Toward a global model of the interactions in low-lying states of methyl cyanide: Rotational and rovibrational spectroscopy of the ${\upsilon }_4=1$ state and tentative interstellar detection of the ${\upsilon }_4={\upsilon }_8=1$ state in Sgr B2(N)

Highlights

• Rotational and rovibrational spectra of CH₃CN associated with ${\upsilon }_4=1$ and ${\upsilon }_4={\upsilon }_8=1$.

• Perturbations in ${\upsilon }_4=1$ analyzed through rotational spectra.

• Model of low-lying states of CH₃CN extended from ${\upsilon }_8$ = 2 to ${\upsilon }_4$ = 1.

• Preliminary single state analysis of ${\upsilon }_4$ = ${\upsilon }_8$ = 1.

• Transitions up to ${\upsilon }_4=1$ detected in Sgr B₂(N), ${\upsilon }_4$ = ${\upsilon }_8=1$ detected tentatively.

Abstract

Rotational spectra of methyl cyanide were recorded newly and were analyzed together with existing spectra to extend the global model of low-lying vibrational states and their interactions to ${\upsilon }_4=1$ at 920 cm⁻¹. The rotational spectra cover large portions of the 36–1439 GHz region and reach quantum numbers J and K of 79 and 16, respectively. Information on the K level structure of CH₃CN is obtained from IR spectra. A spectrum of $2{\nu }_8$ around 717 cm⁻¹, analyzed in our previous study, covered also the ${\nu }_4$ band. The assignments in this band cover 880–952 cm⁻¹, attaining quantum numbers J and K of 61 and 13, respectively.

The most important interaction of ${\upsilon }_4=1$ appears to be with ${\upsilon }_8=3$, $\mathrm{\Delta }K=0,\mathrm{\Delta }l=+3$, a previously characterized anharmonic resonance. We report new analyses of interactions with $\mathrm{\Delta }K=-2$ and $\mathrm{\Delta }l=+1$, with $\mathrm{\Delta }K=-4$ and $\mathrm{\Delta }l=-1$, and with $\mathrm{\Delta }K=-6$ and $\mathrm{\Delta }l=-3$; these four types of interactions connect all l substates of ${\upsilon }_8=3$ in energy to ${\upsilon }_4=1$. A known $\mathrm{\Delta }K=-2,\mathrm{\Delta }l=+1$ interaction with ${\upsilon }_7=1$ was also analyzed, and investigations of the $\mathrm{\Delta }K=+1$, $\mathrm{\Delta }l=-2$ and $\mathrm{\Delta }K=+3,\mathrm{\Delta }l=0$ resonances with ${\upsilon }_8=2$ were improved, as were interactions between successive states with ${\upsilon }_8⩽3$, mainly through new ${\upsilon }_8⩽2$ rotational data.

A preliminary single state analysis of the ${\upsilon }_4={\upsilon }_8=1$ state was carried out based on rotational transition frequencies and on ${\nu }_4+{\nu }_8-{\nu }_8$ hot band data. A considerable fraction of the K levels was reproduced within uncertainties in its entirety or in part, despite obvious widespread perturbations in ${\upsilon }_4={\upsilon }_8=1$.

In addition to the interstellar detection of rotational transitions of methyl cyanide from within all vibrational states up to ${\upsilon }_4=1$, we report the tentative detection of ${\upsilon }_4={\upsilon }_8=1$ toward the main hot molecular core of the protocluster Sagittarius B2(N) employing the Atacama Large Millimeter/submillimeter Array.

Introduction

Methyl cyanide was detected in Sagittarius (Sgr) A and B almost 50 years ago as one of the first molecules observed by radio-astronomical means [1]. Since then, the molecule has been found in very diverse astronomical sources, a fairly detailed overview was given in our previous work on vibrational states ${\upsilon }_8⩽2$ of CH₃CN [2]. We point out that numerous rare isotopologs have been detected as well, which include ¹³CH₃¹³CN [3] and CHD₂CN [4]. More important for the present study is the detection of excited state transitions of CH₃CN up to ${\upsilon }_4=1$ at 920 cm⁻¹ [3]; see Fig. 1 for an overview of the low-lying vibrational states of methyl cyanide and Table 1 for a summary of the vibrational energies, including those of the l substates.

The identification of gaseous methyl cyanide relies mostly on laboratory spectroscopic information; in astronomical sources this is almost exclusively done with rotational spectroscopy from the microwave to the submillimeter region. The first study of the rotational spectrum of CH₃CN, and of its isomer CH₃NC, dates back to the early days of microwave spectroscopy [7]. A detailed account on previous work involving vibrational states up to ${\upsilon }_8=2$ was given in our investigation of these states [2]. A Fermi resonance between ${\upsilon }_8=1^{-1}$ and $2^{+2}$ ($\mathrm{\Delta }l=3$) was identified at $K=13$ and 14 and analyzed by means of rotational spectroscopy. Such resonances occur also between ${\upsilon }_8=2^{-2}$ and $3^{+1}$ at $K=12$ and 13 and between ${\upsilon }_8=2^0$ and $3^{+3}$ at $K=15$. Transitions up to $K=11$ and 13 have been accessed for ${\upsilon }_8=2^{-2}$ and $2^0$, respectively. This particular type of resonance was reported, to the best of our knowledge, for the first time in studies involving the corresponding bending states of propyne [8], [9], [10]. Rotational spectroscopy was instrumental in untangling analogous resonances. A study of such resonances was also reported for CH₃NC in its ${\upsilon }_8⩽2$ states [11]. In the case of CH₃CN, additional resonances of the type $\mathrm{\Delta }{\upsilon }_8=\pm 1,\mathrm{\Delta }K=\mp 2,\mathrm{\Delta }l=\pm 1$ were identified and analyzed in detail for ${\upsilon }_8=1^{-1}$ and $2^0$ ($K=13$ and 11) and ${\upsilon }_8=1^{+1}$ and $2^{+2}$ ($K=15$ and 13). Whereas these resonances caused pronounced perturbations, an analogous resonance between $\upsilon =0$ and ${\upsilon }_8=1^{+1}$ at $K=14$ and 12, respectively, displayed only small perturbations. However, these were strong enough to cause observable cross-ladder transitions between the states, thus connecting strongly these two vibrational states in energy.

The analyses of the next three higher-energy states, ${\upsilon }_4=1$ at 920 cm⁻¹, ${\upsilon }_7=1$ at 1042 cm⁻¹, and ${\upsilon }_8=3$ at 1078 and 1122 cm⁻¹, and their interactions took many years until a fairly comprehensive and sufficiently accurate level was achieved. Interactions involving ${\upsilon }_4=1$ are shown schematically in Fig. 2.

Kondo and Person evaluated the strength of the Coriolis interaction between ${\nu }_4$ and ${\nu }_7$ through intensity perturbations of ${\nu }_4$ in a low-resolution ($\sim$1 cm⁻¹) IR spectrum [12]. Bauer [13] studied the rotational spectra of CH₃CN and CH₃C¹⁵N up to ${\upsilon }_4=1$ (here and in the following, unlabeled atoms refer to ¹²C and ¹⁴N). The ${\upsilon }_4=1$ data with $6⩽J^{″}⩽9$ and $K⩽6$, and with frequencies up to 184 GHz, were published in a journal later [14]. Duncan et al. carried out a comparative study of the IR spectra of several methyl cyanide isotopologs along with a force field calculation [15]. They proposed a strong Fermi interaction between ${\nu }_4$ and $2{\nu }_8^0$, a strong anharmonic resonance between ${\nu }_4$ and $3{\nu }_8^3$, and a moderate anharmonic resonance between ${\nu }_7$ and $3{\nu }_8^1$. Rackley et al. performed a laser Stark investigation of ${\nu }_4$ and ${\nu }_7$ and determined in particular a ${\nu }_4$/${\nu }_7$ interaction parameter, even though they point out that the resonance in ${\nu }_7^{-1}$ occurs at $K\approx 23$ [16]. In addition, they examined the ${\nu }_7^{+1}$/$3{\nu }_8^{+1}$ anharmonic resonance, which is strongest at $K=7$ and 8 [16]. Mori et al. [17] carried out more extensive analyses of CH₃CN IR bands. In addition to the interactions analyzed earlier, they proposed a Fermi resonance between $2{\nu }_8^{-2}$ and ${\nu }_7^{+1}$ at $K=13$ and 14. Mito et al. [18] performed a laser Stark investigation of the ${\nu }_4$ band of CH₃C¹⁵N in order to analyze the resonance with $3{\nu }_8^{+3}$, which they found to be much weaker than proposed by Duncan et al. [15]. They found a crossing between $K=7$ and 8, with the latter farther apart at low J. Wallraff et al. [19] extended the K range of ${\nu }_4$ for CH₃CN and obtained essentially the same results concerning the corresponding resonance. Bocquet et al. [20] recorded submillimeter transitions of methyl cyanide, $J^{″}=19$ up to $K=12$ in the case of ${\upsilon }_4=1$. Cosleou et al. [21] extended the J range of rotational transitions in ${\upsilon }_4=1$ up to 24. They also analyzed the resonance between ${\upsilon }_4=1$ and ${\upsilon }_8=3^{+3}$, located a $\mathrm{\Delta }K=-2,\mathrm{\Delta }l=+1$ interaction between ${\upsilon }_4=1$ and ${\upsilon }_7=1^{+1}$ at $K=6$ and 4, respectively, and proposed a $\mathrm{\Delta }K=+3$, $\mathrm{\Delta }l=0$ interaction between ${\upsilon }_4=1$ and ${\upsilon }_8=2^0$ at $K=5$ and 8, respectively. The most comprehensive and accurate analysis of the ${\nu }_4$, ${\nu }_7$, and $3{\nu }_8$ band system of CH₃CN was presented by Tolonen et al. [5]. They included most of the resonances mentioned before, with the exceptions of the $\mathrm{\Delta }K=0$ and 3, $\mathrm{\Delta }l=0$ interactions between ${\upsilon }_4=1$ and ${\upsilon }_8=2^0$. They introduced a $\mathrm{\Delta }K=+1$, $\mathrm{\Delta }l=-2$ resonance between $K=5$ and 6 of ${\upsilon }_4=1$ and ${\upsilon }_8=2^{-2}$, respectively.

The next five vibrational states are ${\upsilon }_3=1,{\upsilon }_6=1$, ${\upsilon }_4={\upsilon }_8=1,{\upsilon }_7={\upsilon }_8=1$, and ${\upsilon }_8=4$, see Fig. 1 and Table 1. Investigations of the interactions between these states began 50 years ago. Matsuura analyzed the Fermi resonance between ${\nu }_6^{\pm 1}$ and ${({\nu }_7+{\nu }_8)}^{\mp 2}$ [22]. Duncan et al. [23] and later Matsuura et al. [24] included the Coriolis resonance between ${\nu }_6$ and ${\nu }_3$ in their analyses. Paso et al. [6] presented the latest, fairly comprehensive and quite accurate analysis of these bands. Their assignments covered extensive parts of ${\nu }_6$, a fair fraction of ${({\nu }_7+{\nu }_8)}^{\mp 2}$, and some transitions in ${\nu }_3$ ($K=5$ and 6), which gain intensity through the Coriolis resonance with ${\nu }_6$. Information on ${\nu }_4+{\nu }_8,{({\nu }_7+{\nu }_8)}^{\pm 0}$, and $4{\nu }_8^{\pm 2}$ were obtained through various resonances; no information was presented for $4{\nu }_8^0$ and for $4{\nu }_8^{\pm 4}$. A moderately weak anharmonic resonance between ${({\nu }_4+{\nu }_8)}^{-1}$ and ${({\nu }_7+{\nu }_8)}^{+2}$, mainly at $K=5$, and a $\mathrm{\Delta }K=-1$, $\mathrm{\Delta }l=+2$ interaction between ${({\nu }_4+{\nu }_8)}^{-1}$ and ${\nu }_6^{+1}$, mainly at $K=10$ and 9, respectively, were treated in their analysis. Mito et al. [25] studied the ${\nu }_4+{\nu }_8-{\nu }_8$ hot band of CH₃C¹⁵N and analyzed an anharmonic resonance between ${\upsilon }_4={\upsilon }_8=1^{+1}$ and ${\upsilon }_8=4^{+4}$ with largest effect at $K=8$ and a lesser one at $K=9$ and anharmonic resonances between ${\upsilon }_4={\upsilon }_8=1^{-1},{\upsilon }_8=4^{+2}$, and ${\upsilon }_7={\upsilon }_8=1^{+1}$ most strongly perturbed at $K=6$ and less so at $K=5$. Judging from Fig. 6 of Paso et al. [6], the last three resonances occur at the same K values in CH₃CN, with a possible difference in the resonance between ${\upsilon }_4={\upsilon }_8=1^{-1}$ and ${\upsilon }_7={\upsilon }_8=1^{+1}$, which may be strongest in $K=5$.

Approximately 15 years ago, we started our project devoted to recording and analyzing low-lying vibrational states of methyl cyanide. The aims were providing predictions of rotational and rovibrational spectra for radio-astronomical observations and for studies of the atmospheres of Earth and Titan, among others. An additional, but also necessary aim were thorough investigations of perturbations within and between these vibrational states.

In the course of a line-broadening and -shifting study in the ${\nu }_4$ band region of CH₃CN [26], a preliminary analysis of ${\upsilon }_4=1$ and its interactions with other vibrational states was carried out. In addition, assignments were made for the ${\nu }_4+{\nu }_8-{\nu }_8$ hot band and for rotational transitions in ${\upsilon }_4={\upsilon }_8=1$. Subsequently, extended assignments for the ground state rotational spectra of six methyl cyanide isotopologs were based on measurements on a sample of natural isotopic composition [27]. Some time later, we carried out a similar study of three minor isotopologs, CH₃¹³CN, ¹³CH₃CN, and CH₃C¹⁵N, in their ${\upsilon }_8=1$ excited vibrational states [28]. The analysis of CH₃CN vibrational states up to ${\upsilon }_4=1$ was quite advanced about ten years ago [29]. Attempts to introduce ${\upsilon }_7=1$ data into the fit were quite successful, but the inclusion of ${\upsilon }_8=3$ data proved to be more difficult. Data for both states were omitted for the present fit because both states are heavily interacting. Ultimately, our previous account on rotational and rovibrational data of CH₃CN [2] was limited to states with ${\upsilon }_8⩽2$. The omission of ${\upsilon }_4=1$ data at that time was based on large residuals in the $\mathrm{\Delta }K=3$ ground state loops from Ref. [30] and concomitant changes in the purely axial ground state parameters ($A-B,D_K,H_K$). In addition, there were small, but systematic residuals in some K series of the ${\upsilon }_4=1$ rotational data.

In our present study, we have reanalyzed carefully our ${\upsilon }_4=1$ data, recorded additional rotational transitions pertaining to ${\upsilon }_4=1$ and to lower vibrational states as well as to ${\upsilon }_4={\upsilon }_8=1$. We improved the analyses of all known resonances involving ${\upsilon }_4=1$ and those involving states differing in one quantum of ${\upsilon }_8$. Three higher-$\mathrm{\Delta }K$ resonances between ${\upsilon }_4=1$ and ${\upsilon }_8=3$ were also investigated. These findings improve the parameters for ${\upsilon }_4=1$ considerably and for some of the lower states to a lesser extent. We also report a preliminary analysis of ${\upsilon }_4={\upsilon }_8=1$. We use the spectroscopic results obtained for ${\upsilon }_4=1$ and ${\upsilon }_4={\upsilon }_8=1$ in this study to investigate the vibrationally excited methyl cyanide emission in the main hot molecular core embedded in the high-mass star forming protocluster Sagittarius B2(N) observed with the Atacama Large Millimeter/submillimeter Array (ALMA) in the frame of the ReMoCA project [31].

The remainder of this article is outlined as follows: experimental details of the rotational and rovibrational spectra are given in Section 2; Section 3 contains our results with details on the spectroscopy and interactions in low-lying vibrational states of CH₃CN, descriptions of the analyses carried out in the present study, a summary of the data obtained newly as well as those from previous investigations, and the determination of spectroscopic parameters. The astronomical results are described in Section 4; a discussion of the spectroscopic findings is presented in Section 5; Section 6 finally presents conclusions and an outlook from our study.