Circumvention of orbital symmetry restraints by 1,3-H-shifts of enolic radical cations

Focus: McLafferty Rearrangement


The reaction coordinates of 1,3-H-shifts across double bonds are traced by theory for three reactions, CH3C(OH)CH 2 (1) → CH3C(O)CH3 (2), CH2C(OH) 2 (3) → CH3CO2H (4) and CH3C(OH)CH 2 (1) → CH2C(OH)CH 3 (1′), to explore how the need to conserve orbital symmetry influences the pathways for these reactions. In the first and second reactions, prior to the start of the H-transfer the methylene rotates from being in the skeletal plane to being bisected by it. Thus these reactions are neither antarafacial nor suprafacial, but precisely between those possibilities. This stems from a counterbalancing between the need to conserve orbital symmetry and the large distorting forces required to attain an allowed antarafacial transition state. In contrast to the first two reactions, 11′ follows a suprafacial pathway. However, this pathway does not violate conservation of orbital symmetry, as it utilizes lower lying orbitals of appropriate symmetry rather than the antisymmetric uppermost occupied allyl-type orbital. Changes in geometry which presumably produce asymmetric vibrational excitation and the unequal losses of methyl that follow 12, i.e., nonergodic behavior, are also characterized.


Transition State Radical Cation Intrinsic Reaction Coordinate Methylene Hydrogen Complete Active Space 
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Copyright information

© American Society for Mass Spectrometry 2004

Authors and Affiliations

  1. 1.Department of Neurosciences and Cell BiologyUniversity of Texas Medical BranchGalvestonUSA

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