Characterizing centrosymmetric two-ring PAHs using jet-cooled high resolution mid-infrared laser spectroscopy and anharmonic quantum chemical calculations
Résumé
The presence of Polycyclic Aromatic Hydrocarbons (PAH) molecules in the interstellar medium, recently confirmed by the detection of cyano-naphthalenes, renews the interest of extensive spectroscopic and physical-chemistry studies about such large species. The present study reports the jet-cooled rovibrational IR study of three centrosymmetric two-ring PAH molecules, naphthalene (C 10 H 8), [1,5] naphthyridine (C 8 H 6 N 2) and biphenyl (C 12 H 10) in the in-plane ring C-H bending (975-1035 cm-1) and CC ring stretching (1580-1620 cm-1) regions. For the two most rigid PAHs, the accuracy of spectroscopic parameters derived in ground and several excited states (6 for naphthalene and 6 for [1,5] naphthyridine) has significantly improved the literature values. In addition, comparison between experiments and quantum chemical calculations confirms the predictive power of the corrected calculated rotational parameters. The more flexible structure of biphenyl makes particularly challenging the analysis of high resolution jet-cooled spectra of 19 and 23 modes recorded at about 1601 and 1013 cm-1 respectively. The presence of three torsional vibrations below 120 cm-1 together with small values of the rotational constants prevented us to determine the ground and v 19 =1 excited rotational constants independently. In the 23 band region, the presence of two bands rotationally resolved and separated by only 0.8 cm-1 , raises the question of possible splittings due to a large amplitude motion, most probably the torsion of the aliphatic bond between the two phenyl rings.
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