Download MILLIMETER WAVE SPECTRUM OF N

Millimeter Wave Spectrum of
Iso-Propanol
A. MAEDA, I. MEDVEDEV,
E. HERBST and F. C. DE LUCIA
Department of Physics,
The Ohio State University
Iso-Propanol
• Iso-Propanol [(CH3)2CHOH]
– One of the structural isomers of propanol [C3H7OH]:
N-propanol [CH3CH2CH2OH]
Iso-propanol [(CH3)2CHOH]
– Three internal rotors:
Two CH3 tops
One OH top
– Two different structural conformers:
Gauche & Trans
Gauche
• Astrochemical Interest
• Spectroscopic Interest
Trans
OH-Torsional Potential
600
tunneling coupling
500
Gauche
Trans
Gauche’
-1
Energy [cm ]
400
300
gauche (a)
200
trans
gauche (s)
100
0
0
60
120
180
Torsional Angle ( 
240
300
360
Calculated OH torsional potential barrier
and energy levels of iso-propanol
(F. Inagaki, I. Harada and T. Shimanouchi, JMS 46, 381, 1973)
Iso-Propanol
•Astrochemical Interest
– Saturated organic molecule
Important role in hot molecular cores & corinos
– Interstellar Saturated Alcohols
Methanol (CH3OH), Ethanol (C2H5OH)
– Next largest alcohol is
Propanol (C3H7OH) – detectable?
N-propanol; submillimeter-wave observation
Iso-propanol; only microwave data (< 30 GHz) available
Predictions at higher frequency not enough
• Spectroscopic Interest
Iso-Propanol
• Spectroscopic Interest
– Previous studies
Microwave1, Millimeter-wave2,
Far-infrared (OH-torsional fundamental band)3
– Torsion-rotation interaction
for a molecule with an internal rotor
– Relative energy of the trans torsional substate
1. Kondo & Hirota (1970), Hirota (1979), Hirota & Kawashima (2001)
2. Ulenikov et al. (1991) 3. Inagaki, Harada & Shimanouchi (1973)
Experiment --- FASSST
WI04
(Fast Scan Submillimeter-wave Spectroscopic Technique)
• Radiation source
BWOs
sweep very fast
Wide range!
Short time!
• Frequency range
100-370 GHz region
• Measurement
* 200 scans
accumulation
* Up & down sweeps
• Production condition
Commercial iso-propanol 14 mTorr
SO2 (calibration gas) 3 mTorr
• Room temperature
FASSST Spectrum of Iso-Propanol
110-370 GHz region : ~70,000 lines
Assignment with the CAAARS program
Assignment with CAAARS
(Computer Aided Assignment of Asymmetric Rotor Spectra)
Blended b-type R (ΔJ=+1) pure rotational transitions of
(J,Ka,Kc) = (13,0,13) ← (12,1,12) & (13,1,13) (12,0,12)
trans
gauche (a)
gauche (s)
• Assigned lines
— Spectrum
Iso-Propanol in the Ground State
Assignment with CAAARS ~ 7,600 lines
b, c - type rotational transitions within g(s), g(a), trans
a, x - type torsional transitions between g(s) & g(a)
Through J = 68
Kc = 52
x-type: Perturbation allowed transition
↓
ΔKa = 0, ΔKc = 0
between different torsional states
• Assigned lines
— Spectrum
OH-Torsional Potential
600
trans → perturbation free
gauche (s) & gauche (a) → interact with each other
500
-1
Energy [cm ]
400
300
A estimation
8.7 cm-1 ? – Inagaki et al.
1.56 cm-1
200
g (a)
trans
g (s)
100
0
0
60
120
180
Torsional Angle ( 
240
300
Calculated OH torsional potential barrier and energy levels of iso-propanol
(F. Inagaki, I. Harada and T. Shimanouchi, JMS 46, 381, 1973)
360
Analysis with SPFIT
Separate Fits for gauche & trans
• gauche (s) & gauche (a)
– Two-state torsional rotational
Hamiltonian
Heff = HR + HTR + HT
up to
sextic centrifugal
distortion terms
fifth order terms
• trans – Rotational Hamiltonian
for a semi-rigid rotor
HTR (completed through 5th order)
1st
2nd
3rd
4th
Explain gauche (s) & (a) substates very well !
5th
σ; torsional substate (σ ≠ σ’)
Analysis with SPFIT
Separate Fits for gauche & trans
• gauche (s) & gauche (a)
– Two-state torsional rotational
Hamiltonian
Heff = HR + HTR + HT
• trans – HR for a semi-rigid rotor
(Watson type A-reduced Hamiltonian)
Heff = HR
(up to sextic centrifugal distortion)
Perturbation in the trans Substate
• Centrifugal distortion
• Interaction with an excited
vibrational state
These ~320 lines were
excluded from the fit
?
~3 MHz
• Coriolis interaction with gauche
Molecular Constants of Iso-Propanol
in the Ground State / MHz
Prediction for astronomical observation
A. Maeda, I. R. Medvedev, F. C. De Lucia, E. Herbst
ApJ Supplement, accepted
53 parameters for gauche (s) & (a) (~6300 lines)
RMS = 76 kHz
15 parameters for trans (~1500 lines) RMS = 63 kHz
Distribution of Intensity Ratio &
Relative Energy [Baskakov et al. (2006) HCOOH]
Intensity ratio of identical rotational transitions in
different torsional substates
2 -1
2
Mean ΔE(trans, g(s))
=
83
(42)
cm
ET  ' ,      '  i , '

Rint  ' ,    exp  
 2
2
kT

    i ,
• Infrared study
σ’,σ = torsional substates
• Microwave
8.7 cm-1
158 cm-1
Compared 559 lines • Theoretical calculation 55.96 cm-1
in each trans & gauche (s)
Summary
• c.a.7,600 spectra of iso-propanol in the ground state
have been newly assigned and analyzed.
• A prediction has been made accurate enough for
astronomical observation.
• Perturbation was found in trans at J > 50.
• Relative energy of the trans conformer was estimated
from distribution of relative intensity of lines.
Acknowledgement
NASA for its support program
Brenda P. Winnewisser
Manfred Winnewisser
Torsion-Rotation Interaction
for an asymmetric molecule with an internal rotor
• Quade & Lin (1963)
Deuterated Methanol; Effective Hamiltonian with FFAM (FrameworkFixed Axis Method)
• Pearson, Sastry, Herbst, & De Lucia (1996)
Ethanol (J up to 30); HTR expanded up to 5th order terms (no 4th order)
• Duan, Zhang &Takagi (1996), Duan, Wang &Takagi (1999)
Methanol; Higher order HTR terms for a molecule with an internal rotor
derived with sequential contact transformation technique
Present study
HTR complete up to 5th order
Distribution of
Intensity Ratio & Relative Energy
Mean ΔE(g(a), g(s))
= 3.6 (10) cm-1
Comparable to
ΔE(g(a), g(s)) = 1.56 cm-1
556 lines in each
gauche (s) & gauche (a)
Baskakov et al. (2006) HCOOH
Energy Difference / cm-1
Vibrational Excited State
Unassigned ~62,000 lines
3~4 times weaker intensity
— Spectrum
— Unassigned lines