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/

2. Metal benzene sandwiches In 1973 the Nobel prize was awarded to Geoffrey Wilkinson and Ernst Fischer for their work on sandwich compounds. 18 electron rule Fischer, E. O.; Hafner, W. Z. Naturforsch. 1955, 10b, 665. Sidgwick, N. V., The Electronic Theory of Valency. 1st ed.; Oxford University Press: Great Britain, 1927.

3. Previous work: Electronic spectroscopy: Duncan group reported charge transfer photodissociation for metal ion-benzene complexes 1 Mass Spec: Kaya and coworkers found multiple-decker sandwiches of transition metal-benzene complexes 2 Infrared Spec: Lisy and coworkers did infrared (IR) spectroscopy in the C-H stretch region on alkali cations on benzene 3 2. Nakajima, A.; Kaya, K., J. Phys. Chem. A 2000, 104 (2), 176-191 3. Cabarcos, O. M.; Weinheimer, C. J.; Lisy, J. M., J. Chem. Phys. 1999, 110 (17), 8429-8435. 1. Willey, K. F.; Yeh, C. S.; Robbins, D. L.; Duncan, M. A., J. Phys. Chem. 1992, 96 (23), 9106-9111.

4. FELIX IR-REMD (resonance enhanced multiphoton dissociation) used to study Fe, Co, Ni, Ti, V complexes in far-IR ν 19 red-shifted ν 11 blue-shifted Trends not clear due to broad peaks Jaeger, T. D.; van Heijnsbergen, D.; Klippenstein, S. J.; von Helden, G.; Meijer, G.; Duncan, M. A., J. Am. Chem. Soc. 2004, 126 (35), 10981-10991.

5. OPO/OPA OPO/OPA used for C-H stretch region Ar tagging Fermi triad not seen until 3 rd benzene Peak from 1 & 2 benzenes seen under Fermi triad Jaeger, T. D.; Pillai, E. D.; Duncan, M. A., J. Phys. Chem. A 2004, 108 (32), 6605-6610.

6. Al-bz FELIX 730 cm -1  ν 11 (out-of-plane H bend) 990 cm -1  ν 1 not IR active in free benzene (sym C stretch) 1481 cm -1  ν 19 (in-plane C ring distortion) van Heijnsbergen, D.; Jaeger, T. D.; von Helden, G.; Meijer, G.; Duncan, M. A., Chem. Phys. Lett. 2002, 364 (3-4), 345-351. 1420 959 700 992

7. Experimental Nd:YAG laser OPO/OPA range: 2000-4500 cm -1 AgGaSe crystal increases scanning range: 600-2000 cm -1 Aluminum benzene mass spec:

8. FWHM ~50-70 cm -1 FWHM ~8 cm -1 1634 cm -1  sym. in-plane C-H bend, not IR active in bz

9. Al + (bz)Ar 3040 cm -1  C-C stretching 3064 cm -1  C-C stretching and bending 3102 cm -1  C-H stretch Binding energies: Al + (bz) = 30 kcal/mol, V + (bz) = 47.5 kcal/mol 6731486 3048 3079 3101

10. Where does a 2 nd benzene go? Walters, R. S.; Brinkmann, N. R.; Schaefer, H. F.; Duncan, M. A., J. Phys. Chem. A 2003, 107 (38), 7396-7405. Al +  3s 2 S orbital polarizable 1 st ligand polarizes S orbital

11. Symmetric sandwich or not? Fermi triad is seen, but greater red shift New band at 3070 cm -1

12. Comparison to benzene dimer Erlekam, U.; Frankowski, M.; Meijer, G.; von Helden, G., J. Chem. Phys. 2006, 124 (17). “Stem” “Top” For “stem” benzene the weakest peak is seen 3070 cm -1  π H-bond

13. Potential Structures: Theory Al + (bz)Ar Al + (bz) 2 Ar Binding energies of ligand (kcal/mol) Al + B3LYP/6-311+G** V + See ref. Co + See ref. M + -Benzene30.047.558.6 M + -(Benzene) 2 11.632.235.9 M + - Benzene-Ar0.1n/a M + -(Benzene) 2 -Ar0.1n/a Jaeger, T. D.; van Heijnsbergen, D.; Klippenstein, S. J.; von Helden, G.; Meijer, G.; Duncan, M. A., J. Am. Chem. Soc. 2004, 126 (35), 10981-10991.

14. Conclusions The mid and far-infrared spectra of argon tagged Al + (bz) 1,2 is measured. Better resolution than spectra from FELIX and increased range Band shifting clearly seen for the Al + (bz) Al + (bz) 2 appears to have an external benzene Better theory needed for Al + (bz) 2