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 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.

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), Cabarcos, O. M.; Weinheimer, C. J.; Lisy, J. M., J. Chem. Phys. 1999, 110 (17), Willey, K. F.; Yeh, C. S.; Robbins, D. L.; Duncan, M. A., J. Phys. Chem. 1992, 96 (23),

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),

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),

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),

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

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

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

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), Al +  3s 2 S orbital polarizable 1 st ligand polarizes S orbital

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

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

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 + -Benzene M + -(Benzene) 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),

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