THE EVALUATION OF PROTONATION ENERGY OF MOLECULES IN TERMS OF QUANTUM THEORETICAL DESCRIPTORS - Theoretical and Computational Research in the 21st Century - page 210

Cavity sizes of crown ethers correlate generally with trapped cation
radii. Since the extent of ring strain due to formation of intramolecular hy-
drogen bond was employed to measure the PA values, the geometry of the
protonated 1-(2'-methoxy)ethoxy-2-methoxyethane could be a standard
for the extent. Three protonated crown ethers had the glyme-2H + moiety.
The computed PA value of their study of 12-crown-4 was in good agree-
ment with two experimental data [45, 46]. In addition, those of 15-crown-5
and 18-crown-6 were similar to Kebarle's one [46], respectively.
Chandra et al. [47] in 1996 used the density functional theory (DFT)
with different combinations of exchange and correlation functional to study
the proton affi nities of six organic molecules, namely H 2 CO, CH 3 CHO,
CH 3 OH, C 2 H 5 OH, HCOOH, and CH 3 COOH. Complete geometry optimi-
zations were carried out for both the neutral and protonated species with
all combinations of functionals. The calculated proton affi nity values were
then compared with the corresponding experimental values. They com-
bined Perdew's and Becke's exchanges with Proynov's correlation func-
tional, which was the most effective in reproducing the proton affi nity.
The absolute proton affi nities of some carbenes were calculated using
ab initio methods by Josefredo et al. [48] in 1997. The studied species
have a range of absolute proton affi nities from 177.4 to 275.0 kcal mol -1 .
The more basic carbenes (CPh and fl uorenylidene) should react in protic
solvents by PT, whereas the less basic ones (CF 2 , FCOH) should not. The
intermediate species CH 2 , CCl 2 , C(OH) 2 could abstract a proton in neutral
and acidic solutions.
Simple ab initio model for calculating the absolute PA of aromatics was
invoked by Maksic et al. [49] in 1997. In their method, they [49] showed
that a simple scaled HF (ScHF) model was able to describe very well the
ring PA of a vast variety of polysubstituted benzenes, naphthalenes, and
biphenylenes. Its utility in predicting PAs of large alternant aromatics was
illustrated on pyrene and monofl uoropyrenes. The calculated PAs were in
well agreement with the available experimental. Finally, they [49] found
that PAs in polyfl uoropyrenes followed the same simple additivity rule,
based on the independent substituent approximation (ISA), which was ob-
served earlier in smaller alternant aromatic systems.
Maksic et al. [49] in 1997 have shown that the ScHF model was able
to describe very well the ring PA of a wide variety of polysubstituted
benzenes, naphthalenes, and biphenylenes. The average absolute devia-
tion from the full MP2 calculations, which in turn were very close to the