An Empirical IR Frequency Map for Ester C═O Stretching Vibrations.

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• Spectroscopic analysis of carbonyl groups is crucial for understanding molecular structure and dynamics.
• Accurate interpretation of infrared (IR) absorption spectra, particularly for ester carbonyls, requires sophisticated computational models.
• Molecular dynamics (MD) simulations provide insights into molecular behavior but require careful parametrization for spectroscopic predictions.

Purpose of the Study:

• To develop and validate a novel approach for parametrizing spectroscopic maps of carbonyl groups.
• To correlate electric fields from MD simulations with experimental IR absorption spectra, including vibrational frequencies and line shapes.
• To optimize this model for ester carbonyl stretching modes and assess its performance across diverse solvent environments.

Main Methods:

• An exhaustive search of parameter combinations was employed to optimize the model.
• The model was validated against experimental Fourier-transform infrared (FTIR) spectra of ethyl acetate in eight solvents of varying polarities.
• Special attention was given to hydrogen-bonding solvents, requiring independent peak fitting for different hydrogen bond ensembles.

Main Results:

• The optimized electrostatic map, when combined with MD simulations, accurately reproduced experimental C═O IR absorption spectra of ethyl acetate.
• The model achieved a line center Root Mean Square Deviation (RMSD) error of 4.9 cm⁻¹ across 12 solvents.
• The measured spectral line centers spanned a 45 cm⁻¹ range, demonstrating the model's broad applicability.

Conclusions:

• The developed spectroscopic mapping approach provides a reliable method for interpreting IR spectra of ester carbonyl groups.
• This method enhances the predictive power of MD simulations for spectroscopic properties.
• The approach is valuable for analyzing spectra of ester groups in complex and heterogeneous environments, such as biological lipid membranes.