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Molecules 2017, 22, 1059
diagrams (Figure 7) and histograms presented below the results of the cross-validation calculations are
superpositioned in red over the training data.
3. Results
3.1. General Remarks
(1) The experimental values of enthalpies and entropies are temperature-dependent. Any relationship
within these properties or with other ones only make sense if they are referenced to the same
temperature. The usual temperature of reference is 298.15 K, and thus it was ensured in this work
that experimental data from literature were only accepted if they had been either measured at or
adjusted to the standard temperatur of 298.15 K and standard pressure of 100 kPa.
(2) All lists of molecules used in the atom-group parameters evaluations have been collected in
standard SDF files, stored in the supplementary material, ready to be imported by external
chemistry software. The supplementary material also provides the lists of results containing
molecule names, experimental, training and cross-validation values. Beyond this, it also contains
lists of experimental outliers.
3.2. Enthalpy of Vaporization
Experimental data of vaporization enthalpies have essentially been extracted for this work
from the large collection of Acree, Jr. and Chickos [8] and Chickos et al. [10-14], supplemented by
recent data from a number of further authors publishing experimental vaporization values of several
acetophenones [23], aliphatic tertiary amines [24], azidomethyl-N-nitrooxazolidines [25], benzamides [26],
benzocaine [27], bisabolol and menthol [28], crown ethers [29], N,N-dialkyl monoamides [30], fenpropidin
and phencyclidine [31], flavors [32], long-chain fluorinated alcohols [33], whiskey- and metha-lactone [34],
halogenated fluorenes [35], ibuprofen and naproxen [36], imidazo[1,2-a]pyrazine and phthalazine [37],
insect pheromones [38], morpholines [39], organo(thio)phosphates [40], dialkyl phthalates [41], nitrogen
heteroaromatics [42], phenylimidazoles [43], 2-acetylthiophene [44], dicarboxylic n-pentyl esters [45],
and cyclic amines, ethers and alcohols [46]. The result of the atom-group parameters, based on 3581
compounds, is summarized in Table 1. Several tentative calculations with or without inclusion of
certain special groups outlined in Table 2 of the earlier paper [1] revealed a minor improvement of the
goodness of fit upon inclusion of the "atom group" responsible for intramolecular acid-base bonds,
named "H/H Acceptor", as well as of those reserved for saturated and unsaturated pure hydrocarbons,
called "Alkane/No. of C atoms" and "Unsaturated HC/No. of C atoms", which add a correction value
for each carbon atom.
Table 1. Atom Groups and their Contributions (in kJ/mol) for Heat-of-Vaporization Calculations.
Entry Atom Type Neighbours Contribution Occurrences Molecules
1 Const 8.61 3581 3581
2 B C3 21.55 2 2
3 B N2Cl 33.19 1 1
4 B NCl2 28.59 1 1
5 B 02C1 28.23 2 2
6 B OC12 25.53 1 1
7 B S3 76.74 4 4
8 C sp3 H3C 3.07 5380 2388
9 C sp3 H3N 15.65 242 133
10 C sp3 H3N(+) 31.33 2 2
11 C sp3 H30 16.71 372 263
12 C sp3 H3S 14.44 31 25
13 C sp3 H3P 9.04 6 4
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