Semiempirical and DFT Investigations of the Dissociation of Alkyl Halides
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Absolute enthalpies of a wide range of alkyl halides were calculated using the Gaussian 2003 software packaging on a Windows XP computer equipped with a Pentium 4 processor at 2.8 GHz and 512 MB of memory. Five different, but common, methods were used to calculate these enthalpies. These included the AMI and PM3 semi-empirical methods, as well as Hartree-Fock (HF) and B3LYP method. The B3LYP method was also used with the more diffuse 6-31G+(d) basis set, for comparison with the other B3LYP results. The B3LYP method with the 6-31G(d) basis set gave better results in a quantitative sense than all of the other methods, with the exception of the semiempirical PM3 method. The PM3 method produced gas-phase results that were marginally better than those produced by the B3LYP method or at least equivalent within experimental error. In general, the results of all five methods agreed broadly with empirically known trends in carbocation formation from alkyl halides. IEFPCM calculations with water as the solvent showed a very similar trend to the gas-phase calculations.
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