%0 Journal Article %1 hlwoodcock:Skokov1999 %A Skokov, S. %A Wheeler, R. A. %D 1999 %J JOURNAL OF PHYSICAL CHEMISTRY A %K bibtex-import %N 21 %P 4261--4269 %T Oxidative aromatic substitutions: Hartree-Fock/density functional and ab initio molecular orbital studies of benzene and toluene nitrosation %V 103 %X Aromatic nitrosations are prototypes of a recently proposed reaction mechanism-oxidative aromatic substitutions-incorporating ground-state electron transfer prior to the substitution step. Ab initio MO and B3LYP hybrid Hartree-Fock/density-functional (HF/DF) calculations confirm that nitrosation proceeds through initial formation of intermediate electron donor-acceptor (EDA) pi-complexes. Calculated pi-complex geometries and energies agree qualitatively with experimental data and indicate the applicability of HF/DF methods for modeling EDA complexes. Subsequent transformation of (benzene-NO)(+) and (toluene-NO)(+) pi-complexes into N-protonated nitroso-derivatives in B3LYP and MP2 calculations suggest an alternative to the currently proposed mechanism involving pi-complex transformation into Wheland type sigma-complexes (supported by CISD calculations). Kinetic analysis suggests the alternative mechanism is plausible and indicates that proton transfer from the N-protonated nitroso-derivatives to the medium would not be rate limiting. Instead, low nitrosation rates would be assigned to significant potential energy barriers for pi-complex transformation into N-protonated nitroso-derivatives by a novel migratory insertion of nitrogen into the aromatic C-H bond. The insertion step exhibits a large, primary kinetic isotope effect of 11.5, in qualitative agreement with the isotope effect of 8.5 +/- 2.4 previously measured for benzene nitrosation. The difference in B3LYP barrier heights for direct pi-complex conversion to N-protonated nitroso-adducts would also account for the difference in substrate reactivities and regioselectivity in nitrosation reactions. Thus, the conflict between B3LYP and MP2 calculations vs the CISD results cannot be resolved by applying kinetic arguments and must await more definitive work.

@article{hlwoodcock:Skokov1999, abstract = {Aromatic nitrosations are prototypes of a recently proposed reaction mechanism-oxidative aromatic substitutions-incorporating ground-state electron transfer prior to the substitution step. Ab initio MO and B3LYP hybrid Hartree-Fock/density-functional (HF/DF) calculations confirm that nitrosation proceeds through initial formation of intermediate electron donor-acceptor (EDA) pi-complexes. Calculated pi-complex geometries and energies agree qualitatively with experimental data and indicate the applicability of HF/DF methods for modeling EDA complexes. Subsequent transformation of (benzene-NO)(+) and (toluene-NO)(+) pi-complexes into N-protonated nitroso-derivatives in B3LYP and MP2 calculations suggest an alternative to the currently proposed mechanism involving pi-complex transformation into Wheland type sigma-complexes (supported by CISD calculations). Kinetic analysis suggests the alternative mechanism is plausible and indicates that proton transfer from the N-protonated nitroso-derivatives to the medium would not be rate limiting. Instead, low nitrosation rates would be assigned to significant potential energy barriers for pi-complex transformation into N-protonated nitroso-derivatives by a novel migratory insertion of nitrogen into the aromatic C-H bond. The insertion step exhibits a large, primary kinetic isotope effect of 11.5, in qualitative agreement with the isotope effect of 8.5 +/- 2.4 previously measured for benzene nitrosation. The difference in B3LYP barrier heights for direct pi-complex conversion to N-protonated nitroso-adducts would also account for the difference in substrate reactivities and regioselectivity in nitrosation reactions. Thus, the conflict between B3LYP and MP2 calculations vs the CISD results cannot be resolved by applying kinetic arguments and must await more definitive work.}, added-at = {2006-06-16T05:03:46.000+0200}, author = {Skokov, S. and Wheeler, R. A.}, biburl = {https://www.bibsonomy.org/bibtex/2691698cc295cf40878b3ac7b2b8b24f9/hlwoodcock}, citeulike-article-id = {569983}, interhash = {73cb370942d38588355e378f2be9e87f}, intrahash = {691698cc295cf40878b3ac7b2b8b24f9}, journal = {JOURNAL OF PHYSICAL CHEMISTRY A}, keywords = {bibtex-import}, number = 21, pages = {4261--4269}, priority = {2}, timestamp = {2006-06-16T05:03:46.000+0200}, title = {Oxidative aromatic substitutions: Hartree-Fock/density functional and ab initio molecular orbital studies of benzene and toluene nitrosation}, volume = 103, year = 1999 }