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Now, if the electron is removed from the hydrogen atom, there will be
only one proton and some quantized energy level without electron.
10.2
THE PHYSICOCHEMICAL PROCESS OF PROTONATION
When a proton dynamically approaches a molecule, in the physicochemi-
cal process of protonation, from a long distance, it is attracted by the elec-
tron cloud of the molecule equally from all the directions. Thus in the pro-
cess, a proton, acting as an electrophile, starts soaking the electron density
from the entire skeleton of the nucleophile [2]. As a result, the electron
density of the host molecule is reorganized and redistributed. Ultimately,
the proton fixes at a site of lone pair—the preferred site of protonation
in the molecule. However, if there is no lone pair in the structure of the
molecule, the proton remains electrostatically attached to the sphere of the
charge cloud of the molecule. Thus, it is implied that the physicochemical
process of protonation can be covalent as well as electrostatic interaction.
During the process of protonation, the proton induces, concomitant with
the charge density reorganization, a physical process of structural and en-
ergetic changes in the molecule. And the structural and energetic changes
induced by the polarizing power of the proton are expected to be at its
maximum at the gas phase of the molecule.
10.3 PROTON AFFINITY
The proton affinity defined as the negative of the enthalpy change of a pro-
tonation reaction at standard conditions. Brønsted [3] described basicity as
the tendency of a molecule B to accept a proton in the reaction
B + H
+
= BH
+
, ∆H
0
= -PA
(1)
where PA is the proton affinity of the base B, and this is the enthalpy
change of this reaction, and BH
+
is the base-acid (proton) adduct. This
concept of basicity was generalized further and freed from reference to a
specific acid (H
+
) by Lewis [4].
Proton affi nity can be also expressed in terms of individual heats of
formation of the species involved in the adduct formation reaction [5].
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