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)

Spectrum of 13C1-Methyl formate

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.