Visible Photodissociation Spectra of the 1- and 2-Methylnaphthalene Cations: Laser Spectroscopy and Theoretical Simulations

Abstract

The electronic absorption spectra of the two methyl derivatives of the naphthalene cation were measured using an argon tagging technique. In both cases, a band system was observed in the visible range and assigned to the D2 ← D0 electronic transition. The 1-methylnaphthalene(+) absorption bands revealed a red shift of 808 cm(-1), relative to those of the naphthalene cation (14,906 cm(-1)), whereas for 2-methylnaphthalene(+) a blue shift of 226 cm(-1) appeared. A short vibrational progression, similar to the naphthalene cation, was also observed for both isomers and found to involve similar aromatic ring skeleton vibrations. Moreover, insights into the internal rotation motion of the methyl group were inferred, although the spectral resolution was not sufficient to fully resolve the substructure. These measurements were supported by detailed quantum chemical calculations. They allowed exploration of the potential energy curves along this internal coordinate, along with a complete simulation of the harmonic Franck-Condon factors using the cumulant Gaussian fluctuations formalism extended to include the internal rotation.

Anharmonic vibrational spectroscopy of polycyclic aromatic hydrocarbons (PAHs)

“…Therefore, it is suitable for semirigid molecules at not too high vibrational energies. So, it should be appropriate for PAHs, unless they have side groups that give rise to internal rotations, that are poorly represented by a truncated Taylor expansion in normal coordinates 66. We notice that it would be desirable in some cases, to opt for non-cartesian coordinates so that the associated quartic force field would have a better asymptotic behaviour, and the potential would be reasonably described even along large amplitude motions 67,68.…”

“…So, it should be appropriate for PAHs, unless they have side groups that give rise to internal rotations, that are poorly represented by a truncated Taylor expansion in normal coordinates. 66 We notice that it would be desirable in some cases, to opt for non-cartesian coordinates so that the associated quartic force field would have a better asymptotic behaviour, and the potential would be reasonably described even along large amplitude motions. 67,68 Such coordinates, however, are not directly applicable in our case: the Van Vleck approach used here for GVPT2 requires a description of the Hamiltonian in terms of normal coordinates, since it exploits their commutation properties to obtain a formally simple representation of the infinitesimal contact transformations and of the transformed Hamiltonian and dipole moment operators.…”

Photoinduced Electron Transfer in 9‐Substituted 10‐Methylacridinium Ions

“…A similar NIR absorption band was also observed due to ISC from the singlet ET state to the triplet ET state of Acr + ‐1NA (Figure S6 in the Supporting Information). The increase in the NIR band in Figure (and Figure S6 in the Supporting Information) may result from the D 1 ←D 0 dipole‐forbidden transition, which becomes slightly allowed in the triplet ET state, whereas it is forbidden in the singlet ET state. Due to operation of the Pauli principle, the integrated repulsion between electrons of like spin in the triplet is lower than that between electrons of opposite spin in the singlet.…”

“…The increase in the NIR band in Figure 2( and Figure S6 in the Supporting Information) may result from the D 1 ! D 0 dipole-forbidden transition, [50][51][52] which becomes slightly allowed in the triplet ET state, whereas it is forbiddeni nt he singlet ET state. Due to operation of the Pauli principle, the integratedr epulsion between electrons of like spin in the triplet is lower than that between electrons of opposite spin in the singlet.T hus, the unpaired electron in the donor radicalc ation of the triplet ET state may be slightly more delocalized in the AcrC moiety, making the forbidden transition in the NIR region observable.…”

Hydrogen-adduction to open-shell graphene fragments: spectroscopy, thermochemistry and astrochemistry

“…However, these shifts have been shown to be small for similar molecules 49,53,54 and spectra of Ar-tagged molecular cations have proved useful in determining if the observed excitation spectra are relevant to astronomical spectroscopy.…”