Spectroscopic and Computational Studies of Matrix-Isolated iso-CXBr3 (X=F,Cl,Br): Structure, Properties, and Photochemistry of Substituted Iso-Tribromomethanes

Authors

Lisa George, Marquette University
Aimable Kalume, Marquette UniversityFollow
Scott Reid, Marquette UniversityFollow
Brian J. Esselman, University of Wisconsin-Madison
Robert J. McMahon, University of Wisconsin-Madison

Document Type

Article

Publication Date

10-10-2012

Source Publication

Journal of Molecular Structure

Abstract

Iso-polyhalomethanes are important reactive intermediates in the condensed and gas-phase chemistry of halomethanes. Building upon our recent study of iso-bromoform, in this work the substituted iso-tribromomethanes (iso-CXBr₃; X = F, Cl, Br) were characterized by matrix isolation infrared and UV/Vis spectroscopy, supported by ab initio calculations, to further probe the structure, spectroscopy, properties, and photochemistry of these important intermediates. Selected wavelength laser irradiation of CXBr₃ samples in an inert rare gas (typically Ar; mixing ratio 1:500) held at ∼5 K yielded iso-CXBr₃ (XBrC–Br–Br or Br₂C–Br–X). The observed infrared and UV/Vis absorptions are in excellent agreement with computational predictions, and the energies of various stationary points on the CXBr₃ Potential Energy Surfaces (PESs) were characterized computationally using DFT, MP2, and CCSD (T) methods in combination with triple-zeta quality basis sets. These calculations show that the isomers are minima on the PESs that lie ∼200 kJ/mol above the global CXBr₃ minimum, yet are bound by some 50–70 kJ/mol in the gas-phase with respect to the CXBr₂ + Br asymptote. Laser irradiation of the isomers resulted in back photoisomerization to CXBr₃, and intrinsic reaction coordinate (IRC) calculations confirmed the existence of a first order saddle point connecting the two isomers. Calculations of important stationary points on the CXBr₃ PESs show that in the gas-phase the isomerization barrier lies energetically near the threshold for simple bond fission. The iso-CXBr₃ species are significantly stabilized in the condensed phase, due to the high degree of ion-pair character, as revealed by Natural Resonance Theory analysis.

Comments

Accepted version. Journal of Molecular Structure, Vol. 1025, (October 2012): 61–68. DOI: © 2011 Elsevier B.V. Used with permission.

Recommended Citation

George, Lisa; Kalume, Aimable; Reid, Scott; Esselman, Brian J.; and McMahon, Robert J., "Spectroscopic and Computational Studies of Matrix-Isolated iso-CXBr3 (X=F,Cl,Br): Structure, Properties, and Photochemistry of Substituted Iso-Tribromomethanes" (2012). Chemistry Faculty Research and Publications. 217.
https://epublications.marquette.edu/chem_fac/217