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Hydrogen Bonding in Melamine Crystal

Melamine powder (chemical formula: C₃N₆H₆), Its unique physical properties, solubility and solid-state stability are fundamentally determined by its crystal structure and intermolecular interactions, especially hydrogen bonds.

Hydrogen bonding is one of the most critical weak intermolecular forces for organic crystals. Melamine molecules carry multiple amino groups and ring nitrogen atoms, providing abundant sites for intermolecular hydrogen bonding. This article presents single-crystal X-ray diffraction data for melamine and analyzes crystal parameters, bond lengths, bond angles, hydrogen-bond configurations, and π-π stacking effects, and explains how these interactions construct and stabilize the 2D layered crystal structure of melamine.

Experimental Materials and Testing Methods

Instruments and Reagents

The experiment adopted standard analytical-grade melamine as the test sample. Core testing equipment included a Bruker SMART 1000 CCD X-ray single-crystal diffractometer, a Fourier-transform infrared spectrometer, and an elemental analyzer.

Melamine Crystal Preparation

Melamine was dissolved in hot water, mixed with an auxiliary solution, heated and stirred evenly. The mixed solution was cooled naturally and left to evaporate slowly at room temperature. After several days, colorless block-shaped melamine single crystals suitable for structural testing were obtained.
Elemental analysis verified the crystal composition:
  • Calculated value: C 29.00%, H 5.00%, N 67.00%
  • Tested value: C 28.88%, H 5.04%, N 66.78%
The data confirms the pure crystal is composed of single melamine molecules (C₃N₆H₆).

Single-Crystal X-Ray Diffraction Test

Regular, transparent melamine crystals measuring 0.40 mm × 0.30 mm × 0.20 mm were selected for testing. Using Mo-Kα radiation (λ=0.71073 Å) at room temperature (293 K), a total of 2756 diffraction points and 1103 independent diffraction points were collected with the ω-2θ scanning mode. The crystal structure was solved by the direct method and refined by full-matrix least-squares.

Basic Crystal Parameters of Melamine

X-ray diffraction results prove that melamine belongs to the monoclinic crystal system with the P2(1)/c space group. The complete crystal cell parameters are listed below:
  • Lattice parameters: a=7.267(3) Å, b=7.464(3) Å, c=10.567(3) Å
  • Angles: α=90.0°, β=112.26(2)°, γ=90.0°
  • Unit cell volume: V=530.4(3) ų
  • Molecules per unit cell: Z=4
  • Linear absorption coefficient: μ=0.118 mm⁻¹
  • Calculated density: Dc=1.579 g/cm³
  • Reliability factors: R=0.0618, ωR₂=0.1564
The low reliability factors indicate high accuracy of the tested crystal structure data.

Molecular Structure: Bond Lengths and Bond Angles

Melamine is generally a planar molecule with slight structural distortion compared with an ideal aromatic ring. The triazine ring features delocalized electrons, which are reflected in its C-N bond characteristics.

Main Bond Lengths

The C-N bonds on the triazine ring range from 1.335 Å to 1.347 Å. Compared with standard single C-N bonds (1.474 Å) and double C=N bonds (1.265 Å), the bond lengths are between single and double bonds, proving obvious electron delocalization across the entire triazine ring. Typical bond lengths are as follows:
  • N(1)-C(1): 1.336 Å
  • N(2)-C(2): 1.338 Å
  • N(3)-C(3): 1.347 Å
  • N(4)-C(1): 1.351 Å

Main Bond Angles

The internal bond angles of the melamine molecule are concentrated between 114° and 126°. These angle distributions maintain the basic planar configuration of the triazine ring and coordinate the spatial orientation of the amino groups, creating favorable conditions for subsequent intermolecular hydrogen-bond formation. Representative bond angles include:
  • C(1)-N(1)-C(2): 114.11°
  • N(1)-C(1)-N(3): 125.92°
  • N(2)-C(2)-N(1): 125.72°

N-H…N Hydrogen Bonding in Melamine Crystal

As the dominant intermolecular force in melamine crystals, N-H…N hydrogen bonds connect adjacent molecules and build a stable overall structure. All hydrogen bonds involve amino N-H groups as donors and nitrogen atoms on triazine rings or amino groups as acceptors.

Detailed Hydrogen Bond Parameters

Five typical N-H…N hydrogen bonds were identified, with complete geometric data shown below:
D-H … Ad(D-H) / Åd(H…A) / Åd(D…A) / Å∠D-H…A / °
N(4)-H(4A)…N(3)0.862.363.069139.6
N(4)-H(4B)…N(4)0.862.633.411152.3
N(5)-H(5B)…N(1)0.862.183.012162.3
N(6)-H(6A)…N(3)0.862.263.019171.5
N(6)-H(6B)…N(2)0.862.293.054148.6

Classification and Characteristics of Hydrogen Bonds

  1. Weakest hydrogen bond: N(4)-H(4B)…N(4)
    With a D…A distance of 3.411 Å, this is the weakest interaction in the crystal, mainly caused by molecular steric hindrance. Its acceptor is the nitrogen atom from the amino group of an adjacent melamine molecule.
  2. Main stable hydrogen bonds: N-H…N (triazine ring nitrogen)
    Most hydrogen bonds use nitrogen atoms on the triazine ring as acceptors. They have shorter H…A distance and larger bond angles, offering a stronger bonding force. These are the core forces to connect melamine molecules.

2D Layered Structure and π-π Stacking

Construction of 2D Layers

Each melamine molecule forms multiple hydrogen bonds with surrounding molecules. Through cross-linked N-H…N hydrogen bonds, molecules are connected end to end to form a continuous two-dimensional layered structure. Hydrogen bonds act as bridges, stabilizing the relative positions of molecules within the layer.

π-π Stacking Interaction

Along the crystallographic a-axis, adjacent molecular planes are parallel to each other with an average interlayer distance of approximately 3.4 Å. This distance indicates obvious π-π stacking interactions between the triazine rings of upper and lower layers.

Combined effects of intra-layer hydrogen bonds and inter-layer π-π stacking jointly maintain the long-range-ordered crystal structure of melamine, thereby greatly improving the mechanical stability and structural rigidity of the crystal.

conclusion

  1. Melamine crystal belongs to the monoclinic system (P2(1)/c space group), with fully determined lattice parameters obtained from X-ray single-crystal diffraction. The molecule is nearly planar, and the triazine ring has significant electron delocalization.
  2. A large number of N-H…N intermolecular hydrogen bonds exist in the crystal. These hydrogen bonds vary in strength and are the primary force for assembling melamine molecules into 2D layers.
  3. π-π stacking between parallel molecular layers further consolidates the crystal structure. Hydrogen bonding and π-π stacking work synergistically to form the stable layered crystal morphology of melamine.
The study of melamine’s crystal structure and intermolecular forces explains its inherent physical properties such as crystallinity, solubility and solid-state stability. It also provides theoretical support for the design of melamine-based supramolecular compounds, melamine salt materials and crystal modification research.

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