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Exercise 5: Formaldehyde Acetylene

A Simple Cocrystal​

Initial Setup​

  1. Open the GURNEN CIF )
  2. Complete all molecules
  3. Generate Hirshfeld surfaces with HF/3-21G electrostatic potential

Surface Analysis​

  1. Study electrostatic features:
    • Rescale surfaces to ±0.025 au range
    • Observe:
      • Dipolar nature of formaldehyde
      • Quadrupolar acetylene surface

Structure Extension​

  1. Build molecular arrangement:
    • Create adjacent molecules using Generate External Fragment
    • Generate surfaces using Clone Surface
Crystal Engineering

The pattern of electrostatic complementarity supports the original paper's observation that "the packing of formaldehyde with acetylene is such as would be expected by most experts based on crystal engineering principles" [M.T. Kirchner et al., Chem. Eur. J., 16, 2131 (2010)]

Energy Analysis​

  1. Calculate interaction energies:
    • Return to original molecules (Reset Crystal)
    • Generate 3.8 Å clusters
    • Key interactions:
      • C--H···O hydrogen bond: -10.3 kJ/mol
      • Formaldehyde dipole-dipole: -4.7 kJ/mol
      • Displaced acetylene pairs: -4.7 kJ/mol

Energy Framework Analysis​

  1. Examine energy distribution:
    • Generate unit cell molecules
    • Create energy frameworks
    • Use 4 kJ/mol cutoff
    • Observe:
      • Zig-zag network of C--H···O interactions in layers
      • Weaker interlayer connections