Photodissociation Electronic Spectra of Cold Protonated Quinoline and Isoquinoline in the Gas Phase

Article abstract of DOI:10.1021/acs.jpca.7b01301

Photofragmentation electronic spectra of isolated single-isomeric N-protonated quinoline (quinolinium) and isoquinoline (isoquinolinium) ions have been measured at a temperature of ～40 K using a mass-selective, 10 cm^-1 spectral resolution, photodissociation spectrometer. Additionally, ab initio adiabatic transition energies calculated using the RI-ADC(2) method have been employed to assist in the assignment of the spectra. Three electronic transitions having ππ? character were clearly evidenced for both protonated ions within the UV and deep-UV spectral ranges. The corresponding spectra at room temperature were previously reported by Hansen et al., together with TD-DFT calculations and a careful analysis of the possible fragmentation mechanisms. This information will be complemented in the present study by appending better resolved spectra, characteristic of cold ions, in which well-defined vibrational progressions associated with the S₁ ← S₀ and S₃ ← S₀ transitions exhibit clear 0-0 bands at hν_0-0 = 27868 and 42230 cm^-1, for protonated quinoline, and at hν_0-0 = 28043 and 41988 cm^-1, for protonated isoquinoline. Active vibrations in the spectra were assigned with the help of calculated normal modes, looking very similar to those of the structurally related protonated naphthalene. Finally, we have observed that the bandwidths associated with the deep-UV S₃ ← S₀ transition denote a lifetime for the S₃ excited state in the subpicosecond time scale, in contrast with that of S₁.

Full text of DOI:10.1021/acs.jpca.7b01301

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Article
Photodissociation Electronic Spectra of Cold
Protonated Quinoline and Isoquinoline in the Gas-Phase
Géraldine Féraud, Luis Domenianni, Ernesto Marceca, Claude Dedonder-Lardeux, and Christophe Jouvet
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b01301 • Publication Date (Web): 16 Mar 2017
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The Journal of Physical Chemistry
Photodissociation Electronic Spectra of Cold Protonated Quinoline and Isoquinoline in
the Gas-Phase
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Géraldine Férauda,c; Luis Domeniannib1,b2; Ernesto Marcecab1,b2*; Claude Dedonderꢀ
Lardeuxa; Christophe Jouveta*
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a) CNRS, AixꢀMarseille Université, PIIM UMR 7365, Avenue Escadrille NormandieꢀNiémen, 13397 Marseille
Cedex 20, France
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b1) Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, DQIAF, Ciudad Universitaria, 1er
piso, Pab. II, C1428EGA, Buenos Aires, Argentina
b2) CONICETꢀUBA, Inst. Química Física de los Materiales, Medio Ambiente y Energía (INQUIMAE), Ciudad
Universitaria, 3er piso, Pab. II, C1428EGA, Buenos Aires, Argentina
c)
Present address: UPMC, Pierre and Marie Curie University, Paris 6, 4 Place Jussieu, 75005 Paris, France
* corresponding authors: marceca@qi.fcen.uba.ar, christophe.jouvet@univꢀamu.fr
Abstract:
Photofragmentation electronic spectra of isolated singleꢀisomeric Nꢀprotonated quinoline
(quinolinium) and isoquinoline (isoquinolinium) ions have been measured at a temperature of
∼40 K using a massꢀselective, 10 cm−1 spectral resolution, photodissociation spectrometer.
Additionally, abꢀinitio adiabatic transition energies calculated using the RIꢀADC(2) method
have been employed to assist in the assignment of the spectra. Three electronic transitions
having ππ* character were clearly evidenced for both protonated ions within the UV and
deepꢀUV spectral ranges. The corresponding spectra at room temperature were previously
reported by Hansen et al., together with TDꢀDFT calculations and a careful analysis of the
possible fragmentation mechanisms. This information will be complemented in the present
study by appending better resolved spectra, characteristic of cold ions, in which wellꢀdefined
vibrational progressions associated to the S1←S0 and S3←S0 transitions exhibit clear 0−0
bands at hν0−0 = 27868 and 42230 cm−1, for protonated quinoline, and at hν0−0 = 28043 and
41988 cm−1, for protonated isoquinoline. Active vibrations in the spectra were assigned with
the help of calculated normal modes, looking very similar to those of the structurally related
protonated naphthalene. Finally, we have observed that the bandwidths associated with the
deepꢀUV S3←S0 transition denote a lifetime for the S3 excited state in the subpicosecond time
scale, in contrast with that of S1.
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The Journal of Physical Chemistry
Page 2 of 25
1. Introduction:
Polycyclic aromatic nitrogen heterocycles (PANHs) involve a group of compounds
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widely distributed in the Earth´s biosphere1 as a consequence of the emission caused by
natural fires, incomplete combustion of fuels and discharge of industrial effluents, among
other sources.2,3 The toxicity of several PANHs,4 normally exceeding that of the structurally
related polycyclic aromatic hydrocarbons (PAHs), has motivated the development of
analytical methods that, in many cases, must rely on the scarce photophysical information
existing for these molecules.5
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Additionally, many authors have hypothesized6ꢀ8 that PANHs might also exist beyond
the Earth throughout the interstellar medium (ISM), as a result of the reaction between
interstellar nitrogen and PAHs. The presence of PAHs in the ISM has long been postulated to
account for absorption features in the diffuse interstellar bands (DIBs)9ꢀ11 and emission
features in the aromatic infrared bands (AIBs).12 More recently, it was pointed out using
experimental and theoretical arguments that the AIBs’ assignment is improved by considering
nitrogenꢀcontaining PAHs.8,13ꢀ16 Nevertheless, very few gas phase electronic spectra are
available to conclusively establish whether the visible absorption of PANH+s is part of the
observed astronomical DIBs.17 In addition to the radical cations, protonated PANHs (hereafter
PANHH+s) will likely be found in the ionized environments of the ISM due to the high proton
affinity of the aromatic nitrogen expected for these molecules. Evidence supporting this
assumption was given by recent IR measurements15 performed on four smallꢀsize PANHH+s
that were found to reproduce closely the 6.2 ꢀm and 8.6 ꢀm AIBs. However, well resolved
electronic spectra of isolated PANHH+s are still needed18 to explore if the visible absorption
of these ions is a component of the DIBs.
In this work we have measured the photodissociation electronic spectra of cold
protonated quinoline and isoquinoline, the two simplest PANHH+s comprising a pyridinium
cation fused to a sixꢀmembered carbocyclic ring. Both quinoline and isoquinoline are
structurally related to naphthalene, being one CH group of the latter molecule substituted by
one aromatic nitrogen atom at positions 1 and 2, respectively (Scheme 1). Protonation occurs
on the nitrogen sp2 lone electron pair leading to the corresponding quinolinium (QH+) or
isoquinolinium (iQH+) cations, as shown in Scheme 1. Cꢀprotonation is highly disfavored due
to the electron deficiency at the aromatic carbons caused by the inductive polarization of the
electronegative heteroatom.
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The Journal of Physical Chemistry
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Scheme 1. Left, protonated quinoline (quinolinium ion, QH+); right,
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protonated isoquinoline (isoquinolinium ion, iQH+).
Even though the electronic spectroscopic properties of neutral quinolines19ꢀ22 and their
radical cations17 have been extensively studied in the gas phase, less information is available
on the corresponding isolated protonated molecules.23,24 In particular, only one spectrum at
room temperature has been obtained for QH+ and iQH+ using photodissociation action
spectroscopy by Hansen et al.,24 and so far well resolved vibronic spectra of cold ions are still
lacking. A careful analysis of the fragmentation mechanism has been done in the paper by
Hansen et al., and we will use their calculations for further discussion. We will present new
UVꢀvis spectroscopic data for QH+ and iQH+ in which we were able to improve the accuracy
of the electronic band origins, exhibit well resolved vibrational progressions associated to the
low lying excitations, and provide estimations about the lifetime of the excited states based on
the observed line broadenings. All the measurements were performed using a cryogenic ion
trap photodissociation spectrometer in which the ions can be cooled to temperatures of a few
tens of Kelvin. Pumpꢀprobe UV−UV ionꢀdip experiments25 were employed to explore the
contribution of possible tautomers in the spectral bands.
The photochemical processes observed for protonated quinoline and isoquinoline will
be compared with former measurements on protonated naphthalene.26,27 To support the
spectroscopic observations, abꢀinitio calculations were performed on QH+ and iQH+ ground
and excited states.
2. Methods:
Electronic spectra were recorded using the same experimental setꢀup as in previous
work.28,29 Only a brief description of the apparatus will be given here, including the ions’
preparation method and the applied spectroscopic techniques. Parent QH+ and iQH+ ions are
formed in an electrospray source fed by a continuum flow of an acidulated water/methanol
mixture (1:1 in volume) in which the corresponding quinoline is dissolved at a concentration
3
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