submillimeter-wave spectroscopy of 13 c 1 -methyl formate [h 13 cooch 3 ] in the ground state atsuko...
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Submillimeter-wave Spectroscopy of13C1-Methyl formate [H13COOCH3]
in the Ground State
Atsuko Maeda, Ivan Medvedev, Eric Herbst, Frank C. De Lucia,
Department of Physics, The Ohio State University, Columbus, OH
and Peter GronerDepartment of Chemistry, University of Missouri-Kansas City,
Kansas City, MO
(Normal) Methyl formate
• One of abundant interstellar molecules
in hot cores & corinos (Orion KL, Sgr B2)
at Orion KL
Hot Core
Ultra-compact HII region O,B star
T = 100-300 K
n = 10(6)-10(8) cm-3
Cold background
Senent, Villa, Meléndez and Dominguez-Gomez, ApJ. 627, 567 (2005)
E
A
407 MHz(0.0135 cm-1)
CH3 Internal Rotationin Methyl formate
with ZPVE (zero-point vibrational correction)
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
• Ogata et al. (2004) - HCOOCH3: [7-200 GHz] vt = 0 (A, E), 1 (A, E) IAM
• Carvajal et al. (2007) - HCOOCH3: [7-200 GHz] vt = 0 (A, E), 1 (A, E), RAM → WG05
Fewer studies for isotopologues
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
A few b-type transitionsNo prediction
Willaert et al. J. Mol. Struct., 795, 4 (2006)
Detection in interstellar media??
13C
• FASSST spectrometer at OSU110-380 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
Experiment
FASSST Spectrum of 13C1-Methyl formate
~9000 lines with S/N > 5
CAAARS(1)
(Computer Aided Assignment of Asymmetric Rotor Spectra)
(1) Medvedev et al. J. Mol. Struct. 742, 299 (2005)
Transitions of 13C1-Methyl formate
Observed R-transitions, A-state
0
5
10
15
20
25
30
0 10 20 30 40 50 60
J
Ka
Previous
Present
1900 lines → 2200 A transitions2200 lines → 2300 E transitions
a, b-type R, Q, P transitions+
c-type R, Q & x-type R transitions
Observed Q-branch transitions, A-state
0
5
10
15
20
25
30
0 10 20 30 40 50 60
J
Ka
Previous
Present
Effective Rotational Hamiltonian (ErHam) by Groner
Peter Groner, J. Chem. Phys. 107, 4483 (1997)
• Developed for dual or single internal rotorsdimethyl-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
Effective Hamiltonianbased on principal axes system
Hrot forasymmetric rotor
Torsional energy
Coriolis interaction
negligible
where
Molecular Constants of 13C1-Methyl formate
Prediction for astronomical observation
53 parameters for A & E substates (~4800 lines) with RMS = 90 kHz
Comparable with Willaert et al. (2006)
Systematic Deviations at Low Ka
E substate
A substate
Perturbation at Ka = 0, 1 hardly happens
perturbation at high J / Ka affectsthrough least-square fitting
Q-branch transitions (Ka = 0, 1)
Perturbation at High Ka
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 10 20 30 40 50 60
Rotational Quantum Number J
Ob
s-C
alc
. /
MH
z
R branch transitionsin the A substate
Ka ≥ 15
Single state analysis of the A state with a rotational Hamiltonian for asymmetric-top rotors
Perturbation…?
• Perturbation partner
CH3 torsion (ν = 130 cm-1)
COC bending (ν = 318 cm-1)
CO torsion (ν = 332 cm-1)
• No Δv≠0 interaction in the ErHam
• Van Vleck transformation?
Too close to use
→→ interaction term directly0
100
200
300
400
500
600
0 5 10 15 20 25 30 35
J
En
erg
y le
vels
[cm
-1] Ka > ~15
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.