Direct Experimental Observation of the Tetrabromine Cluster Br4 by Synchrotron Photoionization Mass Spectrometry

Pure halogens clusters, especially with even numbered molecular formulae, are elusive gas-phase spectroscopic targets. Photoionization mass spectrometry coupled to tunable vacuum-ultraviolet synchrotron radiation allows for the detection and characterization of the tetrabromine Br₄ cluster. Based on a joint experimental and computational assessment, Br₄ is found to exist as a tetrahedron shape of D_2h symmetry.

Abstract

We present a first spectroscopic characterization of the homoatomic polyhalogen tetrabromine, Br₄, in the gas phase. Photolysis of CHBr₃ at 248 nm is used to generate atomic bromine radicals in a flow tube reactor. Resulting combination products are detected by photoionization mass spectrometry at the Advanced Light Source of the Lawrence Berkeley National Laboratory. Interpretation of the experimental mass spectra is informed by calculated adiabatic ionization energies carried out at the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ and CCSD(T)/aug-cc-pVTZ//cam-B3LYP/6-311++g** levels of theory. Tunable VUV synchrotron radiation enables the collection of the mass-selected photoionization spectra by which Br₄ is assigned using Franck-Condon simulations of a Br₂ dimer with a stretched tetrahedral geometry.