Infrared Photodissociation Spectroscopyof

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

Metal benzene sandwiches

• 1973 Nobel to Wilkinson and Fischer for work on organometallic sandwiches1

1. Fischer, E. O.; Hafner, W. Z. Naturforsch. 1955, 10b, 665.

E. O. Fischer1955

Wilkinson and Woodward1952

Ferrocene Di-benzene chromium

Previous work on metal-benzene ions:

• Duncan group electronic photodissociation1

• Kaya and coworkers multiple-decker sandwiches and photoelectron spectroscopy (PES) on anions2

• Lisy and coworkers infrared (IR)

spectroscopy in C–H stretch region

with alkali metals3

• Duncan et al. FELIX in far IR and IR-OPO in mid IR on transition metals4

• D. S. Yang group ZEKE spectroscopy on transition metals5 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.

2700 2800 2900 3000 3100 3200 3300

V + (bz) 2 Ar

cm -1

V + (bz)Ar

V + (bz) 3

Previous work: OPO/OPA

• OPO/OPA with argon tagging used for C–H stretch region

• Free benzene Fermi resonance1: 3048, 3079, 3101 cm-1

• V+(bz)3 no Ar tagging needed, and free benzene Fermi resonance observed

• 3rd benzene is external

Jaeger, T. D.; Pillai, E. D.; Duncan, M. A., J. Phys. Chem. A 2004, 108, 6605-6610.

ν8+ν19

ν20 ν1+ν6+ ν19

1. Snavely, D. L.; Walters, V.A.; Colson, S.D.; Wiberg, K. B., Chem. Phys. Lett. 1984, 103, 423-429.

Experimental• OPO/OPA range: 600-4500 cm-1

• Binding energies Al+(bz) = 35.2 kcal/mol1, 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.

Al+(bz)Ar• 750 cm-1 ν11 oop H-bend, 77 cm-1 blue

shift

• 990 cm-1 ν1 sym. C stretch, (not IR active in free benzene)

• 1476 cm-1 ν19 in–plane C ring distortion, indicator of charge transfer1, 10 cm-1 red shift

• 1643 cm-1 ν8 C ring stretch (not IR active in free benzene), 33 cm-1 blue shift

• 3033 cm-1 ν20 C–H stretch

• 3097, 3065 cm-1 Fermi resonance: ν20 C–H stretch & ν8+ν19, ν1+ν6+ ν19, respectively

• Theory scaled for each mode

310130483079

673 1486993 1610

7371479

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.

• 3079 cm-1 Fermi resonance caused by addition of second benzene

• 1596 cm-1 ν8 ring stretch, goes from 33 cm-1 blue shift to 14 cm-1 red shift

• 1477 cm-1 ν19 in–plane C ring distortion, 9 cm-1 red shift

• 719 cm-1 ν11 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)2Ar673

1486

993 1610 310130483079

Where does a 2nd benzene go?

1. Bauschlicher, Jr., C. W.; Partridges, H. J. Phys. Chem. 1991, 95, 9694-9698.

• Al+ 3s2

• s orbital polarizable1

• 1st ligand polarizes s orbital1

e-

e-

Walters, R. S.; Brinkmann, N. R.; Schaefer, H. F.; Duncan, M. A., J. Phys. Chem. A 2003, 107, 7396-7405.

Al+(bz)3Ar

• Fourth Fermi resonance disappears

• 1643 cm-1 ν8 ring stretch goes away

• 1478 cm-1 ν19 in–plane C ring distortion, 8 cm-1 red shift

• 723 cm-1 ν11 oop H-bend, 50 cm-1 blue shift, 4 cm-1 blue shift from Al+(bz)2Ar

• Theory indicates ν11 red shifts from Al+

(bz)Ar

• Bond distance increases to 2.9 Å

6731486

310130483079

993 1610

?

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)3Ar for un-shifted bands

Free CO2 band Coordinated CO2 band

Ricks, A. M.; Brathwaite, A. D.; Duncan, M. A. J. of Phys. Chem. A 2013 117 , 1001-1010.

Al+(bz)4

• 1481 cm-1 ν19 in–plane C ring distortion, 5 cm-1 red shift

• Theory predicts a wider ν11 peak due to a 4th external benzene

• No ν11 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 30483079

993 16103101

?

Spectra of Al+(bz)1-3Ar, 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

• ν19 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 ν8 band

• Coordination is not obvious from spectra

• Theory shows a coordination of three

Binding energies: theory

Binding energies of ligand (kcal/mol)

B3LYP/

6-311+G**

MP2/

6-311+G**

B97D/

6-311+G**

Literature1

Al+-Benzene 30.0 35.4 34.1 35 ±2Al+-(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.