how can I distiguish between pi-pi* or n-pi* while doing a TDDFT calculus on gaussian?

Question

Hi I have a system that belongs to the C1 point group which is polycyclic and contains heteroatomes ,when calculating the excited states by TDDFT and after visualising the MOs that are envolved on transitions . How can I distinguish between pi-pi* and n-pi* transition.

Answer 1

There are a few methods, that are equivalent as long as the excited state is either an almost pure pi-pi* state or an almost pure n-pi* state (i.e. no appreciable mixings between these two types of transitions).

1. Plot the difference density between the desired excited state and the ground state. If there is a density depletion in a pi orbital and a density accumulation in a pi* orbital, then it is a pi-pi* state; if the density depletion is in an n orbital instead, then it is an n-pi* state. Pro: applicable even if the state is a heavy mixture of many transitions. Con: some visualization programs do not support this natively.
2. Plot the natural transition orbitals (NTOs) of the desired excited state. If most of the transitions are from pi orbitals, then it is a pi-pi* state; if they are mainly from n orbitals instead, then it is an n-pi* state. Pro: any software that can visualize orbitals can be used, provided that the NTOs are already generated. Con: one must first generate the NTOs, and depending on the computational chemistry software, if one forgot to request for NTOs when writing the input file, one may need to run the calculation again (with the correct keyword(s) added) to get the NTOs.
3. Plot the frontier orbitals to see their character (n, pi, pi* etc.), and assign the excited state based on the canonical MO composition of the state (that are normally printed in the output file of the TDDFT calculation). Pro: any software that can visualize orbitals can be used, and there is no need to perform additional calculations such as the generation of NTOs. Con: when the state is a heavy mixture of many transitions, the amount of work required by this approach is higher than the previous two approaches.