I have ftir from gaussian output file.where ir aand raman are given. However in many research articles value of ir and raman are classified into two portion. Please anyone knows suggest me to how to perform tasks like that
What Gaussian has calculated for you are the vibrational frequencies. From their symmetries, it is possible to analyze whether they will be IR or Raman active by means of group theory: the direct product of the ground state wave function, the operator (dipole or polarizability) and the excited state wavefunction.
When it comes to absolute intensities, the biggest players come from the selection rules:
- for IR, it is the change of dipole for a vibrational mode
- for Raman, it is the change of polarizability
The full equations would be longer, but I a not aware of a software that e.g. evaluates the full Kramers-Heisenberg dispersion formula for Raman spectra. However, some tools like SNF evaluate higher order polarizability contributions, so the most basic one is an electric dipole inducing another electric dipole, but you can also include magnetic or higher electric multipole moments (see the works of Zuber and Hug on Raman optical activity).
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