First high-resolution analysis of the ν21 band of
propane at 921.4 cm-1: Evidence of large amplitude
motion tunneling effects
A. Perrin, F. Kwabia Tchana, J.-M. Flaud
LISA, CNRS, Universités Paris Est Créteil et Paris Diderot, Créteil, France
L. Manceron
CNRS-MONARIS UMR 8233 and Beamline AILES, Synchrotron Soleil, Saint Aubin, France
J. Demaison, N. Vogt
Universität Ulm, Section of Chemical Information Systems, Ulm, Germany
P. Groner
Department of Chemistry, University of Missouri – Kansas City, Kansas City, MO, USA
W. Lafferty
Optical Technology Dividion, National Institute of Standards and Technology, Gaithersburg, MD, USA
International Symposium on Molecular Spectroscopy, June 22-26, 2015
1
Importance of IR Spectroscopy of propane
Gaseous propane, C3H8, is
present in the atmospheres of
Earth (biomass burning)
Giant planets
Some of their moons (Titan)
Principal method
to study abundance & distribution:
High-resolution IR spectroscopy.
Previous high-resolution studies
Sym
Ev / cm-1
91
A1
369.222
261
B2
748.531
92
A1
740.292
261
B2
748.531
191
B1
1338.965
181
B1
1376.850
51
A1
1461.072
51 (A-Corio) 241
171
B1
1462.488
41 (Anh) 51
241
B2
1471.8745
41 (A– Corio)  241
41
A1
1476.384
41  (C-Corio)  171
Tors
Observed Resonances
Ref.
n.o.
a
b
92 (A-Corio) 261
c
d
local
d
171  (B-Corio)  241
a
b
c
d
F. Kwabia Tchana, J.-M. Flaud, W.J. Lafferty, L. Manceron, P. Roy. J. Quant. Spectrosc. Rad. Transf. 111 (2010) 1277–1281
G. Glasser, B. Reissenauer, W. Hüttner, Z. Naturforsch A 44 (1989) 316–24
J.-M. Flaud, F. Kwabia Tchana, W.J. Lafferty and C.A. Nixon, Mol. Phys. 108 (2010) 699–704
J.-M. Flaud, W.J. Lafferty, M. Herman, J. Chem. Phys. 114 (2001) 9361-9366
International Symposium on Molecular Spectroscopy, June 22-26, 2015
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IR Spectrum of propane
Bruker IFS 125HR FT spectrometer
AILES Beamline at SOLEIL
HgCdTe (MCT) detector cooled by liquid N2
Resolution 0.0015 cm-1
SOLEIL–LISA cryo-cell
Optical path length: 45.14 m
Temperature:
142 ± 2 K
Sample pressure:
14.0 ±0.3 Pa
International Symposium on Molecular Spectroscopy, June 22-26, 2015
3
IR Spectrum of propane
Two fundamental bands between 820 and 960 cm-1
a-type band at 921.38 cm-1
ν21 (in-plane CH3 rock)
b-type band at 870.35 cm-1
ν8 (sym CC stretch)
Both bands have split rotational transitions due to interactions between overall and
internal rotations of the methyl groups
ν21 : Summary of
“First high resolution analysis of the ν21 band of propane CH3CH2CH3 at 921.382 cm−1:
Evidence of large amplitude tunneling effects”
A. Perrin et al. (2015) doi:10.1016/j.jms.2015.02.010
ν8 : Preliminary analysis using ERHAM (Effective Rotational Hamiltonian)
P. Groner, J. Chem. Phys. 107 (1997) 4483-4498; J. Mol. Spectrosc. 278 (2012) 52-67
International Symposium on Molecular Spectroscopy, June 22-26, 2015
4
Internal rotation in propane
Point group symmetry at equilibrium:
Molecular symmetry group with 2 LAM’s (CH3 groups):
Each energy level in C2v splits into four components
C2v
G36 or [33]C2v
E  E00  E01  E11  E12 (subscripts refer to σ1σ2)
C2v
G36
Shorthand
AA
(σ1σ2)
(00)
A1
A2
B1
B2
Ka Kc
ee/oo
eo/oe
A1
A3
A4
A2
EE
AE
(01), (10), (11), (22)
(02), (20)
G
E1
G
E2
G
E2
G
E1
EA
(12), (21)
E3
E3
E4
E4
GS nuclear spin weights of rotational energy levels
136
36
64
20
16
120
28
64
12
16
International Symposium on Molecular Spectroscopy, June 22-26, 2015
5
Analysis of ν21 band
“Conventional” analysis
A. Perrin et al. (2015) doi:10.1016/j.jms.2015.02.010
Based on combination differences
GS parameters from [1] kept constant
3 band centers identified for AA, EE & AE+EA substates
Rotational & centrifugal distortion constants for each substate
(J & sextic & octic CD constants kept constant at GS value)
EV
AA
EE
AE+EA
921.3724(38)
921.3821(33)
921.3913(44)
A
0.9831356(3600) 0.9828286(1100) 0.9827518(3700)
B
0.28153860(2200) 0.28153000(1700) 0.28151473(2400)
C
0.24841758(1100) 0.24842073(1200) 0.24842144(1100)
106
a
12.714(560)
a
KJ107
a
-4.51(180)
a
J107
a
2.2756(730)
a
107
a
15.186(550)
a
a Kept at value for EE substate
[1]
B. J. Drouin, J. C. Pearson, A. Walters, V. Lattanzi, J. Mol. Spectrosc. 240 (2006) 227–237.
International Symposium on Molecular Spectroscopy, June 22-26, 2015
6
Simulation
Fig. 1 : Overview of the ν21 band of propane. The distinctive shape of this
typical A-type band is reproduced well. The experimental spectrum is
compared to the calculation performed during this study.
International Symposium on Molecular Spectroscopy, June 22-26, 2015
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Simulation details
Portions of R-branch near 931.4 cm-1 (left) and of P-branch near 915.2 cm-1 (right)
[J, Ka, Kc] = assignment in ν21 state, “d” stand for degenerate Ka.
Non-degenerate Ka : calculated intensities (spin weights ratio) for AA, EE and AE+EA
components lead to reasonable agreement between observed and calculated spectra.
Degenrate Ka : agreement is not good (purple dots for [18,8,d]), triangles for [11,5,d], and
diamonds for [11, 6, d].
.
International Symposium on Molecular Spectroscopy, June 22-26, 2015
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Simulation details
Portion of R branch near 933.2 cm-1 (left) and central part of Q branch (right).
[J, Ka, Kc] = assignment in ν21 state.
Q-branch: The Ka stacks with Ka > 11 are not well reproduced.
International Symposium on Molecular Spectroscopy, June 22-26, 2015
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Origin of torsional splittings
Comparison of torsional splittings E01/EE – E00/AA
GS
ν14
ν27
ν21
E01 –E00 (cm-1)
3.69E-05
-0.0011
-0.0013
0.0097
Torsional splitting in ν21 expected to be comparable to splitting in GS without additional torsional
interaction.
However, it is about 8 times as large as in the torsional excited states ν14 and ν27. Why?
International Symposium on Molecular Spectroscopy, June 22-26, 2015
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Torsional energy levels and splittings for J = 0
Literature
analysis of torsional Raman spectra [1], [2]
ab initio calculations [3]
00
01
11
12
vib
1000
800
New fit of Raman data [1], [2] and splittings from rot. spectra
in GS, ν14 & ν27 [4]
950
ν14 + 3ν27, ν21 FR
925
900
2ν14 + 2ν27, ν8 FR
[3]
[4]
200
825
800
[1]
[2]
400
875
850
3ν14 + ν27
4ν14
600
0
J. R. Durig, P. Groner, and M. G. Griffin, J. Chem. Phys. 66 (1977) 3061-3065; analysis of torsional Raman spectra, no splittings
R. Engeln, J. Reuss, D. Consalvo, J.W.I. Van Bladel, A. Van Der Avoird, V. Pavlov-Verevkin, Chem. Phys. 144 (1990) 81–9; analysis of
torsional Raman spectra
M. Villa, M.L. Senent, M. Carvajal, Phys. Chem. Chem. Phys. 15 (2013) 10258–10269; ab initio methods, only up to 770 cm-1
B. J. Drouin, J. C. Pearson, A. Walters, V. Lattanzi, J. Mol. Spectrosc. 240 (2006) 227–237.
International Symposium on Molecular Spectroscopy, June 22-26, 2015
11
Origin of larger torsional splittings
Comparison of torsional splittings E01/EE – E00/AA
GS
ν14
ν27
ν21
ν14+3ν27
E01 –E00 (cm-1)
3.69E-05
-0.0011
-0.0013
0.0097
0.5234
Fermi resonance ν14 + 3ν27  ν21 could increase such negligible splitting in the observed
direction if ν14+3ν27 had lower energy than ν21 (instead of the predicted higher energy)
Other perturbations
Resonces have been observed particularly for 01/EE state transitions (though not for 00/AA).
Coriolis interaction is allowed for some but not all torsional substates, particularly not for AA.
International Symposium on Molecular Spectroscopy, June 22-26, 2015
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Analysis of ν8 band with ERHAM
Initial assignments by combination difference method (CDM)
Much less satisfactory than for ν21 band (b/c splittings larger than in ν8 band)
Analysis with ERHAM [1]
5000
4000
3000
2000
1000
0
881,32
* Modified to allow prediction & fitting of rovibrational
spectra
* Initial assignments from CDM
5000
* Trial calculation of the most intense high-Ka P- & R-branch 4000
3000
transitions to establish direction of splitting patterns
2000
1000
(6 lines): Ka-degenerate transitions generate 6
0
881,31
characteristic lines, the strongest 3 with equal intensities
(2 01/EE and 1 degenerate 00/AA)
Example for 660 - 551 & 661 – 550 at right:
2 predictions with opposite sign of ε10, bottom: observed spectrum
* Assignments with CAAARS [2] using Loomis-Wood diagrams
6 12,13 - 5 10,11
881,34
881,36
881,38
881,4
881,42
881,33
881,35
881,37
881,39
881,41
[1] P. Groner, J. Chem. Phys. 107 (1997) 4483-4498; J. Mol. Spectrosc. 278 (2012) 52-67
[2] I. R. Medvedev, M. Winnewisser, B. P. Winnewisser,,F. C. De Lucia, E. Herbst., J. Mol. Struct. 742 (2005 229-236
International Symposium on Molecular Spectroscopy, June 22-26, 2015
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Analysis of ν8 band with ERHAM
Current status
4185 transitions assigned, many blends (not fit as blends yet, incl. K a-degeneracy), max J = 30,
max Ka = 15
ρ
0.15611
β / deg
8.68
3959 with non-zero weight
00/AA :1390
01/EE: 2103
11/AE: 393
12/EA: 73
ρ, β, GS constants from rotational spectroscopy [1]
kept fixed
sextic CD constants same as in GS
17 variable parameters
0.00184 cm-1 standard deviation
Many resonances (level crossings) with
unknown dark state
A / MHz
29207.46449
B / MHz
8445.968098
C / MHz
7459.003026
ΔJ / kHz
7.201377
ΔJK / kHz
-26.97966
ΔK / kHz
159.59173
δJ / kHz
1.396750
δK / kHz
3.10017
ε00 / cm-1
ε1-1 / MHz
ε10 / MHz
-0.3682
ε11 / MHz
ε20 / MHz
ε30 / MHz
[A-(B+C)/2]10 / MHz
[(B+C)/2]10 / MHz
[(B-C)/4]10 / MHz
29224.007(78)
8401.424(34)
7422.685(29)
7.359(23)
-20.85(15)
155.71(43)
1.431(19)
12.4(13)
870.3501243(94)
-44.00(82)
-230.5(11)
-76.5(12)
-8.98(85)
-3.87(64)
-0.095(15)
-0.0414(21)
-0.0282(20)
[1] B. J. Drouin, J. C. Pearson, A. Walters, V. Lattanzi, J. Mol. Spectrosc. 240 (2006) 227–237
International Symposium on Molecular Spectroscopy, June 22-26, 2015
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Comparisons of observed and calculated spectrum
Overview
Q-branch J0J – J1,J-1 (J=9 → J=1)
00 01
12 11
Q-branch J1,J-1-J2,J-2 (J=16 → J=8  )
16
15
14
3 4 5 67
00 01 12,11
13
12
11 10 9 8
Circle: level crossing
International Symposium on Molecular Spectroscopy, June 22-26, 2015
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More comparisons
R-branches J+11,J+1-J0J (J=10 → J=13) and J+10,J+1-J1J (J=11 → J=13) [last line also 151,14-142,13)
10
11
230,23-221,22 & 231,23-220,22 (left)
11 & 00 components have
2 degenerate transitions,
intensities should be double!
11
12
66. -55. (middle)
00 01 12 11
12
13
1314
95.-84., 183,16-172,15, 231,22-222,21 (right)
12 01 11 00 01,12
International Symposium on Molecular Spectroscopy, June 22-26, 2015
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Origin of larger torsional splittings
Comparison of torsional splittings E01-E00
E01 –E00 (cm-1)
GS
3.69E-05
ν14
-0.0011
ν27
-0.0013
ν21
0.0097
ν14+3ν27
0.5234
ν8
0.0360
2ν14+2ν27
1.0801
Fermi resonance ν14 + 3ν27  ν21 could increase such negligible splitting in the observed
direction if ν14+3ν27 had lower energy than ν21 (instead of the predicted higher energy)
Similarly, FR 2ν14 + 2ν27  ν8 could increase negligible native splitting in the observed
direction if 2ν14+2ν27 had lower energy than ν8 (instead of the predicted higher energy)
International Symposium on Molecular Spectroscopy, June 22-26, 2015
17
Discussion & conclusion
A start is made!
But a lot more needs to be done.
a) More assignments necessary
to higher J and Ka
more 11/AE and 12/EE components (more difficult b/c they are weaker)
b) More parameters to try, particularly tunneling parameters
c) Identify system of level crossings (01/EE and 12/EA components are much
more susceptible to Coriolis interactions)
d) With enough level crossings, it might be possible to approximate the dark
state 2ν14+2ν27 (and get a better torsional potential function)
.
m) Work out some kinks in this modified version of ERHAM
.
z) Revisit ν21 and try to characterize the dark state ν14+3ν27 .
International Symposium on Molecular Spectroscopy, June 22-26, 2015
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Thank you
International Symposium on Molecular Spectroscopy, June 22-26, 2015
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