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Photoionization Spectroscopy O Photoionization spectroscopy of CH3C3N in the vacuum-ultraviolet range N. Lamarre, C. Falvo, C. Alcaraz, B. Cunha de Miranda, S. Douin, A. Flütsch, C. Romanzin, J.-C. Guillemin, Séverine Boyé-Péronne, B. Gans To cite this version: N. Lamarre, C. Falvo, C. Alcaraz, B. Cunha de Miranda, S. Douin, et al.. Photoionization spectroscopy of CH3C3N in the vacuum-ultraviolet range. Journal of Molecular Spectroscopy, Elsevier, 2015, 315, pp.206-216. 10.1016/j.jms.2015.03.005. hal-01138635 HAL Id: hal-01138635 https://hal-univ-rennes1.archives-ouvertes.fr/hal-01138635 Submitted on 4 Nov 2015 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Photoionization spectroscopy of CH 3C3N in the vacuum-ultraviolet range N. Lamarre a, C. Falvo a, C. Alcaraz b,c, B. Cunha de Miranda b, S. Douin a, A. Fl utsch¨ a, C. Romanzin b, J.-C. Guillemin d, a, a, S. Boy e-P´ eronne´ ∗, B. Gans ∗ aInstitut des Sciences Mol´eculaires d’Orsay, Univ Paris-Sud; CNRS, bat 210, Univ Paris-Sud 91405 Orsay cedex (France) bLaboratoire de Chimie Physique, Univ Paris-Sud; CNRS UMR 8000, bat 350, Univ Paris-Sud 91405 Orsay cedex (France) cSynchrotron SOLEIL, L'Orme des Merisiers, St. Aubin, B.P. 48, 91192, Gif-sur-Yvette cedex, (France) dInstitut des Sciences Chimiques de Rennes, Ecole´ Nationale Sup´erieure de Chimie de Rennes, CNRS UMR 6226, 35708 Rennes cedex 7, (France) Abstract Using vacuum-ultraviolet (VUV) synchrotron radiation, threshold and dissociative photoionization of cyanopropyne (CH 3C3N) 1 1 in the gas phase have been studied from 86000 cm − up to 180000 cm − by recording Threshold-PhotoElectron Spectrum (TPES) + 2 + 2 + 2 + + and PhotoIon Yield (PIY). Ionization energies of the four lowest electronic states X˜ E, A˜ A1, B˜ E and C˜ of CH 3C3N are derived from the TPES with a better accuracy than previously reported. The adiabatic ionization potential of CH 3C3N is measured 1 as 86872 20 cm − . A description of the vibrational structure of these states is proposed leading to the first determination of the vibrational± frequencies for most modes. The vibrational assignments of the X˜ + state are supported by density functional theory calculations. In addition, dissociative photoionization spectra have been recorded for several cationic fragments in the range 12- 1 + 15.5 eV (96790-125000 cm − ) and they bring new information on the photophysics of CH 3C3N . Threshold energies for the + + + + + cationic dissociative channels leading to CH 2C3N , CHC 3N , HC 3H , HCNH and CH 3 have been measured for the first time and are compared with quantum chemical calculations. Keywords: cyanopropyne, methylcyanoacetylene, 2-butynenitrile, CH 3C3N, synchrotron radiation, TPES, dissociative photoionization, DFT calculations 1. Introduction radical on propyne (a gas observed in the ISM as well as in the atmosphere of Titan) has been proposed. Corresponding studies Cyanopropyne CH -C C-C N (or 2-butynenitrile) is the 3 ≡ ≡ have been performed via a neutral-neutral reaction under single methylated derivative of cyanoacetylene HC 3N, an important collision conditions in a crossed molecular beam experiment to astrophysical species. Although the chemical and physical yield the expected product and the cyanoallene isomer [10, 11, properties of HC 3N have been extensively studied (see for in- 12]. However other approaches starting from cyanoacetylene stance Ref. [1] and references therein), cyanopropyne has been might also be proposed for its synthesis in these media. much less investigated. Less ubiquitous than HC 3N, which has been observed in several molecular clouds of the Interstel- Cyanopropyne is a prolate symmetric top which has a neutral ground state configuration lar Medium (ISM), in Titan and in comets, cyanopropyne was 2 2 2 2 2 4 4 2 4 detected in 1984 in the molecular cloud TMC-1 and more re- [core](1a1) (2a1) (3a1) (4a1) (5a1) (1e) (2e) (6a1) (3e) , ˜ 1 cently in Sgr B2 [2, 3]. Twelve years after its first detection, an leading to a X A1 state in the C 3v point group. This molecule has 12 vibrational modes: 6 symmetric modes of a 1 symmetry isomer, the cyanoallene (2,3-butadienenitrile, H 2C=C=CHCN) was found in TMC-1 [4] but in lower amounts: a 0.25 ratio (ν1 to ν6) and 6 degenerate modes of e symmetry ( ν7 to ν12 ). compared to cyanopropyne, as it has been often observed for The vibrational structure of the electronic ground state has been thermodynamically less stable compounds [5]. studied by infrared spectroscopy [13, 14, 15]. The rotational Even if it has never been unambiguously detected up to now structure of the vibronic ground state [16, 17, 18] and of the v12 in the atmosphere of Titan, the presence of cyanopropyne was fundamental vibrational level [19] have also been determined found in several atmosphere simulations of this satellite using by microwave spectroscopy. a plasma discharge in N 2-CH4 mixtures at low pressures [6, 7, Concerning the electronic spectroscopy of cyanopropyne, 8]. Note that Vuitton et al. reported the detection of a C 4H3N Bruston et al. recorded a UV absorption spectrum at room- species in the atmosphere of Titan by mass spectrometry [9] but temperature between 185 and 250 nm and made the first vi- did not specify the corresponding molecular structure. bronic analysis of the transition to the first valence state [20]. To explain its formation in the ISM as well as how it could On the other hand, the highly-excited electronic stucture is be formed in the atmosphere of Titan, the addition of the CN still to be unveiled since no vacuum-ultraviolet (VUV) absorp- tion nor multiphoton excitation spectra of CH 3-C C-C N have been reported to date. ≡ ≡ ∗corresponding authors: [email protected]; severine.boye- [email protected] As for the cyanopropyne cation, very little information is cur- Preprint submitted to Journal of Molecular Spectroscopy March 12, 2015 rently available. This information has been obtained by He(I) molecules were introduced in a very low-pressure chamber 6 and He(II) photoelectron spectroscopy [21, 22, 23] in the gas ( 2 10 − mbar) at room temperature where they were ex- phase or by neon matrix absorption spectroscopy [24]. From cited≈ by× the VUV synchrotron radiation. Ions produced by the these studies, ionization energies of the six lowest electronic direct or dissociative ionization processes were mass-selected states have been extracted and only a few vibrational frequen- and guided towards the micro channel plate detector using a cies of the three lowest electronic states of the cation have been first quadrupolar mass filter, two octopolar guides and a sec- measured with consequent error bars. ond quadrupolar mass filter. The photoelectrons with near Finally, only three theoretical studies have been reported to zero-kinetic energy (so-called threshold photoelectrons) were our knowledge: in 1982, Niessen et al. calculated ionization collected in the opposite direction and selected by geometri- energies of CH 3C3N by a Green's function method [25], in the cal and temporal discrimination criteria. The photon resolu- 90's Siam et al. optimized the geometry of the neutral species tion of the TPES was approximately 4 meV below 15.5 eV using Hartree-Fock calculations [26] and Moliner et al. ob- and 10 meV above 15.5 eV. The corresponding overall reso- tained the rotational constant and the dipole moment of the lution of the spectra, including the resolution of the photons vibronic ground state with second-order-Møller-Plesset (MP2) and of the photoelectrons, was measured at 10 meV (around 1 and density-functional-theory (DFT) calculations [27]. 80 cm − ) below 15.5 eV and about 15 meV above 15.5 eV in The present work is focused on the CH 3-C C-C N cationic the present experimental conditions. For the dissociative ion- species which might be involved in the ion-neutral≡ ≡ chemistry ization study, the fragment ion yields were recorded at lower of astrophysical media, as for instance the ISM and the up- resolution (15 meV). The photon energy scale of all spectra was 1 per atmosphere of Titan. The first part of this paper is de- calibrated with an accuracy of 1 meV (8 cm − ) using the argon voted to an accurate description of the vibronic structure of and neon absorption lines observed in the ion signals due to the cation reached by direct ionization from the neutral ground their presence in the beamline gas filter. Note that these rare state. Vibrational assignment of the X˜ + spectrum is guided by gas absorptions were not fully corrected by the normalisation exploratory density functional theory (DFT) calculations. In procedure since the photon flux was recorded with a di fferent the second part of this paper, dissociative ionization is stud- spectral resolution. ied through observation of appearance energies for the di fferent + cationic fragmentation channels of CH 3C3N . They are com- pared with estimated thresholds obtained by DFT calculations. 3. Theoretical calculations All electronic structure calculations were carried out using 2. Experiment the Gaussian09 suite of programs [32]. Equilibrium geometries and vibrational harmonic frequencies were computed for the 2-Butynenitrile (cyanopropyne) has been prepared in a two- neutral and cation electronic ground states using DFT with the step sequence starting from ethyl butynoate (Alfa-aesar) used as hybrid functional B97-1 [33], in combination with the basis set a precursor of 2-butynamide, as previously reported [28].
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