Rui C. Santos

A Review on Prediction Methods for Molar Enthalpies of Vaporization of Hydrocarbons: The ELBA Method as the Best Answer

A review on prediction methods for molar enthalpies of vaporization at T = 298.15 K of hydrocarbons is presented. A new method is proposed and compared with six of the most common used ones from the literature. This new method, the extended Laidler bond additivity (ELBA), was applied to the prediction of standard molar enthalpies of vaporization of hydrocarbons (alkanes, alkenes, alkynes, polyenes, poly-ynes, cycloalkanes, cycloalkenes, benzene compounds, biphenyl compounds, and polyphenyl compounds) at T = 298.15 K. A total of 260 experimental standard molar enthalpies of vaporization at T = 298.15 K were used for the parameters optimization. Comparison between the experimental values and those calculated using ELBA led to an average absolute difference of 0.35 kJ mol−1, corresponding to an average relative error of 0.92%. In addition, this new method proves to be better than the ones used for comparison with an independent set of 83 experimental standard molar enthalpies of vaporization at T = 298.15 K.

Energetics of nonbonded ortho interactions in alkylbenzenes.

The gas-phase enthalpies of formation for a set of ortho-substituted alkylbenzenes were obtained from CCSD(T*)-F12 and W1-F12 calculations. Most values are in keeping with available experimental data. The gas-phase enthalpies of formation of 1-ethyl-2-propylbenzene, 1-ethyl-2-isopropylbenzene, 1,2-diisopropylbenzene, 1,2,4-triethylbenzene, and 1,2,4,5-tetraethylbenzene, for which no experimental data are available, were determined as -46.0, -46.8, -68.7, -76.9, and -116.8 kJ mol(-1), respectively (estimated error bar ±4 kJ mol(-1)). The whole set of experimental and theoretical values are in good agreement with the estimates obtained using the extended Laidler bond additivity (ELBA) method. This agreement supports the approximation used in ELBA that interactions between ortho alkyl groups (other than tertiary alkyl groups) have roughly the same magnitude as a methyl-methyl interaction.

The Thermochemistry of Cubane 50 Years after Its Synthesis: A High-Level Theoretical Study of Cubane and Its Derivatives

The gas-phase enthalpy of formation of cubane (603.4 ± 4 kJ mol(-1)) was calculated using an explicitly correlated composite method (W1-F12). The result obtained for cubane, together with the experimental value for the enthalpy of sublimation, 54.8 ± 2.0 kJ mol(-1), led to 548.6 ± 4.5 kJ mol(-1) for the solid-phase enthalpy of formation. This value is only 6.8 kJ mol(-1) higher than the 50-year-old original calorimetric result. The carbon-hydrogen bond dissociation enthalpy (C-H BDE) of cubane (438.4 ± 4 kJ mol(-1)), together with properties relevant for its experimental determination using gas-phase ion thermochemistry, namely the cubane gas-phase acidity (1704.6 ± 4 kJ mol(-1)), cubyl radical electron affinity (45.8 ± 4 kJ mol(-1)), cubane ionization energy (1435.1 ± 4 kJ mol(-1)), cubyl radical cation proton affinity (918.8 ± 4 kJ mol(-1)), cubane cation appearance energy (1099.6 ± 4 kJ mol(-1)), and cubyl ionization energy (661.2 ± 4 kJ mol(-1)), were also determined. These values were compared with those calculated for unstrained hydrocarbons (viz., methane, ethane, and isobutane). The strain energy of cubane (667.2 kJ mol(-1)) and cubyl radical (689.4 kJ mol(-1)) were independently estimated via quasihomodesmotic reactions. These values were related via a simple model to the C-H BDE in cubane. Taking into account the accuracy of the computational method, the comparison with high-precision experimental results, and the data consistency afforded by the relevant thermodynamic cycles, we claim an uncertainty better than ±4 kJ mol(-1) for the new enthalpy of formation values presented.

Known Mean, Unknown Maxima? Testing the Maximum Knowing Only the Mean

In the quantitative group testing problem, the use of the group mean to identify if the group maximum is greater than a prefixed threshold (infected group) is analyzed, using n independent and identically distributed individuals. Under these conditions, it is shown that the information of the mean is sufficient to classify each group as infected or healthy with low probability of misclassification when the underline distribution is a unilateral heavy-tailed distribution.

Energetics of tert-Butoxyl Addition Reaction to Norbornadiene: A Method for Estimating the π-Bond Strength of a Carbon-Carbon Double Bond

The energetics of tert-butoxyl radical addition reaction to norbornadiene was investigated by time-resolved photoacoustic calorimetry (TR-PAC). The result, together with the C-O bond dissociation enthalpy (BDE) in the addition product, allowed us to calculate the pi-bond dissociation enthalpy in norbornadiene. Quantum chemistry (QC) methods were also used to obtain several enthalpies of reaction of the addition of oxygen-centered radicals to alkenes. The pi-bond dissociation enthalpies in these molecules were calculated by a procedure similar to that used in the case of norbornadiene and were compared with the pi-BDE values obtained by the method proposed by Benson. These two different approaches yield similar values for the pi-BDEs in alkenes, indicating that the addition method proposed in the present study is a valid way to derive that quantity. The influence of strain in the pi-BDEs of cyclic alkenes was investigated and allowed us to justify the difference between the pi-BDE in norbornene and norbornadiene. Finally, the thermochemistry of the addition and abstraction reactions involving these two molecules and tert-butoxyl radical was analyzed.

Testing the Maximum by the Mean in Quantitative Group Tests

Group testing, introduced by Dorfman in 1943, increases the efficiency of screening individuals for low prevalence diseases. A wider use of this kind of methodology is restricted by the loss of sensitivity inherent to the mixture of samples. Moreover, as this methodology attains greater cost reduction in the cases of lower prevalence (and, consequently, a higher optimal batch size), the phenomenon of rarefaction is crucial to understand that sensitivity reduction. Suppose, with no loss of generality, that an experimental individual test consists in determining if the amount of substance overpasses some prefixed threshold l. For a pooled sample of size n, the amount of substance of interest is represented by \(\left (Y _{1},\cdots \,,Y _{n}\right )\), with mean \(\overline{Y }_{n}\) and maximum M n . The goal is to know if any of the individual samples exceeds the threshold l, that is, M n > l. It is shown that the dependence between \(\overline{Y }_{n}\) and M n has a crucial role in deciding the use of group testing since a higher dependence corresponds to more information about M n given by the observed value of \(\overline{Y }_{n}\).

Pseudo-convex mixtures

Allowing weights w∈]−1;1]\{0}, pseudo-convex mixtures increase usual mixtures flexibility. The finite mixtures with negative components are investigated for distribution families closed under minimization. The main purpose is to define these mixtures and to study their properties.

Food energy values: example of an experimental determination, using combustion calorimetry

The energies involved in the combustion, under atmosphere of oxygen, of breakfast cereals and dehydrate powdered whole milk samples, were determined by combustion calorimetry. This practical work, in the field of human nutrition, involved the characterization of the nutritional composition and the combustion of samples of the two foods that are part of the alimentary diet, namely, at breakfast. The obtained results allowed to assess the energy value printed in the foods labels and discuss the way those values are estimated.

Thermodynamically sick molecules: searching for defective experimental enthalpies of formation values using empirical and quantum chemistry methods

A revised parameterization of the extended Laidler bond additivity method and quantum chemistry calculations were independently used to assess the standard molar enthalpies of formation of 20 non-polycyclic hydrocarbons in the gas phase. The detected discrepancies between predicted and experimental values are discussed, illustrating how this methodology can be useful in curing thermochemical data.

Pseudo-convex mixtures generated by shape-extended stable distributions for extremes

A shape-extended definition of min-stable (and max-stable) laws is provided and applied to obtain pseudo-convex mixtures. The main properties of the obtained class of distributions are derived, and a particular case generated by the exponential distribution is investigated.