Yr. Luo et Jl. Holmes, HOMOLYTIC AND HETEROLYTIC X-C BOND-ENERGIES .2. HETEROLYTIC BOND-ENERGIES IN UNSATURATED ORGANIC CHAIN COMPOUNDS, Journal of physical chemistry, 98(40), 1994, pp. 10368-10372
The heterolytic bond dissociation energies (BDEs), DH degrees(X(-)-R()), of X-C bonds at the P-position, relative to the pi bond in unsatur
ated chain compounds, can be estimated by the equation DH degrees(X(-)
-R(+)) DH degrees(X(-)-C+(CH3)(m)H-3-m) - 1.4 gamma(+) + Delta V-nb E(s)(+) kcal/mol, where R(+) represents the unsaturated chain cation (
allylic, propargylic, cyanated, or others) and X is a wide variety of
univalent atoms or substituents. The first three terms on the right si
de of the above expression are the same as those used previously for s
aturated compounds; tn is the degree of methyl substitution with m 1,
2, or 3 for primary, secondary, or tertiary cations; DH degrees(X(-)-C
+(CH3)(m)H-3-m) are the heterolytic BDEs of model compounds; gamma(+)
is the total number of framework atoms at and beyond the gamma-positio
n, relative to the formal charge center; Delta V-nb is the steric comp
ression relief due to the bond fission. The last term, E(s)(+), is a n
ew parameter, the interaction energy between the empty orbital (at the
formal charge site) and the pi orbital in the unsaturated cations. E(
s)(+) has the value -13.4, 3.5, and 33.4 kcal mol(-1) for allylic, pro
pargylic, and cyanated cations, respectively. The negative and positiv
e E,+ values respectively describe the stabilization and destabilizati
on of the given unsaturated cation, relative to its saturated analogue
. Agreement between experimental and estimated values is generally ver
y good; cases of significant disagreement are critically evaluated.