The Astrophysical Journal, 777:120 (8pp), 2013 November 10 C 2013.
doi:10.1088/0004-637X/777/2/120
The American Astronomical Society. All rights reserved. Printed in the U.S.A.
THE CM-, MM-, AND SUB-MM-WAVE SPECTRUM OF ALLYL ISOCYANIDE AND RADIOASTRONOMICAL OBSERVATIONS IN ORION KL AND THE SgrB2 LINE SURVEYS 2 ´ I. Haykal1 , L. Margul`es1 , T. R. Huet1 , R. A. Motyienko1 , P. Ecija , E. J. Cocinero2 , F. Basterretxea2 , 2 2 3 4 ˜ , A. Lesarri , J. C. Guillemin , B. Tercero5 , and J. Cernicharo5 ´ , F. Castano J. A. Fernandez 1
Laboratoire de Physique des Lasers, Atomes, et Mol´ecules, UMR CNRS 8523, Universit´e de Lille 1, F-59655 Villeneuve d’Ascq C´edex, France;
[email protected] 2 Departamento de Qu´ımica F´ısica, Facultad de Ciencia y Tecnolog´ıa, Universidad del Pa´ıs Vasco, Barrio Sarriena s/n, E-48940 Leioa, Spain 3 Departamento de Qu´ımica F´ısica y Qu´ımica Inorg´ anica, Facultad de Ciencias, Universidad de Valladolid, E-47011 Valladolid, Spain 4 Institut des Sciences Chimiques de Rennes, Ecole Nationale Sup´ erieure de Chimie de Rennes, CNRS, UMR 6226, All`ee de Beaulieu, CS 50837, F-35708 Rennes Cedex 7, France 5 Centro de Astrobiolog´ıa (CSIC-INTA), Laboratory of Molecular Astrophysics, Department of Astrophysics, Ctra. De Ajalvir, km 4, E-28850 Torrej´on de Ardoz, Madrid, Spain Received 2013 March 28; accepted 2013 June 23; published 2013 October 22
ABSTRACT Organic isocyanides have an interesting astrochemistry and some of these molecules have been detected in the interstellar medium (ISM). However, rotational spectral data for this class of compounds are still scarce. We provide laboratory spectra of the four-carbon allyl isocyanide covering the full microwave region, thus allowing a potential astrophysical identification in the ISM. We assigned the rotational spectrum of the two cis (synperiplanar) and gauche (anticlinal) conformations of allyl isocyanide in the centimeter-wave region (4–18 GHz), resolved its 14 N nuclear quadrupole coupling (NQC) hyperfine structure, and extended the measurements into the millimeter and submillimeter-wave (150–900 GHz) ranges for the title compound. Rotational constants for all the monosubstituted 13 C and 15 N isotopologues are additionally provided. Laboratory observations are supplemented with initial radioastronomical observations. Following analysis of an extensive dataset (>11000 rotational transitions), accurate ground-state molecular parameters are reported for the cis and gauche conformations of the molecule, including rotational constants, NQC parameters, and centrifugal distortion terms up to octic contributions. Molecular parameters have also been obtained for the two first excited states of the cis conformation, with a dataset of more than 3300 lines. The isotopic data allowed determining substitution and effective structures for the title compound. We did not detect allyl isocyanide either in the IRAM 30 m line survey of Orion KL or in the PRIMOS survey toward SgrB2. Nevertheless, we provided an upper limit to its column density in Orion KL. Key words: ISM: molecules – line: identification – methods: laboratory: molecular – methods: observational – submillimeter: ISM Online-only material: color figures, machine-readable tables allyl isocyanide leads in very good yields to the corresponding allyl cyanide, in good agreement with theoretical calculations (Lattelais et al. 2010), providing evidence for the higher thermodynamic stability of cyanide by about 90 kJ mol−1 . On the other hand, the photolysis of some nitriles results in the corresponding isocyanides being the main products (Coupeaud et al. 2007). Among the compounds with a C4 H5 N formula, only attempts to detect the pyrrole have been reported (Myers et al. 1980; Kutner et al. 1980), but this most stable isomer was never found in the flash vacuum thermolysis products of many other isomers including allyl cyanide and allyl isocyanide. The presence of huge amounts of hydrogen isocyanide and some inorganic isocyanides (AlNC, MgNC, SiNC) in the ISM prompted us to imagine the formation in this medium of organic isocyanides that are not isomers of more stable detected isomers. In this hypothesis, and following a previous report on diisocyanomethane (Motiyenko et al. 2012), we present the microwave (MW) spectrum of this allyl isocyanide synthesized long ago. The laboratory observations have been followed by initial radioastronomical observations.
1. INTRODUCTION The discovery of new complex organic molecules in the interstellar medium (ISM) is an exciting challenge for astronomers and chemists to obtain a better understanding of the composition and chemistry of the ISM (Guillemin 2010). However, the low number of molecules still detected in the ISM leads one to wonder about candidates that may not satisfy all the chemistry rules imagined for the ISM on the basis of previously observed species. Isocyanides are isomers of the corresponding cyanides that differ in terms of their connection via the nitrogen atom. The preparation of the first isocyanide, the four-carbon allyl compound (H2 C=CH–CH2 NC), was reported by Lieke (1859). The first detection in the ISM of an organo isocyanide, methyl isocyanide (CH3 NC), was obtained 28 yr ago (Larson et al. 1985). Up until now, only the ethynyl derivative (HC≡C–NC) has been added to the list (Kawaguchi et al. 1992), although the microwave spectra of several isocyanides isomeric of detected nitriles have been recorded (Gripp et al. 2000; Chang et al. 1988; McNaughton et al. 1988; Møllendal et al. 2011). Isocyanides are thermodynamically less stable than the corresponding nitrile isomers, so the minimum energy principle (Lattelais et al. 2009) would imply that they are less populated in the ISM unless different routes of formation can be found. As an example, in laboratory experiments, the flash vacuum thermolysis of
2. EXPERIMENT Allyl isocyanide was synthesized on the gram scale, as recently reported (Chrostowska et al. 2012). The sample was first 1
The Astrophysical Journal, 777:120 (8pp), 2013 November 10
Haykal et al.
Figure 2. Principal inertial axes and atom labeling for the cis (left) and gauche (right) conformations of allyl isocyanide. (A color version of this figure is available in the online journal.)
and ab initio methods. Initial geometry optimizations used the recent Truhlar’s M06-2X functional (Zhao & Truhlar 2008), empirically accounting for dispersion interactions, and the Pople 6-311++G(d,p) and 6-311++G(3df,2pd) triple-ζ basis set. These calculations were extended to the second-order MP2 (frozen core) perturbation method and the same basis set. Finally, additional calculations used MP2 and Dunning’s (Peterson & Dunning 2002) augmented correlation-consistent (aug-cc) double- (pVDZ), triple- (pVTZ) and quadruple-ζ (pVQZ) basis sets in order to check the consistency of the theoretical predictions and the efficiency of the computational methods in terms of cost benefit efforts. All calculations used Gaussian09 (Frisch et al. 2009).
Figure 1. Potential curve along the torsion path of allyl isocyanide, showing the two most stable cis (synperiplanar, global minimum) and gauche (anticlinal) molecular conformations. (A color version of this figure is available in the online journal.)
analyzed in the 4–18 GHz centimeter-wave region using a supersonic jet Fourier-transform microwave spectrometer (Grabow & Caminati 2009) built at the University of the Basque Country. This spectrometer has been described elsewhere (Cocinero et al. 2010), so only some experimental details are given here. Briefly, the sample was vaporized in a heating pulsed-nozzle and near-adiabatically expanded with a carrier gas (Ne, 1–3 bar), achieving strong rovibrational cooling (Trot < 5 K, ground state) and sub-Doppler resolution. The molecules in the jet were polarized with short (μs), low-power (150 GHz) and submillimeter region up to 940 GHz in order to derive accurate centrifugal distortion parameters and astrophysically relevant data. As illustrated in Figure 4, the roomtemperature spectrum is very crowded. Strong lines associated with the ground state of the two conformers and with the first excited states of the most stable conformer are clearly observed. Extensive spectral measurements were then compiled for the ground state of both conformations. For conformer1, the experimental dataset comprised more than 7100 rotational transitions, spanning angular momentum quantum numbers between J = 6 and J = 99. In the case of conformer2, more than 2900 transitions (J = 5–100) were assigned. The final analysis of the rotational transitions proceeded in two steps. Since the 14 N hyperfine effects collapse for the high-J millimeter-wave transitions, this dataset was fit to a semi-rigid rotor Hamiltonian with no nuclear quadrupolar terms excluding the centimeter-wave transitions. Following several tests with both the asymmetric (A) and symmetric (S) reductions, we collect in Table 1 the results for the A-reduction (Ir representation). For both conformers, all quartic and sextic centrifugal distortion constants were determined. Some of the octic constants, accounting for minor centrifugal contributions, could also be additionally determined. In a second step, the centimeter-wave transitions were refit alone to determine final values of the NQC tensor, fixing all rotational and centrifugal distortion constants to the values of the millimeter-wave fit. In this process, the diagonal tensor elements were accurately determined but the off-diagonal terms were constrained to zero. The resulting hyperfine constants are also collected in Table 1. The complete list of measurements is given in Tables 2 and 3.
Figure 3. 1 MHz section of the microwave spectrum of allyl isocyanide, illustrating the 14 N nuclear quadrupole coupling hyperfine structure of the synperiplanar conformation (hyperfine components labeled with quantum numbers F = I + J ). (A color version of this figure is available in the online journal.)
(J ← J ) μb transitions, but no μc lines were observed. As illustrated in Figure 3, all transitions exhibited small (
