Molecular symmetry group analysis of the low-wavenumber torsions and vibration-torsions in the S1 state and ground state cation of p-xylene: An investigation using resonance-enhanced multiphoton ionization (REMPI) and zero-kinetic-energy (ZEKE) spectroscopy

Abstract

For the first time, a molecular symmetry group (MSG) analysis has been undertaken in the investigation of the electronic spectroscopy of p-xylene (p-dimethylbenzene). Torsional and vibration-torsional (vibtor) levels in the S1 state and ground state of the cation of p-xylene are investigated using resonance-enhanced multiphoton ionization (REMPI) and zero-kinetic-energy (ZEKE) spectroscopy. In the present work, we concentrate on the 0–350 cm−1 region, where there are a number of torsional and vibtor bands and we discuss the assignment of this region. In Paper II [W. D. Tuttle et al., J. Chem. Phys. 146, 124309 (2017)], we examine the 350–600 cm−1 region where vibtor levels are observed as part of a Fermi resonance. The similarity of much of the observed spectral activity to that in the related substituted benzenes, toluene and para-fluorotoluene, is striking, despite the different symmetries. The discussion necessitates a consideration of the MSG of p-xylene, which has been designated G72, but we shall also designate [{3,3}]D2h and we include the symmetry operations, character table, and direct product table for this. We also discuss the symmetries of the internal rotor (torsional) levels and the selection rules for the particular electronic transition of p-xylene investigated here.

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

“…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].…”

Vibrational and vibrational-torsional interactions in the 0–600 cm−1 region of the S1 ← S spectrum of p-xylene investigated with resonance-enhanced multiphoton ionization (REMPI) and zero-kinetic-energy (ZEKE) spectroscopy

“…13, we present analysis of the 0-600 cm -1 region of the REMPI spectrum of the symmetrically disubstituted molecule, p-xylene (pXyl), which has two methyl groups located on opposite sides of the phenyl ring. The presence of a second substituent modifies the vibrations significantly, in the same way that the vibrations of monosubstituted benzenes are significantly different to those of benzene; 6 this affects both the form of the vibrations as well as their wavenumbers.…”

“…This will be seen to be particularly useful in the present work, owing to: errors and inconsistencies in, and inappropriateness of the use of Herein, we examine the S1  S0 electronic transition in pXyl using REMPI and ZEKE spectroscopy. Except for the origin, we have discussed the 0-300 cm -1 region in a separate publication, 13 just noting here that this region consists of torsion and vibtor transitions. We shall initially discuss the ZEKE spectrum recorded via the S1  S0 origin, and then move onto the 300-600 cm -1 section of the spectrum, which contains a number of low-wavenumber vibrations.…”

“…As discussed in our accompanying paper, 13 to consider the spectroscopy of pXyl involving the torsional levels requires consideration of the molecular symmetry. The molecular symmetry group (MSG) of pXyl may be designated [3,3]D2h which may be seen to be isomorphic with the G72 MSG, which has been discussed by Bunker and Jensen.…”

“…52 It proves more straightforward to use G72 labels in discussing the spectroscopy of pXyl, with the details of the MSG notation being given in ref. 13. The torsional levels of pXyl, may loosely be described as linear combinations of single-rotor levels of each methyl group, but there are different linear combinations for different levels each of which has a definite G72 symmetry -see ref.…”