Calculation of the Vibrational Stark Effect Using a First-Principles QM/MM Approach

Area of Science:

• Computational chemistry
• Molecular modeling
• Spectroscopy

Background:

• Accurately describing the electric environment in condensed phases is crucial for molecular force fields.
• The CHARMM additive force field's ability to model aqueous solutions needs validation.

Purpose of the Study:

• To test and validate the CHARMM force field's capability in describing the electric environment of aqueous solutions.
• To calculate the vibrational Stark effect (VSE) using combined quantum mechanics/molecular mechanics (QM/MM) methods.

Main Methods:

• Employed first-principles methodology with correlated electronic structure techniques.
• Calculated Stark shifts between gas and solvent phases, and between different solvent environments.
• Utilized three VSE probes with acetonitrile or fluorine functionalities.
• Incorporated anharmonic corrections for frequency calculations.

Main Results:

• Achieved reasonable agreement with experimentally determined Stark shifts when using the CHARMM force field.
• Demonstrated the necessity of anharmonic corrections for accurate frequency calculations.
• Computed the electric field along the CN bond for acetonitrile.
• Determined a theoretical Stark tuning rate for acetonitrile that aligns well with experimental values.

Conclusions:

• The CHARMM additive force field, with anharmonic corrections, provides a reasonable description of the electric environment in aqueous solutions for VSE calculations.
• The QM/MM approach is effective for calculating VSE and Stark tuning rates, showing good agreement with experimental data.