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[59]. The model was applied to the gas-phase acid-base equilibria of al-
kyl alcohols. PT was simulated as the motion of a charged particle in an
applied external potential defi ned by the chemical environment of the
proton, and represented by the difference in PA of the conjugated bases
RO- and CH3O-; the latter was taken as reference. The electronic chemi-
cal potential of transfer accounted for both the amount and direction of
charge transfer (CT). The relative acidity for a short series of alkyl alco-
hols was determined by the difference in proton affi nity (∆PA) = PA(RO
-
)
− PA(CH3O
-
)) of the conjugated bases.
PAs of the four smallest cyclic amines—aziridine (C
2
H
5
N), azetidine
(C
3
H
7
N), pyrrolidine (C
4
H
9
N), and piperidine (C
5
H
11
N), were calculated
by Vayner et al. [60]
using
ab initio
methods and density functional cal-
culations.
The methods varied in their ability to calculate accurate ∆
f
H
's,
with MP2 calculations being the most accurate and HF calculations the
least. Most methods were able to predict the proton affi nities well, typi-
cally to within 30 kJ/mol (or about 5%).
The relative gas-phase acidity of halosubstituted acetic acids CH
2
X-
CO
2
H, CHX
2
CO
2
H, and CX
3
CO
2
H (X)F, Cl, and Br) was analyzed in
terms of global and local descriptors of reactivity by Pérez et al. [61]. The
model was based on the analysis of proton-transfer equilibria with refer-
ence to acetic acid CH
3
CO
2
H. The relative acidity pattern displayed by
the series was rationalized in terms of the hard and soft acids and bases
principle. The relative stability between the neutral species and the cor-
responding anions is in agreement with the maximum hardness principle.
Charge transfer between the conjugated bases present in the proton-trans-
fer equilibria was correctly accounted for by using a classical ion-transport
model that introduces the electronic chemical potential of transfer. The lo-
cal reactivity analysis based on regional Fukui functions and local softness
was able to display a good correlation with the experimental gas-phase
acidity within the series.
Silva et al. [62] in 2000 used the thermodynamical cycle to calculate
the absolute pKa values for Brønsted acids in aqueous solution. The po-
larizable continuum model (PCM) was used to describe the solvent, and
absolute pKa values were computed for different classes of organic com-
pounds: aliphatic alcohols, thiols, and halogenated derivatives of carbox-
ylic aliphatic acids. The model was competent to furnish pKa values in
good agreement with the experimental results for some classes of com-
pounds.
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