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The following text is from Solomons, Fryhle and Snyder Organic Chemistry Third Edition, chapter 1 "The Basics: Bonding and Molecular Structure", page 13, topic 1.5 "Resonance Theory", sub topic 1.5B "Schematic Energy Level Diagrams of Some Mesomeric Molecules/Ions":

Energy level diagrams of some resonating structures are given as follows. (Note: The downward arrow marks the conventional mesomeric energies and indicates energy level).

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It is given that the downward arrows in examples 1, 2, 3, and 4 mark the conventional mesomeric energies. What is meant by "conventional mesomeric energy"? I searched for this word on Google as well as on this site, but I couldn't find any relevant information.

What is the purpose of the downward arrows in the above energy level diagrams? Is that to depict the structure with least energy or the one which is the most stable? I thought this was the reason until I saw example 5. The arrow goes from the structure which has the highest stability to the one which has the least stability. Is there any specific reason for this upward arrow or is it just a misprint? I was unable to see any other energy level diagrams with these arrows, so is this an outdated convention followed by the book?

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    $\begingroup$ The downward arrow has no meaning. it is just a way of showing the name of the preferred structure. And "conventional" mesomeric energy means "qualitatively determined" $\endgroup$ – Maurice Jan 15 at 11:27
  • $\begingroup$ @Maurice: Thank you for your comment. Could you tell, what does the upward arrow in the last example mean? $\endgroup$ – Guru Vishnu Jan 15 at 11:31
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    $\begingroup$ The upward arrow is drawn in the figure 5 just to explain that energy is needed to pass from the pure covalent molecule to the excited dipole. The same arrow could have been drawn in the figures 3 and 4. $\endgroup$ – Maurice Jan 15 at 16:11
  • $\begingroup$ @Maurice: Could you transform your comments to an answer? I'm satisfied with your explanation and I could mark the answer as accepted. Thank you. $\endgroup$ – Guru Vishnu Feb 4 at 14:02

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