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Neutral hydrocarbons like cyclopentadiene with an odd number of carbon atoms and one intervening CH2 group in the ring are not aromatic. Cyclopentadiene with 4 π electrons does not satisfy the 4n + 2 π electron rule. Additionally, the intervening CH2 group is sp3 hybridized and lacks a vacant p orbital, thereby interrupting the overlap of p orbitals in a continuous manner and preventing the delocalization of π electrons throughout the ring.
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Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
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π-π stacking between polyaromatic hydrocarbon sheets beyond dispersion interactions.

Nadeesha J Silva1, Francisco B C Machado2, Hans Lischka3

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High-level ab initio calculations reveal that two-dimensional polyaromatic hydrocarbon (PAH) sheets exhibit significant non-planarity and strong intersheet binding. These findings align with experimental data for graphite defoliation energies.

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Area of Science:

  • Computational chemistry
  • Materials science
  • Physical chemistry

Background:

  • Polycyclic aromatic hydrocarbons (PAHs) are fundamental building blocks in materials science.
  • Understanding the electronic and structural properties of large PAHs is crucial for their applications.
  • Accurate theoretical methods are needed to model the behavior of these complex molecules.

Purpose of the Study:

  • To investigate the structural and energetic properties of quasi one-dimensional acenes and two-dimensional PAH sheets.
  • To compare theoretical calculations with experimental data for polyaromatic hydrocarbons.
  • To evaluate the performance of different computational methods, including ab initio and density functional theory (DFT), in describing PAH structures.

Main Methods:

  • High-level ab initio calculations, including coupled cluster and explicitly correlated methods.
  • Second-order Møller-Plesset perturbation theory using spin scaling (SOS-MP2).
  • Geometry optimizations for sandwich (AA) and slipped parallel (AB) dimer structures.
  • Consideration of basis set superposition effects.
  • Limited investigation of DFT with empirical dispersion corrections (e.g., D3 method).

Main Results:

  • Two-dimensional PAH sheets display significant biconcave deviations from planarity.
  • Computed intersheet binding energies are comparable to experimental graphite defoliation energies.
  • SOS-MP2 calculations predict non-planar structures, while most DFT variants result in quasi-planar graphene sheet models.
  • The D3 DFT method shows a tendency towards underbinding.

Conclusions:

  • High-level ab initio methods accurately predict the non-planar geometry and strong intersheet binding of large PAHs.
  • Discrepancies between ab initio and DFT results highlight the importance of method selection for studying PAH structures.
  • The findings provide valuable insights into the behavior of PAHs relevant to materials science and condensed matter physics.