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Correlation of proton affi nity and HOMO energy of neutral and nega-
tively charged bases were given by Cerofolini et al. [63] in 2002. High-
level quantum mechanical calculations were performed to verify the ex-
tent to which the proton affi nities of closed-shell molecules or anions were
related to the energy E HOMO of the highest occupied molecular orbital. The
accuracy of calculations was verifi ed comparing the calculated PAs with
the experimental ones (when available). The data in their analysis cor-
roborated that the PA was a decreasing function of |E HOMO | modulated by
the atomic number of the proton-hosting atom.
A quadratic confi guration interaction methods were invoked by Miller
et al. [64] to obtain the optimize geometry and harmonic vibrational fre-
quencies of neutral and protonated acetic acid. The proton affi nity of ace-
tic acid was calculated at the QCISD(T)/6-311++G(3df,3pd)//QCISD/6-
311+G(2d,2p) level of theory. The value obtained in the calculation was
187.9±0.7 kcal/mol, which was in excellent agreement with the experi-
mental value of 187.3 kcal/mol. Thermochemical calculations using the
CBS-Q, CBS-QB3, CBS-APNO, G2, and G2(MP2) model chemistries
also displayed excellent agreement with the experimental PA, which sug-
gested that these model chemistries would provide accurate PAs for the
carboxylic acids that were too large to treat with high-level ab initio meth-
od.
The experimental PA of diborane (B2H6), based on an unstable species,
B 2 H + 7 which is observed only at low temperatures. Invoking the Gauss-
ian-3 method and other high-level compound ab initio methods, Betowski
et al. [65] calculated the PA of diborane to check its experimental value.
The calculated value of the PA of diborane in their study was thus reported
at 147.7 kcal/mol, compared with the experimental value of 147.4 kcal/
mol. However, the experimental value was found to be based on two val-
ues, each of which were found to be held in error by 12 kcal/mol, but in
opposite directions.
Beelen et al. [66] studied on the PAs of furan, the methylphenols,
and the related anisoles both experimentally and theoretically. The study
showed that the average literature value for the proton affi nity of furan of
803 kJ mol -1 was too low. It was recommended to alter the value to 812 kJ
mol -1 in keeping with the experimental results obtained in the study and
the outcome of the G3(MP2) calculations.
Rao et al. [67] employed a b initio quantum chemical calculations,
G3B3, second-order MP (MP2), and the hybrid density functional method
 
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