An Accurate and Universal Method for the Anharmonic Vibrational Analysis of the Raman Active C≡C Stretch of Terminal Alkynes

The terminal alkyne C≡C stretch has a large Raman scattering cross section in the “silent” region for biomolecules. This has led to many Raman tag and probe studies using molecules with this moiety. Computational investigation of these systems is vital to aid in the interpretation of the results. In this work, we develop a localized normal mode discrete variable representation (DVR) method for computing terminal alkyne vibrational frequencies and transition isotropic polarizabilities which can easily and accurately be applied to any terminal alkyne molecule. The errors of localization to the terminal alkyne moiety, anharmonic normal mode isolation, and discretization of the Born-Oppenheimer potential energy surface are quantified and found to oppose each other. This results in a method with low error compared to other anharmonic vibrational methods like VPT2 and experiment. Several density functionals are tested using the method, and TPSS-D3 is found to perform surprisingly well. Additionally, diffuse functions are found to be important for the accuracy of computed frequencies. Finally, the computation of vibrational properties like transition isotropic polarizabilities and the universality of the normal mode atomic displacements across molecules are demonstrated.