THE EVALUATION OF PROTONATION ENERGY OF MOLECULES IN TERMS OF QUANTUM THEORETICAL DESCRIPTORS - Theoretical and Computational Research in the 21st Century - page 206

result when protonation occurred at nonbonded electron pairs on A and B
are similar, except for the C- and O-protonated forms of C≡O. However,
the HF barrier to PT which separated the isomers is signifi cantly lowered
by electron correlation. Correlation also lowered the barrier to PT for ions
H m AAH n+1 + except for N 2 H + . Zero-point vibrational energy corrections
were also evaluated and found to lead to signifi cant decreases in computed
protonation energies.
Lohr [31] conceived for the fi rst time the protofelicity as a protonic
counterpart of electronegativity. Such a new structural descriptor involv-
ing the physicochemical process of protonation depends on both number
of electrons and number of protons in the molecule. The method relies on a
concept of a charge-dependent electronegativity is extended to protons by
the use of a polynomial representation of molecular energies as a function
of the number of protons and the number of electrons. The relationships
provide an organizing principle for gas-phase acidity and basicity data by
expressing succinctly the interdependence of the energies of proton-trans-
fer and electron-transfer reactions. Application is made to the energetics of
strongly hydrogen-bonded systems.
In 1984, the accident in Bhopal, India, in which heavy casualties re-
sulted from the release of a large quantity of methyl isocyanate, CH 3 NCO,
considerable attention focused on its chemistry. Methyl isothiocyanate,
CH 3 NCS, and methyl thiocyanate, CH 3 SCN, which are closely related to
CH 3 NCO as far as their toxicity and chemical properties were concerned,
had been the subject of an ICR study [32]. The gas-phase ion chemistry of
CH 3 NCO, CH 3 NCS, and CH 3 SCN was investigated by pulsed ICR tech-
niques, and their proton affi nities were determined as being 184.5 ± 0.5,
193.0 ± 0.4, and 192.6 ± 0.5 kcal/mol, respectively, by Karpas et al. [33].
The main reaction of the molecular ion in the three compounds was pro-
duction of the protonated molecule. The CH 2 X + ions, where X = NCO,
NCS, or SCN, were unreactive toward the parent molecule. The fragment
ions CH 2 Y + , where n = 0-3 and Y = 0 or S, reacted by charge transfer
or PT. Those protonated molecules reacted very slowly with their parent
compounds. Although protonated dimers were observed, their production
was ineffi cient. Ab initio calculations at the SCF level were used to de-
termine the structures of the neutral and protonated molecules. The cal-
culated proton affi nities, 188.5, 188.6, and 193.7 kcal/mol for CH 3 NCO,
CH 3 SCN, and CH 3 NCS, respectively, were in good agreement with the
experimental values.