Transcript talk_ohio_2006_p041_2.ppt

Fritz-Haber-Institut
der
Max-Planck-Gesellschaft
A First Experimental Value for the B1u C-H
Stretch Fundamental Mode in Benzene
Undine Erlekam
Fritz-Haber-Institut
Vibrational Modes in Benzene
der
Max-Planck-Gesellschaft
•
20 symmetry unique vibrations
•
4 modes
- 2 A1g
- 7 E2g
- 13 B1u
- 20 E1u
•
strong Fermi interactions between
the B1u mode and the E2g E1u
combination band
no reliable experimental value for
the B1u mode
•
value of 3057 cm-1 derived from
product rules is questionable
•
recent anharmonic theory:
ν13 (B1u) 2990 - 3090 cm-1
in the C-H stretch region:
Raman active
not optically active
not optically active
IR active
JCP, 112, 248, (2000)
JPCA, 108, 3085, (2004)
IR activation via dimerization
Fritz-Haber-Institut
der
Max-Planck-Gesellschaft
Benzene dimer
• weakly bound Van der Waals complex:
binding energy between 500 and 1000 cm-1
•
•
•
•
distorted T-shaped structure
top is freely rotating around its C6 axis
top: C6v symmetry
stem: C2v symmetry
monomer
top
stem
D6h
C6v
C2v
B1u
B1
B1
E1u
E1
A1+B1
E2g
E2
A1+B1
A1g
A1
A1
Fritz-Haber-Institut
IR ion dip spectroscopy
der
Max-Planck-Gesellschaft
REMPI spectrum
experimental set up
excitation scheme
•
•
mixture of C6H6 and C6D6 (0.3%) in
Helium
000 transition symmetry forbidden in
benzene monomer but weakly allowed in
benzene dimer
IR spectra of benzene dimer
Fritz-Haber-Institut
der
Max-Planck-Gesellschaft
E1u E2g
c6v
c2v
?
A1g
monomer
top
stem
D6h
C6v
C2v
B1u
B1
B1
E1u
E1
A1+B1
E2g
E2
A1+B1
A1g
A1
A1
Anharmonic DFT calculations
Fritz-Haber-Institut
der
Max-Planck-Gesellschaft
anharmonic†
anharmonic†
(scaled)
(C6H6)*(C6D6)
13
(B1u)
2993.2
3011.2
3012.4
3015
-
20
(E1u)
3030.1
3048.3
3044.0
3045.6
3047.0
a
7
(E2g)
3034.8
3053.0
3056.2
3057.3
3056.7
a
2
(A1g)
3053.2
3071.5
3070.9
3073.9 b
†B3LYP
C6H6
cc-pVTZ
a Mat Fys Skr Dan Vid Selsk 1, 7 (1959)
b J.Mol.Spec., 82, 126 (1987)
Conclusions
Fritz-Haber-Institut
der
Max-Planck-Gesellschaft
• benzene dimer consists of two inequivalent subunits
IR spectra for the two moieties are different
• due to symmetry reduction all four fundamental modes of
the benzene monomer are found in the benzene dimer
• a first experimental value for the B1u fundamental mode of
the benzene monomer is determined as 3015  5 cm-1
for further information see:
The Journal of Chemical Physics 124, 171101 (2006)
An experimental Value for the B1u C-H stretch mode in benzene
Undine Erlekam, Marcin Frankowski, Gerard Meijer and Gert von Helden
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195, Berlin, Germany
Acknowledgements:
Gert von Helden, Marcin Frankowski, Gerard Meijer