Quantitative Structure-Property Relationships of Acenes
By Charles Xie ✉
Acenes are a family of polycyclic aromatic hydrocarbon molecules, as opposed to monocyclic molecules such as cycloalkanes. They are formed by joinining benzene rings in a linear way, following the general formula C4n+2H2n+4. Like the linear alkanes and cycloalkanes, their boiling points also increase with the number of carbon atoms. With AIMS, you can study this quantitative structure-property relationship (QSPR). In general, a good QSPR model must meet the so-called OECD Principles, which requires the model to have 1) a defined endpoint; 2) an unambiguous algorithm; 3) a defined domain of applicability; 4) appropriate measures of goodness-of-fit; 5) robustness and predictivity; and 6) a mechanistic interpretation, if possible.
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Aside from acenes, other hydrocarbons such as linear alkanes and cycloalkanes also observe this relationship.
You can follow the instruction in our article about linear alkanes to build a regressional QSPR model for acenes using a training set and then examine its validity with a test set. It should not be difficult to develop a QSPR model that meets the first five OECD principles.
QSPR modeling gives us a tool to analyze the data, but it does not provide any explanation about the result by itself. To make sense of the result, we still need to resort to fundamentals in chemistry. Equipped with the power of molecular dynamics simulations, AIMS allows us to design and conduct computational experiments to check a QSPR model and/or find a theoretical explanation (the sixth OECD principle).
Molecular Dynamics Simulations
The following two simulations allow you to compare the boiling points of benzenes and octacenes on a qualitative basis.
Benzenes (C6H6)
There are 13 benzene molecules (156 atoms in total) in this simulation. The temperature is initially set to be 373K (100°C). A greatly exaggerated gravitational field is applied to keep the molecules at the bottom of the container when they condense. As you can see, at this temperature, these benzenes are in a gaseous state. The atoms are colored by their kinetic energy — blue represents low kinetic energy and red high kinetic energy. The dashed lines represent the intermolecular interactions.
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Note that it may take a while for a molecular dynamics simulation to produce a desired emergent behavior (e.g., condensation), especially on low-grade computers, when you change a simulation parameter such as lowering the temperature.
Octacenes (C34H20)
There are three cyclodecane molecules (162 atoms in total) in this simulation. The temperature is initially set to be 373K (100°C). As is in the case of the benzenes above, a greatly exaggerated gravitational field is applied to keep the molecules near the bottom of the container when they condense. As you can see, at this temperature, these octacenes are in a condensed state occupying the lower part of the container. If you drag the mouse to rotate the container and let it stay in a certain orientation, you can observe that these molecules will start falling to the bottom of the container, but they manage to stick together while moving down. This is not the case with the benzenes in the above simulation.
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While the boiling points of benzenes and octacenes predicted by our molecular dynamics simulations may not agree exactly with the experimental data (80°C and 745°C, respectively), it is clear that the simulations show that octacenes have a higher boiling point than benzenes. The higher boiling point of octacenes may originate from the fact that they have more intermolecular interactions to hold them together, as indicated by the dashed lines shown in the above windows.
Compared with linear alkanes and cycloalkanes, the planarity of acenes allows for more efficient stacking between molecules in the solid or liquid state, thereby increasing the strength of their intermolecular forces and making their boiling points higher than molecules of similar weight from the other two families of hydrocarbons. For example, naphthalene (C10H8) has a boiling point of 218°C, while octane (C9H20) has a boiling point of 151°C. Their molecular weights are both 128 g/mol. As an exercise, you can try to build a QSPR model that relates boiling point and molecular planarity.