THE EVALUATION OF PROTONATION ENERGY OF MOLECULES IN TERMS OF QUANTUM THEORETICAL DESCRIPTORS - Theoretical and Computational Research in the 21st Century - page 200

10.4 EFFECT OF PROTONATION

The protonated form of the molecule is totally a new chemical species
having different charge distribution of the atomic sites within the mol-
ecule, polarizability, dipole moment, bond length, and bond angle. Proton-
ating or deprotonating a molecule or ion alters many chemical properties,
in addition to the change in the charge and mass, hydrophilicity, reduction
potential, optical properties, etc. are among others.
In 1972, Hehre et al. [6] studied the protonation of benzene theoreti-
cally. They [6] introduced the simple ab initio molecular orbital theory to
study the possible geometries and energies of protonated benzene. Using
the STO-3G minimal basis set, the optimum form of the ion was found to
be one in which the proton was bonded to a ring carbon that was assumed
approximately tetrahedral coordination. The form in which the proton
bridges a carbon-carbon bond was found to less favored, and structures
corresponding to the edge and face protonation were found to be poorer
still. The calculated proton affi nity of benzene was in good agreement with
experiment, and a theoretical value was proposed for the energy required
for the proton to migrate from one carbon to another through a bridge
intermediate.
An experimental and theoretical investigation on the effect of proton-
ation on the molecular structure and reactivity of a typical Merocyanine
dye were investigated by Abdel-Halim et al. [7] in 1993.
The molecular reactivity of 1-methyl-2-(4-hydroxystyryl) pyridini-
umbetaine (B) was affected upon protonation in both ground and excited
states. In aqueous solution, the protonated trans form was photochemi-
cally active and isomerizes to give the cis form. The quantum yields Φ tc
and Φ ct were determined. The rate constant and the thermodynamic param-
eters of the reverse cis-trans thermal reaction, from the unprotonated cis
form, were also calculated. Due to the irreversibility of the thermal reac-
tion, a complete molecular reaction cycle was performed in one direction.
The excited molecule exhibited more acidic character than in the ground
state. To correlate with the experimental work, ASED-MO (atom superpo-
sition and electron delocalization molecular orbital) [8] calculations were
applied for both unprotonated and protonated forms. The photochemical
isomerization and the thermal reactions were discussed in terms of chang-
es of the geometrical structure from the quinonoid to the benzenoid form
upon protonation in both ground and excited states. Upon excitation, it