THE EVALUATION OF PROTONATION ENERGY OF MOLECULES IN TERMS OF QUANTUM THEORETICAL DESCRIPTORS - Theoretical and Computational Research in the 21st Century - page 214

B3LYP to compute the PAs of 24 heterocyclic amines. A range of basis
sets starting from double-polarization quality to triple-quality basis set
with augmented diffuse and polarization function were employed in their
calculation. Experimental values were used to calibrate the performance
of various theoretical models. For the given series of compounds, the per-
formance of B3LYP/6-31++G** and G3B3 levels of theory were in ex-
cellent agreement with the experimental results and the deviations were
found to be minimum i.e., like the experimental error.
Dinadayalane et al. [68] tried more to compute the accurate PAs of
naturally occurring amino acids. Systematic quantum chemical studies of
HF and MP2 methods, and B3LYP functional, with a range of basis sets
were employed to evaluate PA values of all naturally occurring amino ac-
ids. The B3LYP and MP2 in conjunction with 6-311 + G (d,p) basis set cal-
culated the PA values. The computed values were in very good agreement
with the experimental results, with an average deviation of ~1 kcal/mol.
High-level calculations of PA and electron affi nity (EA) of CH 2 X - and
H2CHCHX - (with X = F, Cl, Br, and I) systems were obtained by Morgon
[69] in 2006. The methodology employed in the calculation of PA and
EA based on (completely renormalized-coupled cluster with single and
double and perturbative triple excitation) CR-CCSD[T]/B1// MP2/B0 and
CCSD(T)/B1//MP2/B0 levels, respectively. The differences between cal-
culated results of PA and EA and the experimental values were in the range
of 0.2- 4.5 kJ mol -1 and 0.01 to 0.10 eV, respectively.
Gas-phase PAs of a series of 25 small, aliphatic carbanions were com-
puted using different Gaussian-3 methods: G3, G3(B3LYP), G3(MP2),
and G3(MP2, B3LYP) and complete basis set extrapolation methods:
CBS-4M,CBS-Q,CBS-QB3, and CBS-APNO by Danikiewiczin [70] in
2009 . The results were compared with critically selected experimental
data. The analysis of the result showed that for the majority of the stud-
ied molecules, all compound methods (Gaussian-3 and CBS), except for
CBS-4M, gave comparable results, which differed no more than ±2 kcal
mol -1 from the experimental data. As an additional proof, the result ob-
tained by these two methods a compared with the values obtained using
CCSD(T) ab initio method with large basis set. It was also found that
some of the published experimental data were erroneous and should be
corrected. The results described in this work show that for the majority
of the studied compounds PA values calculated using compound methods
could be used with the same or even higher confi dence as the experimental