1. Infrared Photodissociation Spectroscopy of Aluminum Benzene Cation Complexes Nicki Reishus, Biswajit Bandyopadhyay and Michael A. Duncan Department of Chemistry, University of Georgia, Athens, GA 30602 www.arches.uga.edu/~maduncan/ nreishus@uga.edu; maduncan@uga.edu

2. 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

3. 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, 9106-9111. 2.Nakajima, A.; Kaya, K., J. Phys. Chem. A 2000, 104, 176-1913. 3.Cabarcos, O. M.; Weinheimer, C. J.; Lisy, J. M., J. Chem. Phys. 1999, 110, 8429-8435. 4.M.A. Duncan, Int. J. Mass Spectrom. 2008, 272, 99. 5.B. R. Sohnlein, Y. Lei and D.-S. Yang, J. Chem. Phys. 2007, 127, 114302/1-114302/10.

4. 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, 6605-6610. 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, 423-429.

5. Experimental OPO/OPA range: 600-4500 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, 5277-5283.

6. 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 3101 3048 3079 673 1486 9931610 737 1479 3121 981 730 990 1610 1481 van Heijnsbergen, D.; Jaeger, T. D.; von Helden, G.; Meijer, G.; Duncan, M. A., Chem. Phys. Lett. 2002, 364, 345-351. 1.Chaquin, P.; Costa, D.; Lepetit, C.; Che, M. J. Phys. Chem. A 2001 105, 4541-4545.

7. 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 673 1486 993 1610 3101 3048 3079

8. Where does a 2 nd benzene go? 1.Bauschlicher, Jr., C. W.; Partridges, H. J. Phys. Chem. 1991, 95, 9694-9698. 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, 7396-7405.

9. 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 673 1486 3101 3048 3079 993 1610 ?

10. 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 2013 117, 1001-1010.

11. Al + (bz) 4 1481 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? 673 1486 3048 3079 993 1610 3101 ?

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

13. 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

14. Binding energies: theory Binding energies of ligand (kcal/mol) B3LYP/ 6-311+G** MP2/ 6-311+G** B97D/ 6-311+G** Literature 1 Al + -Benzene30.035.434.135 ±2 Al + -(Benzene) 2 11.6-19.6- Al + -(Benzene) 3 4.5--- Al + -(Benzene) 4 2.8--- 1. Dunbar, R. C.; Klippenstein, S. J.; Hrusak, J.; Stockigt, D.; Schwarz, H. J. Am. Chem. Soc. 1996, 118, 5277-5283.