Vibrations of the low energy states of toluene ($\tilde X$X̃ 1A1 and $\tilde A$Ã 1B2) and the toluene cation ($\tilde X$X̃ 2B1)

Abstract

We commence by presenting an overview of the assignment of the vibrational frequencies of the toluene molecule in its ground (S 0 ) state. The assignment given is in terms of a recently proposed nomenclature, which allows the ring-localized vibrations to be compared straightforwardly across different monosubstituted benzenes. The frequencies and assignments are based not only on a range of previous work, but also on calculated wavenumbers for both the fully hydrogenated (tolueneh 8 ) and the deuterated-methyl group isotopologue (α 3 -toluene-d 3 ), obtained from density functional theory (DFT), including artificial-isotope shifts. For the S 1 state, one-colour resonance-enhanced multiphoton ionization (REMPI) spectroscopy was employed, with the vibrational assignments also being based on previous work and time-dependent density functional theory (TDDFT) calculated values; but also making use of the activity observed in two-colour zero kinetic energy (ZEKE) spectroscopy. The ZEKE experiments were carried out employing a (1 + 1 ) ionization scheme, using various vibrational levels of the S 1 state with an energy <630 cm -1 as intermediates; as such we only discuss in detail the assignment of the REMPI spectra at wavenumbers <700 cm -1 , referring to the assignment of the ZEKE spectra concurrently. Comparison of the ZEKE spectra for the two toluene isotopologues, as well as with previously reported dispersed-fluorescence spectra, and with the results of DFT calculations, provide insight both into the assignment of the vibrations in the S 1 and D 0 + states, as well as the couplings between these vibrations. In particular, insight into the nature of a complicated Fermi resonance feature at ∼460 cm -1 in the S 1 state is obtained, and Fermi resonances in the cation are identified. Finally, we compare activity observed in both REMPI and ZEKE spectroscopy for both toluene isotopologues with that for fluorobenzene and chlorobenzene.

Resonance-enhanced multiphoton ionization (REMPI) spectroscopy of p–chlorofluorobenzene

“…However, the observed isotopic shifting of the bands suggests the higher wavenumber, weaker contribution at~347 cm -1 is the 11 1 transition for p 35 ClFB, and the lower one, whose maximum is uncertain because of the overlap, is due to 14 2 . Of course, there is the possibility that D11 and 2D14 are interacting, but evidence from other studies suggests that this is weak [4], [6], [8]. Since the D11 vibration shifts, while the D14 overtone is not expected to, it seem that in p 37 ClFB these two transitions are now coincident (and the fact that they are also supports the very weak interaction suggested.)…”

“…Thus, we have recorded resonance-enhanced multiphoton ionization (REMPI) spectra of the three monohalobenzenes, fluorobenzene (FBz), chlorobenzene (ClBz) and bromobenzene (BrBz) [1], [2], [3], toluene (Tol) [4], [5], para-fluorotoluene (pFT) [6], [7] and para-xylene (pXyl) [8], [9]. Central to establishing the common activity, is the use of consistent labelling schemes, which we have described for the monosubstituted benzenes [10] and the para-disubstituted benzenes [11].…”

Torsion and vibration-torsion levels of the S1 and ground cation electronic states of para-fluorotoluene

“…In line with the conclusions from ref. 14, we can reasonably expect to see transitions involving vibtor 8 levels arising from the combination of the m = 3(-) torsional level with vibrations of b1 (a2) symmetry and, as will be seen, other torsional levels also may appear with vibrations of other symmetries (see Table II, and additional comments below).…”

“…8,9 In the present work, we expect many facets of the interactions in toluene to carry over into pFT, with the proviso that the vibrational levels are expected to shift, but that the energies of the unperturbed torsional levels should be to be very similar to those of toluene. In line with the conclusions from ref.…”

“…Unfortunately, we did not record ZEKE spectra when exciting via the 165 cm -1 feature when we performed the toluene study, 8 but we did excite through other Fermi resonance features and their different make-up was inferred from the varying intensities in the ZEKE spectra. 8,9 In the present work, we expect many facets of the interactions in toluene to carry over into pFT, with the proviso that the vibrational levels are expected to shift, but that the energies of the unperturbed torsional levels should be to be very similar to those of toluene.…”