagents and confirmed the importance of lithium hydride as an intermediate species. 1. Introduction Organolithium compounds have a widespread use in organic synthesis,'S2 and a characterization of their structural and electronic properties is of fundamental interest for an under- standing of their reactivity. Various mono- and polylithiated organic compounds can be readily prepared e~perimentally,~.~ but apart from a few known crystal structures (e.g., CH3,4Li5a.b and C2H5LiSC) and the structural determination of C2H2Li in an Ar matrix (IR6 and ESR7), only the most general information concerning the structures of these species is available. The fact that organolithium compounds have a strong tendency to associate into oligomeric units' has prevented a detailed structural determination in many cases. A characterization of the single molecule properties as the basis for an understanding of the more complex real systems thus mainly relies on theoretical investigations. Due to their unusual properties, such as their special ability of stabilizing unconventional forms of carbon (planar tetracoordinated carbonse or perpendicular olefinsEb),these molecules have fascinated theoreticians for more than two decades.8 A theoretical structural characterization and, in particular, the search for optimal geometries has turned out to be far from trivial. A large number of studies on different systems8 have shown that organolithium compounds tend to adopt highly unorthodox structures that cannot easily be derived from the classical hydrocarbon analogues. Even for moderately complex isolated molecules in the gas phase, a comprehensive charac- terization of the potential energy surface (PES), e.g. the localization of the lowest-energy structure(s), is a time-consum- ing and demanding task.9 The large number of geometrical possibilities as well as the occurrence of low-symmetry isomers and competing electronic states (see e.g. ref zyx 8g) has so far precluded an extensive search for many systems. Ab initio molecular dynamics (MD) simulations based on density functional theory" allow for an efficient minima search via simulated annealing techniques25and have proven to be a valuable tool in the investigation of disordered and amorphous * Author to whom correspondence should be addressed. @ Abstract published in Advance ACS Abstracts, December 1, 1994. (1) Wakefield, B. J. The Chemistry of Organolithium Compounds; Pergamon: Oxford, U.K., 1974. (2) Elschenbroich, Ch.; Salzer, A. Organometallics; Weinheim, New York, 1992. (3) See, e.g.: (a) West, R.; Rochow, E. G. zyxwvutsr J. Org. Chem. 1953,18, 1739- 1742 (1,4-dilithiobutane). (b) Applequist, D. E.; Saurbom, E. C. J. Org. Chem. 1972, 37, 1676 zyxwvutsrq (1,l-dilithiocyclopropene). (c) Momson, J. A,; Chung, C.; Lagow, R. J. J. Am. Chem. SOC. 1975, 97, 5015 (1,l- dilithiopropene). (c) hiester, W.; West, R.; Chwang, T. L. J. Am. Chem. SOC. 1976, 98, 8413. (d) Chung, C.; Lagow, R. J. J. Chem. SOC., Chem. Commun. 1978, 1078 (C2Li4). (4) (a) Seetz, J. W. F. L.; Schat, G.; Akkerman, 0. S.; Bickelhaupt, F. J. Am. Chem. SOC. 1982, 104, 3651-3655. (b) Maercker, A,; Graile, T.; Demuth, W. Angew. Chem., Inr. Ed. Engl. 1987, zyxwvutsrq 26, 1032-1034. (5) (a) Weiss, E.; Lucken, E. A. C. J. Organomet. Chem. 1964, 2, 197. (b) Weiss, E.; Hencken, G. Ibid. 1970, 21, 265. (c) Dietrich, H. Acta Crystallogr. 1963, 16, 681. (6) Manceron, L.; Andrews, L. J. Am. Chem. SOC. 1985,107,563-568. (7) Manceron, L.; Schimpf, A.; Bomemann, T.; Rosendahl, R.; Faller, 0002-7863/95/1517-0042$09.00/0 F.; Stockman, H.-J. Chem. Phys. 1993, 169, 219-229. (8) (a) Apeloig, Y.; Schleyer, P. v. R.; Binkley, J. S.; Pople, J. A,; Jorgensen, W. L. Tetrahedron Left. 1976, 3923 (LiZC2). (b) Apeloig, Y.; Schleyer, P. v. R.; Binkley, J. S.; Pople, J. A. J. Am. Chem. SOC. 1976, 98, 4332-4334 (1,l-dilithioethane). (c) Streitwieser, A., Jr.; Williams, J. E., Jr.; Alexandratos, S.; McKelvey, J. M. hid. 1976, 98, 4778. (d) Collins, J. B.; Dill, J. D.; Jemmis, E. D.; Apeloig, Y.; Schleyer, P. v. R.; Seeger, R.; Pople, J. A. Ibid. 1976,98, 5419-5427. (e) Jemmis, E. D.; Poppinger, D.; Schleyer, P. v. R.; Pople, J. A. Ibid. 1977, 99, 5796-5798 (Li3C4). zy (0 Laidig, W. D.; Schaefer, H. F. Ibid. 1978, 100, 5972. (g) Nagase, S.; Morokuma, K. bid. 1978, 100, 1661-1666. (h) Jemmis, E. D.; Chan- drasekhar, J.; Schleyer, P. v. R. Ibid. 1979, IOI, 2048-2056. (i) Laidig, W. D.; Schaefer, H. F. Ibid. 1979, 101, 7184-7188. (i) Schleyer, P. v. R.; Kos, A. J.; Kaufmann, E. bid. 1983,105,7617. (k) Disch, R. L.; Schulman, J. M.; Ritchie, J. P. Ibid. 1984, 105, 6246. (1) Schleyer, P. v. R. J. Phys. Chem. 1990, 94, 5560-5563 (Li2C2 and Li4C4). (m) Dorigo, A. E.; Van Eikema Hommes, N. J. R.; Krogh-Jespersen, K.; Schleyer, P. v. R. Angew. Chem., Inr. Ed. Engl. 1992, 12, 1602-1603 (Li4C4). (n) Nguyen, M. T.; Ha, T. K.; Yoshimine, M. Mol. Phys. 1992, 77, 921-936 (LiC2H2). (9) Boulton, E.; Schaefer, H. F., 111; Laidig, W. D.; Schleyer, P. v. R. J. Am. Chem. SOC. 1994, 116, 9602-9612. (10) Streitwieser, A., Jr. Acc. Chem. Res. 1984, 17, 353-357. (11) Car, R.; Parrinello, M. Phys. Rev. Lefr. 1985, 55, 2471-2473. 0 1995 American Chemical Society