Dehydrocyclooctatetraene. Photoelectron Spectroscopy of the C8H6 Anion

The photoelectron spectrum of the dehydrocyclooctatetraene negative ion, C8H6, is reported. The spectrum strongly resembles that previously reported for the cyclooctatetraene anion, indicating that the structure of C8H6 is very similar to that of C8H8. Two electronic states of dehydrocyclooctatetraene are observed in the photoelectron spectrum. The lowest energy feature is assigned to singlet 1,3,5-cyclooctatrien-7-yne, while the higher energy band corresponds to a triplet state of dehydrocyclooctatetraene. The electron affinity of C8H6 is found to be 1.044 ( 0.008 eV, and the energy difference between the singlet and triplet states is 0.708 ( 0.006 eV. Vibrational activity is observed in the photoelectron spectrum and assigned using a simple potential energy surface. Stretching of the triple bond in cyclooctatrienyne is found to have a frequency of 2185 cm-1, essentially what is expected for a triple bond within an eight-membered ring. Ab initio and density functional molecular orbital calculations on dehydrocyclooctatetraene and the corresponding ions are reported. Cyclooctatrienyne is calculated to have a planar or pseudoplanar structure, consistent with assignments based on peak widths in the photoelectron spectrum. A continuing challenge for chemists is the preparation and characterization of cyclic molecules that contain a triple bond. Although they are typically highly strained, many cyclic alkynes have been prepared and characterized.1 Cyclooctyne is the smallest unsubstituted cycloalkyne that can be isolated,2,3 but spectroscopic evidence for systems as small as cyclopentyne has been provided.4 In many cases, the sole evidence for the existence of the intermediate is the products obtained in cycloaddition reactions with diene trapping reagents. In this paper, we report our studies of dehydrocyclooctatetraene (DHCOT), a cyclic alkyne that has been trapped in this fashion. Many previous studies have been carried out on DHCOT, studies primarily designed to investigate the extent of planarity in the molecule. Semiempirical MO calculations have been used to investigate the possible electronic states of DHCOT. Using MINDO/3, Huang et al.5 found that introducing a triple bond into cyclooctatetraene (COT) to form 1,3,5cyclooctatrien-7-yne, 1, leads to planarization of the normally tub-shaped molecule.6 The highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals of 1 are shown in Chart 1. The HOMO is a bonding b1 π orbital, and is similar to the b1u orbital in planar COT.7,8 The LUMO of 1 is an antibonding a2 orbital, and resembles the b2u orbital in planar COT. From these orbitals, it is possible to form additional electronic states of DHCOT. For example, double occupancy of the a2 orbital gives 1,2,3,5,7-cyclooctapentaene, 2, which has a cyclic cumulene structure. Calculations on 1, 2, and additional DHCOT isomers were carried out by using MNDO9 and MNDO/CI10 protocols. In all cases, 1 was found to be the lowest energy monocyclic state, with 2 higher in energy by 9 to 57 kcal/mol.9,10 A B2 state of DHCOT, 3, is formed by single occupation of each orbital. There have not been any previous studies of this electronic state. Experimental evidence for the existence of 1 is inferred from studies of reaction products and mechanisms. Krebs11 has shown that the reaction of bromocyclooctatetraene with potassium tert-butoxide in the presence of tetraphenylcyclopentadienone or phenyl azide leads to the formation of cyclooctatetraene derivatives as shown in eqs 1 and 2, respectively. If the reaction is carried out without the trapping agents, the products shown in eq 3 are obtained. These results indicate the presence of † Present address: Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX 79409-1061. (1) Krebs, A.; Wilke, J. Top. Curr. Chem. 1983, 109, 189 and references therein. (2) Blomquist, A. T.; Liu, L. H. J. Am. Chem. Soc. 1953, 75, 2153. (3) Wittig, G.; Krebs, A. Chem. Ber. 1961, 94, 3260. (4) Chapman, O. L.; Gano, J.; West, P. R.; Regitz, M.; Maas, G. J. Am. Chem. Soc. 1981, 103, 7033. (5) Huang, N. Z.; Mak, T. C. W.; Li, W.-K. Tetrahedron Lett. 1981, 38, 3765. (6) Bastiansen, O.; Hedberg, L.; Hedberg, K. J. Chem. Phys. 1957, 27, 1311. (7) Hrovat, D. A.; Borden, W. T. J. Am. Chem. Soc. 1992, 114, 5879. (8) Wenthold, P. G.; Hrovat, D.; Borden, W. T.; Lineberger, W. C. Science 1996, 272, 1456. (9) Glidewell, C.; Lloyd, D. Tetrahedron 1984, 40, 4455. (10) Dewar, M. J. S.; Merz, K. M., Jr. J. Am. Chem. Soc. 1985, 107, 6175. (11) Krebs, A. Angew. Chem., Int. Ed. Engl. 1965, 4, 953. Chart 1 7772 J. Am. Chem. Soc. 1997, 119, 7772-7777 S0002-7863(97)00934-7 CCC: $14.00 © 1997 American Chemical Society cyclooctatrienyne, 1. Products corresponding to trapping of 2 were not reported for these reactions,11,12 which suggests that 2 either does not exist or is much higher in energy than 1. Additional instances of the trapping of 1 have subsequently been reported.13-17 Although 1 itself is too reactive to be isolated, stable annulated derivatives, such as 4, have been prepared and characterized.18-21 These derivatives are essentially planar, with large CCC bond angles about the acetylenic carbons. The bond lengths between the dehydrocarbons were found by using X-ray crystallography to be 1.211 ( 0.008 and 1.22 ( 0.010 Å for 4a20 and 4b,21 respectively, indicating a high degree of triple bond character. Upon cursory inspection, planar 1 would appear to resemble its 6-member-ring counterpart, o-benzyne. Both molecules are planar and fully conjugated. However, while the π system of o-benzyne is aromatic and delocalized,22 1 is not aromatic, and it has alternating single and double bonds.9,10 We have now used negative ion photoelectron spectroscopy to study the properties of DHCOT. We have found that the lowest energy form of the DHCOT ion consists of attachment of an electron to 1, and, upon detachment of the ion, both 1 and 3 are formed. The photoelectron spectrum of the negative ion of DHCOT, 1-, is significantly different than that of the o-benzyne ion23 and, rather, is very similar to the photoelectron spectrum of the cyclooctatetraene negative ion, COT-.8 The CtC stretching frequency in 1 is found to be 2185 cm-1, much higher than that in o-benzyne but consistent with what is expected for a normal triple bond. Finally, the singlet-triplet splitting in DHCOT is obtained, and found to be comparable to that in planar COT. Experimental Section The photoelectron spectrometer and experimental procedures used to carry out this work have been described in detail previously,24 and only a summary is provided here. Atomic oxygen ions, O-, are created by a microwave discharge in O2 seeded in approximately 0.5 Torr of helium in a liquid nitrogen cooled flowing afterglow apparatus. Cyclooctatetraene is added through ring inlets downstream from the discharge source. The Oion abstracts H2 from COT to make C8H6 ions.25 A small portion of the ions in the flowing afterglow are extracted through a 1-mm orifice in a nosecone into a differentially pumped chamber, where they are focused, accelerated to 735 eV, mass selected by using a Wien velocity filter (M/∆M ≈ 40), and decelerated to 40 eV. The ion beam is crossed with the 351-nm output of an argon ion laser in a build-up cavity described in detail previously.24 Photodetached electrons are energy analyzed by using a hemispherical analyzer, which has an resolution of ca. 8 meV, and detected using positionsensitive detection. The photoelectron spectrum depicts the number of electrons detected as a function of electron binding energy, the difference between the laser photon energy (3.531 19 eV) and the electron kinetic energy. The absolute energy scale is calibrated by using the position of the P2 + er P3/2 peak in the spectrum of O(EA(O) ) 1.461 12 eV).26 A small energy scale compression factor is determined by comparing the measured relative peak positions in the spectrum of tungsten ion with the known term energies of the tungsten atom.27 The extent of the scale compression is less than 1%, and absolute photoelectron energies can be obtained to an accuracy of (0.003 meV. Materials. All reagents were purchased from commercial suppliers and were used as received. Cyclooctatetraene (98%) was obtained from Aldrich. Gas purities were He (99.995%) and O2 (99%). Results and Discussion In this section, we present the photoelectron spectrum of the DHCOT anion, 1-, along with a vibrational analysis. However, before doing so, we provide a description of the structure of the electronic states of DHCOT and the negative ions derived from them. An understanding of the electronic structure is essential in order to interpret the photoelectron spectrum. Electronic Structure Calculations. We have used ab initio and density functional calculations to investigate the geometries and energies of planar 1, 2, and 3.28 The geometries calculated at the Becke3LYP/6-31G* and R(O)HF/6-31G* levels of theory are shown in Figure 1. As expected, 1 and 2 clearly contain alternating single and double bonds, while the bond lengths calculated for 3 are essentially midway between those in 1 and 2. The bond angles do not differ significantly among the three (12) Krebs, A.; Byrd, D. Liebigs Ann. Chem. 1967, 707, 66. (13) Sanders, D. C.; Marczak, A.; Melendez, J. L.; Schecter, H. J. Org. Chem. 1987, 52, 5622. (14) Harmon, C. A.; Streitweiser, A., Jr. J. Org. Chem. 1973, 38, 549. (15) Lankey, A. S.; Ogliaruso, M. A. J. Org. Chem. 1971, 36, 3339. (16) Elix, J. A.; Sargent, M. V.; Sondheimer, F. J. Am. Chem. Soc. 1970, 92, 963. (17) Elix, J. A.; Sargent, M. V. J. Am. Chem. Soc. 1969, 91, 4734. (18) Wong, H. N. C.; Garratt, P. J.; Sondheimer, F. J. Am. Chem. Soc. 1974, 96, 5604. (19) Wong, H. N. C.; Sondheimer, F. Tetrahedron 1981, 37(S1), 99. (20) de Graff, R. A. G.; Gorter, S.; Romers, C.; Wong, H. N. C.; Sondheimer, F. J. Chem. Soc., Perkin Trans. 2 1981, 478. (21) Chan, T.-L.; Mak, T. C. W.; Poon, C.-D.; Wong, H. N. C.; Jia, J. H.; Wang, L. L. Tetrahedron 1986, 42, 655. (22) Sheiner, A. C.; Schaefer, H. F., III Chem. Phys. Lett. 1991, 177, 471. (23) Leopold, D. G.; Stevens-Miller, A. E.; Lineberger, W. C. J. Am. Chem. Soc. 1986, 108, 1379. (24) Ervin, K. M.; Lineberger, W. C. In AdVances in Gas Phase Ion Chemistry; Adams, N. G., Babcock, L. M., Eds.; JAI Press: Greenwich, 1992; Vol. 1, p 121. (25) Lee, J.; Grabowski, J. J. Chem. ReV. 1992, 92, 1611. (26) Neumark, D. M.; Lykke, K. R.; Andersen, T.; Lineberger, W. C. Phys. ReV. A 1985, 32, 1890. (27) Moore, C. E. Atomic Energy LeVels; US GPO: Washington, 1952; Circular No. 467. (28) Gaussian 94, Frisch, M. J.; Trucks, G. W.; Schlegal, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Kieth, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; HeadGordon, M.; Gonzalez, C.; Pople, J. A., Gaussian, Inc.: Pittsburgh, PA, 1994. Photoelectron Spectroscopy of the C8H6 Anion J. Am. Chem. Soc., Vol. 119, No. 33, 1997 7773