The torsional fundamental band and high-J rotational spectra of the ground, first and second excited torsional states of acetone
Graphical abstract
Introduction
Acetone ((CH3)2CO) is of substantial astrophysical interest as an ubiquitous molecule in the interstellar medium [1], [2], [3], [4] having also a rich and dense spectrum. Transitions belonging to the ground [1], [2] as well as the first excited torsional state [3] have been detected in interstellar medium. Therefore, one important aim of the present investigation is to provide reliable predictions for astronomical observations of acetone at millimeter and submillimeter wavelengths. Note that acetone is also of interest for atmospheric monitoring science (see for example [5]) as one of the most abundant oxygenated volatile organic compounds in the troposphere where it influences the oxidizing capacity of the atmosphere [6]. Acetone presents as well a strong intrinsic spectroscopic interest as it is a well-suited molecule to test the performances of the methods applied to the analysis of internal rotation phenomena of two equivalent methyl rotors. In this study we test the performance of the recently developed PAM_C2v_2tops program [7] with respect to analysis of highly excited rotational states of the acetone molecule.
The microwave, millimeter and sub-millimeter wave spectra of acetone have been extensively studied for more than 60 years, as documented in a series of papers by Groner [8], [9], [10]. The rotational spectra of the vibrational and torsional ground state (ν12,ν17) = (0,0) [8], the first excited state of the lower torsional mode (ν12,ν17) = (1,0) [9], and the first excited state of the higher torsional mode (ν12,ν17) = (0,1) [10] have been investigated, which based on the energy ordering considerations were referred as the ground, the first torsional excited state, and the second torsional excited state, respectively. Recently Groner and coworkers performed a microwave-microwave double-resonance spectroscopy study of acetone focusing on the (ν12,ν17) = (0,1) torsional excited state [11]. Our current work is a continuation of our study of the acetone spectrum using the recently developed program (PAM_C2v_2tops) for fitting the high-resolution torsion–rotation spectra of molecules with two equivalent methyl rotors and C2v symmetry at equilibrium [7]. At the first stage of our study [7] the PAM_C2v_2tops program was applied to the available literature data [8], [9], [10]. At the second stage we performed an investigation of the millimeter wave spectrum of acetone that covers the frequency range from 34 to 150 GHz and the range of rotational J quanta up to 60 [12]. At the third stage of the work presented here, we performed an analysis of the high resolution spectrum of the fundamental torsional band (ν12,ν17) = (0,1) ← (0,0) of acetone, which was recorded on the AILES beamline of the synchrotron SOLEIL, and expanded the frequency range of our microwave study up to 940 GHz. The accompanying expansion of the range of rotational J quantum number coverage up to 90 provides a new test of PAM_C2v_2tops program performance in the case of highly excited rotational states.
Section snippets
Experimental details
The absorption spectrum of acetone in the frequency ranges of 34–150 GHz and 315–400 GHz was recorded using the automated millimeter wave spectrometer of the Institute of Radio Astronomy of NASU. This spectrometer is built according to so-called «classical» scheme of absorption spectrometers and its detailed description can be found in Ref. [13]. A passive Schottky multiplier (X3) from Virginia Diodes Inc. was used to reach the 315–400 GHz range. The measurements in the frequency range between
Theoretical model
The program employed in the current study (PAM_C2v_2tops [7]) uses following general expression for the Hamiltonian:where subscripts A and B denote the two methyl tops, the quantities in brackets are quantum
Assignments and fit
We started our analysis from the results of Ref. [12] where the weighted standard deviation of 0.78 was achieved for the dataset consisting of 7 FIR and 12,128 microwave line frequencies with J ≤ 60 and Ka ≤ 35 that was fitted using 99 parameters of the Hamiltonian model that assumes two equivalent methyl rotors and C2v symmetry at equilibrium (PAM_C2v_2tops program). Assigning and fitting of the new microwave data using the PAM_C2v_2tops program proceeded in a fairly conventional iterative way
Discussion
One of the questions that arises in connection with the obtained set of parameters concerns the presence of a rather large number of parameters included in the final fit. On one side the present fit can be evaluated using a ratio of 117/(3 × 4) = 9.75 parameters per “asymmetric rotor spectrum”, where the 3 in the denominator represents the three torsional states under study and the 4 in the denominator arises because each vibration–rotation level in acetone splits into four components due to
Conclusion
We have presented a new study of the rotational spectra in the lowest three torsional states of acetone ((CH3)2CO) employing a model that makes use of an explicit two-dimensional potential function for molecules with two equivalent methyl rotors and C2v symmetry at equilibrium. We expanded the frequency range of the microwave studies up to 940 GHz and the range of the rotational quantum number J coverage up to 90. Also for the first time we present an analysis of the high-resolution spectrum of
Acknowledgment
The work was done under support of the Volkswagen foundation. The assistance of the Science and Technology Center in Ukraine, Ukraine is acknowledged (STCU partner project P686). A portion of this research was performed at the Jet Propulsion Laboratory, California Institute of Technology under contract with the National Aeronautics and Space Administration.
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