Vibrations of the p-chlorofluorobenzene cation.

David J Kemp,Adrian M Gardner,William D Tuttle,Benjamin T Speake,Laura E Whalley,Timothy G Wright

doi:10.1039/c8cp01274a

Abstract

The vibrations of the ground state cation (X[combining tilde]2B1) of para-chlorofluorobenzene (pClFB) have been investigated using zero-electron-kinetic-energy (ZEKE) spectroscopy. ZEKE spectra were recorded using different vibrational levels of the S1 state as intermediate levels, for which assignments were put forward in an earlier paper [W. D. Tuttle, A. M. Gardner, and T. G. Wright, Chem. Phys. Lett., 2017, 684, 339]. These different intermediate levels dramatically modify the Franck-Condon factors for the ionization step. The adiabatic ionization energy (AIE) for pClFB was measured as 72 919 ± 5 cm-1, and analysis of the vibrational structure in the ZEKE spectra allowed further interrogation of the assignments of the REMPI spectrum. Assignment of the vibrational structure has been achieved by comparison with corresponding spectra of related molecules, via quantum chemical calculations, and via shifts in bands between the spectra of the 35Cl and 37Cl isotopologues. In this way it was possible to assign twenty out of the thirty vibrational modes of the ground state pClFB+ cation. Additionally, evidence for Fermi resonances between some vibrational levels was found in the S1 state, but no large-scale intramolecular vibrational redistribution (IVR) was seen in the spectra here. Finally, we discuss trends in AIE shifts for benzenes with one or two halogen atoms or methyl substituents.

Highlights

In recent work from our group, we have focused on the study of vibrations of substituted benzenes in the S1 state and cation, with a particular interest in identifying common activity across related molecular species
We reported the resonance-enhanced multiphoton ionization (REMPI) spectrum of para-chlorofluorobenzene[15] where we made assignments based on expected activity and quantum chemical calculation of the harmonic vibrational wavenumbers, together with shifts for the p35ClFB and p37ClFB isotopologues
REMPI spectrum In Figure 1(a) we show an overview of the REMPI spectra of p35ClFB up to ~1400 cm-1

Summary

Introduction

In recent work from our group, we have focused on the study of vibrations of substituted benzenes in the S1 state and cation, with a particular interest in identifying common activity across related molecular species. These S1 vibrational levels are of overall a1 symmetry, and the REMPI bands are consistent with activity in the S1 S0 excitation spectra of paradisubstituted benzenes,[6,9,29] with the assignment of these discussed in more detail in Ref. 15.

Results

Conclusion