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Quantum Chemical Investigation on Indole: Vibrational Force Field and Theoretical Determination of Its Aqueous pKa Value
journal contribution
posted on 2013-08-08, 00:00 authored by Andrea Pietropolli Charmet, Giuseppe Quartarone, Lucio Ronchin, Claudio Tortato, Andrea VavasoriIndole and its derivatives are molecules
which play important roles
in different fields, from biology to pharmacology. Here we report
a thorough investigation on the anharmonic force fields of indole
as well as the ab initio determinations of its gas phase basicity
and aqueous pKa value. For the geometry
optimizations, the calculations have been performed using both density
functional (DFT) and second-order Møller–Plesset (MP2)
levels of theory employing different basis sets. Anharmonic force
fields have been obtained employing both the B3LYP and the B97-1 functionals
and an hybrid approach: the best agreement to the experimental data
has been determined employing the B3LYP functional combined with the
recently developed N07D basis set (mean unsigned error, MUE, of 5.1
cm–1 and a root-mean-square error, RMSE, of 7.2
cm–1). Gas phase basicity and proton affinity have
been computed employing several computational schemes, namely the
G3 and G4 Gaussian models, the complete basis set (CBS) extrapolation
methods of Petersson and co-workers, several DFT calculations, and
different hybrid extrapolation schemes based on combining single-point
energy calculations performed at MP2 as well as at coupled cluster
level of theory with single, double and perturbative triple excitations,
CCSD(T). Regarding the aqueous pKa computations,
two implicit solvation models (SMD and SM8) have been employed to
determine the free energy of solvation and the corresponding pKa value.