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Title: Spectroscopic and computational studies of matrix-isolated iso-CHBr{sub 3}: Structure, properties, and photochemistry of iso-bromoform

Abstract

Iso-polyhalomethanes are known reactive intermediates that play a pivotal role in the photochemistry of halomethanes in condensed phases. In this work, iso-bromoform (iso-CHBr{sub 3}) and its deuterated isotopomer were characterized by matrix isolation infrared and UV/visible spectroscopy, supported by ab initio and density functional theory calculations, to further probe the structure, spectroscopy, and photochemistry of this important intermediate. Selected wavelength laser irradiation of CHBr{sub 3} isolated in Ar or Ne matrices at {approx}5 K yielded iso-CHBr{sub 3}; the observed infrared and UV/visible absorptions are in excellent agreement with computational predictions, and the energies of various stationary points on the CHBr{sub 3} potential energy surface were characterized computationally using high-level methods in combination with correlation consistent basis sets. These calculations show that, while the corresponding minima lie {approx}200 kJ/mol above the global CHBr{sub 3} minimum, the isomer is bound by some 60 kJ/mol in the gas phase with respect to the CHBr{sub 2}+ Br asymptote. The photochemistry of iso-CHBr{sub 3} was investigated by selected wavelength laser irradiation into the intense S{sub 0}{yields} S{sub 3} transition, which resulted in back photoisomerization to CHBr{sub 3}. Intrinsic reaction coordinate calculations confirmed the existence of a first-order saddle point connecting the two isomers, which liesmore » energetically below the threshold of the radical channel. Subsequently, natural bond orbital analysis and natural resonance theory were used to characterize the important resonance structures of the isomer and related stationary points, which demonstrate that the isomerization transition state represents a crossover from dominantly covalent to dominantly ionic bonding. In condensed phases, the ion-pair dominated isomerization transition state structure is preferentially stabilized, so that the barrier to isomerization is lowered.« less

Authors:
; ; ;  [1]; ;  [2]
  1. Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53233 (United States)
  2. Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706 (United States)
Publication Date:
OSTI Identifier:
22038717
Resource Type:
Journal Article
Journal Name:
Journal of Chemical Physics
Additional Journal Information:
Journal Volume: 135; Journal Issue: 12; Other Information: (c) 2011 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0021-9606
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ABSORPTION; BROMOFORM; DENSITY FUNCTIONAL METHOD; INFRARED SPECTRA; ISO; ISOMERIZATION; LASER RADIATION; MATRIX ISOLATION; PHOTOCHEMISTRY; POTENTIAL ENERGY; SURFACES

Citation Formats

George, Lisa, Kalume, Aimable, Wagner, James, Reid, Scott A., Esselman, Brian J., and McMahon, Robert J. Spectroscopic and computational studies of matrix-isolated iso-CHBr{sub 3}: Structure, properties, and photochemistry of iso-bromoform. United States: N. p., 2011. Web. doi:10.1063/1.3640887.
George, Lisa, Kalume, Aimable, Wagner, James, Reid, Scott A., Esselman, Brian J., & McMahon, Robert J. Spectroscopic and computational studies of matrix-isolated iso-CHBr{sub 3}: Structure, properties, and photochemistry of iso-bromoform. United States. doi:10.1063/1.3640887.
George, Lisa, Kalume, Aimable, Wagner, James, Reid, Scott A., Esselman, Brian J., and McMahon, Robert J. Wed . "Spectroscopic and computational studies of matrix-isolated iso-CHBr{sub 3}: Structure, properties, and photochemistry of iso-bromoform". United States. doi:10.1063/1.3640887.
@article{osti_22038717,
title = {Spectroscopic and computational studies of matrix-isolated iso-CHBr{sub 3}: Structure, properties, and photochemistry of iso-bromoform},
author = {George, Lisa and Kalume, Aimable and Wagner, James and Reid, Scott A. and Esselman, Brian J. and McMahon, Robert J.},
abstractNote = {Iso-polyhalomethanes are known reactive intermediates that play a pivotal role in the photochemistry of halomethanes in condensed phases. In this work, iso-bromoform (iso-CHBr{sub 3}) and its deuterated isotopomer were characterized by matrix isolation infrared and UV/visible spectroscopy, supported by ab initio and density functional theory calculations, to further probe the structure, spectroscopy, and photochemistry of this important intermediate. Selected wavelength laser irradiation of CHBr{sub 3} isolated in Ar or Ne matrices at {approx}5 K yielded iso-CHBr{sub 3}; the observed infrared and UV/visible absorptions are in excellent agreement with computational predictions, and the energies of various stationary points on the CHBr{sub 3} potential energy surface were characterized computationally using high-level methods in combination with correlation consistent basis sets. These calculations show that, while the corresponding minima lie {approx}200 kJ/mol above the global CHBr{sub 3} minimum, the isomer is bound by some 60 kJ/mol in the gas phase with respect to the CHBr{sub 2}+ Br asymptote. The photochemistry of iso-CHBr{sub 3} was investigated by selected wavelength laser irradiation into the intense S{sub 0}{yields} S{sub 3} transition, which resulted in back photoisomerization to CHBr{sub 3}. Intrinsic reaction coordinate calculations confirmed the existence of a first-order saddle point connecting the two isomers, which lies energetically below the threshold of the radical channel. Subsequently, natural bond orbital analysis and natural resonance theory were used to characterize the important resonance structures of the isomer and related stationary points, which demonstrate that the isomerization transition state represents a crossover from dominantly covalent to dominantly ionic bonding. In condensed phases, the ion-pair dominated isomerization transition state structure is preferentially stabilized, so that the barrier to isomerization is lowered.},
doi = {10.1063/1.3640887},
journal = {Journal of Chemical Physics},
issn = {0021-9606},
number = 12,
volume = 135,
place = {United States},
year = {2011},
month = {9}
}