Explaining the Broad Raman Peak for Alkynes Solvated in Triethylamine

The terminal alkyne C≡C stretch has a large Raman scattering cross section in the “silent” region for biomolecules. Experimental work taking advantage of this property provide an impetus for the development of theoretical tools addressing the vibration. In prior work, we have developed a localized normal mode method for computing terminal alkyne vibrational frequencies using a discrete variable representation of the potential energy surface. Using this method and molecular dynamics simulations, we interpret the unusually broad Raman spectrum of alkynes solvated in triethylamine. Energy decomposition analysis is performed on alkyne-triethylamine dimers to determine that like charge transfer, electrostatics, and Pauli exclusion have large effects on the frequency. Molecular dynamics simulations are performed and uncover that the terminal alkyne hydrogen interacts strongly with the triethylamine nitrogen when the alkynes are solvated in triethylamine. Using this data, a spectroscopic map for terminal alkynes in triethylamine is developed and used to compute Raman spectra. We find that the broad experimental spectra result from the combination of a population of alkynes associated with the solvent nitrogens and a population not associated with those nitrogens. This work sets the stage for investigations of alkynes in more complex environments.