Theoretical thermochemistry of homolytic C-C and C-Cl bond dissociations in unbranched perchloroalkanes
- Florida State Univ., Tallahassee, FL (United States)
Proper description of the dispersion interactions that attenuate the closed-shell repulsions among chlorine atoms in n-perchloroalkanes requires an accurate treatment of long-range electron correlation effects. The presently known density functionals, which do not correctly account for such effects, grossly underestimate thermodynamic stability of these molecules, yielding the standard enthalpy {Delta}H{degree}{sub C-C} of C-C bond dissociation in n-C{sub 4}Cl{sub 10} as low as 20 kcal/mol. In contrast, the predictions of the CBS-4 scheme fail to reproduce the weakening of the C-C bonds that is experimentally observed in the higher members of the C{sub n}Cl{sub 2n+2} homologous series. For the CCl{sub 4} and C{sub 2}Cl{sub 6} the most reliable estimates of {Delta}H{degree}{sub C-C} and {Delta}H{degree}{sub C-Cl} are provided by the G2 And G2MP2 methods. The MP2/6-311G{sup **} level of theory (without ZPEs and finite-temperature corrections) appears at present to be the only viable, though quite inaccurate, theoretical approach to theoretical thermochemistry of larger chlorocarbons. At that level of theory, {Delta}H{degree}{sub C-C} is predicted to decrease by ca. 10 kcal/mol upon the addition of each -CCl{sub 2}- unit, convincingly explaining the thermal lability of higher n-perchloroalkanes. Similar conclusions are reached by examining the estimates of {Delta}H{degree}{sub C-C} derived from the CBS-4 standard enthalpies with the E{sub CBS} energy term excluded.
- Sponsoring Organization:
- USDOE, Washington, DC (United States)
- DOE Contract Number:
- FG02-97ER14758
- OSTI ID:
- 320834
- Journal Information:
- Journal of Physical Chemistry A: Molecules, Spectroscopy, Kinetics, Environment, amp General Theory, Vol. 102, Issue 48; Other Information: PBD: 26 Nov 1998
- Country of Publication:
- United States
- Language:
- English
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