Observation of Borirene from Crossed Beam Reaction of Boron Atoms with Ethylene

Nicolas Galland and Yacine Hannachi Laboratorie de Physico-Chimie Moléculaire (CNRS UMR 3803) UniVersité Bordeaux 1,351 Cours de la Libération, 330405 Talence Cedex, France ReceiVed April 26, 2000 The small-ring Hückel 2π aromatic compound borirene, (CH)2BH, has received considerable attention since Volpin et al.1 suggested that trivalent boron could replace a carbon atom of the isoelectronic cyclopropenyl cation C3H3. Ab initio studies of borirene and its derivatives demonstrated the aromatic character of the molecule, which was found to have about 70% of the resonance energy of cyclopropenyl cation and was predicted to be thermodynamically stable.2 Isolation of borirene, however, eluded chemists for a long time, and only substituted borirenes were prepared in the laboratory.3 More recently, matrix isolation spectroscopy allowed identification of the borirene molecule formed from reaction of laser ablated boron with ethylene;4 the experimental observation of the aliphatic isomers as additional reaction products in the matrix study5 stimulated theoretical calculations on the structures and energetics of singlet C2H3B species and on the possible reaction pathways.5,6 The most stable isomer of gross formula C2H3B was found to be borirene (whose aromatic character was confirmed) followed by ethynylborane (H2BCCH) and borallene (H2CCBH) which are 27 and 102 kJ mol-1, respectively, higher in energy.6 The two other possible isomers, C2H3B and H2CBCH, were found to be much less stable.6 Andrews et al. suggested the reaction pathways be B addition to the double C-C bond or B insertion into one of the C-H bond;4,5 however, the effect of matrix trapping, the poorly defined energy and the unknown composition of the laser-ablated boron reagent do not allow a definite assignment of the reaction mechanism. Investigation at the molecular level, in a collision-free environment where it is possible to observe the consequences of a single reactive event, can provide a direct insight into the reaction mechanism.7,8 Surprisingly, experimental investigation of B atom reactions at the microscopic level has practically eluded the array of the elaborate experimental techniques devised in the field of reaction dynamics, although such techniques have been successfully used to investigate the reaction dynamics of light, secondrow atoms (Li, C, N, O, and F).7 We recall that, among those techniques, the crossed molecular beam (CMB) method with mass spectrometric detection turned out to be particularly suitable for investigating reactions giving polyatomic products which are not a priori predictable and whose spectroscopic properties are unknown.7,8 The application of such a technique has one stringent prerequisite, that is it must be possible to produce a beam of the unstable (atomic or radical) species of sufficient intensity to carry out angular and velocity distribution measurements of the reaction products.8 In our laboratory, we have recently succeeded in generating a pulsed beam of boron atoms by laser ablation of a boron rod9 and undertaken a systematic investigation of B(2P) reactions with simple unsaturated hydrocarbons of potential practical interest. The aim is also to gain an insight into the chemical behavior of this atomic species which is still unexplored. In this contribution, we report the first account on the dynamics of a ground-state B(2P) reaction, namely that with ethylene. By using the CMB technique with mass spectrometric detection and combining our results with electronic structure calculations, our study gives a clear evidence of formation of borirene as primary reaction products under collision free conditions. We have performed a first scattering experiment at a collision energy, Ec, of 17.6 kJ mol-1 using the 35” universal CMB apparatus.10 Two well-collimated, supersonic beams of the reagents are crossed at 90° in a scattering chamber maintained in the 10-7-mbar range. The reaction products are detected by a rotatable electron impact quadrupole mass spectrometer, contained in an ultrahigh-vacuum (<8 × 10-13 mbar) chamber. Product velocity distributions are obtained using the time-of-flight (TOF) technique at different laboratory scattering angles and for different mass-to-charge ratios (m/e) of the ionized products. Characterization on axis of the pulsed boron beam (obtained by seeding the ablated boron in neat helium) shows that no boron clusters were present. A chopper wheel located after the ablation zone and the skimmer of the primary source selected a slice of the boron beam with a peak velocity of 1611 ( 15 m s-1 and speed ratio of 8.7 ( 0.2. The second pulsed beam of ethylene was obtained by expanding 500 Torr of pure C2H4; peak velocity and speed ratio were 895 ( 15 m s-1 and 13.0 ( 0.5, respectively. Reaction products were detected at m/e ) 38, 37, 36, 35, and 34 corresponding to the ions C2H3B, C2H2B/C2H3B, C2HB/C2H2B, C2B/C2HB, and C2B, respectively. The laboratory distributions of the different ions were found to be superimposable, which unambiguously indicates8 that the only detected product is C2H3B and that it partly fragments to daughter ions in the electron impact ionizer. No radiative association to C2H4B was detected because under single collision conditions, differently from the case of matrix experiments, the initially formed adduct fragments due to its high energy content. Because of the best signal-to-noise ratio, all the final measurements were carried out at m/e ) 37; the product laboratory angular distribution is shown in Figure 1 (top). Quantitative information on the reaction dynamics is obtained by moving from the lab coordinate system to the CM one and analyzing the product angular, T(θ), and translational energy, P(E′T), distributions into which the CM product flux can be factorized.7 The solid line superimposed on the experimental results in Figure 1 is the calculated curve when using the best-fit CM functions. A convenient way to summarize the dynamical features of the reactive event is to report the CM product flux contour map, where the intensity is given as a function of product CM velocity, u, and scattering angle, θ (Figure 2). The shape of our CM angular distribution is isotropic, that is with the same intensity in the whole angular range, as well visible † Visiting scientist; permanent address: Dipartimento di Chimica, Università di Perugia, 06123 Perugia, Italy. ‡ Also: Department of Physics, Technical University Chemnitz, 09107 Chemnitz, Germany; and Department of Physics, National Taiwan University, Taipei, 106, Taiwan, ROC. (1) Volpin, M. E.; Koreshkov, Yu. D.; Dulova, V. G.; Kursanov, D. N. Tetrahedron 1962, 18, 107-122. (2) Krogh-Jespersen, K.; Cremer, D.; Dill, J. D.; Pople, J. A.; Schleyer, P. v. R. J. Am. Chem. Soc. 1981, 103, 2589-2594. Byun, Y.-G.; Saebo, S.; Pittman, C. U. J. Am. Chem. Soc. 1991, 113, 3689-3696. (3) Eisch, J. J.; Shafii, B.; Rheingold, A. L. J. Am. Chem. Soc. 1987, 109, 2526. (4) Lanzisera, D. V.; Hassanzadeh, P.; Hannachi, Y.; Andrews, L. J. Am. Chem. Soc. 1997, 119, 12402-12403. (5) Andrews, L.; Lanzisera, D. V.; Hassanzadeh, P.; Hannachi, Y. J. Phys. Chem. A 1998, 102, 3259-3267. (6) Galland, N.; Hannachi, Y.; Lanzisera, D. V.; Andrews, L. Chem. Phys. 1998, 230, 143-151. (7) (a) Casavecchia, P. Rep. Prog. Phys. 2000, 63, 355-414. (b) Casavecchia, P.; Balucani, N.; Volpi, G. G. Annu. ReV. Phys. Chem. 1999, 50, 347376. (8) Lee, Y. T. In Atomic and Molecular Beam Methods; Scoles, G., Ed.; Oxford University Press: New York, 1987; Vol. 1, pp 553-568. (9) Balucani, N.; Asvany, O.; Lee, Y. T.; Kaiser, R. I. Manuscript in preparation. (10) We rather find an intermediate where the B atom attacks only one C atom via a barrier of 10-20 kJ mol-1 to form a BCH2-CH2 radical. The most favorable reaction channel of the latter is subsequent ring closure (barrier ∼2 kJ mol-1) to borirane. 11234 J. Am. Chem. Soc. 2000, 122, 11234-11235