Infrared Photodissociation Spectroscopy of Aluminum Benzene Cation Complexes Nicki Reishus, Biswajit Bandyopadhyay and Michael A. Duncan Department of Chemistry, University of Georgia, Athens, GA

Metal benzene sandwiches 1973 Nobel to Wilkinson and Fischer for work on organometallic sandwiches 1 1.Fischer, E. O.; Hafner, W. Z. Naturforsch. 1955, 10b, 665. E. O. Fischer 1955 Wilkinson and Woodward 1952 FerroceneDi-benzene chromium

Previous work on metal-benzene ions: Duncan group  electronic photodissociation 1 Kaya and coworkers  multiple-decker sandwiches and photoelectron spectroscopy (PES) on anions 2 Lisy and coworkers  infrared (IR) spectroscopy in C–H stretch region with alkali metals 3 Duncan et al.  FELIX in far IR and IR-OPO in mid IR on transition metals 4 D. S. Yang group  ZEKE spectroscopy on transition metals 5 1.Willey, K. F.; Yeh, C. S.; Robbins, D. L.; Duncan, M. A., J. Phys. Chem. 1992, 96, Nakajima, A.; Kaya, K., J. Phys. Chem. A 2000, 104, Cabarcos, O. M.; Weinheimer, C. J.; Lisy, J. M., J. Chem. Phys. 1999, 110, M.A. Duncan, Int. J. Mass Spectrom. 2008, 272, B. R. Sohnlein, Y. Lei and D.-S. Yang, J. Chem. Phys. 2007, 127, / /10.

Previous work: OPO/OPA OPO/OPA with argon tagging used for C–H stretch region Free benzene Fermi resonance 1 : 3048, 3079, 3101 cm -1 V + (bz) 3 no Ar tagging needed, and free benzene Fermi resonance observed 3 rd benzene is external Jaeger, T. D.; Pillai, E. D.; Duncan, M. A., J. Phys. Chem. A 2004, 108, ν 8 +ν 19 ν 20 ν 1 +ν 6 + ν 19 1.Snavely, D. L.; Walters, V.A.; Colson, S.D.; Wiberg, K. B., Chem. Phys. Lett. 1984, 103,

Experimental OPO/OPA range: cm -1 Binding energies  Al + (bz) = 35.2 kcal/mol 1, Al + (bz)Ar = 0.8 kcal/mol (MP2/6-311+G** ) Theory: B3LYP/6-311+G** Aluminum benzene mass spec: 1. Dunbar, R. C.; Klippenstein, S. J.; Hrusak, J.; Stockigt, D.; Schwarz, H. J. Am. Chem. Soc. 1996, 118,

Al + (bz)Ar 750 cm -1  ν 11 oop H-bend, 77 cm -1 blue shift 990 cm -1  ν 1 sym. C stretch, (not IR active in free benzene) 1476 cm -1  ν 19 in–plane C ring distortion, indicator of charge transfer 1, 10 cm -1 red shift 1643 cm -1  ν 8 C ring stretch (not IR active in free benzene), 33 cm -1 blue shift 3033 cm -1  ν 20 C–H stretch 3097, 3065 cm -1  Fermi resonance: ν 20 C–H stretch & ν 8 +ν 19, ν 1 +ν 6 + ν 19, respectively Theory scaled for each mode van Heijnsbergen, D.; Jaeger, T. D.; von Helden, G.; Meijer, G.; Duncan, M. A., Chem. Phys. Lett. 2002, 364, Chaquin, P.; Costa, D.; Lepetit, C.; Che, M. J. Phys. Chem. A ,

3079 cm -1  Fermi resonance caused by addition of second benzene 1596 cm -1  ν 8 ring stretch, goes from 33 cm -1 blue shift to 14 cm -1 red shift 1477 cm -1  ν 19 in–plane C ring distortion, 9 cm -1 red shift 719 cm -1  ν 11 oop H-bend, 46 cm -1 blue shift, 31 cm -1 less than Al + (bz)Ar Bond distance 2.5 Å  2.8 Å cm -1 Al + (bz) 2 Ar

Where does a 2 nd benzene go? 1.Bauschlicher, Jr., C. W.; Partridges, H. J. Phys. Chem. 1991, 95, Al +  3s 2 s orbital polarizable 1 1 st ligand polarizes s orbital 1 e-e- e-e- Walters, R. S.; Brinkmann, N. R.; Schaefer, H. F.; Duncan, M. A., J. Phys. Chem. A 2003, 107,

Al + (bz) 3 Ar Fourth Fermi resonance disappears 1643 cm -1  ν 8 ring stretch goes away 1478 cm -1  ν 19 in–plane C ring distortion, 8 cm -1 red shift 723 cm -1  ν 11 oop H-bend, 50 cm -1 blue shift, 4 cm -1 blue shift from Al + (bz) 2 Ar Theory indicates ν 11 red shifts from Al + (bz)Ar Bond distance increases to 2.9 Å ?

Vib. of external ligands are usually un-shifted External ligands cause un-shifted ligand peaks to appear But there is no evidence in Al + (bz) 3 Ar for un-shifted bands Free CO 2 bandCoordinated CO 2 band Ricks, A. M.; Brathwaite, A. D.; Duncan, M. A. J. of Phys. Chem. A ,

Al + (bz) cm -1  ν 19 in–plane C ring distortion, 5 cm -1 red shift Theory predicts a wider ν 11 peak due to a 4 th external benzene No ν 11 cm -1 peak observed (likely because of diss. energy) Again no evidence of un-shifted bands Different IR intensities for bonded vs external bands? ?

Spectra of Al + (bz) 1-3 Ar, Al + (bz) 4 do not show evidence for an external benzene ? cm -1 ?

Conclusions Best quality IR spectra yet measured for a metal ion benzene system ν 19 shows there is not much charge transfer between the Al + and benzene Theory predicts a consistent red shift for the ~700 cm -1 band, but experiments show a change in relative shift from red to blue Theory does not predict the ν 8 band Coordination is not obvious from spectra Theory shows a coordination of three

Binding energies: theory Binding energies of ligand (kcal/mol) B3LYP/ G** MP2/ G** B97D/ G** Literature 1 Al + -Benzene ±2 Al + -(Benzene) Al + -(Benzene) Al + -(Benzene) Dunbar, R. C.; Klippenstein, S. J.; Hrusak, J.; Stockigt, D.; Schwarz, H. J. Am. Chem. Soc. 1996, 118,