THE EVALUATION OF PROTONATION ENERGY OF MOLECULES IN TERMS OF QUANTUM THEORETICAL DESCRIPTORS - Theoretical and Computational Research in the 21st Century - page 201

seemed that the molecular polarity character decreased in the case of B
but increases for BH + . Protonation was found as an exothermic, downhill
reaction in the ground state and as an endothermic, uphill reaction in the
excited state. It was shown that the lowest electronic transition is π - π* ,
which was higher for BH + than for B.
A theoretical study of the effects of protonation and deprotonation on
bond dissociation energies were introduced by Boyd et al. [9] Ab initio
molecular orbital calculations indicated that the bond dissociation ener-
gies (BDEs) for hemolytic cleavage of CX bonds (X = C, N, O, F) were
increased by protonation of the corresponding alkyl, amine, alcohol, or
fl uoride functional groups; the effect of deprotonation of these groups was
rather small for saturated species; whereas for unsaturated ones, deproton-
ation leads to large increases in the CX BDEs. The effects on the CC BDEs
in CCX compounds were quite systematic: protonation of X increased the
CC BDE, whereas the converse holds for deprotonation. Two types of cor-
relation between bond lengths and homolytic bond dissociation energies
were observed. First, protonation and deprotonation lead to a normal cor-
relation for the adjacent CC bonds. But the bond length decreased with the
increase in BDE. Protonation, however, gave an anomalous correlation for
the CX bonds that the bond length and BDE both increased in the same
way. These observations were rationalized in terms of electronegativity,
resonance stabilization, and competing heterolytic dissociation.
Protonation in series of aldehydes and ketones of formula R 1 -CO-R 2
(with R 1 , R 2 =H, Me, Et, Pr, and Bu) was studied by using the theory of
atoms in molecules to examine the atomic and bond properties of the car-
bonyl group and its relationship to the energy involved in the protonation
process [10]. Based on the results, aldehydes, methyl keones, and the re-
mainder dialkylketones studied exhibit three different behaviors. Small
differences were resulted on the protonation of the C=O bond. Although
the atomic charge of the carbonyl group hardly changes, the proton bond-
ed to the oxygen exhibits high positive charge after protonation. Atomic
contributions to the total energy depend on molecular size.
10.5 PROTONATION ITS IMPORTANCE IN CHEMISTRY,
BIOCHEMISTRY, AND BIOLOGY
The proton affinities (PAs) of molecules have been used as the primary
information of many chemicophysical processes and to characterize the