Consistent_assignment

Report
Unravelling the assignments of the
vibrations of the monosubstituted
benzenes
Adrian M. Gardner and Timothy G. Wright
67th International Symposium on Molecular Spectroscopy
Ohio State University
19th June 2012
Previous Assignment Schemes
1.
•
•
Mulliken Notation
Vibrations are listed in order of symmetry.
Numbered in each symmetry group in decreasing frequency.
2.
•
Wilson Notation
Vibrations of substituted benzene molecules are labelled in terms of
benzene vibrations.
3.
•
Varsányi Notation
Normal modes of vibration suggested explicitly for each type of
substitution.
Previous Assignment Schemes
Wilson Mode1 15
~1350 cm-1
Varsányi Mode2 15
~300 cm-1
Calculated lowest frequency b2 mode of fluorobenzene.1
1Calculated
at the B3LYP/aVDZ level. 2G. Varsányi , Assignments of the Vibrational Spectra of Seven
Hundred Benzene Derivatives, (Wiley, New York, 1974) Vol. 2.
Fluorobenzene
•Vibrational frequencies have been calculated at the B3LYP/aVDZ level for the S0
electronic state of fluorobenzene.
•The agreement with experimentally determined frequencies is excellent.
•However, the assignment of the calculated normal vibrational modes more
complicated...
•Comparison to the calculated normal modes to those determined by Varsányi in
some cases is easy.
Varsányi mode 6b
Calculated 6b mode
Fluorobenzene
•Vibrational frequencies have been calculated at the B3LYP/aVDZ level for the S0
electronic state of fluorobenzene.
6a
1
12
•The agreement with experimentally determined frequencies is excellent.
•However, the assignment of the calculated normal vibrational modes is more
complicated...
Varsányi normal modes
•Comparison to the calculated
normal
modes
to-1those
determined
-1
~500 cm
~800 cm
~1000
cm-1 by Varsányi in
some cases is easy...
•But in many cases this is very difficult...
Calculated normal mode
814 cm-1
Fluorobenzene
•Through comparisons of the vibrational wavefunctions of benzene and fluorobenzene
it is clear that the Wilson notation approach also fails in many cases.
•Pugliesi et al.3 recently investigated this using a Duschinsky mixing approach.
3. I. Pugliesi, N. M. Tonge, and M. C. R. Cockett, J. Chem. Phys., 129, 104303, (2008)
A New Assignment Scheme
• Use the calculated normal modes of fluorobenzene as a basis for a
labelling scheme for monosubstituted benzene vibrations.
• We propose each normal mode is denoted Mi where i is the number of the
mode.
Chlorobenzene
Matrix representing the Duschinsky matrixa elements for benzene/chlorobenzene.
aCalculated
using FC-LabII. I. Pugliesi and K. Müller-Dethlefs, J. Phys. Chem. A 110, 4657 (2006), a free download of the
software can be found at http://www.fclab2.net/.
Chlorobenzene
Matrix representing the Duschinsky matrixa elements for fluorobenzene/chlorobenzene.
aCalculated
using FC-LabII 7I. Pugliesi and K. Müller-Dethlefs, J. Phys. Chem. A 110, 4657 (2006), a free download of the
software can be found at http://www.fclab2.net/.
Why Fluorobenzene?
Other Monosubstituted Benzene Molecules
Toluene
15 amu
Aniline
16 amu
Phenylacetylene
25 amu
Benzonitrile
26 amu
Other Monosubstituted Benzene Molecules
Conclusions
• A new assignment scheme has been proposed for monosubstitued
benzene molecules based on the normal vibrational modes of
fluorobenzene.
• This allows a significantly more consistent picture of the
vibrational assignments of monosubtitued benzene molecules to
made.
• This system is currently being extended to di- and tri-substituted
benzene molecules.
Acknowledgements
Prof. Tim Wright
Dr. Alistair Green
A. M. Gardner and T. G. Wright, J. Chem. Phys., 135, 114305 (2011)

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