Tech Blog

Melamine Under External Electric Fields: Physical Properties & Infrared Spectrum Characteristics

Melamine powder (C3H6N6) is an industrial raw material. Its illegal addition to food has caused severe health crises, making research on its molecular properties crucial for detection and degradation.

Research Methodology & Experimental Basis

Computational Approach

Theoretical Framework: Density Functional Theory (DFT) with the B3PW91/6-31+(d,p) basis set—validated to yield results closest to experimental values (average error = 0.00324 Å for bond lengths).

External Electric Field: Applied along the N-C bond direction (1C-7N), with intensity ranging from 0 to 0.010 a.u. (1 a.u. = 5.14225×10¹¹ V·m⁻¹).

Calculated Indicators: Bond length, total energy, dipole moment, HOMO-LUMO energy gap, infrared (IR) spectrum, and N-C bond dissociation potential energy surface (PES).

Model & Validation

Molecular Model: Optimized ground-state structure of melamine (Figure 1) constructed via Gaussian 09W and GaussView 5.0.

Validation: Bond length calculations at B3PW91/6-31+(d,p) showed minimal deviation from experimental data, confirming computational reliability.

Key Effects of External Electric Fields on Melamine

Changes in Physical Properties

Bond Length Variation

External electric fields induce differential changes in melamine’s bond lengths:

N-H Bond: Gradually shortens (from 1.00529 Å to 1.00377 Å at 0.010 a.u.), increasing bond stability.
N-C Bonds: Show opposite trends—
N7−C1 bond lengthens (from 1.35316 Å to 1.37131 Å), weakening bond energy and facilitating cleavage;
N5−C1 bond shortens (from 1.34135 Å to 1.33478 Å), enhancing stability.

Total Energy & Dipole Moment

Total Energy: Decreases linearly with increasing electric field—from -446.3683 Hartree (0 a.u.) to -446.3743 Hartree (0.010 a.u.), indicating enhanced molecular stability in stronger fields.

Dipole Moment: Monotonically increases from 0 Debye (non-polar molecule) to 3.0305 Debye (0.010 a.u.), as external fields induce charge separation, strengthening molecular polarity.

0	-446.36830301	1.00529	1.35316	0.0000
0.002	-446.36853299	1.00481	1.35675	0.6084
0.004	-446.36924999	1.00447	1.36026	1.2147
0.006	-446.37044323	1.00417	1.36382	1.8197
0.008	-446.37211312	1.00396	1.36753	2.4253
0.010	-446.37425803	1.00377	1.37131	3.0305

Modulation of Orbital Energy Levels

HOMO-LUMO Gap: Narrows significantly with increasing electric field—from 6.84 eV (0 a.u.) to 5.52 eV (0.010 a.u.) (Table 2).

Orbital Energy Changes:
LUMO energy decreases (enhancing electron-accepting ability), with a faster rate at higher fields.
HOMO energy increases (weakening electron-binding ability), leading to reduced energy gap and higher chemical reactivity—facilitating molecular excitation and chemical reactions.

0	-0.24267	0.00889	6.84
0.004	-0.24174	-0.00320	6.49
0.008	-0.24094	-0.02529	5.85
0.010	-0.23998	-0.03701	5.52

Infrared Spectrum Redshift

External electric fields induce a significant redshift in melamine’s IR spectrum:

Key Vibrations: The NH symmetric stretch (NH sym-str) and the NH asymmetric stretch (NH asym-str) exhibit decreasing frequencies with increasing field strength.

Mechanism: Field-induced charge redistribution weakens NH bond vibration energy, shifting absorption peaks to lower wavenumbers (redshift) without altering IR intensity—providing a theoretical basis for electric field-assisted IR detection of melamine.

Reduced Dissociation Barrier

N-C Bond Cleavage: The potential energy surface (PES) of N-C bond dissociation gradually loses its bound state as the electric field increases.

Dissociation Tendency: The N7−C1 bond (which lengthens with field strength) exhibits a reduced dissociation barrier, making it more prone to cleavage—offering insights for electric field-assisted melamine degradation.

Practical Implications & Applications

Enhanced Detection Technology

The field-induced IR redshift and dipole moment changes can optimize melamine detection methods (e.g., IR spectroscopy), improving sensitivity and selectivity for trace melamine in food or environmental samples.

Targeted Degradation Strategies

The weakened N7−C1 bond and reduced dissociation barrier suggest potential for electric field-assisted melamine degradation—guiding the development of efficient, low-cost pollution control technologies.

Computational Chemistry Reference

The DFT-based methodology and systematic data provide a framework for studying other toxic molecules under external fields, advancing environmental chemistry research.

conclusion-Melamine Under External Electric Fields

Melamine Under External Electric Fields (0~0.010 a.u.) exert profound, predictable effects on melamine’s molecular properties: differential bond length changes, reduced total energy, enhanced polarity, narrowed energy gap, IR redshift, and easier N-C bond cleavage. These findings bridge computational chemistry and practical applications, providing critical theoretical support for melamine detection optimization and degradation technology development.

As concerns about melamine pollution persist, this research opens new avenues for environmental monitoring and pollution control—proving that external electric fields can be a powerful tool for manipulating toxic molecular behavior.