Conformational study of eugenol by density functional theory method and matrix-isolation infrared spectroscopy

Area of Science:

• Computational chemistry
• Molecular spectroscopy
• Physical organic chemistry

Background:

• Eugenol (4-allyl-2-methoxyphenol) is a widely used compound with potential for various applications.
• Understanding its conformational landscape is crucial for predicting its behavior and properties.
• Previous studies have not fully characterized all possible conformers of eugenol.

Purpose of the Study:

• To computationally identify all possible conformers of eugenol.
• To experimentally characterize the dominant conformers using FTIR spectroscopy.
• To investigate conformational dynamics and stability in low-temperature matrices.

Main Methods:

• Density Functional Theory (DFT) calculations using the B3LYP/6-311++G(2d,2p) level of theory.
• Fourier Transform Infrared (FTIR) spectroscopy of eugenol isolated in solid argon and xenon matrices.
• Analysis of potential energy surfaces to determine conformer stability and interconversion barriers.

Main Results:

• Twelve conformers of eugenol were identified, grouped into syn-anti (SA), antianti (AA), and antigauche (AG) types.
• The three lowest-energy SA conformers, stabilized by intramolecular hydrogen bonds, constitute 99.8% of the population.
• FTIR spectroscopy unequivocally identified three SA conformers in solid argon and xenon matrices.
• Evidence of conformational cooling was observed, with less stable SA conformers converting to the most stable SAA+ form.

Conclusions:

• The syn-anti conformers are the dominant and most stable forms of eugenol under the studied conditions.
• Experimental FTIR data confirm the theoretical predictions regarding eugenol's conformational preferences.
• Low energy barriers allow for interconversion between SA conformers, leading to the prevalence of the most stable form at higher deposition temperatures.