Millimeter Wave Spectrum ofIso-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 topsOne OH top

– Two different structural conformers:Gauche & Trans

• Astrochemical Interest• Spectroscopic Interest

Gauche

Trans

600

500

400

300

200

100

0

Ene

rgy

[cm

-1]

360300240180120600Torsional Angle (

OH-Torsional Potential

Calculated OH torsional potential barrier and energy levels of iso-propanol

(F. Inagaki, I. Harada and T. Shimanouchi, JMS 46, 381, 1973)

tunneling coupling

Gauche Trans Gauche’

gauche (a) trans

gauche (s)

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?

• Spectroscopic Interest

N-propanol; submillimeter-wave observationIso-propanol; only microwave data (< 30 GHz) availablePredictions at higher frequency not enough

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 (Fast Scan Submillimeter-wave Spectroscopic Technique)

• Radiation source BWOs

sweep very fast

• 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

Wide range! Short time!

• Room temperature

WI04

110-370 GHz region : ~70,000 lines

FASSST Spectrum of Iso-Propanol

Assignment with the CAAARS program

• Assigned lines— Spectrum

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)

Assignment with CAAARS(Computer Aided Assignment of Asymmetric Rotor Spectra)

Assignment with CAAARS ~ 7,600 lines

Iso-Propanol in the Ground State

• Assigned lines— Spectrum

b, c - type rotational transitions within g(s), g(a), transa, x - type torsional transitions between g(s) & g(a)

Through J = 68Kc = 52

x-type: Perturbation allowed transition ↓

ΔKa = 0, ΔKc = 0between different torsional states

600

500

400

300

200

100

0

Ene

rgy

[cm

-1]

360300240180120600Torsional Angle (

OH-Torsional Potential

Calculated OH torsional potential barrier and energy levels of iso-propanol(F. Inagaki, I. Harada and T. Shimanouchi, JMS 46, 381, 1973)

1.56 cm-1

trans → perturbation freegauche (s) & gauche (a) → interact with each other

g (a)

g (s)

trans

A estimation

8.7 cm-1 ? – Inagaki et al.

Analysis with SPFITSeparate Fits for gauche & trans

• gauche (s) & gauche (a)

– Two-state torsional rotational

Hamiltonian

Heff = HR + HTR + HT

• trans – Rotational Hamiltonian

for a semi-rigid rotor

up tosextic centrifugal fifth order termsdistortion terms

HTR (completed through 5th order)

σ; torsional substate (σ ≠ σ’)

1st

2nd

3rd

4th

5th

Explain gauche (s) & (a) substates very well !

Analysis with SPFITSeparate 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

• Coriolis interaction with gauche

• Interaction with an excited vibrational state

These ~320 lines wereexcluded from the fit

?

~3

MH

z

Molecular Constants of Iso-Propanol

in the Ground State / MHz

53 parameters for gauche (s) & (a) (~6300 lines) RMS = 76 kHz

15 parameters for trans (~1500 lines) RMS = 63 kHz

Prediction for astronomical observation

A. Maeda, I. R. Medvedev, F. C. De Lucia, E. HerbstApJ Supplement, accepted

2

,

2',

2

2'

int

,'exp,'

i

iT

kT

ER

Distribution of Intensity Ratio & Relative Energy [Baskakov et al. (2006) HCOOH]

Intensity ratio of identical rotational transitions in different torsional substates

σ’,σ = torsional substates

Compared 559 lines in each trans & gauche (s)

Mean ΔE(trans, g(s)) = 83 (42) cm-1

• Infrared study 8.7 cm-1

• Microwave 158 cm-1

• Theoretical calculation 55.96 cm-1

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.

AcknowledgementNASA for its support program

Brenda P. Winnewisser

Manfred Winnewisser

Torsion-Rotation Interactionfor an asymmetric molecule with an internal rotor

• Quade & Lin (1963) Deuterated Methanol; Effective Hamiltonian with FFAM (Framework-Fixed 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 studyHTR 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

Baskakov et al. (2006) HCOOH

556 lines in eachgauche (s) & gauche (a)

Energy Difference / cm-1

Unassigned ~62,000 lines3~4 times weaker intensity

Vibrational Excited State

— Spectrum— Unassigned lines