Spin-coupled studies of reaction mechanisms

Spin-coupled studies of reaction mechanisms
David L Cooper at the University of Liverpool and Peter Karadakov at the University of York.

Abstract from "Aromatic versus Diradical Character in the Transition States of the Cope Rearrangements of 1,5-Hexadiene and its Cyano Derivatives" by J.J. Blavins, D.L. Cooper and P.B. Karadakov, J. Phys. Chem. A 108, 194-202 (2004):

Using geometries optimized at the (U)B3LYP/6-31G(d) level, ab initio modern VB calculations are used to quantify directly the relative importance of aromatic, diradical and bis-allyl contributions to the transition states of gas-phase Cope rearrangements of 1,5-hexadiene and various cyano derivatives. The main effect on the character of the transition state of substituting radical-stabilizing cyano groups is found to be indirect, via the geometry, with shorter inter-allyl separations favoring diradical character and larger ones aromaticity. In the case of the parent Cope reaction, the weights of the aromatic and diradical components are comparable at the transition state. We find that bis-allyl character only becomes important for transition states with rather large inter-allyl separations.

Abstract from "Modern valence bond description of the electronic mechanisms of S_N2 identity reactions." by J.J. Blavins, D.L. Cooper and P.B. Karadakov, J. Phys. Chem. A 108, 914-20 (2004):

Series of ab initio modern valence bond calculations, based on spin-coupled (SC) theory, along the MP2(fc)/6-31G(d,p) minimum energy paths, are used to examine the electronic rearrangements that take place during the gas-phase S_N2 identity reactions of Cl^- with RCl, where R is methyl, ethyl or t-butyl. The corresponding reaction of F^- with CH₃F is also considered. The SC descriptions of the two CH₃X + X^- reactions (X = F or Cl) are found to be qualitatively similar, but there is a significantly larger extent of bond formation at the transition state for the fluorine case, and the electronic rearrangements also start much sooner. Comparing CH₃Cl + Cl^- and CH₃CH2Cl + Cl^-, the SC calculations suggest that the electronic structure reorganization is largely unaffected by the presence of the additional methyl group. The description of the transition state for the corresponding gas-phase S_N2 identity reaction of (CH₃)₃CCl is found to be radically different: it is held together by predominantly ionic interactions and most closely resembles a carbocation 'clamped' between two chloride ions.

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