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
Metal benzene sandwiches
•
1973 Nobel to Wilkinson and Fischer for work on organometallic
sandwiches1
Ferrocene
Wilkinson and Woodward
1952
Di-benzene chromium
E. O. Fischer
1955
1. Fischer, E. O.; Hafner, W. Z. Naturforsch. 1955, 10b, 665.
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.
2.
3.
4.
5.
Willey, K. F.; Yeh, C. S.; Robbins, D. L.; Duncan, M. A., J. Phys. Chem. 1992, 96, 9106-9111.
Nakajima, A.; Kaya, K., J. Phys. Chem. A 2000, 104, 176-1913.
Cabarcos, O. M.; Weinheimer, C. J.; Lisy, J. M., J. Chem. Phys. 1999, 110, 8429-8435.
M.A. Duncan, Int. J. Mass Spectrom. 2008, 272, 99.
B. R. Sohnlein, Y. Lei and D.-S. Yang, J. Chem. Phys. 2007, 127, 114302/1-114302/10.
Previous work: OPO/OPA
ν +ν
19
8
ν20

•
OPO/OPA with argon tagging used for C–H
stretch region
•
Free benzene Fermi resonance1: 3048,
3079, 3101 cm-1
ν1+ν6+ ν19

+
V (bz)3
+
V (bz)2Ar
•
V+(bz)3 no Ar tagging needed, and free
benzene Fermi resonance observed
•
3rd
+
V (bz)Ar
benzene is external
2700
2800
2900
3000
3100
3200
3300
-1
cm
Jaeger, T. D.; Pillai, E. D.; Duncan, M. A., J. Phys. Chem. A 2004, 108,
6605-6610.
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
673
•
750
 ν11 oop H-bend, 77
blue shift
•
990 cm-1 ν1 sym. C stretch, (not IR
active in free benzene)
cm-1
cm-1
•
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
•
730 993
1610
1481
990
1610
3079
3048 3101
3121
737
1479
Theory scaled for each mode
981
1. Chaquin, P.; Costa, D.; Lepetit, C.; Che, M. J. Phys.
Chem. A 2001 105, 4541-4545.
1486
van Heijnsbergen, D.;
Jaeger, T. D.; von
Helden, G.; Meijer,
G.; Duncan, M. A.,
Chem. Phys. Lett.
2002, 364, 345-351.
Al+(bz)2Ar
•
3079
 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
673
1486
1610
993
cm-1
3079
3048 3101
Where does a 2nd benzene go?
•
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.
1. Bauschlicher, Jr., C. W.; Partridges, H. J. Phys. Chem. 1991, 95, 9694-9698.
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
673
993
1486
1610
?
3079
3101
3048
Vib. of external ligands are usually un-shifted
•
•
External ligands cause un-shifted ligand
peaks to appear
Free CO2 band
Coordinated CO2 band
But there is no evidence in Al+(bz)3Ar for
un-shifted bands
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
673
993
?
•
Different IR intensities for bonded vs
external bands?
1486
1610
3079
3048 3101
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
Literature1
Binding energies B3LYP/
of ligand
6-311+G**
(kcal/mol)
MP2/
B97D/
6-311+G**
6-311+G**
Al+-Benzene
30.0
35.4
34.1
35 ±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.