Evaluating aliphatic CF, CF2, and CF3 groups as vibrational Stark effect reporters

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• Fluorination is crucial in molecular design, impacting solvation and electrostatic interactions.
• The carbon-fluorine (C-F) bond's behavior in solution is key to understanding these effects.
• Vibrational spectroscopy offers direct insights into molecular electrostatics via the vibrational Stark effect (VSE).

Purpose of the Study:

• To investigate the VSE of different aliphatic fluorination patterns (mono-, di-, trifluorination).
• To compare the VSE response of aliphatic C-F bonds with aromatic ones.
• To determine the electric field sensitivity and orientation of C-F bonds in various fluorinated groups.

Main Methods:

• Utilized density functional theory (DFT) for electronic structure calculations.
• Employed molecular dynamics (MD)-assisted solvatochromic analysis in solvents of varying polarity.
• Determined Stark tuning rates and difference dipole vector orientations for v(C-F) normal modes.

Main Results:

• Obtained Stark tuning rates ranging from 0.2-0.8 cm⁻¹/(MV/cm).
• Found CFaliphatic groups exhibit the lowest electric field sensitivity, while CF3aliphatic groups show the highest.
• Observed that Stark tuning rate vectors can be significantly tilted relative to the molecular symmetry axis, enabling multi-dimensional electrostatic mapping.

Conclusions:

• Aliphatic fluorination patterns show distinct responses to electric fields, with varying sensitivities.
• The orientation of the Stark tuning vector provides a nuanced view of molecular electrostatics beyond the static dipole.
• Accurate modeling of fluorinated molecules necessitates multipolar and/or polarizable molecular dynamics force fields.