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Cryogenic Photochemical Synthesis and Electronic Spectroscopy of Cyanotetracetylene Urszula Szczepaniak, Robert Kolos, Marcin Gronowski, Michele Chevalier, JeanClaude Guillemin, Michal Turowski, Thomas Gage Custer, and Claudine Crepin J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b07849 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 6, 2017
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The Journal of Physical Chemistry
Cryogenic Photochemical Synthesis and Electronic Spectroscopy of Cyanotetracetylene Urszula Szczepaniaka, b *, Robert Kołosb, Marcin Gronowskib, Michèle Chevaliera, Jean-Claude Guilleminc, Michał Turowskib, †, Thomas Custerb, and Claudine Crépina a
Institut des Sciences Moléculaires d’Orsay (ISMO), CNRS, Univ. Paris-Sud, Université Paris-
Saclay, F-91405 Orsay, France, b
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224
Warsaw, Poland, c
Institut des Sciences Chimiques de Rennes, École Nationale Supérieure de Chimie de Rennes,
CNRS, UMR 6226, 11 Allée de Beaulieu, CS 50837, 35708 Rennes Cedex 7, France *
tel. +48 22 343 3353 ; e-mail :
[email protected],
[email protected] Present addresses † University of Colorado at Boulder, Department of Chemistry and Biochemistry, 215 UCB, Boulder, CO 80309-0215
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ABSTRACT HC9N is a molecule of astrochemical interest. In this study, it was produced in cryogenic Ar and Kr
matrices
from
UV-photolyzed
diacetylene/cyanodiacetylene
mixtures.
Its
strong
phosphorescence was discovered and served for the identification of the compound. Vibrationally-resolved phosphorescence excitation spectra gave insight into excited singlet electronic states. Two electronic systems were observed: around 26 000 – 34 000 cm-1 and 35 000 – 50 000 cm-1. Energies of the second excited singlet and the lowest triplet state were derived from analysis of these systems. Vibrational and electronic spectroscopic features were assigned with the assistance of density functional theory calculations. Some trends concerning the electronic spectroscopy of HC2n+1N family molecules are presented.
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The Journal of Physical Chemistry
INTRODUCTION Monocyanopolyynes (HC2n+1N) constitute the most prominent homologous series of astrochemically-relevant molecules. They have been detected via microwave rotational transitions in many extraterrestrial sources up to n = 4. Cyanoacetylene, HC3N, the smallest neutral molecule possessing both –C≡C– and –C≡N structural motifs, was first observed in a star-forming region Sagittarius (Sgr B2(N))1 and then in other Galactic2–4 and extragalactic5 radio sources. It has also been detected in the atmosphere of Titan6,7 and in Hale-Bopp comet8. Along with cyanoacetylene, longer members of the HC2n+1N series (n = 2, 3) have been found in several molecular clouds.9 HC9N (cyanotetraacetylene, 1-cyano-octa-1,3,5,7-tetrayne or nona2,4,6,8-tetraynenitrile) was first observed in 1978 at millimeter wavelengths in the dense interstellar cloud HCl-2.10 It has also been found in expanding circumstellar envelopes (IRC +10216
11,12
and CRL 2688 12). As observations of the n = 5 species, HC11N, in IRC +10216
and in the Taurus Molecular Cloud14 have been challenged, first in IRC +10216
14
13
and now in
TMC-1,15 HC9N is currently considered the longest member of the family to have been observed in space. The astrochemical synthesis of monocyanopolyynes probably involves the reactions of CN• or C2n+1N• radicals with polyynic hydrocarbons HC2mH. Cherchneff et al. proposed the following pathway for formation of monocyanopolyynes around IRC +10216: HC2H + C2n+1N• → HC2n+3N + H•16,17. Loomis et al.15 suggested that cyclization reactions are responsible for non-detection of n >4 cyanopolyynes. The astronomical detections of HC9N were based on microwave spectroscopy combined with quantum chemical studies of molecular structure.18–21 Density functional theory (B3LYP) investigations22 have suggested that HC2n+1N compounds should be linear and polyynic (having alternating single and triple bonds) at least up to n = 8. Vichietti and Haiduke predicted IR spectra of long cyano-23 (HC2nCN) and isocyanopolyynes24 (HC2nNC) and suggest that the central triple bond stretching mode is the one producing the strongest band. 3 ACS Paragon Plus Environment
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Gas-phase spectroscopic studies of HC9N are limited to microwave measurements in which minute amounts of the substance are transiently formed in a discharge plasma using acetylene with acrylonitrile25 or dicyanoacetylene with cyanoacetylene26 as seed gases. These investigations allowed derivation of the molecular geometry. Independently, electronic absorption and NMR spectra of HC9N in organic solvents have been reported 27–32. In particular, Wakabayashi et al.31 obtained HC2n+1N (n = 3-6) by laser ablation of isotope-enriched (99 % 13
C) graphite in liquid acetonitrile followed by chromatographic purification.
Very small
amounts of the compound were then characterized by UV absorption and NMR spectroscopy. Isotopic experiments proved that the carbon atoms of HC9N came both from graphite and solvent. The molecule has also appeared among the products of graphite bombardment with high energy H2 and N2 beams.33 In order to extend the spectroscopic characterization of cyanotetraacetylene, we aimed at synthesizing the molecule in noble gas matrices. Cryogenic photochemical syntheses of cyanopolyacetylenes have already been demonstrated for cyanodiacetylene (HC5N)34,35 and cyanotriacetylene (HC7N)36. Cyanodiacetylene appeared in solid Ar doped with HC3N/C2H2 mixtures34 irradiated using a H2 discharge lamp emitting in the far-UV. It was also seen in Ar, Kr, N2, and pH2 solids containing HC3N alone and photolyzed at 193 nm using a laser.35 Cyanotriacetylene was detected in solid Ar doped with a mixture of either HC3N and C4H2 (diacetylene) or HC5N and C2H2,36 and photolyzed either by excimer laser radiation or an H2 discharge lamp. The HC7N molecule has also been produced by electric discharges in a mixture of HC3N and Kr just prior to trapping on a cold window.37 In all of these cryogenic approaches, the cyanopolyacetylenes generated were found to emit strong phosphorescence which has proved to be the most sensitive spectroscopic indicator of their presence. Here we verified that a mixture of C4H2 and HC5N precursors could couple to produce HC9N during UV-assisted, lowtemperature synthesis. The spectroscopic identification has been supported using density functional theory calculations of electronic and vibrational energy levels. 4 ACS Paragon Plus Environment
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The Journal of Physical Chemistry
EXPERIMENT The precursor molecules were prepared using the methods developed by Trolez & Guillemin38 (HC5N) and Khlifi et al.39 (C4H2). HC5N was alternately exposed to vacuum at T
