Calculation of Five Thermodynamic Molecular Descriptors by Means of a General Computer Algorithm Based on the Group-Additivity Method: Standard Enthalpies of Vaporization, Sublimation and Solvation, and Entropy of Fusion of Ordinary Organic Molecules and Total Phase-Change Entropy of Liquid Crystals Page: 2 of 41
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Molecules 2017, 22, 1059
wanted to verify their experimental results by means of the predictions. Among the many approaches,
from the most elaborate ones such as the time-consuming ab initio methods to the fastest semiempirical
self-consistent field procedures, one has turned out to be the most versatile and accurate and is not
even quantum-theory-related: the atomic group-additivity method. A recent paper  demonstrated
its versatility in that it enabled the calculation of mutually totally unrelated descriptors such as heat
of combustion, solubility, refractivity, polarizability and toxicity by means of one single computer
algorithm. This approach marks the endpoint, so to speak, of the various earlier group-additivity
methods focusing on specific fields of application such as the prediction of the logPo/w values [2,3],
the molar refractivity , the molecular polarizability [5,6], or-closer to the present goal-the
"simultaneous" evaluation of the logP, the aqueous solubility and the brain/blood distribution ratio
logBB using individual parameter sets . It is no secret, however, that the unsuccessful attempts
in paper  to reliably predict just the latter descriptor, logBB, put a damper on the expectation of
a universal applicability of the present atomic group-additivity method. Yet, the exceptionally high
prediction quality for the heat of combustion values across the entire structural spectrum of compounds
presented in paper -showing a cross-validated correlation coefficient of better than 0.9999 for 1965
compounds-at least gave rise to the hope that this method might successfully be extended to further
The standard enthalpies of vaporization and sublimation were the first targets to be examined,
not only because of their importance in chemical and environmental science, but also because a great
deal of groundwork had already been done by Acree, Jr. and Chickos , who collected a large number
of experimental vaporization and sublimation data covering more than a century. Several attempts
to estimate the standard enthalpies of vaporization and sublimation have already been published:
Roux et al.  evaluated the standard phase-change enthalpies of molecules from their experimental
phase-change enthalpies at any given temperatures using their estimated heat capacity at room
temperature. In cases where the number of experimental data was insufficient, they extrapolated the
data from compounds with known experimental values. This estimation method, however, was limited
to the vaporization enthalpy of liquid hydrocarbons. Similarly, Chickos et al. [10,11] estimated the
vaporization enthalpies of larger even-numbered linear n-alkanes from a series of smaller ones [12,13]
using their temperature dependence of the gas chromatographic retention time. A further indication
of the potential applicability of the group-additivity method to predict the heats of vaporization and
sublimation was found in the high correlation of the chain length of the homologues of saturated and
unsaturated fatty acids with their experimental values .
Determination of the enthalpy of solvation has recently been based on the Abraham solute
parameters model [15-18], the model consisting of a linear equation of five parameters relating to
the molecule's excess molar refraction, the polarity/dipolarity, solute hydrogen-bond acidity and
hydrogen-bond basicity, and the McGowan (i.e., molecular) volume. These parameters have been
derived from the molecular structure of a series of compounds using multilinear regression analysis
and artificial neural networks . Earlier, Cabani et al.  described a group-contribution method
for the estimation of the enthalpy, Gibbs free energy and heat capacity of liquids of non-ionic solutes
in water, limiting the method for the calculation of the group contributions to compounds with not
more than one heteroatom and then applying correction parameters for molecules containing more
than one heteroatom.
The entropy of fusion (often-and more logically-called entropy of phase change or even
better: entropy of melting) of ordinary organic molecules as well as its special manifestation with
liquid crystals, called total phase-change entropy, generally mean the entropy of the transition of a
molecule from its most stable crystalline form to the isotropic melt. While for ordinary molecules
this transition in most cases occurs in one step or two consecutive steps upon addition of thermal
energy, this process is much more complex with liquid crystals in that they know several intermediate,
semi-crystalline phases melting at considerably different temperatures. In the first case, occurrence of
more than one melting step may be explained by polymorphism of the crystalline form, their various
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Naef, Rudolf & Acree, William E. (William Eugene). Calculation of Five Thermodynamic Molecular Descriptors by Means of a General Computer Algorithm Based on the Group-Additivity Method: Standard Enthalpies of Vaporization, Sublimation and Solvation, and Entropy of Fusion of Ordinary Organic Molecules and Total Phase-Change Entropy of Liquid Crystals, article, June 25, 2017; Basel, Switzerland. (https://digital.library.unt.edu/ark:/67531/metadc984102/m1/2/: accessed June 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.