THE EVALUATION OF PROTONATION ENERGY OF MOLECULES IN TERMS OF QUANTUM THEORETICAL DESCRIPTORS - Theoretical and Computational Research in the 21st Century - page 209

point energy (ZPE) contribution to the PA was estimated at the HF/6-31
G* level of approximation. It appeared that the contribution of the ZPE
to relative ∆PA proton affi nities was negligible. They showed that in cal-
culation, the empirical ZP energies for the protonated species the simple
additivity rule could be the better option. The energetically most favorable
site of the proton attack was para to the OH substitution in accordance
with the experimental fi nding. Performance of the MP2 (fc) 6-31G** +
ZPE (HF/6-31G*) model in reproducing protonation at the oxygen atom
was verifi ed in some medium-sized alcohols and ethers. The calculated PA
values were in good agreement with the measured data.
A mass spectrometric and computational study of the protonation and
methylation of H 2 NOH was reported by Angelelli et al. [42] in 1995. The
FT-ICR “bracketing” technique and by the kinetic method based on the
unimolecular fragmentation of proton-bound were employed to evaluate
the gas-phase PA of H 2 NOH, and the PA value obtained in these experi-
mental procedure was 193.7 ± 2 kcal mol -1 at 298 K, which was in excel-
lent agreement with the 193.8 kcal mol -1 PA computed at the GAUSSIAN
1 level of theory for the N atom.
The computed PAs of the N atom of CH 3 NHOH and H 2 NOCH 3 , that
is, 203.9 and 201.0 kcal mol -1 , respectively, were also in good agreement
with the values obtained from FT-ICR equilibrium measurements, 205.1
± 2 and 202.7 ± 2 kcal mol -1 , respectively, whereas the PAs calculated for
the O atom were much lower, 175.1 and 179.8 kcal mol -1 , respectively.
Smith et al. [43] introduced the G2(MP2, SVP) method to calculate the
proton affi nity of some molecules.
The G2(MP2, SVP) method was shown to reproduce proton affi nities
for a set of reference molecules to within the G2 target accuracy of 10 kJ
mol -1 but at signifi cantly lower computational cost.
Ab initio calculations of 12-crown-4, 15-crown-5, and 18-crown-6 and
their protonated species were performed by Wasada et al. [44] (Wasada
1996) in 1996. As a reference molecule, 1-(2'-methoxy)ethoxy-2-me-
thoxyethane (glyme-2) was also examined. Three protonated crown ethers
was found to have quite similar moieties to that of glyme-2H + . The ring
strain due to formation of intramolecular hydrogen bonds was a measure
of PA values. The computed result supports Meot-Ner's and Sharma's PA
values [45, 46] of 12-crown-4. Those of 15-crown-5 and 18-crown-6 re-
ported by Sharma [46] were very close to the theoretical values.