1. Adrian Gardner, William Tuttle, Peter Groner and Timothy Wright
Torsional, Vibrational and Vibration-torsional Levels in the S1 and Ground Cationic states of p-Xylene
Adrian Gardner, William Tuttle, Peter Groner and Timothy Wright
International Symposium on Molecular Spectroscopy, 19th-23rd June 2017

2. Introduction ʺC2vʺ D2h IVR rate increased by x40 [1]
[1] P. J. Timbers, C. S. Parmenter and D. B. Moss, J. Chem. Phys., 100, 1028 (1994).

3. Introduction ʺC2vʺ D2h ʺD2hʺ IVR rate increased by x40 [1] ? ?
[1] P. J. Timbers, C. S. Parmenter and D. B. Moss, J. Chem. Phys., 100, 1028 (1994).

4. Experimental Techniques
1 + 1 REMPI
1 + 1ʹ ZEKE-PFI

5. Electronic Spectrum: Torsional Transitions

6. Electronic Spectrum: Torsional Transitions
Previous work assigned observed bands in terms of transitions on one or the other methyl group. [2,3]
[2] P. J. Breen, et al. J. Chem. Phys., 87, 1917 (1987). [3] Held et al., J. Phys. Chem. A, 102, 9625 (1998)

7. Electronic Spectrum: Torsional Transitions
We disagree, and believe a full molecular symmetry group analysis is required to assign the transitions…remember 15 minutes ago(!) and see [4]
[4] A.M. Gardner, W.D. Tuttle, P. Groner, T.G. Wright, J. Chem. Phys., 146, (2017)

8. Electronic Spectrum: Vibration-Torsion Transitions
M20 (b1)
D20 (b1)

9. Electronic Spectrum: Vibration-Torsion Transitions
M20 (b1)
D20 (b1)

10. Electronic Spectrum: Vibration-Torsion Transitions
M20 (b1)
D20 (b1)

11. Ground State  All molecules are in the ground electronic state
Molecules are “cooled” to the zero point vibrational level in a supersonic jet expansion
What about the torsional level?
It is possible to show that there are four sets of torsional levels which cannot interconvert within the jet expansion…

12. Ground State  All molecules are in the ground electronic state
Molecules are “cooled” to the zero point vibrational level in a supersonic jet expansion
What about the torsional level?
It is possible to show that there are four sets of torsional levels which cannot interconvert within the jet expansion…
But it is not trivial…

13. Ground State  All molecules are in the ground electronic state
Molecules are “cooled” to the zero point vibrational level in a supersonic jet expansion
What about the torsional level?
 It is possible to show that there are four sets of torsional levels which cannot interconvert within the jet expansion…
 But it is not trivial…
 We have done it [4]
[4] A.M. Gardner, W.D. Tuttle, P. Groner, T.G. Wright, J. Chem. Phys., 146, (2017)

14. Ground State  All molecules are in the ground electronic state
Molecules are “cooled” to the zero point vibrational level in a supersonic jet expansion
What about the torsional level?
 It is possible to show that there are four sets of torsional levels which cannot interconvert within the jet expansion…
 But it is not trivial…
 We have done it [4]
{0,0} (a1ʹ)
{0,1} (gʺ)
{1,1} (e3ʹ)
{1,-1} (e1ʹ)
(Remembering this notation represents a set of symmetrized internal rotor wavefunctions)
[4] A.M. Gardner, W.D. Tuttle, P. Groner, T.G. Wright, J. Chem. Phys., 146, (2017)

15. Ground State  All molecules are in the ground electronic state
Molecules are “cooled” to the zero point vibrational level in a supersonic jet expansion
What about the torsional level?
 It is possible to show that there are four sets of torsional levels which cannot interconvert within the jet expansion…
 But it is not trivial…
 We have done it [4]
{0,0} (a1ʹ)
{0,1} (gʺ)
{1,1} (e3ʹ)
{1,-1} (e1ʹ)
(Remembering this notation represents a set of symmetrized internal rotor wavefunctions)
 Transitions may originate from any of these sets of levels…much more complicated than aaathe single rotor case we saw earlier.
[4] A.M. Gardner, W.D. Tuttle, P. Groner, T.G. Wright, J. Chem. Phys., 146, (2017)

16. Electronic Spectrum: Let’s do some assigning

17. Electronic Spectrum: Let’s do some assigning

18. Electronic Spectrum: Let’s do some assigning
{0,4} can interact with D20{0,1}
{1,4}- can interact with D20{1,1}
Nothing of the correct symmetry for {1,-4}-

19. Electronic Spectrum: Let’s do some assigning
D20{0, 3(-)}- can interact with the {0, 6(+)}+ [analogous to the interact in pFT]
Both D20{3, 1} and D20{-3, 1} can interact with {6,1} and {-6, 1}

20. Electronic Spectrum: Let’s do some assigning?

21. Electronic Spectrum: Let’s do some assigning?
2D20 interacting with D19 levels in pXyl

22. E = m2F Free Rotor Hindered Rotor m m 6+ ±6 6- V6/4 ±5 ±5
Energy / cm-1
E = m2F ±4 ±4 3+ ±3
V6/2 3- ±2 ±2 ±1 ±1

23. Electronic Spectrum: m=3 level
Δm = 0 transition are expected to be the most intense (symmetry allowed), followed by Δm = ±3 transitions
(torsion-electronic coupling), followed by Δm = ±6.
𝑚 0 3(+) transition much more intense than the 𝑚 0 3(−) transition.
m=3(-) level shown to lie below the m=3(+) level for all three molecules → staggered conformer “favoured”…
[6] J. R. Gascooke, E. A. Virgo, W. D. Lawrance, J. Chem. Phys., 143, (2015)

24. m=3 level: what about the cation?
Has been suggested that both the m = 3(+) lies below the m = 3(-) level in the toluene cation → eclipsed geometry “preferred". [5]
m = 3(+)
m = 3(-)
ZEKE Spectrum via S1 origin:
m = 3(+) observed.
ZEKE Spectrum via S1 m = 3(+) level:
m = 3(+) and 3(-) observed.
ZEKE spectra of toluene taken from [5]
[5] K.-T. Lu, G. C. Eiden, J. C. Weisshaar, J. Phys. Chem., 96, 9742 (1992)

25. m=3 level: what about the cation?
pFT
ZEKE Spectra
REMPI Spectra

26. m=3 level: what about the cation?
Has been suggested that both the m = 3(+) lies below the m = 3(-) level in the toluene cation → eclipsed geometry preferred. [5]
m = 3(+)
ZEKE Spectra
m = 3(-)
REMPI Spectra
ZEKE spectra of toluene taken from [5]
[5] K.-T. Lu, G. C. Eiden, J. C. Weisshaar, J. Phys. Chem., 96, 9742 (1992)

27. m=3 level: what about the cation?
Has been suggested that both the m = 3(+) lies below the m = 3(-) level in the toluene cation → eclipsed geometry preferred. [5]
m = 3(-)
ZEKE Spectra
m = 3(+)
REMPI Spectra
ZEKE spectra of toluene taken from [5]
[5] K.-T. Lu, G. C. Eiden, J. C. Weisshaar, J. Phys. Chem., 96, 9742 (1992)

28. m=3 level: what about the cation?
pFT
ZEKE Spectra
m = 3(-) observed via S1 origin owing to torsion-electronic coupling
It lies below the m = 3(+) level →staggered geometry “preferred”
REMPI Spectra

29. m=3 level: what about the cation?
pXyl
pFT
ZEKE Spectra
REMPI Spectra

30. Conclusions
Discussed the similarities of the electronic spectra of pFT and pXyl.
Demonstrated the importance of assigning observed transitions in the full G72 MSG.
Qualitatively explained observed splitting…quantitative analysis is forthcoming.

31. Conclusions
Discussed the similarities of the electronic spectra of pFT and pXyl.
Demonstrated the importance of assigning observed transitions in the full G72 MSG.
Qualitatively explained observed splitting…quantitative analysis is forthcoming.
Shown that the m = 3(+) level lies above the m = 3(-) level in the cation of both molecules (and demonstrated that this is likely true in toluene also).
This shows the staggered conformer is “preferred” however determining the barrier to rotation requires deconvolution of couplings. [6]
Owing to the large amplitude (if not free rotation!) discussion of “preferred” conformation is largely unphysical.
[6] J. R. Gascooke, E. A. Virgo, W. D. Lawrance, J. Chem. Phys., 143, (2015)

32. Future Work ? ? ? ? C2v ʺC2vʺ D2h ʺC2vʺ ʺD2hʺ
IVR rate increased by x40 [1]
ʺC2vʺ
D2h
ʺC2vʺ
ʺD2hʺ ? ?
[1] P. J. Timbers, C. S. Parmenter and D. B. Moss, J. Chem. Phys., 100, 1028 (1994).

33. Acknowledgements This work has been supported by:
Prof. Warren Lawrance
Dr. Jason Gascooke
Prof. Katharine Reid
Dr. Julia Davies
Laura Whalley
Prof. Timothy Wright
Prof. Peter Groner
William Tuttle
This work has been supported by:
Grant no. L021366/1