Analysis of Rotational Transitions of Methyl Formate in the Ground and First Excited Torsional States

Abstract

The microwave spectrum of methyl formate has been observed in the 7−200 GHz region, and new 437 lines have been assigned to the first excited A torsional substate. Both excited state lines and ground state lines reported previously were analyzed simultaneously on the basis of an internal axis method Hamiltonian. A total of 3514 lines were fitted to a 10th-order reduced Hamiltonian model involving 67 molecular parameters to a 1σ standard deviation of 179 kHz.

The Millimeter‐ and Submillimeter‐Wave Spectrum of13C1‐Methyl Formate (H13COOCH3) in the Ground State

“…In particular, the effective Hamiltonian of the ERHAM approach is different from that used in the previous studies of the rotational spectrum of methyl formate (Oesterling et al 1999;Karakawa et al 2001;Odashima et al 2003;Ogata et al 2004;Willaert et al 2006), which were based on the rho axis method (RAM; Hougen et al 1994;Bunker & Jensen 1998), in which a component of the reference axis system, here designated a , is chosen parallel to r so that the bĴb and cĴc terms in the Hamiltonian vanish. 3 As a result, the RAM Hamiltonian has only one cross term,Ĵ aĵ , which is diagonal in the symmetric-top quantum number K and can be added to the pure torsional terms of the Hamiltonian , which then becomes K-dependent.…”

“…Following Curl (1959), numerous spectroscopic studies of its rotational 1 Also Departments of Astronomy and Chemistry. spectrum were undertaken for normal methyl formate (Brown et al 1975;Bauder 1979;Demaison et al 1983;Plummer et al 1984Plummer et al , 1986Oesterling et al 1999;Karakawa et al 2001;Odashima et al 2003;Ogata et al 2004). The most recent studies (Odashima et al 2003;Ogata et al 2004) have extended the analysis to include the first excited torsional state.…”

Global fit of rotational transitions of methyl formate (HCOOCH3) in the ground and first excited torsional states

“…Observed transition frequencies were reproduced with torsion-rotation Hamiltonian models to an accuracy of microwave spectroscopy ($100 kHz) [8,9]. For excited torsional states, we recently assigned 437 rotational lines to the first excited torsional state (v t ¼ 1), although all the assigned lines were limited to A-species [10]. For Especies lines affected more strongly by the methyl torsion, we failed to assign any transition definitely because the number of assigned transitions was insufficient for combination relations.…”

Torsionally Excited Methyl Formate in Orion KL

“…However, the assignment of the torsionally excited state is generally more difficult, because the internal rotation makes the spectra very congested, the splitting due to A/E torsional symmetry becomes larger than the ground state, and the Boltzmann distribution makes the signal weaker. Recent studies (Odashima et al 2003;Ogata et al 2004) report the successful assignment of laboratory spectra of methyl formate in the first torsionally excited state ( ) that lies near v p 1 t 135 cm Ϫ1 (∼200 K) above the ground state. In this study, we give the first report that about 20 U lines previously detected in Orion KL are identified as methyl formate in its first torsionally excited state.…”