Submillimeter-wave Spectroscopy of 13 C 1 -Methyl formate [H 13 COOCH 3 ] 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 Groner Department 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 = K n = 10(6)-10(8) cm -3 Cold background

Senent, Villa, Meléndez and Dominguez-Gomez, ApJ. 627, 567 (2005) E A 407 MHz ( cm -1 ) CH 3 Internal Rotation in Methyl formate with ZPVE (zero-point vibrational correction)

Previous Rotational Studies of Methyl formate Curl (1959) - HCOOCH 3, DCOOCH 3, HCOOCH 2 D, H 13 COOCH 3, HCOO 13 CH 3, HC 18 OOCH 3, HCO 18 OCH 3 [10-30 GHz] v t = 0 (A, E), H R Bauer (1979) - HCOOCH 3 : [5-58 GHz] v t = 0 (A, E), IAM DeMaison et al. (1983) - HCOOCH 3 : [ GHz] v t = 0 (A, E) Plummer et al. (1984) - HCOOCH 3 : [ GHz] v t = 0 (A), H R Plummer et al. (1986) - HCOOCH 3 : [ GHz] v t = 0 (E), PAM Oesterling et al. (1995) - DCOOCH 3 : [-680 GHz] v t = 0 (A, E), RAM Oesterling et al. (1999) - HCOOCH 3 : [-380 GHz] v t = 0 (A, E), RAM Karakawa et al. (2001) - HCOOCH 3 Odashima et al. (2003) - HCOOCH 3 : [7-200 GHz] v t = 0 (A, E), 1 (A), IAM Ogata et al. (2004) - HCOOCH 3 : [7-200 GHz] v t = 0 (A, E), 1 (A, E) IAM Carvajal et al. (2007) - HCOOCH 3 : [7-200 GHz] v t = 0 (A, E), 1 (A, E), RAM → WG05 Fewer studies for isotopologues

Previous Study of 13 C 1 -Methyl formate Willaert et al. (2006) [7-62, & GHz] Ground A, E states J max = 58 (K max = 24) RAM Hamiltonian A few b-type transitions No prediction Willaert et al. J. Mol. Struct., 795, 4 (2006) Detection in interstellar media?? 13 C

FASSST spectrometer at OSU GHz with three BWOs 100 scans accumulation up & down-ward sweeps → averaged SO 2 lines + ring cavity for frequency calibration Commercial H 13 COOCH 3 (Cambridge Isotope Laboratories, Inc.) A few mTorr in 8 m-long aluminum cell under room temperature Experiment

FASSST Spectrum of 13 C 1 -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 13 C 1 -Methyl formate

Transitions of 13 C 1 -Methyl formate 1900 lines → 2200 A transitions 2200 lines → 2300 E transitions a, b-type R, Q, P transitions + c-type R, Q & x-type R transitions

Effective Rotational Hamiltonian (ErHam) by Groner Peter 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

Effective Hamiltonian based on principal axes system H rot for asymmetric rotor Torsional energy Coriolis interaction negligible where

Molecular Constants of 13 C 1 -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 K a E substate A substate Perturbation at K a = 0, 1 hardly happens perturbation at high J / K a affects through least-square fitting Q-branch transitions (K a = 0, 1)

Perturbation at High K a R branch transitions in the A substate Ka ≥ 15 Single state analysis of the A state with a rotational Hamiltonian for asymmetric-top rotors

Perturbation…? Perturbation partner CH 3 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 directly K a > ~15

Conclusions 3000 lines in GHz were newly assigned to 4800 transitions of 13 C 1 -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 13 C 1 -methyl formate has been made. The first excited torsional state of 13 C 1 -methyl formate is now being investigated.