Skip to main content

ORIGINAL RESEARCH article

Front. Astron. Space Sci., 02 February 2022
Sec. Astrochemistry
This article is part of the Research Topic RNA World Hypothesis and the Origin of Life: Astrochemistry Perspective View all 13 articles

Rotational Rest Frequencies and First Astronomical Search of Protonated Methylamine

Philipp C. SchmidPhilipp C. Schmid1Sven ThorwirthSven Thorwirth1Christian P. EndresChristian P. Endres2Matthias Tpfer&#x;Matthias Töpfer1lvaro Snchez-MongeÁlvaro Sánchez-Monge1Andreas SchwrerAndreas Schwörer1Peter SchilkePeter Schilke1Stephan SchlemmerStephan Schlemmer1Oskar Asvany
Oskar Asvany1*
  • 1I. Physikalisches Institut, Universität zu Köln, Köln, Germany
  • 2Max Planck Institute for Extraterrestrial Physics, Garching bei München, Germany

We report first laboratory rest frequencies for rotational transitions of protonated methylamine, CH3NH3+, measured in a cryogenic 22-pole ion trap machine and employing an action spectroscopy scheme. For this prolate symmetric top molecule thirteen transitions between 80 and 240 GHz were detected in the ground vibrational state, covering JK = 2K − 1K up to JK = 6K − 5K with K = 0, 1, 2. Some transitions exhibit noticeable structure that is attributed to internal rotation splitting. As the CN radical and several of its hydrogenated and protonated forms up to methylamine, CH3NH2, are well known entities in the laboratory and in space, protonated methylamine, CH3NH3+, is a promising candidate for future radio astronomical detection.

1 Introduction

The cyano radical, CN, was one of the first molecules detected in the interstellar medium (McKellar, 1940). It is astrochemically linked to its hydrogenated and protonated forms, most of which have been detected in the interstellar medium, typically in the galactic center source Sgr B2, such as HCN (Snyder and Buhl, 1971), HNC (Snyder and Buhl, 1972; Zuckerman et al., 1972), HCNH+ (Ziurys and Turner, 1986), and H2C = NH (Godfrey et al., 1973). The terminal product of this hydrogenation series is methylamine, CH3NH2, which has been well characterized by laboratory spectroscopy (Ohashi et al., 1987; Ilyushin and Lovas, 2007; Motiyenko et al., 2014) and was first detetcted in Sgr B2 and Orion A in 1974 by Kaifu et al. (1974) and Fourikis et al. (1974).

In interstellar environments, molecules may also occur in their protonated forms, generated by a proton transfer from a proton donor like H3+, e.g. CO + H3+HCO+ + H2. Many of the simpler protonated species have been detected in the interstellar medium, such as HCO+ (Buhl and Snyder, 1970), N2H+ (Turner, 1974), HCNH+ (Ziurys and Turner, 1986) and HOCO+ (Thaddeus et al., 1981), whereas many of the more complex ones so far are only suspected to be present. Examples are CH3NH3+, as a product of proton transfer to methyl amine (see, e.g., the KIDA database; Wakelam et al., 2015) or from radiative association of NH3 and CH3+ (Herbst, 1985), but also protonated methanol, CH3OH2+ (Jusko et al., 2019), and protonated methane, CH5+ (Asvany et al., 2012; Asvany et al., 2015). In particular the latter two molecular ions have not yet been searched for in space because their laboratory microwave spectra are predicted to be quite irregular and still not known.

Following up on our recent work on the high-resolution rotational spectra of CN+ (Thorwirth et al., 2019a) and CH2NH2+ (Markus et al., 2019), in this study, we finally focus on the very last member of the CN hydrogenation/protonation chain, protonated methylamine, CH3NH3+. Upon protonation, methylamine, CH3NH2, an asymmetric top molecule featuring two internal large-amplitude motions (internal rotation and inversion), is converted into a much simpler symmetric top of C3v point group symmetry, in which the CH3 and the NH3 groups assume a staggered configuration at the global energy minimum (see inset in Figure 1). Literature on the spectroscopy of CH3NH3+ is extremely sparse, with only one experimental paper presenting low-resolution infrared (IR) features of the Ar-tagged species (Michi et al., 2003), and one work reporting ab initio values for its IR vibrational frequencies (Zeroka and Jensen, 1998). Therefore, the spectroscopic study in the present work was complemented with new high-level quantum-chemical predictions of the molecular structure and force field (see Section 3). The ion trap experiment is briefly described in Section 2. The experimentally derived rotational transitions of CH3NH3+ as well as the ground state spectroscopic parameters are summarized in Section 4. A first astronomical search of CH3NH3+ towards Sgr B2(N) and Sgr B2(M) is finally presented in Section 5.

FIGURE 1
www.frontiersin.org

FIGURE 1. Measurements of the JKJK (K = 0, 1, 2) rotational transitions of CH3NH3+ (red trace), recorded as depletion signal of the normalized CH3NH3+He counts. The simulations (green sticks indicate nitrogen quadrupole hyperfine structure (hfs) and their convolution is given as black traces), based on a symmetric rotor model (Table 1, fit I), indicate that the hyperfine splitting is not resolved and that discrepancies between the simulated and measured spectra, in particular the blue-shifted shoulders for K = 0 and 1 (red trace), are most probably due to the neglect of torsional motion in the simulation.

2 Experimental Methods

The rotational transitions of CH3NH3+ have been measured using an action spectroscopic method which exploits the rotational state dependence of the ternary attachment of He atoms to cations at low temperature (Brünken et al., 2014; Brünken et al., 2017; Doménech et al., 2017; Jusko et al., 2017; Doménech et al., 2018a; Doménech et al., 2018b; Thorwirth et al., 2019a; Asvany et al., 2021). The experiment was performed in the 4 K ion trapping machine COLTRAP described by Asvany et al. (2010, 2014). The ions were generated in a storage ion source by electron impact ionization (Ee ≈ 26–30 eV) of the precursor gas mixture. This mixture consisted of methylamine, CH3NH2 (Aldrich Chem. Corporation, CAS 74-89-5, 98%), and helium (Linde 5.0) which were admitted to the ion source via two separate leakage valves. CH3NH3+ is generated by a reaction of the type CH3NH2+ + CH3NH2 CH3NH3+ + CNH4. A pulse of several ten thousand mass-selected parent ions (m = 32 u) was injected into the 22-pole ion trap filled with about 1014 cm−3 He at 4 K. At the beginning of the trapping time lasting 800 ms, CH3NH3+-He complexes formed by three-body collisions with He. The resonant absorption of the cw millimeter-wave radiation by the trapped cold CH3NH3+ cations is detected by observing the decrease of the number of CH3NH3+-He complexes. A rotational line is thus recorded by repeating these trapping cycles (1 Hz) and counting the mass-selected CH3NH3+-He complexes (m = 36 u) as a function of the millimeter-wave frequency. This millimeter-wave radiation was supplied by a multiplier chain source (Virginia Diodes, Inc.), covering the ranges 80–125 and 170–1100 GHz. This source was driven by a synthesizer (Rohde&Schwarz SMF100A), which was referenced to a rubidium atomic clock. The beam of the millimeter-wave source has been directed toward the ion trap via an elliptical mirror and a thin diamond vacuum window. The 160 GHz line of CH3NH3+ was measured with a different multiplier chain (Radiometer Physics GmbH), covering the range of 110–170 GHz.

3 Computational Methods

As no previous high-resolution experimental data were available for CH3NH3+, spectroscopic searches and analysis were based on high-level quantum-chemical calculations performed here at the CCSD(T) level of theory (Raghavachari et al., 1989). Equilibrium geometries were calculated using analytic gradient techniques (Watts et al., 1992) and Dunning’s correlation-consistent basis sets as large as cc-pwCVQZ (Peterson and Dunning, 2002). Anharmonic force fields to evaluate the zero-point vibrational contributions 12iαiA,B,calc (= ΔA0, ΔB0) to the equilibrium rotational constants were calculated using analytic second-derivative techniques (Gauss and Stanton, 1997; Stanton and Gauss, 2000) followed by additional numerical differentiation to calculate the third and fourth derivatives needed for the anharmonic force field (Stanton et al., 1998; Stanton and Gauss, 2000). These calculations were carried out using the frozen core (fc) approximation in combination with the cc-pVTZ basis set (Dunning, 1989).

All calculations were performed using the CFOUR program (Matthews et al., 2020) and strategies summarized elsewhere (Puzzarini et al., 2010). The computed spectroscopic parameters are listed in Table 1 that also provides scaled (best-estimate) parameters obtained from a comparison of experimental and calculated values of isoelectronic ethane, C2H6 (see, e.g., Martinez et al. (2013) for a similar procedure used in the vinyl acetylene/protonated vinyl cyanide family of isoelectronic species). Additional results from the calculations are given in the Supplementary Material. An estimate of the torsional barrier height was obtained using the energy difference between the staggered and the eclipsed forms of CH3NH3+ calculated at the CCSD(T)/cc-pwCVQZ level and under consideration of harmonic zero-point vibrational contributions calculated at the CCSD(T)/cc-pVTZ level. This procedure results in a barrier of V3 = 1.98 kcal/mol. A similar calculation of ethane, C2H6, yields 2.45 kcal/mol to be compared against an experimental value of 3.00 kcal/mol (Borvayeh et al., 2008). Using the ethane exp/calc-ratio of V3 for the purpose of scaling, a best estimate value of 2.42 kcal/mol (846 cm−1) for the torsional barrier in CH3NH3+ is obtained.

TABLE 1
www.frontiersin.org

TABLE 1. Calculated and experimental spectroscopic parameters (in MHz, unless noted otherwise) of CH3NH3+. Two experimental fits to the measured data are presented, one (fit I) ignoring the torsional motion, and one (fit II) including torsional motion. Numbers in parentheses are one standard deviation in units of the last digit.

4 Laboratory Results and Spectroscopic Parameters

Using frequency predictions based on the new high-level quantum-chemical calculations, the lines shown in Figure 1 were found subsequently during targeted spectroscopic survey scans. The spectra were recorded in individual measurements in which the frequency was stepped in an up-and-down manner several times. The frequency steps were typically 2 kHz, except for the JK = 4K ← 3K measurement at 159 GHz, for which 10 kHz steps were applied. Such individual measurements were repeated typically ten times. In the depiction of Figure 1, all available spectra were accumulated and rebinned to a stepwidth of 10 kHz. This deep integration was necessary due to the comparably small signal strength (maximum depletion upon photon absorption is on the order of 2%), and due to somewhat noisy signal counts, which had its origin in ion source instabilities caused by the sticky consistency of the methylamine precursor. Also, during the measurements, care was taken to avoid power broadening. Thus, the linewidths were expected to be dominated by the Doppler broadening due to the kinetic temperature of the ions in the trap (nominal temperature T = 4 K with some residual heating to typically 8 K), and a possible contribution due to non-resolved hyperfine splitting. The line patterns detected for a given rotational transition were in line with those expected for a prolate symmetric top molecule and at first sight, no further spectroscopic complexity, as could be expected for resolved nitrogen quadrupole hyperfine structure or torsional motion between the CH3 and NH3 subunits, was detected in individual measurements at our experimental conditions. For all individual measurements, the detected lines were fitted to Gaussian functions, from which line centers and their uncertainties were determined, and the first results were very similar to the frequencies quoted in Table 2 (Model I).

TABLE 2
www.frontiersin.org

TABLE 2. Frequencies of pure rotational transitions (in MHz) of CH3NH3+, obtained by fitting multiple Gaussians to the spectra in Figure 1. The uncertainties (1 σ) are given in parentheses in units of the last significant digits. The assignment of Model I assumes a standard symmetric top Hamiltonian whereas that of Model II assumes a symmetric top featuring torsional splitting, see text for details.

The measured frequencies of the pure rotational lines collected in Table 2 (Model I) were first fit with a standard symmetric rotor Hamiltonian using the PGOPHER program (Western, 2017), yielding the set of parameters for the ground state given in Table 1 (fit I). Since the transitions of a symmetric rotor obey the ΔK = 0 selection rule, A and DK cannot be determined experimentally, and only B0, DJ and DJK were derived in the least-squares fitting procedure and are presented in Table 1. Computed parameters are also included in Table 1 for comparison. As can be seen, the overall agreement between the calculated and scaled best-estimate values and the experimental values is very good. A simulated stick spectrum and its convolution, based on fit I of Table 1 and accounting for the hyperfine structure due to the quadrupole moment of the N14 nucleus (with spin I = 1), are included in Figure 1. As the computed value eQq(N14) = +158 kHz is very small (see also Table 1), it is not surprising that the hyperfine structure is not resolved in our experiment.

Closer inspection after co-addition of all spectra revealed some noticeable blue-shifted shoulders, as discernible in the accumulation of Figure 1. These are particularly evident in the spectra at 119 (J = 3K − 2K) and 159 GHz (J = 4K − 3K), but are also discernible in the weaker spectrum at 199 GHz (J = 5K − 4K). In the spectrum at 119 GHz (second panel in Figure 1 and close-up in Figure 2) both blue shoulders have an offset of about +170 kHz relative to the main peaks (K = 0 and 1), and this offset is found to increase for the higher frequency transitions at 159 (+230 kHz) and 199 GHz (+290 kHz). While a detailed analysis is hampered by the overall poor signal-to-noise ratio of the spectra, a first analysis was performed here under the assumption that the spectra are affected by internal rotation, an effect that has rarely been observed previously for symmetric top molecules in their ground vibrational states (see, e.g., Ozier and Moazzen-Ahmadi, 2007, and references therein). As indicated in Figure 2, the structure of the J = 3K − 2K spectrum may be decomposed into four features, two strong ones (the putative K = 0 and 1 components used in the first fitting procedure in Table 1, fit I) each of which is accompanied by a weaker satellite that is found blue-shifted relative to the main component. Similar patterns are observed for the J = 4K − 3K and J = 5K − 4K transitions. Ideally, a suitable model Hamiltonian should be able to reproduce both the magnitude of the torsional splitting as well as the intensities of the individual spectroscopic components. First, the magnitude of the torsional splitting to be expected in CH3NH3+ was estimated based on the so-called hybrid approach described elsewhere (Wang et al., 2001) using the dominant parameters (V3, ρ = Iα/Ic, F = A/(ρ × (1 − ρ)), F3J, F3K) as calculated here and complemented with parameters taken from isoelectronic ethane (Borvayeh et al., 2008). Using this approach, the splitting was predicted in very good agreement with the experimental values. In analogy to other closely related symmetric top molecules such as methyl silane, CH3SiH3, each rotational transition of CH3NH3+ with K = 1, 2 is expected to split in up to three components (σ = 0, +1, −1) whereas transitions with K = 0 only split into two, σ = 0, ±1 (Pelz et al., 1992; Ozier and Moazzen-Ahmadi, 2007). The symmetries and nuclear spin weights under consideration of torsional splitting that are needed for intensity estimates have been given, for example, in Pelz et al. (1992). From this, it is concluded that in the CH3NH3+-spectra two strong torsional components of the K = 1-transitions (σ = 0, 1) are too close in frequency to be spectroscopically resolved and thus only one strong (superposition of σ = 0 and 1) and one weaker torsional component (σ = − 1) are observed. The K = 0 splitting is also (partly) resolved and both components assigned, a stronger (σ = 0) and a weaker one (σ = ±1), respectively. For the K = 2 transitions, only one component (σ = 0) can be assigned in the spectra obtained here with some confidence. Using this spectroscopic knowledge, the accumulated spectra as depicted in Figure 1 were refitted using multiple Gaussian components, and the final frequencies and their assignments are listed in Table 2, Model II. With this improved spectroscopic assignment, a second fit was performed (using an in-house program and assuming a uniform frequency uncertainty of 15 kHz) leading to an alternative set of molecular parameters given in Table 1 (fit II). In this approach, owing to the limited amount of spectroscopic information available to constrain the internal rotation problem more rigorously, several parameters required in the theoretical description were kept fixed at values calculated quantum-chemically (ρ, F, V3). In analogy to fit I, B0, DJ, and DJK were varied in the least squares adjustment and additionally F3J was released, resulting in a good fit result and agreement with the fit I and C2H6 parameter sets. If released also, V3 cannot be determined statistically from the present data set and changing its value from the scaled calculated value of 2.42 kcal/mol to the unscaled value of 1.98 kcal/mol has little impact on the overall fit quality but decreases the value of F3J significantly to −87(3) MHz. Based on this new set of parameters and taking into consideration the proper spin statistical effects (e.g., Pelz et al., 1992), new simulations of spectra have been obtained that are shown in Figure 3. The agreement between the experimental spectra and the simulations is rather compelling and hence speaking very much in favor of internal rotation as to the cause of the peculiar lineshapes and splittings observed.

FIGURE 2
www.frontiersin.org

FIGURE 2. Zoom into the transition 3K ← 2K (K = 0, 1) highlighting internal rotation splitting in CH3NH3+. Four Gaussian features have been assumed in performing a line profile fit. The red vertical bars show the location of the five possible σ torsional components labeled at the top with the length of the bars indicating their contribution from spin statistical effects, see text for details. The gray horizontal markers indicate the torsional splitting of some 170 kHz observed in this particular transition (cf. transition frequencies collected in Table 1, Model II).

FIGURE 3
www.frontiersin.org

FIGURE 3. Same as Figure 1 but with torsional splitting included into the simulation (black line, Table 1, fit II). Owing to the very small magnitude of eQq(14N) calculated here, quadrupole hyperfine structure has not been considered in this simulation.

5 Interstellar Search

The CH3NH3+ spectral line transitions reported in Table 2 have frequencies >40 GHz and constitute good targets to be searched for with facilities like ALMA (Atacama Large Millimeter/sub-millimeter Array; ALMA Partnership et al., 2015). While the low-energy transitions are not covered by current ALMA bands, the J = 4K → 3K and JK = 5K → 4K lines can be observed with the new ALMA band 5 receiver (covering a frequency range from 159 to 211 GHz), and the JK = 6K → 5K transitions are observable with the commonly used band 6 receiver (211–276 GHz).

The detection of species closely related to protonated methylamine such as HCN, HCNH+ or CH3NH2 in the star-forming region Sgr B2 (e.g., Schilke et al., 1991; Belloche et al., 2013) motivates the search for protonated methylamine in this region. We have made use of the ALMA spectral line survey published in Sánchez-Monge et al. (2017, see also Schwörer et al., 2019). The observations target the star-forming objects Sgr B2(N) and Sgr B2(M) and cover the whole ALMA band 6 with a spectral resolution of 0.7 km s−1 and an angular resolution of 0.′′4 (corresponding to 3300 au at the distance of the source). This high angular resolution allows to resolve the regions in more than 40 different dense cores (see Sánchez-Monge et al., 2017). Figure 4 presents the spectra extracted towards different selected cores in both regions (see source coordinates in Tables 1, 2 of Sánchez-Monge et al., 2017). The spectra are obtained after averaging the emission inside the 3σ polygon that defines the source size (between 0.′′5 and 0.′′7). We mark with vertical lines the location of the CH3NH3+ transitions.

FIGURE 4
www.frontiersin.org

FIGURE 4. ALMA spectra towards seven dense cores in the Sgr B2(N) and (M) regions. Data from Sánchez-Monge et al. (2017). The baseline for each spectrum corresponds to a brightness temperature of 0 K, however, the spectrum of each source has been manually shifted by 60 K (with respect to the source below) for an easier visualization of the data. The intensities of some spectra have been divided by the factor indicated to the right. Grey solid and dashed vertical lines mark the transitions of CH3CN (JK = 13K − 12K), CH313CN (JK = 13K − 12K) and SO2 (JKa,Kc=217,15226,16). The blue vertical lines mark the 6K → 5K transitions of CH3NH3+. The top panel shows the expected synthetic spectrum for CH3NH3+ as observed with ALMA at 0.′′4 angular resolution and considering a column density of 5 × 1014 cm−2, a temperature of 225 K, and a linewidth of 5 km s−1.

We have selected objects in different evolutionary stages and with different physical properties. Cores AN02, AN17 and AM03 correspond to dense cores dominated by dust but with a rich chemistry. In particular, core AN02 has been extensively studied in the literature in the search of new chemical species (e.g., Belloche et al., 2014). We have also included objects in which an embedded Hii region has been found (e.g., AN10, AM06, AM15). The presence of the UV radiation from embedded massive stars ionizing the gas and resulting in Hii regions can enhance the protonation of methylamine in the photon-dominated region around the Hii region. As shown in Figure 4, no obvious features are detected at the frequencies of the tabulated CH3NH3+ transitions, suggesting a low abundance of protonated methylamine in Sgr B2. We have determined an upper limit of 5×1014 cm−2 to the column density of protonated methylamine. This is estimated based on a 3-sigma noise upper limit and assuming a gas temperature of 200–300 K (as derived from other molecular species in Sgr B2, Schwörer et al., 2019). This translates into an upper limit of the fractional abundance of ∼10–10. The high densities of the Sgr B2 region (∼ 105–108 cm−3; see also Schmiedeke et al., 2016) can rapidly attenuate the radiation field and result in a low production of heavy ions like CH3NH3+. Moreover, the J = 6K → 5K transitions are located close to bright CH3CN and CH313CN (JK = 13K → 12K) transitions and a strong SO2 feature which may hinder the detection of the weaker CH3NH3+ in chemically rich, embedded objects. Observations at other frequency ranges, as well as in other astronomical sources associated with highly ionized and less dense gas, may help in the detection of this species in space.

6 Conclusion and Outlook

In the present study, the pure rotational spectrum of protonated methyl amine, CH3NH3+, was observed for the first time. It is worthwhile to recall that upon protonation of CH3NH2, forming CH3NH3+, the complex dynamical behaviour of the former molecular system is largely simplified, because the inversion at the NH2 group in CH3NH2 is eliminated and only the torsional motion between CH3 and NH3 has to be potentially considered for the pure rotational spectrum of CH3NH3+ in its ground vibrational state. In this study, a total of five pure rotational transitions from J = 2K − 1K to J = 6K − 5K and K up to 2 were identified. As indicated through complementary quantum-chemical calculations, explicit treatment of nuclear quadrupole hyperfine structure from the presence of the nitrogen nucleus was found to be negligible. While the strongest spectroscopic features in the spectrum can be assigned and fitted reasonably well in a straightforward fashion using a standard symmetric-top Hamiltonian, peculiar lineshapes and weak substructure identified in the spectra upon close inspection indeed required extension of the theoretical treatment to account for internal rotation. This refined model description permits very convincing reproduction of the experimental line profiles. A more comprehensive treatment of the internal rotational problem in CH3NH3+ would certainly benefit from extension of the present work to a higher degree of rotational excitation but also from detection of vibrational satellites of CH3NH3+ in torsionally excited states.

Future work towards the spectroscopic characterization of protonated amines offers many possibilities, not only in the millimeter-wave regime. For example, the low-resolution vibrational spectra of CH3NH3+ and also protonated ethyl amine, C2H5NH3+, were observed recently in the range from 700 to 1750 cm−1 (Thorwirth et al., 2019b) using the FELion ion trap apparatus (Jusko et al., 2019). For the purpose of millimeter-wave radio astronomical searches of CH3NH3+ the data presented here should already suffice. Although the column density of CH3NH3+ in the interstellar medium may be some orders of magnitude lower than that of CH3NH2, the more favorable partition function of CH3NH3+ will support radio astronomical detectability. In the search towards SgrB2 presented in this work CH3NH3+ was unfortunately not found.

Data Availability Statement

The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation.

Author Contributions

PS: measurement, manuscript writing MT: measurement ST: quantum chemical computations, data evaluation, manuscript writing CE: data evaluation, manuscript writing AS-M: data evaluation, manuscript writing AS: data evaluation, manuscript writing PS: supervision SS: supervision, manuscript writing OA: measurement, data evaluation, manuscript writing, supervision.

Funding

This work has been supported via Collaborative Research Centre 956, sub-projects A6 and B2, funded by the Deutsche Forschungsgemeinschaft (DFG, project ID 184 018 867), as well as DFG SCHL 341/15-1 (Gerätezentrum “Cologne Center for Terahertz Spectroscopy”).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Acknowledgments

The authors gratefully acknowledge the work done over the last years by the electrical and mechanical workshops of the I. Physikalisches Institut. We thank Frank Lewen for assistance using the mm-wave setup. CE is grateful for support from the Max Planck Society.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fspas.2021.805162/full#supplementary-material

References

Asvany, O., Bielau, F., Moratschke, D., Krause, J., and Schlemmer, S. (2010). Note: New Design of a Cryogenic Linear Radio Frequency Multipole Trap. Rev. Sci. Instrum. 81, 076102. doi:10.1063/1.3460265

PubMed Abstract | CrossRef Full Text | Google Scholar

Asvany, O., Brünken, S., Kluge, L., and Schlemmer, S. (2014). COLTRAP: A 22-pole Ion Trapping Machine for Spectroscopy at 4 K. Appl. Phys. B 114, 203–211. doi:10.1007/s00340-013-5684-y

CrossRef Full Text | Google Scholar

Asvany, O., Krieg, J., and Schlemmer, S. (2012). Frequency Comb Assisted Mid-Infrared Spectroscopy of Cold Molecular Ions. Rev. Sci. Instrum. 83, 093110. doi:10.1063/1.4753930

PubMed Abstract | CrossRef Full Text | Google Scholar

Asvany, O., Markus, C. R., Roucou, A., Schlemmer, S., Thorwirth, S., and Lauzin, C. (2021). The Fundamental Rotational Transition of NO+. J. Mol. Spectrosc. 378, 111447. doi:10.1016/j.jms.2021.111447

CrossRef Full Text | Google Scholar

Asvany, O., Yamada, K. M. T., Brünken, S., Potapov, A., and Schlemmer, S. (2015). Experimental Ground-State Combination Differences of CH5+. Science 347, 1346–1349. doi:10.1126/science.aaa3304

PubMed Abstract | CrossRef Full Text | Google Scholar

Belloche, A., Garrod, R. T., Müller, H. S. P., and Menten, K. M. (2014). Detection of a Branched Alkyl Molecule in the Interstellar Medium: Iso-Propyl Cyanide. Science 345, 1584–1587. doi:10.1126/science.1256678

PubMed Abstract | CrossRef Full Text | Google Scholar

Belloche, A., Müller, H. S. P., Menten, K. M., Schilke, P., and Comito, C. (2013). Complex Organic Molecules in the Interstellar Medium: IRAM 30 m Line Survey of Sagittarius B2(N) and (M). Astron. Astrophys. 559, A47. doi:10.1051/0004-6361/201321096

CrossRef Full Text | Google Scholar

Borvayeh, L., Moazzen-Ahmadi, N., and Horneman, V.-M. (2008). The ν12ν9 Band of Ethane: A Global Frequency Analysis of Data from the Four Lowest Vibrational States. J. Mol. Spectrosc. 250, 51–56. doi:10.1016/j.jms.2008.04.009

CrossRef Full Text | Google Scholar

Brünken, S., Kluge, L., Stoffels, A., Asvany, O., and Schlemmer, S. (2014). Laboratory Rotational Spectrum of l-C3H+ and Confirmation of its Astronomical Detection. Astrophys. J. Lett. 783, L4. doi:10.1088/2041-8205/783/1/l4

CrossRef Full Text | Google Scholar

Brünken, S., Kluge, L., Stoffels, A., Pérez-Ríos, J., and Schlemmer, S. (2017). Rotational State-dependent Attachment of He Atoms to Cold Molecular Ions: An Action Spectroscopic Scheme for Rotational Spectroscopy. J. Mol. Spectrosc. 332, 67–78. doi:10.1016/j.jms.2016.10.018

CrossRef Full Text | Google Scholar

Buhl, D., and Snyder, L. E. (1970). Unidentified Interstellar Microwave Line. Nature 228, 267–269. doi:10.1038/228267a0

PubMed Abstract | CrossRef Full Text | Google Scholar

Doménech, J. L., Jusko, P., Schlemmer, S., and Asvany, O. (2018a). The First Laboratory Detection of Vibration-Rotation Transitions of 12CH+ and 13CH+ and Improved Measurement of Their Rotational Transition Frequencies. Astrophys. J. 857, 61. doi:10.3847/1538-4357/aab36a

CrossRef Full Text | Google Scholar

Doménech, J. L., Schlemmer, S., and Asvany, O. (2017). Accurate Frequency Determination of Vibration-Rotation and Rotational Transitions of SiH+. Astrophys. J. 849, 60. doi:10.3847/1538-4357/aa8fca

PubMed Abstract | CrossRef Full Text | Google Scholar

Doménech, J. L., Schlemmer, S., and Asvany, O. (2018b). Accurate Rotational Rest Frequencies for Ammonium Ion Isotopologues. Astrophys. J. 866, 158. doi:10.3847/1538-4357/aadf83

CrossRef Full Text | Google Scholar

Dunning, T. H. (1989). Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron through Neon and Hydrogen. J. Chem. Phys. 90, 1007–1023. doi:10.1063/1.456153

CrossRef Full Text | Google Scholar

ALMA Partnership Fomalont, E. B., Vlahakis, C., Corder, S., Remijan, A., Barkats, D., et al. (2015). The 2014 ALMA Long Baseline Campaign: An Overview. Astrophys. J. Lett. 808, L1. doi:10.1088/2041-8205/808/1/L1

CrossRef Full Text | Google Scholar

Fourikis, N., Takagi, K., and Morimoto, M. (1974). Detection of Interstellar Methylamine by its 202 → 110Aa-State Transition. Astrophys. J. Lett. 191, L139. doi:10.1086/181570

CrossRef Full Text | Google Scholar

Gauss, J., and Stanton, J. F. (1997). Analytic CCSD(T) Second Derivatives. Chem. Phys. Lett. 276, 70–77. doi:10.1016/s0009-2614(97)88036-0

CrossRef Full Text | Google Scholar

Godfrey, P. D., Brown, R. D., Robinson, B. J., and Sinclair, M. W. (1973). Discovery of Interstellar Methanimine (Formaldimine). Astrophys. Lett. 13, 119.

Google Scholar

Herbst, E. (1985). The Rate of the Radiative Association Reaction between CH3+ and NH3 and its Implications for Interstellar Chemistry. Astrophys. J. 292, 484–486. doi:10.1086/163179

CrossRef Full Text | Google Scholar

Ilyushin, V., and Lovas, F. J. (2007). Microwave Spectra of Molecules of Astrophysical Interest. Xxv. Methylamine. J. Phys. Chem. Ref. Data 36, 1141–1276. doi:10.1063/1.2769382

CrossRef Full Text | Google Scholar

Jusko, P., Brünken, S., Asvany, O., Thorwirth, S., Stoffels, A., van der Meer, L., et al. (2019). The FELion Cryogenic Ion Trap Beam Line at the FELIX Free-Electron Laser Laboratory: Infrared Signatures of Primary Alcohol Cations. Faraday Discuss. 217, 172–202. doi:10.1039/c8fd00225h

PubMed Abstract | CrossRef Full Text | Google Scholar

Jusko, P., Stoffels, A., Thorwirth, S., Brünken, S., Schlemmer, S., and Asvany, O. (2017). High-resolution Vibrational and Rotational Spectroscopy of CD2H+ in a Cryogenic Ion Trap. J. Mol. Spectrosc. 332, 59–66. doi:10.1016/j.jms.2016.10.017

CrossRef Full Text | Google Scholar

Kaifu, N., Morimoto, M., Nagane, K., Akabane, K., Iguchi, T., and Takagi, K. (1974). Detection of Interstellar Methylamine. Astrophys. J. Lett. 191, L135–L137. doi:10.1086/181569

CrossRef Full Text | Google Scholar

Markus, C. R., Thorwirth, S., Asvany, O., and Schlemmer, S. (2019). High-Resolution Double Resonance Action Spectroscopy in Ion Traps: Vibrational and Rotational Fingerprints of CH2NH2+. Phys. Chem. Chem. Phys. 21, 26406–26412. doi:10.1039/c9cp05487a

PubMed Abstract | CrossRef Full Text | Google Scholar

Martinez, O., Lattanzi, V., Thorwirth, S., and McCarthy, M. C. (2013). Detection of Protonated Vinyl Cyanide, CH2CHCNH+, a Prototypical Branched Nitrile Cation. J. Chem. Phys. 138, 094316. doi:10.1063/1.4793316

CrossRef Full Text | Google Scholar

Matthews, D. A., Cheng, L., Harding, M. E., Lipparini, F., Stopkowicz, S., Jagau, T.-C., et al. (2020). Coupled-Cluster Techniques for Computational Chemistry: The CFOUR Program Package. J. Chem. Phys. 152, 214108. doi:10.1063/5.0004837

CrossRef Full Text | Google Scholar

McKellar, A. (1940). Evidence for the Molecular Origin of Some Hitherto Unidentified Interstellar Lines. Publ. Astron. Soc. Pac. 52, 187. doi:10.1086/125159

CrossRef Full Text | Google Scholar

Michi, T., Ohashi, K., Inokuchi, Y., Nishi, N., and Sekiya, H. (2003). Infrared Spectra and Structures of (CH3NH2)nH+ (n = 1-4). Binding Features of an Excess Proton. Chem. Phys. Lett. 371, 111–117. doi:10.1016/s0009-2614(03)00217-3

CrossRef Full Text | Google Scholar

Motiyenko, R. A., Ilyushin, V. V., Drouin, B. J., Yu, S., and Margulès, L. (2014). Rotational Spectroscopy of Methylamine up to 2.6 THz. Astron. Astrophys. 563, A137. doi:10.1051/0004-6361/201323190

CrossRef Full Text | Google Scholar

Ohashi, N., Takagi, K., Hougen, J. T., Olson, W. B., and Lafferty, W. J. (1987). Far-Infrared Spectrum and Ground State Constants of Methyl Amine. J. Mol. Spectrosc. 126, 443–459. doi:10.1016/0022-2852(87)90249-9

CrossRef Full Text | Google Scholar

Ozier, I., and Moazzen-Ahmadi, N. (2007). “Internal Rotation in Symmetric Tops,” in Advances in Atomic, Molecular, and Optical Physics. Editors P. R. Berman, C. C. Lin, and E. Arimondo (Amsterdam: Elsevier), 54, 423–509. doi:10.1016/s1049-250x(06)54007-8

CrossRef Full Text | Google Scholar

Pelz, G., Mittler, P., Yamada, K. M. T., and Winnewisser, G. (1992). Millimeter-Wave Spectra and Revised Spectroscopic Parameters of Methylsilane. J. Mol. Spectrosc. 156, 390–402. doi:10.1016/0022-2852(92)90240-o

CrossRef Full Text | Google Scholar

Peterson, K. A., and Dunning, T. H. (2002). Accurate Correlation Consistent Basis Sets for Molecular Core-Valence Correlation Effects: The Second Row Atoms Al-Ar, and the First Row Atoms B-Ne Revisited. J. Chem. Phys. 117, 10548–10560. doi:10.1063/1.1520138

CrossRef Full Text | Google Scholar

Puzzarini, C., Stanton, J. F., and Gauss, J. (2010). Quantum-Chemical Calculation of Spectroscopic Parameters for Rotational Spectroscopy. Int. Rev. Phys. Chem. 29, 273–367. doi:10.1080/01442351003643401

CrossRef Full Text | Google Scholar

Raghavachari, K., Trucks, G. W., Pople, J. A., and Head-Gordon, M. (1989). A Fifth-Order Perturbation Comparison of Electron Correlation Theories. Chem. Phys. Lett. 157, 479–483. doi:10.1016/s0009-2614(89)87395-6

CrossRef Full Text | Google Scholar

Sánchez-Monge, Á., Schilke, P., Schmiedeke, A., Ginsburg, A., Cesaroni, R., Lis, D. C., et al. (2017). The Physical and Chemical Structure of Sagittarius B2. II. Continuum Millimeter Emission of Sgr B2(M) and Sgr B2(N) with ALMA. Astron. Astrophys. 604, A6. doi:10.1051/0004-6361/201730426

CrossRef Full Text | Google Scholar

Schilke, P., Walmsley, C. M., Millar, T. J., and Henkel, C. (1991). Protonated HCN in Molecular Clouds. Astron. Astrophys. 247, 487.

Google Scholar

Schmiedeke, A., Schilke, P., Möller, T., Sánchez-Monge, Á., Bergin, E., Comito, C., et al. (2016). The Physical and Chemical Structure of Sagittarius B2. I. Three-Dimensional Thermal Dust and Free-Free Continuum Modeling on 100 au to 45 pc Scales. Astron. Astrophys. 588, A143. doi:10.1051/0004-6361/201527311

CrossRef Full Text | Google Scholar

Schwörer, A., Sánchez-Monge, Á., Schilke, P., Möller, T., Ginsburg, A., Meng, F., et al. (2019). The Physical and Chemical Structure of Sagittarius B2. IV. Converging Filaments in the High-Mass Cluster Forming Region Sgr B2(N). Astron. Astrophys. 628, A6. doi:10.1051/0004-6361/201935200

CrossRef Full Text | Google Scholar

Snyder, L. E., and Buhl, D. (1972). Detection of Several New Interstellar Molecules. Ann. N. Y. Acad. Sci. 194, 17–24. doi:10.1111/j.1749-6632.1972.tb12687.x

PubMed Abstract | CrossRef Full Text | Google Scholar

Snyder, L. E., and Buhl, D. (1971). Observations of Radio Emission from Interstellar Hydrogen Cyanide. Astrophys. J. Lett. 163, L47. doi:10.1086/180664

CrossRef Full Text | Google Scholar

Stanton, J. F., and Gauss, J. (2000). Analytic Second Derivatives in High-Order many-body Perturbation and Coupled-Cluster Theories: Computational Considerations and Applications. Int. Rev. Phys. Chem. 19, 61–95. doi:10.1080/014423500229864

CrossRef Full Text | Google Scholar

Stanton, J. F., Lopreore, C. L., and Gauss, J. (1998). The Equilibrium Structure and Fundamental Vibrational Frequencies of Dioxirane. J. Chem. Phys. 108, 7190–7196. doi:10.1063/1.476136

CrossRef Full Text | Google Scholar

Thaddeus, P., Guélin, M., and Linke, R. A. (1981). Three New 'nonterrestrial' Molecules. Astrophys. J. 246, L41–L45. doi:10.1086/183549

CrossRef Full Text | Google Scholar

Thorwirth, S., Schreier, P., Salomon, T., Schlemmer, S., and Asvany, O. (2019a). Pure Rotational Spectrum of CN+. Astrophys. J. Lett. 882, L6. doi:10.3847/2041-8213/ab3927

CrossRef Full Text | Google Scholar

Thorwirth, S., Schmid, P. C., Töpfer, M., Brünken, S., Asvany, O., and Schlemmer, S. (2019b). “Spectroscopic Studies of Protonated Amines: CH3NH3+ and C2H5NH3+,” in International Symposium on Molecular Spectroscopy, 74th meeting, Champaign-Urbana, IL, U.S.A., June 17-21, Talk WF04.

Google Scholar

Turner, B. E. (1974). U93.174 - A New Interstellar Line with Quadrupole Hyperfine Splitting. Astrophys. J. Lett. 193, L83. doi:10.1086/181638

CrossRef Full Text | Google Scholar

Wakelam, V., Loison, J. C., Herbst, E., Pavone, B., Bergeat, A., Beroff, K., et al. (2015). The 2014 KIDA Network for Interstellar Chemistry. Astrophys. J. Suppl. Ser. 217, 20. doi:10.1088/0067-0049/217/2/20

CrossRef Full Text | Google Scholar

Wang, S.-X., Schroderus, J., Ozier, I., Moazzen-Ahmadi, N., McKellar, A. R. W., Ilyushyn, V. V., et al. (2001). Infrared and Millimeter-Wave Study of the Four Lowest Torsional States of CH3CF3. J. Mol. Spectrosc. 205, 146–163. doi:10.1006/jmsp.2000.8235

PubMed Abstract | CrossRef Full Text | Google Scholar

Watts, J. D., Gauss, J., and Bartlett, R. J. (1992). Open-Shell Analytical Energy Gradients for Triple Excitation many-body, Coupled-Cluster Methods: MBPT(4), CCSD+T(CCSD), CCSD(T), and QCISD(T). Chem. Phys. Lett. 200, 1–7. doi:10.1016/0009-2614(92)87036-o

CrossRef Full Text | Google Scholar

Western, C. M. (2017). Pgopher: A Program for Simulating Rotational, Vibrational and Electronic Spectra. J. Quant. Spectrosc. Radiat. Transf. 186, 221–242. doi:10.1016/j.jqsrt.2016.04.010

CrossRef Full Text | Google Scholar

Zeroka, D., and Jensen, J. O. (1998). Infrared Spectra of Some Isotopomers of Methylamine and the Methylammonium Ion: A Theoretical Study. J. Mol. Struct. THEOCHEM 425, 181–192. doi:10.1016/s0166-1280(97)00109-7

CrossRef Full Text | Google Scholar

Ziurys, L. M., and Turner, B. E. (1986). HCNH(+) - A New Interstellar Molecular Ion. Astrophys. J. 302, L31–L36. doi:10.1086/184631

PubMed Abstract | CrossRef Full Text | Google Scholar

Zuckerman, B., Morris, M., Palmer, P., and Turner, B. E. (1972). Observations of Cs, HCN, U89.2, and U90.7 in NGC 2264. Astrophys. J. Lett. 173, L125. doi:10.1086/180931

CrossRef Full Text | Google Scholar

Keywords: rotational spectroscopy, protonated methylamine, ion trap, astrochemistry, symmetric top molecule

Citation: Schmid PC, Thorwirth S, Endres CP, Töpfer M, Sánchez-Monge Á, Schwörer A, Schilke P, Schlemmer S and Asvany O (2022) Rotational Rest Frequencies and First Astronomical Search of Protonated Methylamine. Front. Astron. Space Sci. 8:805162. doi: 10.3389/fspas.2021.805162

Received: 29 October 2021; Accepted: 22 December 2021;
Published: 02 February 2022.

Edited by:

André Canosa, UMR6251 Institut de Physique de Rennes (IPR), France

Reviewed by:

Roman Motiyenko, Université de Lille, France
Anthony Remijan, National Radio Astronomy Observatory, United States

Copyright © 2022 Schmid, Thorwirth, Endres, Töpfer, Sánchez-Monge, Schwörer, Schilke, Schlemmer and Asvany. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Oskar Asvany, asvany@ph1.uni-koeln.de

Present Address: Matthias Töpfer, Vinnolit GmbH & Co. KG, Hürth, Germany

Disclaimer: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.