1. A Combined Gigahertz and Terahertz Synchrotron-based Fourier Transform Infrared Spectroscopic Investigation of Ortho-D-phenol: Tunneling Switching S. Albert,1, 2 Z. Chen,1 C. Fábri,1 R. Prentner1, M. Quack1 and D. Zindel 1Physical Chemistry, ETH Zurich, CH-8093 Zurich, Switzerland 2 Swiss Light Source, Paul-Scherrer-Institute, CH-5332 Villigen, Switzerland
Tunneling switching seen
in small molecules so far
Here: a large molecule
Why? Important for experiments on molecular parity violation, see also talk WG11
Tunneling switching in
asymmetric potentials
Delocalized wave functions
+ “forbidden transitions”
Localized wave functions
+ “Allowed transitions” Δ
-OH potential, textbook symmetric double well potential with the two minima corresponding to the two equivalent structures.
Pure rotational transitions connecting the two halves have been observed in MW studies, we conducted the first rotationally resolved IR study of the bands shown by arrows.
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2. -OH torsion in phenol : Tunneling→ ←
-OH potential, textbook symmetric double well potential with the two minima corresponding to the two equivalent structures.
Pure rotational transitions connecting the two halves have been observed in MW studies, we conducted the first rotationally resolved IR study of the bands shown by arrows.
E. Mathier, D. Welti, A. Bauder and Hs. H. Günthard, J. Mol.Spectrosc.37, (1971)
C. Tanjaroon, S. G. Kukolich, J. Phys.Chem.A 113, (2009)
T. Pedersen, N. W. Larsen and L. Nygaard, J. Mol. Struc. 4, 59 (1969)
MW:
IR:
S. Albert, Ph. Lerch, R. Prentner and M. Quack, Angew. Chem. Int. Ed.52, (2013)
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3. Introduction of an asymmetry in the effective potential (with symmetric Born-Oppenheimer potential) by zero point energy effects
Torsional potential of D-phenol predicted using the Quasiadiabatic Channel
Reaction Path Hamiltonian (RPH) approach
cm-1
What happens when the symmetry of the potential is removed in D-phenols?
Asymmetry raised from isotopic substitution. Two conformers, depending on relative positions to of the OH to D.
Theory predicts that tunneling quenched, at gs and v=1, resumption from v=2 and v=3
To validate the theory, conformation by spectroscopy is needed.
BO potential (dashed)
Lowest adiabatic channel potential (bold)
S. Albert, Ph. Lerch, R. Prentner and M. Quack, Angew. Chem. Int. Ed.52, (2013)
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4. Aims
To characterize the asymmetry in the effective –OH torsional potential energy function of D-phenols.
Rotational and vibrational levels to be probed through high resolution GHz and THz spectra.
Important goal is to observe and understand the tunneling switching dynamics in the higher excited states as prototypical case also for asymmetry due to parity violation in chiral molecules.
Theory:
pure-rotational and rovibrational transitions
One at a time,
B. Fehrensen, D. Luckhaus and M. Quack, Chem. Phys. Lett. 300, 312 (1999), Chem. Phys. 338, 90 (2007)
R. Prentner, M. Quack, J. Stohner and M. Willeke, J. Phys. Chem. A 119, (2015)
Theoretical background on the quasiadiabatic channel reaction path Hamiltonian approach:
H.W. Miller, N.C. Handy and J.E. Adams, J. Phys.Chem, 72, 99 (1980)
L. Hofacker, Z. Naturforsch.A 18, 607 (1963)
R. A. Marcus, J. Chem. Phys. 43, 1598 (1965)
J. T. Hougen, P.R. Bunker and J.W.C. Johns, J. Mol. Spectrosc. 34, 136 (1970)
M. Quack and J. Troe, Ber. Bunsenges. Phys.Chem. 78, 240 (1974)
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5. Tunneling Switching in m-D-phenol
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S. Albert, Z. Chen, C. Fábri, P. Lerch, R. Prentner and M. Quack, Mol.Phys., 114, (2016).

6. GHz setup at ETH Zürich Frequency multiplier 75-110 GHz Detector
Lock-in amplifier
Second-harmonic lock-in detection
MW synthesizer
250kHz-20GHz
2.5 m gaseous cell
M. Suter and M. Quack, Appl. Opt. 54, (2015)
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7. Overview spectra: 90-110 GHz
J’’
25 μbar
294 K
Overview of the room-temp spectra.
With vib sat. expected, G.S lines stand out. Strong clusters are visible corresponding to a diff J progressions.
GHz
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8. Two sets of patterns line up nicely, due to the presence of two different conformers.
Clusters on the lower frequency side has 1/5 of the g.s intensity, also two sets on top of each other, are assigned to the first excited torsional state.
FWHM ≈ 200 kHz
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9. Spectroscopic parameters of the ground state and the first excited torsional state of o-D-Phenol (GHz study)
Watson’s A-reduction, Ir-representation
Anti
Syn
v=0
vT =1 ( cm-1)
vT =1 ( cm-1)
Rotational constants / MHz A
(12)
(22)
(12)
(43) B
(81)
(39)
(84)
(66) C
(81)
(53)
(86)
(63)
Centrifugal distortion constants / kHz ΔJ
(56)
(31)
(60)
(36) ΔK
(26)
0.540(45)
(27)
0.39(13)
ΔJK
(10)
(10) δj
(15)
(16) δk
(12)
(13)
drms / kHz
13.08
22.74
13.52
25.88
No. of transitions
487 71
490 66
Well determined, include distortion constants
No perturbation, good agreement with the theory
Vt=2, difficulty
1Values without parenthesis held fixed to the respective values of the v = 0.
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10. Synchrotron-based high resolution FTIR setup at the Swiss Light Source
unapodized instrument resolution
of cm-1 (16 MHz)
Bruker IFS 125 HR with 11 chambers, high resolution, for congested spectra
Synchrotron advantage is known in the range of FIR/THz which is where the band of interest is
S. Albert, K.K. Albert, Ph. Lerch, M. Quack, Faraday Discussions, 150, (2011)
S. Albert, K. K. Albert and M. Quack, High Resolution Fourier Transform Infrared Spectroscopy, in Handbook of High-Resolution Spectroscopy, Vol. 2 (Eds. M. Quack and F. Merkt), John Wiley & Sons, Ltd, Chichester, pp (2011)
S. Albert, Ph. Lerch and M. Quack, Chem. Phys. Chem. 14, (2013)
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11. Overview spectra: 240-340 cm-1 (7.195-10.193 THz)
Frequency /THz
6.8952
7.7946
8.3942
8.9938
9.5934
Loomis-Wood plots showing patterns of the fundamental
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12. Torsional fundamental
~8000 transitions assigned
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13. Spectroscopic parameters of the ground state and the first excited torsional state of o-D-Phenol (THz study)
Watson’s A-reduction, Ir-representation
Anti
Syn
v=0
vT =1 ( (53) cm-1)
vT =1 ( (67) cm-1)
Rotational constants / cm-1 A
(23)
(13)
(19)
(17) B
(26)
(21)
(24)
(26) C
(43)
(25)
(49)
(31)
Centrifugal distortion constants / 10-6 cm-1 ΔJ
(17)
(72)
(24)
(10) ΔK
(32)
(25)
(53)
(38)
ΔJK
(49)
(31)
(77)
(46) δj
(96)
(57)
(13)
(78) δk
(59)
(69)
(13)
(10)
drms / kHz
No. of transitions
1234(Ground state comb. diff.)
7379 (Anti+Syn)
1275(Ground state comb. diff.)
Well determined, include distortion constants
No perturbation, good agreement with the theory
Vt=2, difficulty
1Values without parenthesis held fixed to the respective values of the v = 0.
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14. Torsional fundamental
Two Q branches well separated
No signs of perturbations
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15. Overview spectra: 240-340 cm-1 (7.195-10.193 THz)
Frequency /THz
6.8952
7.7946
8.3942
8.9938
9.5934
Loomis-Wood plots showing patterns of the hot band
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16. First torsional hot band
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17. First torsional hot band: Perturbation
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18. Spectroscopic parameters of the second excited torsional state of m-D-Phenol
Anti
Syn
vT =1
vT =2
Band origin
(37)
(28)
Rotational constants / cm-1 A
(14)
(14)
(16)
(95) B
(16)
(22)
(19)
(45) C
(38)
(30)
(43)
(72)
Centrifugal distortion constants / 10-6 cm-1 ΔJ
(12)
(15)
(17) ΔK
(25)
(18)
(31)
(28)
ΔJK
(38)
(19)
(46)
(43) δj
(60)
(73) δk
(42)
(53)
drms / cm-1
No. of transitions
1686 (Upper state comb. diff. from the fundamental)
2132 (Anti+Syn)
1245 (Upper state comb. diff. from the fundamental)
F, first evident to connect the two structures, would not be there if there is no tunneling
1Values without parenthesis held fixed to the respective values of the vT =1.
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19. Forbidden Transitions??? Δ(Anti-Syn)=0.00203（7）cm-1 vT = 2 vT = 1
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20. The torsional polyad in o-D-phenol
RPH
Exp.
Exp.
cm-1
cm-1
cm-1
cm-1
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21. Summary We have investigated the –OH torsion of
phenols using high resolution spectroscopy
The assignment of pure rotational and
rovibrational spectra led to the accurate
determination of vibrational levels of the
torsion polyads and assignment of the two isomers
Particularly, tunneling-switching has been proven and analyzed
in m-D-phenol by the observation of “forbidden” transitions and resonances involving upper states but for o-D-phenol only preliminary evidence could be obtained so far for this effect .
The effective spectroscopic Hamiltonian parameters provided here can be used to improve effective potentials for this prototypical system
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22. Acknowledgement
The group of Martin Quack at ETH Zürich:
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23. All low energy levels in o-D-phenolIR IR
harmonic frequencies calculated at B3LYP/cc-pVTZ level
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24. Assignment of the -syn and -anti structuresor ?
v=0
vT =1
Exp.
Calc.(RPH)
∆A / (MHz)
4.08
3.95
3.84
4.38 ∆B
-0.48
-0.58
-0.49
-0.54 ∆C
0.21
0.22
0.19
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25. “Forbidden” tunneling-rotation-vibration transitions in m-D-phenol
vT = 2
vT = 1
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26. Vibrational satellites: ν11 mode
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27. Vibrational satellites: ν11 mode
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28. Vibrational satellites: ν11 mode
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29. Vibrational satellites: ν11 mode
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