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Exercise 2: Benzene and Acetylene

Dispersion Interactions Dominate​

Initial Setup​

  1. Open the BENZEN CIF )
  2. Complete the molecule (initially shows just unique atoms)
  3. Generate the Hirshfeld surface (HS) with HF/3-21G electrostatic potential mapped on it

Fingerprint Analysis​

  1. Generate the 'fingerprint plot'
    • Note the differences from acetic acid
    • Observe the absence of hydrogen bonding 'spikes'
    • Look for C--H···π 'wings'
tip

To see the C--H···π feature, filter the plot to show C inside and H outside, including reciprocal contacts

Electrostatic Analysis​

  1. Adjust the electrostatic potential surface:
    • Rescale to ±0.025 au range
    • Observe:
      • Electronegative regions above the ring (carbon π electrons)
      • Corresponding electropositive H atoms

Interaction Analysis​

  1. Examine the C--H···π interaction:
    • Use Generate External Fragment
    • Create molecular pairs using Clone Surface
    • Note the electrostatic complementarity between adjacent molecules

Energy Calculations​

  1. Calculate interaction energies:
    • Select a molecular pair
    • Use Calculate Energies
    • Select Energies from user-defined wavefunction
    • Expected result: -11.1 kJ/mol
    • Note the dominant role of dispersion energy

Energy Framework Analysis​

  1. For the cluster analysis:
    • Select a molecule and Calculate Energies
    • This creates a 3.8 Å cluster
    • Computes energies for 3 unique molecular pairs
    • Generate energy frameworks:
      • Select Display / Energy frameworks
      • Compare Coulomb (EeleE_\text{ele}), Dispersion (EdisE_\text{dis}), and Total (EtotE_\text{tot}) energies

Lattice Energy​

  1. Calculate lattice energy:
    • 3.8 Å cluster result: -47.2 kJ/mol
    • 8 Å cluster (15 pairs): -62.4 kJ/mol
    • Compare with experimental sublimation enthalpy (~45 kJ/mol)

Acetylene Analysis​

Energy Calculations​

  1. Repeat the analysis for ACETYL03:
    • All 12 nearest neighbor molecules: -3.3 kJ/mol each
    • Lattice energy estimates:
      • First shell: -19.8 kJ/mol
      • 8 Å cluster: -23.6 kJ/mol
note

This result is close to a converged lattice sum for this non-dipolar molecule