The torsional fundamental band and high-J rotational spectra of the ground, first and second excited torsional states of acetone

https://doi.org/10.1016/j.jms.2019.06.008Get rights and content

Highlights

  • A new global study of the mm, submm and FIR spectra of acetone.

  • First analysis of the high resolution spectrum of the fundamental torsional band of acetone.

  • Performance of PAM_C2v_2tops program with respect to high-J states is tested.

Abstract

We present a new global study of the millimeter, submillimeter and far-infrared (FIR) spectra involving the three lowest torsional states of acetone ((CH3)2CO). New microwave measurements have been carried out between 34 and 940 GHz using spectrometers in IRA NASU (Ukraine), and PhLAM Lille (France). The FIR spectrum of acetone has been recorded on the AILES beamline of the SOLEIL synchrotron facility. The new data involving torsion–rotation transitions with J up to 90 and Ka up to 52 were combined with previously published measurements and analyzed using a model developed recently to study the high resolution spectra of molecules with two equivalent methyl rotors and C2v symmetry at equilibrium (PAM_C2v_2tops program). The final fit included 117 parameters to give an overall weighted root-mean-square deviation of 0.85 for the dataset consisting of 29,584 microwave and 1116 FIR line frequencies belonging, respectively, to the three lowest torsional states (ν1217) = (0,0), (1,0), (0,1) and to the observed fundamental band associated with the methyl-top torsion mode (ν1217) = (0,1) ← (0,0). The high values of rotational quantum numbers involved in this study provide an opportunity to test the performance of the PAM_C2v_2tops program approach for the case of highly excited rotational states.

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 (ν1217) = (0,0) [8], the first excited state of the lower torsional mode (ν1217) = (1,0) [9], and the first excited state of the higher torsional mode (ν1217) = (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 (ν1217) = (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 (ν1217) = (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:H=1/4knpqr1r2s1s2t1t2Cknpqr1r2s1s2t1t2×{J2kJznJxpJyq×[pAr1pBr2cos(3s1αA)cos(3s2αB)sin(3t1αA)sin(3t2αB)+-1(n+q)pBr1pAr2cos(3s1αB)cos(3s2αA)sin(3t1αB)sin(3t2αA)]+[-1(n+q)sin(3t2αA)sin(3t1αB)cos(3s2αA)cos(3s1αB)pAr2pBr1+sin(3t2αB)sin(3t1αA)cos(3s2αB)cos(3s1αA)pBr2pAr1]×JyqJxpJznJ2k},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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