VUV Absorption Spectra of Gas-Phase Quinoline in the 3.5–10.7 eV Photon Energy Range

Sydney Leach (1) , Nykola C. Jones (2) , Søren V. Hoffmann (2) , Sun Un (3)

1 LERMA (UMR_8112) - Laboratoire d'Etude du Rayonnement et de la Matière en Astrophysique
2 Department of Physics and Astronomy [Aarhus]
3 I2BC - Institut de Biologie Intégrative de la Cellule

The absorption spectrum of quinoline was measured in the gas phase between 3.5 and 10.7 eV using a synchrotron photon source. A large number of sharp and broad spectral features were observed, some of which have plasmon-type collective π-electron modes contributing to their intensities. Eight valence electronic transitions were assigned, considerably extending the number of π–π* transitions previously observed mainly in solution. The principal factor in solution red-shifts is found to be the Lorentz–Lorenz polarizability parameter. Rydberg bands, observed for the first time, are analyzed into eight different series, converging to the D0 ground and two excited electronic states, namely, D3 and D4, of the quinoline cation. The R1 series limit is 8.628 eV for the first ionization energy of quinoline, a value more precise than previously published. This value, combined with cation electronic transition data, provides precise energies, respectively, 10.623 and 11.355 eV, for the D3 and D4 states. The valence transition assignments are based on density functional theory (DFT) calculations as well as on earlier Pariser–Parr–Pople (P–P–P) self-consistent field linear combination of atomic orbitals molecular orbital results. The relative quality of the P–P–P and DFT data is discussed. Both are far from spectroscopic accuracy concerning electronic excited states but were nevertheless useful for our assignments. Our time-dependent DFT calculations of quinoline are excellent for its ground-state properties such as geometry, rotational constants, dipole moment, and vibrational frequencies, which agree well with experimental observations. Vibrational components of the valence and Rydberg transitions mainly involve C–H bend and C═C and C═N stretch modes. Astrophysical applications of the vacuum UV absorption of quinoline are briefly discussed.

Domaines: Chimie théorique et/ou physique

Sydney Leach, Nykola C. Jones, Søren V. Hoffmann, Sun Un. VUV Absorption Spectra of Gas-Phase Quinoline in the 3.5–10.7 eV Photon Energy Range. Journal of Physical Chemistry A, 2018, William M. Jackson Festschrift, 122 (27), pp.5832-5847. ⟨10.1021/acs.jpca.8b04629⟩. ⟨hal-01957160⟩