1. Gas Phase Infrared Spectroscopy of Mass Selected Carbocations Department of Chemistry University of Georgia Athens Georgia, 30602 www.arches.uga.edu/~maduncan Allen M. Ricks, Gary E. Douberly, Michael A. Duncan

2. Protonated Molecular Complexes, Metal Ion Ligand Complexes, and Carbocations H + (CO 2 ) n -Ohio State 2007 H + (C 2 H 2 ) n -Ohio State 2007 H + (N 2 ) n -TI101 M + (L) n -M108, M109 Carbocations C 2 H 3 +, C 2 H 5 +, C 3 H 3 +, C 3 H 5 +, C 4 H 9 +,C 6 H 7 +, C 7 H 9 +, C 10 H 9 +, IR Spectroscopy probes fundamental bonding interactions and structures.  Ligand vibrational shifts  Structures, isomers  Coordination spheres  Solvation  Intracluster reactions Allyl Protonated Benzene, 2-Propenyl Protonated Toluene, Protonated Naphthalene Propargyl Cyclopropenyl Protonated Acetylene Protonated Ethylene Coming Up Next!

3. Experimental Setup 1-2.5 kV discharge through a supersonic expansion of pure Ar with precursor seeded in Mass selection of Ar tagged species in first flight tube Intersection of ions and photodissociation laser in reflectron Spectra obtained via monitoring photodissociation as a function of laser wavelength Can continuously tune 700 - 4600 cm -1

4. Obtaining Spectra on a Zero Background hνhν

5. C 3 H 3 + Cation Two low energy isomers Cyclic C 3 H 3 + is the smallest aromatic hydrocarbon 2π electrons satisfies 4n+2 rule (n=0) Cyclic neutral C 3 H 2 ubiquitous in the Interstellar Medium (ISM) 1 C 3 H 3 + proposed progenitor of C 3 H 2 via dissociative electron ion recombination 1 C 3 H 3 + + e - C 3 H 2 +H m/z=39 (C 3 H 3 + ?) detected in Halleys comet using cation mass spectrometry 2 [1] D. Smith, Chem. Rev, 1992, 92, 1473 [2] A. Korth, M. L. Marconi, D. A. Mendis, F. R. Krueger, A. K. Richter, R. P. Lin, D. L. Mitchell, K. A. Anderson, C. W. Carlson, H. Rème, J. A. Sauvaud, C. d'Uston, Nature, 1989, 337, 5. Expt 3 E = +25 kcal/mol Theor 4 E = +23.4 kcal/mol [3] F. P. Lossing, J. Can. Chem., 1972, 50, 3973. [4] D e CCSD(T) cc-pVTZ

6. Presence of Two Isomers Why both isomers present? Similar spectra seen when generating C 3 H 3 + from t-butyl chloride Similar isomer ratios obtained by Dopfer et. al. 1 and Adams et. al. 2 using dissociative proton transfer to allene to generate C 3 H 3 + Linear isomer seems to dominate at higher temperatures 2 Contour on 3092 cm -1 gives rotational temp of ~25 K Precursor: propargyl bromide [1] a) D. Roth, O. Dopfer, Phys. Chem. Chem. Phys., 2002, 4, 4844. b) O. Dopfer, D. Roth, J. Maier, Int. J. Mass. Spec., 2002, 218, 281 c) O. Dopfer, D. Roth, J. Maier, J. Am. Chem. Soc, 2002, 124, 494. [2] J. McLain, V. Poterya, C. D. Molek, D. M. Jackson, L. M. Babcock, N. G. Adams, J. Phys. Chem. A., 2005, 109, 5119.

7. Barrier to C 3 H 3 + Rearrangement Large barrier to isomer interconversion due to C-C bond breaking Linear C 3 H 3 + dominates at higher temperature while cyclic C 3 H 3 + seems to dominate at lower temperature 1 There must be another route to make the cyclic isomer Feasible? Need cyclic precursor! [1] J. McLain, V. Poterya, C. D. Molek, D. M. Jackson, L. M. Babcock, N. G. Adams, J. Phys. Chem. A., 2005, 109, 5119. singlet triplet

8. C 3 H 5 + : Two Isomers Again Present Bowers et al. suggested 1 two isomeric forms of C 3 H 5 + 18 kcal/mol barrier between isomers 2 Our IR spectra here (from ethylene discharge) indicate about 1:1 ratio. allyl (0.0) C 2v propargyl (+8.0) kcal/mol C s [1] J. Am. Chem. Soc. 102, 4830 (1980); [2] D. A. Fairley, D. B. Milligan, L. M. Wheadon, C. G. Freeman, R. G. A. R. Mablagan, and M. H. McEwan, Int. J. Mass. Spec., 1999, 185, 253

9. IR spectra from discharge with cyclopropyl bromide or allyl bromide produce more allyl and less 2-propenyl. N 2 tagging works just like Ar tagging. ethylene discharge cyclopropyl bromide discharge allyl bromide discharge allyl 2-propenyl Shifting isomer concentration by choice of precursor. allyl 2-propenyl

10. ~1585 cm -1 marker band for the allylic CCC asymmetric stretch.

11. C5H9+C5H9+ + ~1585 cm -1 marker band for the Allylic CCC asymmetric stretch. Persistent in the spectra of methyl substituted allyl cations Other species with the allyl motif…….

12. Acknowledgements The NSF for funding Assistant Prof. Gary Douberly (Fall ’08)