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CF_2XCF_2X and CF_2XCF_2• Radicals (X = Cl, Br, I): Ab Initio and DFT Studies and Comparison with Experiments

Ihee, Hyotcherl and Kua, Jeremy and Zewail, Ahmed H. (2001) CF_2XCF_2X and CF_2XCF_2• Radicals (X = Cl, Br, I): Ab Initio and DFT Studies and Comparison with Experiments. Journal of Physical Chemistry A, 105 (14). pp. 3623-3632. ISSN 1089-5639. doi:10.1021/jp004035x.

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1,2-dihalotetrafluoroethanes (CF_2XCF_2X, X = I, Br and Cl) and halotetrafluoroethyl radicals (CF_2XCF_2•, X = I, Br, and Cl) have been studied by ab initio molecular-orbital techniques using restricted Hartree−Fock and Density functional theory (DFT-B3PW91). For the optimized HF geometries, we carried out local MP2 calculations to account for electron correlation effects. Each CF_2XCF_2X molecule and CF_2XCF_2• radical has two conformational minima (anti and gauche) and two rotational transition structures in the rotational energy surface along the C−C bond. The rotational barriers of the  radicals are smaller than those of the parent molecules due to the absence of the nonbonded interaction between two halogen atoms. In contrast, the conformational energy difference between two stable rotamers (anti and gauche) of each radical is larger than that in the corresponding parent molecules. This stabilizing effect on the anti conformers of the radicals is rationalized in terms of hyperconjugation between the radical center and the σ^*(C−X) molecular orbital. The dissociation energies for breaking the first and second C−X bonds of CF_2XCF_2X were also calculated and compared with available experimental data. The CF_2XCF_2• radicals show dramatically different behavior compared with haloethyl radicals (CH_2XCH_2•). The CF_2XCF_2• radical has two minima and two saddle points, whereas CH_2XCH_2• radical has only one minimum and one saddle point in the rotational energy surface. In addition, the bridged structures are not stable for CF_2XCF_2• radicals in contrast with CH_2XCH_2• radicals. The origin of these differences is attributed to differences in the environment of the radical center. The calculated structures of the CF_2ICF_2• radical were used in interpreting a recent experimental observation (Cao et al. Proc. Natl. Acad. Sci. 1999, 96, 338) and are compared with quantitative results from a new experiment (Ihee et al. Science 2001, 291, 458) using the ultrafast electron diffraction technique.

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Additional Information:© 2001 American Chemical Society. Received: October 31, 2000; In Final Form: February 6, 2001. Publication Date (Web): March 13, 2001. This research was funded by a grant (WAG) from the NSF (Grant No. CHE 95-12279) and by contracts (AZ) with the Air Force Office of Scientific Research and the Office of Naval Research. The facilities of the MSC used for these studies are also supported by grants from DOE-ASCI, NSF-MRI, ARO-MURI, Exxon, Chevron Corp., Seiko-Epson, Dow Chemical, Avery Dennison, 3M, the Beckman Institute, Asahi Chemical, and BP Amoco.
Funding AgencyGrant Number
NSFCHE 95-12279
Air Force Office of Scientific Research (AFOSR)UNSPECIFIED
Office of Naval Research (ONR)UNSPECIFIED
Department of Energy (DOE)UNSPECIFIED
Chevron CorporationUNSPECIFIED
Caltech Beckman InstituteUNSPECIFIED
Issue or Number:14
Record Number:CaltechAUTHORS:20160819-112043058
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Official Citation:CF2XCF2X and CF2XCF2• Radicals (X = Cl, Br, I):  Ab Initio and DFT Studies and Comparison with Experiments Hyotcherl Ihee, Jeremy Kua, William A. Goddard III, and Ahmed H. Zewail The Journal of Physical Chemistry A 2001 105 (14), 3623-3632 DOI: 10.1021/jp004035x
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:69790
Deposited By: Ruth Sustaita
Deposited On:22 Aug 2016 23:26
Last Modified:11 Nov 2021 04:19

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