Consistent assignment of the vibrations of para-disubstituted benzene molecules Anna Andrejeva,...
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Consistent assignment of the vibrations of para-disubstituted benzene molecules Anna Andrejeva, Alison J. Lee and Timothy G. Wright 69 th International
Consistent assignment of the vibrations of para-disubstituted
benzene molecules Anna Andrejeva, Alison J. Lee and Timothy G.
Wright 69 th International Symposium on Molecular Spectroscopy
University of Illinois at Urbana-Champaign RC07/ P516
Slide 2
Introduction Wilson 1 nomenclature assigns the vibrations of a
substituted benzene molecule in terms of benzene vibrations
Varsanyi 2 scheme proposes a series of normal vibrational modes for
a series of different categories of substituted benzenes arbitrary
division of light and heavy substituents Mulliken 3 scheme numbers
the vibrations of a molecule according to their symmetry and
decreasing wavenumber. Creates difficulty e.g. toluene and
fluorobenzene Gardner-Wright 4 - new nomenclature where ring-
localised vibrations having the same atomic motion are given the
same label, allowing these vibrations to be compared
straightforwardly across different substituted benzenes 1.E. B.
Wilson Jr., Phys. Rev., 45, 706 (1934) 2.G.Varsanyi, Assignments of
the vibrational Spectra of Seven Hundred Benzene Derivatives,Wiley,
New York, 1974, Vol. I and II 3.R. S. Mulliken, J. Chem. Phys.,
23,1997 (1955) 4.A. M. Gardner and T. G. Wright, 135, 114305
(2011)
Slide 3
Monosubstituted benzenes Gardner-Wright scheme A. M. Gardner
and T. G. Wright, 135, 114305 (2011) Para-disubstituted
benzenes
Slide 4
Benzene vs pDFB Figure 1. Calculated Duschinsky matrix elements
for benzene S 0 vs p-difluorobenzene S 0. Benzene vs
p-difluorobenzene vibrational mode comparison via a generalised
Duschinsky approach. This shows whether a vibration is a pure
Wilson mode or not. Vibrational modes are highly mixed, indicating
use of Wilson notation describing substituted benzene vibrations is
questionable. B3LYP/ aug-cc-pVTZ -S 1 TD-DFT - Scaling factor
0.97
Slide 5
Figure 2. Calculated Duschinsky matrix elements for
fluorobenzene S 0 vs chlorobenzene S 0. Fluorobenzene is viewed as
prototypical system Gardner-Wright scheme Fluorobenzene vibrational
modes serve as a model upon which benzene derivative vibrations are
labelled. Vibrations having the same atomic motion are given the
same label Each mode corresponding to a vibration is given the
symbol M i Figure 3. Calculated Duschinsky matrix elements for
fluorobenzene S 0 vs p-difluorobenzene S 0.
Slide 6
Wavenumber / cm -1 Why p- Difluorobenzene? A1A1 A2A2 B2B2 B1B1
Substituent mass / amu
Slide 7
New Labelling Scheme - p D i p-Difluorobenzene vibrational
modes serve as a model upon which benzene derivative vibrations are
labelled. Nearly complete diagonalisation occurs Each mode
corresponding to a vibration is given the symbol p D i Ordered by
symmetry then decreasing wavenumber Substituents treated as a point
mass
Slide 8
Wavenumber / cm -1 Substituent mass / amu A1A1 A2A2 B1B1 B2B2
Comparison to experimental data 5 5. A. Stojiljkovi and D.H.
Whiffen, Spectrochimica Acta, 12, 47 (1958)
Slide 9
What about excited states? Can assign corresponding vibrational
modes with confidence Calculated Duschinsky matrix elements for
p-DFB S 0 and p-DFB S 1 Calculated Duschinsky matrix elements for
p- DFB S 0 and p-FT S 1
Slide 10
Spectra Comparison Wavenumber / cm -1 (1+1) REMPI spectrum from
V. L. Ayles, C. J. Hammond, D. E. Bergeron, Q. J. Richards and T.
G. Wright, J. Chem. Phys., 126, 244304 (2007) Adapted LIF spectrum
from A. E. W. Knight and S. H. Kable, J. Chem. Phys., 88, 7139
(1988)
Slide 11
Take home message Have applied Gardner-Wright scheme
successfully to para- disubstituted benzene molecules yielding a
new labelling scheme for para-disubstituted benzene molecules p D i
The same vibrational motions have the same p D i label for all
para- disubstituted benzene molecules regardless of the energy
ordering or changes in molecular symmetry, allowing straightforward
comparison between species Over horizon: study more extensively S 1
states of para-disubstituted benzene systems the effect of the meta
and ortho substituent positions may have on the vibrational
modes.
Slide 12
Acknowledgements Prof. Tim Wright for the processor time on
their HPC I. Pugliesi and C. Schriever for developing program for
DTA Project Studentship Alison Jasmin Lee