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

Abstract

We investigate the low-energy transitions (0-570 cm -1 ) of the S1 state of para-fluorotoluene (pFT) using a combination of resonance-enhanced multiphoton ionization (REMPI) and zerokinetic-energy (ZEKE) spectroscopy and quantum chemical calculations. By using various S1states as intermediate levels, we obtain zero-kinetic-energy (ZEKE) spectra. The differing activity observed allows detailed assignments to be made of both the cation and S1 lowenergy levels. The assignments are in line with the recently-published work on toluene from the Lawrance group [J. Chem. Phys. 143, 044313 (2015)], which considered vibration-torsion coupling in depth for the S1 state of toluene. In addition, we investigate whether two bands that occur in the range 390-420 cm -1 are the result of a Fermi resonance; we present evidence for weak coupling between various vibrations and torsions that contribute to this region. This work has led to the identification of a number of misassignments in the literature, and these are corrected.2

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

“…In Figure 1 we show an overview of the REMPI spectra of p 35 ClFB together with the corresponding spectra of pFT and pDFB for comparison -regions of the latter two spectra have been presented in previous work [6], [7], [8]. The origin of the S1  S0 transition is measured as 36275±2 cm -1 , and is the same for both isotopologues, within 1 cm -1 as judged from the position and band profiles.…”

“…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.)…”

“…(It is also possible that there is some minor contribution to the -31 cm -1 band from the pClFB-Ar complex -a typical shift for such complexes. 6 )…”

“…We note that the calculated wavenumbers have been scaled by 0.97 to correct both for anharmonicity and other weaknesses in the method; additionally, as we have commented on previously [3], [6], [8] the lowest wavenumber a2 vibration (D14 here) does not appear to be described well for substituted benzenes and we do not report values for this vibration, instead relying on the appearance of the spectrum to assign transitions involving this vibration. Further comments on the calculations will be made below.…”

“…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].…”