Submillimeterwave Spectroscopy of 13C1Methyl formate H13COOCH3 in the Ground State

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Submillimeter-wave Spectroscopy of13C1-Methyl
formate H13COOCH3in the Ground State

• Atsuko Maeda, Ivan Medvedev,
• Eric Herbst, Frank C. De Lucia,
• Department of Physics, The Ohio State University,
  Columbus, OH
• and Peter Groner
• Department of Chemistry, University of
  Missouri-Kansas City, Kansas City, MO

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(Normal) Methyl formate

• One of abundant interstellar molecules
• in hot cores corinos (Orion KL, Sgr B2)

T 100-300 K
Ultra-compact HII region
O,B star
n 10(6)-10(8) cm-3
Cold background
Hot Core
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CH3 Internal Rotationin Methyl formate
with ZPVE (zero-point vibrational correction)
E
407 MHz (0.0135 cm-1)
A
Senent, Villa, Meléndez and Dominguez-Gomez, ApJ.
627, 567 (2005)
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Previous Rotational Studies of Methyl formate

• Curl (1959) - HCOOCH3, DCOOCH3, HCOOCH2D,
  H13COOCH3, HCOO13CH3,
• HC18OOCH3, HCO18OCH3 10-30 GHz vt 0 (A, E),
  HR
• Bauer (1979) - HCOOCH3 5-58 GHz vt 0 (A, E),
  IAM
• DeMaison et al. (1983) - HCOOCH3 220-240 GHz
  vt 0 (A, E)
• Plummer et al. (1984) - HCOOCH3 140-550 GHz vt
  0 (A), HR
• Plummer et al. (1986) - HCOOCH3 140-300 GHz vt
  0 (E), PAM
• Oesterling et al. (1995) - DCOOCH3 -680 GHz vt
  0 (A, E), RAM
• Oesterling et al. (1999) - HCOOCH3 -380 GHz vt
  0 (A, E), RAM
• Karakawa et al. (2001) - HCOOCH3
• Odashima et al. (2003) - HCOOCH3 7-200 GHz vt
  0 (A, E), 1 (A), IAM

Fewer studies for isotopologues
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Previous Study of 13C1-Methyl formate

• Willaert et al. (2006)
• 7-62, 300-322 570-610 GHz
• Ground A, E states
• Jmax 58 (Kmax 24)
• RAM Hamiltonian

13C
A few b-type transitions No prediction
Detection in interstellar media??
Willaert et al. J. Mol. Struct., 795, 4 (2006)
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Experiment

• FASSST spectrometer at OSU
• 110-380 GHz with three BWOs
• 100 scans accumulation
• up down-ward sweeps ? averaged
• SO2 lines ring cavity for frequency calibration
• Commercial H13COOCH3
• (Cambridge Isotope Laboratories, Inc.)
• A few mTorr in 8 m-long aluminum cell
• under room temperature

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FASSST Spectrum of 13C1-Methyl formate
9000 lines with S/N gt 5
CAAARS(1) (Computer Aided Assignment of
Asymmetric Rotor Spectra)
(1) Medvedev et al. J. Mol. Struct. 742, 299
(2005)
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Spectrum of 13C1-Methyl formate
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Transitions of 13C1-Methyl formate
a, b-type R, Q, P transitions c-type R, Q
x-type R transitions
1900 lines ? 2200 A transitions 2200 lines ? 2300
E transitions
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Effective Rotational Hamiltonian (ErHam) by
GronerPeter Groner, J. Chem. Phys. 107, 4483
(1997)

• Developed for dual or single internal rotors
• dimethyl-ether, 3-methyl-1,2-butadiene, aceton,
  ethyl methyl ether, dimethyl diselenide, methyl
  cabamate etc.
• Principal axes system (PAS) or
• Rho axis system (RAS)
• Torsional energy, rotational constants are given
  as Fourier series

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Effective Hamiltonianbased on principal axes
system
Hrot for asymmetric rotor
Coriolis interaction
negligible
Torsional energy
where
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Molecular Constants of 13C1-Methyl formate
53 parameters for A E substates (4800 lines)
with RMS 90 kHz
Comparable with Willaert et al. (2006)
Prediction for astronomical observation
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Systematic Deviations at Low Ka
Perturbation at Ka 0, 1 hardly
happens perturbation at high J / Ka
affects through least-square fitting
A substate
E substate
Q-branch transitions (Ka 0, 1)
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Perturbation at High Ka
Single state analysis of the A state with a
rotational Hamiltonian for asymmetric-top rotors
R branch transitions in the A substate
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Perturbation?

• Perturbation partner
• CH3 torsion (? 130 cm-1)
• COC bending (? 318 cm-1)
• CO torsion (? 332 cm-1)
• No ?v?0 interaction in the ErHam
• Van Vleck transformation?
• Too close to use
• ?? interaction term directly

Ka gt 15
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Conclusions

• 3000 lines in 110-380 GHz were newly assigned to
  4800 transitions of 13C1-methyl formate in the
  ground state.
• Most of transitions were fitted satisfactorily
  within 100 kHz.
• Perturbation from excited torsional states?
• Prediction of transition frequencies of
  13C1-methyl formate has been made.
• The first excited torsional state of 13C1-methyl
  formate is now being investigated.