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The question is quite simple — how to interpret NBO results of second order perturbation theory analysis? As I have read several papers the most important interactions are $\pi\rightarrow\pi^{*}$, $LP\rightarrow\sigma^{*}$ and $LP\rightarrow\pi^{*}$. However, how to check whether they corresponds to resonance or hyperconjugation effect?

In the paper of Milenković et al. (10.20450/mjcce.2019.1333), for structure below: enter image description here

A following data of NBO-analysis was gathered: enter image description here

The authors claim that:

The most important interaction (n-π*) energy, related to resonance in the molecule, is electron donation from the LP2(O) atom orbitals to the antibonding acceptor π*(C–C) of the phenyl ring (LP2(O5) → π*(C5–C6) (159.2 kJ mol–1)). This large interaction energy indicates hyperconjugation between the electron-donating oxygen atom and the phenyl ring.

Later similarly:

Besides LP2(O) → π*(C–C) and LP2(O) → σ*(C–C) interactions, strong intra- molecular hyperconjugative interactions are formed by orbital overlap between π(C–C) → π*(C–C) bond orbitals, resulting in intramolecular charge transfer (ICT), which causes stabilization of the system.

The problem for me is how to distinguish hyperconjugation from resonance? I was sure that, quoting wiki –

... hyperconjugation is the interaction of the electrons in a sigma (σ) orbital (e.g. C–H or C–C) with an adjacent unpopulated non-bonding p or antibonding σ* or π* orbitals to give a pair of extended molecular orbitals. Thus, why would LP2(O) → π*(C–C) be hyperconjugation if it is rather resonance?


I have also found somewhere about Intramolecular Charge Transfer. Could someone briefly explain how it can be understood and found in this analysis?

Kind regards

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