THE EVALUATION OF PROTONATION ENERGY OF MOLECULES IN TERMS OF QUANTUM THEORETICAL DESCRIPTORS - Theoretical and Computational Research in the 21st Century - page 204

PA(B) − HA(B + ) = IP(H) − IP(B), (6)

where IP(B) is the ionization potential of species B. As defined in Eq.
(3), the hydrogen affinity is simply the hydrogen bond strength in the ion,
D(B + -H).
With the help of the above model (Eq. 6), Beauchamp et al. [21] ob-
served that the gas-phase proton affi nities of H 2 S and H 2 O were 164 ± 4
kcal/mol and 178 ± 2 kcal/mol, respectively.
Thus ion-cyclotron single- and multiple-resonance techniques gave
a simple means for identifying ion -molecule reactions and examining
certain aspects of their energetics even in complex mixtures. This simple
method by which proton affi nities of water and hydrogen sulfi de were de-
termined could directly be applicable to determine the proton affi nities of
other molecules.
In 1969, Haney et al. [22] invoked mass spectrometric method of deter-
mination of PA and estimated the heat of a PT reaction.
Aue et al. [23] in 1971, for the fi rst time, tried to evaluate the accurate
quantitative relative gas phase basicity introducing the equilibrium tech-
niques. They defi ned the gas phase basicity (GB) of B as the negative of
the free energy (∆G ° ) for reaction 3, whereas the proton affi nity (PA) is the
negative of the enthalpy (∆H ° ) for reaction 3. The HA

B + H +

BH +

(7)

B + + H

BH +

(8)

PA(B) = −IP(B) + HA(B .+ ) +IP(H ) (9)

of B + is the negative enthalpy for reaction 8 and is equivalent to the bond
dissociation energy of BH + . Then, the proton affinity of base B, PA(B),
then can be written as Eq. (9) above.
From their PAs and from known adiabatic IPs [24], accurate relative
hydrogen affi nities were calculated for the fi rst time. They [23] used this
technique to evaluate the accurate quantitative relative gas-phase basici-
ties of amines.
Long et al. [25] utilized the bracketing technique of gaseous PT reac-
tion to evaluate the proton affi nity for some acids, esters, and alcohols.
They observed that the each methyl substituent on the α -carbon of the ac-
ids, esters, and alcohols increased the proton affi nity by about 15 cal/mol.