The 103–360 GHz rotational spectrum of benzonitrile, the first interstellar benzene derivative detected by radioastronomy
Graphical abstract
Introduction
Since the reported infrared detection of benzene in the interstellar medium (ISM) by Cernicharo et al. [1], there has been increased motivation to detect analogous aromatic molecules more reliably by radioastronomy [2], [3]. Two small aromatic molecules, cyclopropenylidene [4], [5] and cyclopropenone[6], have already been detected in the ISM. Due to its strong dipole moment (μa = 4.5 D) [7], being a derivative of benzene, and the fact that numerous nitriles have been detected in the ISM [8], benzonitrile (C6H5CN, depicted in Fig. 1) provided a very attractive target for direct detection in the ISM by means of its rotational transitions and radioastronomy.
Indeed, benzonitrile has been detected in the ISM [9], [10] by observation of nine hyperfine-resolved rotational transitions in Taurus Molecular Cloud 1 (TMC-1) [11] from 18 to 23 GHz using the Green Bank Telescope. These astronomical observations in the cm-wave region motivate a refinement of the benzonitrile spectroscopic constants and an extension of the mm-wave broadband spectrum. Despite the availability of instruments that can observe mm-wave spectra, such as the Atacama Large Millimeter Array (ALMA), nearly all of the mm-wave data previously available for benzonitrile have been in a relatively narrow band of 152–160 GHz [12] and no astronomical searches in the mm-wave region have been reported.
Benzonitrile is a prolate (κ = −0.85) asymmetric rotor with C2v symmetry and its substantial dipole moment along its a-inertial axis results in intense aR-type absorptions. The rotational spectrum of benzonitrile has been studied extensively in the cm-wave region (1–30 GHz) [7], [9], [13], [14], [15], [16], [17], [18]. Erlandsson and Lide independently published the first rotational spectral analyses in 1954 [13], [14]. Bak et al. later published infrared (IR) and cm-wave analyses of benzonitrile, producing a complete substitution structure based on data for the parent isotopologue and all possible mono-substituted isotopologues [15], [19]. This structure was refined by Casado et al. with the addition of several Q-branch transitions for most of the mono-substituted isotopologues [16]. Green and Harrison refined the analysis of the experimental IR spectrum, accounting also for the lowest wavenumber modes [20], while Csaszar and Fogarasi carried out a computed harmonic force field analysis [21]. Wlodarczak et al. extended the observed spectroscopic transitions into the mm-wave region (up to 160 GHz), providing a substantial improvement in the determination of the centrifugal distortion constants [12]. The rotational constants, hyperfine coupling constants, and dipole moment were well-determined by supersonic expansion Fourier-transform microwave (FTMW) spectroscopy [7]. In the course of the study by Dahmen et al., the rotational constants for the heavy isotopologues of benzonitrile were refined, resulting in an updated substitution structure [22]. Most recently, Rudolph et al. determined the semi-experimental equilibrium structure of benzonitrile using B3LYP-calculated vibration-rotation interaction (αi) constants and the previously published rotational constants for ten isotopologues [23].
Parallel to the laboratory spectroscopic work, several authors have suggested mechanisms of formation and postulated the presence of benzonitrile in extra-terrestrial bodies and the ISM [24], [25], [26], [27], [28]. Khare et al. demonstrated that an electric discharge of a mixture of methane, ammonia, and water generated benzonitrile at a range of temperatures (150–600 °C) [24]. While not directly analogous to the ISM, these experiments were designed to show that complex organic molecules could be generated from simple gases under conditions observed in the Jovian planets. Balucani et al. explored the formation of benzonitrile and a hydrogen atom using a crossed molecular beam collision of a cyano radical and benzene, concluding that the barrierless reaction was favorable by 95 kJ/mol [25]. Woods et al. suggested that, in the proto-planetary nebula CRL 618, the concentration of benzonitrile, generated primarily via the reaction between benzene and cyano radical, was similar to that of benzene [26]. Additionally, Woon explored this reaction computationally (MP2 and B3LYP) and concluded that benzonitrile formation dominated the chemistry of benzene and cyano radical across a wide range of temperatures and pressures relevant for Titan’s atmosphere [27]. Most recently, Parker et al. observed benzonitrile after mixing nitrosobenzene and vinyl cyanide in a pyrolytic reactor [28]. They proposed that phenyl radical is generated in situ along with cyanovinyl radical, which subsequently decomposes to the cyano radical and reacts with the phenyl radical to form benzonitrile. Thus, benzonitrile has been a clear target for radioastronomical detection due to its favorable dipole and ability to be generated from cyano radical [29] and benzene, [1], [30], [31] which have both been detected in the ISM.
In the current study, we performed new measurements of the rotational spectrum of benzonitrile in the 103–360 GHz frequency region. Extensive measurements were performed for the ground state, and for the two lowest energy vibrational states. The rotational transitions in these two states have been assigned for the first time and were found to be only partially amenable to a single-state model. As has been observed with other single-ring aromatic compounds and larger polycyclic aromatic hydrocarbons (PAHs) [32], [33], [34], these two lowest energy vibrational states are strongly coupled. In the case of quinoline [34], phenylacetylene [32], and the current work, transitions in these two states were fitted with a two-state model that ultimately accounted for all observable transitions to within the measurement accuracy. The coupling of the two lowest-energy vibrational modes appears to be the norm for these aromatic molecules and should be considered when investigating these species and preparing for their radioastronomical observations.
Section snippets
Experimental methods
Commercially available samples of benzonitrile were used without further purification for all measurements. The 103–136 GHz (low) and 185–207 GHz (mid) portions of the spectrum were recorded with the millimeter-wave spectrometer in Warsaw that uses source modulation and second derivative detection. The low-frequency segment was recorded using a synthesizer-driven 12 × VDI multiplier source [35] and the mid-frequency range was collected using a phase-locked direct generation BWO source [32].
Computational methods
The CCSD/ANO1 [41] geometry optimization and VPT2 anharmonic frequency calculations were performed using CFOUR 2.0 [42]. The geometry optimization utilized analytic gradients [43], [44] with the frozen core approximation. Numerical differentiation of the analytic second derivatives at displaced points was used to obtain cubic force constants [45], [46], which allowed calculation of vibration-rotation interaction constants (αi values). A geometry optimization using OPT = tight with an ultrafine
Ground state
The benzonitrile spectrum in the 103–136 GHz, 185–206.5 GHz, and 250–360 GHz ranges is fairly dense and features multiple vibrational satellites. While the assignment, measurement, and least-squares fitting of three states are discussed in this work, tentative assignments have been made also for three more fundamentals and several additional overtones and combination bands. A sample region of the experimental spectrum at the onset of the submillimeter region is shown in Fig. 2, with an
Conclusions
In this paper, we report the results of extensive measurements on a broadband rotational spectrum of benzonitrile. The spectrum was newly recorded with the use of several different mm-wave spectrometers and around 3000 rotational transitions have been measured in the ground state and in each of the two lowest excited vibrational states. A global single-state fit of all available data is reported for the ground state, while transitions in the newly assigned v22 = 1 and v33 = 1 states revealed
Acknowledgments
We gratefully acknowledge funding from the National Science Foundation for support of this project (NSF-1664912) and for support of shared Departmental computing resources (NSF-CHE-0840494).
Declarations of interest
None.
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