Development of a Spectroscopic Map to Explain the Broad Raman Peak for Alkynes Solvated in Triethylamine

Area of Science:

• Chemical Physics
• Spectroscopy
• Computational Chemistry

Background:

• Terminal alkynes possess a distinct C≡C stretch vibration with a large Raman scattering cross-section.
• This property is valuable for spectroscopic applications, particularly in the "silent" region for biomolecules.
• Developing theoretical tools is crucial for interpreting vibrational spectra of terminal alkynes.

Purpose of the Study:

• To interpret the unusually broad Raman spectrum of alkynes solvated in triethylamine.
• To develop theoretical tools for computing terminal alkyne vibrational frequencies.
• To establish a spectroscopic map for terminal alkynes in solution.

Main Methods:

• Localized normal mode method with discrete variable representation (DVR) for frequency computation.
• Molecular dynamics (MD) simulations to study alkyne-solvent interactions.
• Energy decomposition analysis (EDA) on alkyne-triethylamine dimers.

Main Results:

• Charge transfer, electrostatics, and Pauli repulsion significantly influence alkyne vibrational frequencies.
• MD simulations revealed strong, persistent interactions (3-10 ps) between the terminal alkyne hydrogen and triethylamine nitrogen.
• A spectroscopic map was developed, correlating spectral features with alkyne populations.

Conclusions:

• The broad experimental Raman spectra are attributed to a combination of interacting and non-interacting alkyne populations.
• This work provides a foundation for studying alkynes in complex systems like proteins and on nanomaterial surfaces.
• Theoretical and computational methods are effective for understanding vibrational spectroscopy of solvated molecules.