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Combined experimental and DFT investigation of the structural, spectroscopic and electronic properties of 2-hydroxy-5-(naphthalen-1-yldiazenyl)benzaldehyde

This study combines experimental spectroscopy (FTIR, ¹H NMR, UV-Vis) and DFT calculations to characterize the structural, electronic, and spectroscopic properties of 2-hydroxy-5-(naphthalen-1-yldiazenyl)benzaldehyde, revealing its hydrogen bonding, solvent-dependent absorption shifts, and potential as a metal-coordinating ligand.

Original authors: Tahir Javadzade, Rustam Rustamov

Published 2026-08-27

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Tahir Javadzade, Rustam Rustamov

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Molecules are not just static collections of atoms; they are dynamic structures with distinct shapes, electrical personalities, and the ability to interact with light. Among the vast family of organic compounds, a specific group known as azo dyes stands out for their vivid colors and their ability to absorb light energy. These molecules act like tiny antennas, capturing light and converting it into electronic excitement. Their behavior is governed by how their atoms are arranged and how electrons move through their long, connected frameworks. Scientists are deeply interested in these properties because they determine how a molecule might be used, whether as a dye, a sensor, or a component in new electronic materials. To understand these complex behaviors, researchers often combine two powerful approaches: direct observation through experiments and detailed simulation through computer modeling. This dual strategy allows them to see not only what a molecule looks like in a test tube but also to map out the invisible forces and electron movements that dictate its character.

In a recent study, researchers Tahir Javadzade and Rustam Rustamov from Khazar University applied this combined approach to a specific molecule called 2-hydroxy-5-(naphthalen-1-yldiazenyl)benzaldehyde. This compound is a complex structure featuring a naphthalene ring, an azo bridge, and a benzaldehyde group, all linked together. The team wanted to understand exactly how this molecule is built, how it vibrates, how it interacts with light, and how its electrons are distributed. To do this, they first synthesized the molecule in the laboratory and then subjected it to a battery of tests. They used infrared spectroscopy to listen to the molecule's vibrations, nuclear magnetic resonance to examine the magnetic environment of its hydrogen atoms, and ultraviolet-visible spectroscopy to see how it absorbs light. Parallel to these physical experiments, they used a sophisticated computer method called density functional theory to build a virtual model of the molecule. This digital twin allowed them to calculate the precise arrangement of atoms, the energy of its electrons, and how it would theoretically respond to light and electric fields.

The physical and virtual investigations revealed a molecule with a very specific and stable shape. The computer model showed that the molecule lies almost flat, with its various ring systems and bridges forming a continuous, planar surface. This flatness is crucial because it allows electrons to flow freely across the entire structure, creating a highway for electrical charge. A key feature of this stability is a hidden handshake within the molecule itself: a hydrogen atom from a hydroxyl group reaches across to touch an oxygen atom on a nearby aldehyde group. This internal connection locks the molecule into a rigid shape, preventing it from flopping around. The experimental data supported this picture perfectly. The infrared spectrum, which acts like a fingerprint of molecular vibrations, matched the computer's predictions, confirming that the molecule exists in this specific, locked conformation. Furthermore, the magnetic resonance data showed that the hydrogen involved in this internal connection was highly deshielded, a tell-tale sign that it was indeed engaged in a strong interaction with its neighbor.

When the researchers turned their attention to how the molecule interacts with light, they found a fascinating discrepancy between the real world and the computer simulation. In the laboratory, the molecule absorbed light most strongly at a wavelength of 500 nanometers, which corresponds to a deep orange-red color. However, the computer model, which simulated the molecule in isolation, predicted that it would absorb light at a shorter wavelength of 430 nanometers, a color closer to blue-violet. The researchers explained that this difference arises because the real-world experiment was conducted in a liquid solvent, acetone. The solvent molecules surround the excited molecule and stabilize it, effectively stretching the energy gap and shifting the color toward the red end of the spectrum. Despite this shift, both the experiment and the simulation agreed on the fundamental nature of the event: the molecule undergoes a massive internal shift of electrons, moving charge from one end of the structure to the other when it absorbs light. This confirms that the molecule functions as a donor-acceptor system, where one part pushes electrons and another part pulls them.

The study also mapped out the electrical landscape of the molecule to see where it would be most likely to grab onto other atoms or ions. The analysis showed that the oxygen atoms, particularly the one in the carbonyl group and the one in the hydroxyl group, are rich in negative electrical potential. These areas act like magnets for positively charged species, suggesting that the molecule could easily bind to metal ions. This finding is significant for potential applications in coordination chemistry, where such molecules might serve as ligands to hold metal centers in place. The researchers calculated the energy difference between the molecule's highest occupied electron level and its lowest unoccupied level to be 3.26 electron volts. This energy gap is a measure of how easily the molecule can be electronically excited, and it aligns with the molecule's ability to absorb visible light. The study concluded that the combination of experimental data and computer modeling provided a complete and consistent picture of the molecule's structure and behavior, validating the use of these theoretical tools to predict the properties of complex organic dyes.

Technical Summary: Combined Experimental and DFT Investigation of 2-Hydroxy-5-(naphthalen-1-yldiazenyl)benzaldehyde

Problem Statement
While the synthesis and basic structural data of 2-hydroxy-5-(naphthalen-1-yldiazenyl)benzaldehyde have been previously reported, a comprehensive analysis correlating its experimental spectroscopic data with detailed quantum-chemical computations was lacking. Azo compounds with hydroxyl and aldehyde substituents exhibit complex electronic behaviors influenced by intramolecular interactions, such as hydrogen bonding and $\pi$ -electron delocalization. Understanding the precise relationship between the molecule's structural geometry, vibrational properties, proton environments, and electronic transitions is essential for interpreting its spectroscopic performance and potential applications in coordination and analytical chemistry.

Methodology
The study employed a dual approach combining experimental spectroscopy with Density Functional Theory (DFT) calculations:

Experimental: The compound was synthesized via an azo coupling reaction between 1-naphthylamine and salicylaldehyde. Structural and electronic properties were characterized using:
- FTIR Spectroscopy: Recorded in the 4000–400 cm⁻¹ range using KBr pellets.
- UV-Vis Spectroscopy: Measured in acetone solution.
- ¹H NMR: Data was compared against previously reported literature values for the compound in CDCl₃.
Computational: Theoretical investigations were conducted using the Gaussian 09 software package with the following parameters:
- Level of Theory: B3LYP hybrid functional with the 6-311++G(d,p) basis set.
- Geometry Optimization: Performed without symmetry constraints to identify the local minimum on the potential energy surface.
- Vibrational Analysis: Harmonic frequency calculations were used to assign FTIR bands.
- NMR Shielding: Calculated using the Gauge-Independent Atomic Orbital (GIAO) method with the SMD implicit solvent model for chloroform.
- Electronic Excitation: Time-dependent DFT (TD-DFT) was used to simulate UV-Vis spectra and analyze excitation energies.
- Electronic Structure: Frontier Molecular Orbital (FMO) analysis, Molecular Electrostatic Potential (MEP) mapping, and global reactivity descriptors were computed to characterize electronic properties.

Key Contributions and Results

Structural Geometry and Intramolecular Interactions:
The optimized geometry revealed an almost planar conjugated system comprising the naphthalene moiety, azo bridge, and substituted benzene ring. A critical finding was the confirmation of an intramolecular O–H···O=C hydrogen bond between the phenolic hydroxyl and the aldehyde carbonyl oxygen. This interaction forms a six-membered pseudo-ring, resulting in a rigid molecular conformation with an O–H···O angle of 141.64° and an H···O distance of 1.876 Å.
FTIR Spectroscopic Correlation:
There was significant agreement between the calculated harmonic frequencies and experimental FTIR data.
- The experimental band at 3047 cm⁻¹ was assigned to aromatic C–H stretching.
- The C=O stretching vibration was observed at a lower frequency experimentally than theoretically, a discrepancy attributed to the intramolecular hydrogen bond weakening the carbonyl bond.
- Bands in the 1575–1536 cm⁻¹ range were identified as coupled vibrations of the azo (N=N), aromatic (C=C), and C–N frameworks, rather than isolated modes.
- Low-frequency bands (653–878 cm⁻¹) were successfully assigned to out-of-plane aromatic C–H bending.
¹H NMR Analysis:
GIAO-calculated shielding constants qualitatively reproduced the experimental chemical shift trends. The phenolic hydroxyl proton showed the highest deshielding (experimental $\delta$ 11.37 ppm; calculated shielding 21.34 ppm), consistent with the strong intramolecular hydrogen bond. The aldehydic proton was the second most deshielded ( $\delta$ 10.11 ppm). The aromatic region displayed complex splitting patterns due to non-equivalent proton environments, which the theoretical model successfully differentiated.
UV-Vis and Electronic Transitions:
- Experimental: A prominent absorption maximum was observed at 500 nm (2.48 eV) in acetone.
- Theoretical: TD-DFT predicted an intense transition at 430 nm (2.88 eV), representing a blue shift of approximately 70 nm relative to the experiment.
- Interpretation: The authors attribute this bathochromic shift (red shift) in the experimental data primarily to solvent effects stabilizing the charge-transfer excited state, which the gas-phase/simplified solvent model calculations did not fully capture. Both methods confirm a strong low-energy transition driven by the extended $\pi$ -conjugation.
Frontier Molecular Orbitals (FMO) and Reactivity:
- HOMO-LUMO Gap: The energy gap was calculated at 3.26 eV (HOMO: -6.10 eV; LUMO: -2.84 eV).
- Orbital Distribution: The HOMO is delocalized over the naphthalene-azo-phenolic framework, while the LUMO shows increased contribution from the azo linkage and the electron-withdrawing aldehyde-substituted ring. This distribution supports a donor- $\pi$ -acceptor mechanism involving intramolecular charge transfer.
- Reactivity Descriptors: The molecule exhibits an electrophilicity index of 6.12 eV and a global hardness of 1.63 eV, indicating a capacity to accept electronic charge, consistent with its electron-deficient aldehyde and azo regions.
Molecular Electrostatic Potential (MEP):
MEP mapping identified the carbonyl oxygen and phenolic oxygen as the most electron-rich (negative potential) regions, marking them as likely sites for metal coordination or interaction with electrophilic species. Conversely, the phenolic hydrogen and peripheral aromatic hydrogens displayed positive potential.

Significance and Claims
The paper asserts that the combined experimental and theoretical approach provides a reliable framework for interpreting the structural and spectroscopic attributes of this hydroxyazo molecule. The study successfully validates the computational model's ability to reproduce experimental trends in vibrational frequencies, proton shielding, and electronic absorption characteristics, despite inherent limitations in reproducing exact experimental values due to solvent and anharmonicity effects.

The authors conclude that the molecule possesses substantial $\pi$ -electron delocalization and distinct electron-rich oxygen centers, establishing a valuable platform for its future utilization in coordination chemistry and analytical applications. The work emphasizes that theoretical calculations are most effective for interpreting experimental trends and assigning spectral features rather than merely reproducing exact numerical values.

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