The gauche Effect. A Theoretical Study of the Topomerization (Degenerate Racemization) and Tautomerization of Methoxide Ion Tautomer

Abstract

Non-empirical double zeta quality molecular orbital calculations on −CH2OH as a function of the C—O bond length (r), the rotational angle about the C—O bond (θ), and the pyramidal angle at carbon [Formula: see text] are described. From the stretching potential curve, E(r), it is shown that dissociation of −CH2OH proceeds to give CH2 and OH−. The rotation–inversion surface, [Formula: see text], has two types of minima; in both cases the most favorable pyramidal angle at carbon is 105°. The lower minimum corresponds to a structure (the Y conformation) having the hydroxyl proton on the external bisector of the HCH angle. The higher minimum is 6.67 kcal/mol less stable and corresponds to a structure (the W conformation) having the hydroxyl proton on the internal bisector of the HCH angle. The relationship of these results to the gauche effect is discussed and it is noted that at certain internuclear distances the nuclear–nuclear repulsion term (Enucl) may overcome the tendency of adjacent electron pairs and polar bonds to exist preferentially in that conformation which has the maximum number of gauche interactions between these electron pairs or polar bonds.The topomerization of −CH2OH, i.e., the conformational transformation from one Y conformation into another, proceeds, via the W conformation as an intermediate, by two separate events, viz. rotation about the C—O bond, having a barrier of 10.58 kcal/mol, and pyramidal inversion at carbon, with a barrier of 20.52 kcal/mol. Some factors governing the relative importance of rotation and inversion in degenerate racemization are discussed.In its ground electronic state CH3O− is 22.18 kcal/mol more stable than −CH2OH. However, in the low-lying excited states all conformations of −CH2OH are stabilized relative to CH3O−. The most stable excited state structure of −CH2OH corresponds to the energy maximum for rotation–inversion of the ground electronic state.