Isotopes molecular orbitals from

The polarized-TT frontier molecular orbital (PPFMO) method has been employed to study protonation and sulphenylation of sugar-related dihydrofurans and tetrahydropy-rans. The predictions are consonant with the experimental observations62. Contrary to expectations, the proton-catalysed addition of alcohols to glycals, such as 30, has been shown by isotope labelling (2H) not to be anii-diaxial addition. This observation has been rationalized by the initial attack by deuteron from the bottom, giving ion 31, and by the anomeric effect favouring axial substituent at C-l (32)63. 

Molecular orbital calculations of the w-electron distribution in pyridine predict that more 4- than 2-aminopyridine should be formed in the Tschitschibabin reaction.4 The fact that no 4-aminopyridine can be detected when the two positions are allowed to compete for a deficiency of sodamide (see, e.g., Abramovitch et al 268) has led to the suggestion that the observed orientation in this reaction depends on the relative ease of elimination of a hydride ion from C-2 and C-4 and not upon the initial mode of addition (which, by implication, must take place predominantly at C-4 as predicted by the molecular orbital calculations).4 This hypothesis necessitates that the addition step be rapidly reversible and that the second stage, the elimination of hydride ion, be the rate-determining one (Scheme VII). Although it seems reasonable to assume that the hydride ion eliminations are the slow steps in this reaction, the fact that no deuterium isotope effect was observed in the reaction of 3-picoline-2d and of pyridine-2d with sodamide implies that the first stage must be virtually irreversible,268 as was found also in the case of the addition of phenyllithium to pyridine.229 The addition stage must, therefore, be the product-... 

The observed deuterium isotope effects on the preferences of occupation site can be interpreted in terms of a zero point vibrational energy (ZPVE) of the C—H bond stretching vibration. The singly occupied molecular orbital (SOMO) is delocalised from the centre carbon to the hydrogens making the corresponding C—bond weaker and longer. To a first approximation the ZPVE is proportional to where k is the force constant and m is the mass of... 

As a prelude to further analysis, it is useful to review one important property of molecular orbitals. As noted in Chapter 1, symmetry-correct molecular orbitals must be either symmetric or antisymmetric with respect to the full symmetry of the basis set of atomic orbitals that are used to construct the molecular orbitals. In the analysis of orbital symmetries, we will need to consider only the number of molecular S5nnmetry elements that are sufficient to distinguish between allowed and forbidden pathways. Also, it is not necessary to consider here the minor perturbation of molecular orbital symmetry that results from isotopic or alkyl substitution. In other words, to a first approximation the basis set orbitals of any conjugated diene are considered to be the same as those for 1,3-butadiene. Figtue 11.13 shows the... 

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