1. FTIR Synchrotron Spectroscopy of
Two Asymmetric C-H Stretching Bands of Methyl Mercaptan (CH3SH):
A Perplexity of Perturbations
Two asymmetric C-H stretching modes
as functions of torsion and bend angles

2. Ronald M. Lees, Li-Hong Xu, Elias M. Reid
Centre for Laser, Atomic and Molecular Sciences (CLAMS),
Dept. of Physics, University of New Brunswick, Saint John, NB
Bishnu P. Thapaliya, Mahesh B. Dawadi, David S. Perry Department of Chemistry, University of Akron, Ohio
Sylvestre Twagirayezu Brookhaven National Laboratory, Dept. of Chemistry, Upton, NY
Brant E. Billinghurst Canadian Light Source, University of Saskatchewan, Saskatoon, SK

3. Background and Motivation
CH3SH is a significant species for Sulphur chemistry in planetary atmospheres, star-forming regions and interstellar clouds, precise high resolution laboratory data are needed for interpretation of new astronomical survey spectra - MW & THz
The large-amplitude internal rotation in methyl mercaptan makes the vibrational energy manifold both complex and interesting, with strong torsion-mediated interactions coupling the different vibrational modes. Data on vibrational analysis and assignments for CH3SH are remarkably sparse - IR
Perturbations among high-lying ground-state torsional levels and the vibrational fundamental, overtone and combination states can give clues to the torsion-vibration energy structure & dynamics - Intramolecular Vibrational Energy Redistribution

4. Vibrational Modes of CH332SH
Description nobs / cm-1
A' n1 CH asym stretch 3015
n2 CH sym stretch 2948
n3 SH stretch 2605
n4 CH3 asym bend 1453
n5 CH3 sym bend 1332
n6 CH3 in-plane rock 1072
n7 SH bend 802
n8 CS stretch 710
A" n9 CH asym stretch 3015
n10 CH3 o-o-p bend 1444
n11 CH3 o-o-p rock 956
n12 CH3 torsion ~200
This is the table of vibration modes
Vibrational Coupling
to Torsional Ladder
A1 (1 r r r6)
E (2 r r r6) Symmetrized basis vibrations
E ( r r6) in G6 with A1 & E basis vibrations in G6
Wavenumbers from I. W. May and E. L. Page, Spectrochim. Acta. 24A (1968)

5. Two band centres are very close!
Methyl Mercaptan C-H Stretching Bands
Two band centres are very close!
½ of the traditional DK = ± 1 bands are missing!
Symmetric
C-H Stretch
Asymmetric
C-H Stretches
l-doubling!
D(K - l) = 0
Q-Branches
(K – 1) ¬ K
Dl = -1
(A' symmetry)
Q-Branches
K ® (K + 1)
Dl = +1
(A" symmetry)
A1 ( )
E ( )
E ( ) … …
For symmetric molecules: delta(K-l) = 0
So, K -> K+1 l -> l (since GS l = 0, so, 0 -> +1)
K -> K-1 l -> l ( 0 -> -1)
Symmetry is based on selection rules. High frequency band (with lower energy), has a b-type selection rule, so A’
Low frequency band (with higher energy), has a c-type selection rule, so A”

6. Two Asymmetric C-H Stretch Band Centres are Very Close!
½ of the traditional DK = ± 1 bands are missing!
Symmetries are deduced from A+/- selection rules
l-doubling!
D(K - l) = 0
Asymmetric
C-H Stretches
State I
E ( )
l = -1
(A' symmetry)
State II
E ( )
l = +1
(A" symmetry)
Higher energy state
Lower energy state
K+1
K-1
Observed
DK = + 1
Not observed
DK = - 1
Assignments cannot be confirmed by traditional ^ band DK = ± 1 scheme;
and can only be confirmed by GS a-type differences and asymmetry splittings.
K+2
Ground
State
K+1
K-1
K-2 K
l = 0

7. K = 3 ¬ 2 A Q Branch in High Resolution
(local perturbations)
K′ = 3 J
To the lower energy state (A") J
K″ = 2
c-type selection rule is assumed 11 K
3- ¬ 2-
3+ ¬ 2+ 12 14 15 21 14 21 17 16 15 16 J 11 13 20 22 20 19 18 22 13 12 17 18 19 23 24 23 24
The A+ and A- selection rules mean that the upper state for this DK = +1 transition is the A" vibrational component of the C-H stretch.

8. 1 ¬ 0 A Q-Branch in High Resolution
To the lower energy state (A")
K′ = 1 J
R-P differences are confirmed by GS energy differences.
Q-branch J values are uncertain, as Q and R/P transitions access different +/- components. P Q R
J-dependence is slow Þ small upper state K=1 splitting
J+1 J
J-1 +
c-type selection rule is assumed
K″ = 0

9. K = 1 ¬ 2 A Q Branch in High Resolution
K′ = 1 and K″ =2 asymmetry splittings are similar in size!
To the higher energy state (A') 7 8 9 10 11 12 13 14
+ ¬ - 6 7 8 9 10 11 12 13 14 15 16
- ¬ +
b-type selection rule is assumed J
K″ = 2
K′ = 1
I finally managed to assign the K = 1<2 A Q-branch by recognizing doublet structure in the P branch and then predicting the Q branch, which turned out to fit beautifully. The attached powerpoint figure shows the structure. The A+ and A- selection rules mean that the upper state for this deltaK=-1 transition is the A' vibrational component of the C-H stretch. Previously, the K=1<0 A Q-branch structure had indicated the upper state for the deltaK=+1 transitions had to be the A" component. The K=1A K-doublet splitting is very small in the upper state.
The A+ and A- selection rules mean that the upper state for this DK = -1 transition is the A' vibrational component of the C-H stretch.

10. Asymmetric C-H Stretches – J, K Reduced Energies
From the A+/- K-doublet selection rules, we deduce that
the higher energy state with DK = -1 is the A’ of asymm. CH-stretch
the lower energy state with DK = +1 is the A” of asymm. CH-stretch
A′ higher energy state
from DK = -1 subbands
A″ lower energy state
from DK = +1 subbands

11. CH3OH CH3SH Lower E state Higher E state (A”) (A’) E0 3009.5 3010.4
A-B 2.94 DK
r (= Itop/Imol) z l
Asymm. C-H stretch states (J reduced) A′ A″
Normal A-E pattern??
Fig. 1 Methanol reduced energies. (This is Fig. 5 from Sylvestre Twagirayezu, Xiaoliang Wang, David S. Perry, Justin L. Neill, Matt T. Muckle, Brooks H. Pate, Li-Hong Xu, IR and FTMW-IR Spectroscopy and Vibrational Relaxation Pathways in the CH Stretch Region of CH3OH and CH3OD, J. Phys. Chem. A, 115, (2011),
Inverted A-E pattern A′
E = Eo - a1 cos[2p/3 ((1-r)K + t)]
- a2 cos[4p/3 ((1-r)K + t)] (A - B) K2 - DK K4 - AzKl
To be done, after band origins are better determined.
For the tau plots, eventually we want fitted tau curves, but data points indicating A, E1, E2 symmetry, like the methanol plots.  Given the strong perturbations to the CS stretch band, I am surprised that the tau curves for the CH stretched are as regular as they are.
Ground state (J, K reduced)
t = 0 1 2
s = E1 E2 A
Normal A-E pattern means, a1 is positive

12. CH3OH CH3SH A′ A″ A′ Ground state
Fig. 1 Methanol reduced energies. (This is Fig. 5 from Sylvestre Twagirayezu, Xiaoliang Wang, David S. Perry, Justin L. Neill, Matt T. Muckle, Brooks H. Pate, Li-Hong Xu, IR and FTMW-IR Spectroscopy and Vibrational Relaxation Pathways in the CH Stretch Region of CH3OH and CH3OD, J. Phys. Chem. A, 115, (2011),
To be done, after band origins are better determined.
For the tau plots, eventually we want fitted tau curves, but data points indicating A, E1, E2 symmetry, like the methanol plots.  Given the strong perturbations to the CS stretch band, I am surprised that the tau curves for the CH stretched are as regular as they are.
Ground state

13. Summary
Asymmetric C-H stretching bands in the FTIR synchrotron spectrum of CH332SH have been analyzed at high resolution, and term values and substate origins have been determined.
The two C-H stretching band centers are very close and display qualitatively new features, since the separation between the A′ and A″ asymmetric C-H stretching states is of the same order as the splittings due to the torsion and molecular asymmetry. The two states appear to behave as a symmetric rotor l-doublet, with only one of the |DK| = |Dl | transitions strongly allowed.
Perturbations from the regular oscillatory level pattern as well as a host of J-localized resonances are evidenced from the spectrum, implying a complex network of interactions among the vibrational fundamental modes, torsional combination states, and high-nt torsional ground-state levels.