THE EVALUATION OF PROTONATION ENERGY OF MOLECULES IN TERMS OF QUANTUM THEORETICAL DESCRIPTORS - Theoretical and Computational Research in the 21st Century - page 207

With some exceptions, Dewar et al. [34] effi ciently invoked the AM1
semiempirical molecular orbital model to calculate the PAs for 60 com-
pounds and deprotonation enthalpies (DPEs) for 80 compounds. Intermo-
lecular hydrogen bonding in protonated bifunctional bases is also effec-
tively reproduced.
In the case of small anions with the AM1 study, some problems were
encountered. Systematic errors were accompanied on the introduction of
methyl or phenyl substituents at anionic centers. Systematic error was also
found in the extension of alkyl chains by addition of methene groups and
in substitution of amine and hydroxyl groups for methyl groups bonded to
secondary or tertiary carbons in neutral molecules. The errors involved in
the deprotonation of alcohols and protonation of amines were not, how-
ever, totally systematic. As a result, the relative DPEs of alcohols and PAs
of amines were not accurately reproduced by AM1 study.
Dewar et al [34] also observed that AM1was able to perform well as
compare to the high-level ab initio procedures. But the fact is that they
[34] made the comparisons with few simple cases . Also ab initio pro-
cedure was not tested. To carry out with full geometry optimization at
that level of calculation, it became very expensive for large molecules.
Since the accuracy achieved by AM1 was suffi cient for the results to be
chemically useful and since it was used to study reactions of quite large
molecules at moderate cost, AM1 was considered as an aid in interpreting
PT in chemistry and biochemistry.
Ab initio molecular orbital calculations using the STO-3G, STO-3G**,
3-21G, and 6-31G** basis sets were performed for the series HCOOH,
CH 3 COOH, CH 3 OH, and C 2 H 5 OH by Nagy-Felsobuki et al. [35] in 1990.
Structural calculations of the neutrals and predictions of the structure of
the respective cations were discussed and reported. Relative proton affi ni-
ties were calculated. Only at the 6-31G**//6-31G**, HF level of theory
were effective to evaluate the relative proton affi nities and trend for this
simple organic acid/base series were in agreement with the experimental
value. The STO-3G//STO-3G model was the most effective of the other
models.
Smith et al. [36] in 1993 invoked the ab initio molecular orbital calcu-
lations at the G2 level to obtain absolute values for the proton affi nities of
a variety of prototypical small molecules. A total of 31 molecules were ex-
amined in their study. The results were compared with the standard proton
affi nity scale of that time and with more advanced revisions of that scale.