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Revisiting the Maximum Hardness Principle: A Quantitative Analysis on Reaction Datasets.

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The maximum hardness principle (MHP) generally guides chemical reactions toward greater stability, though its validity depends on calculation methods and reaction type. Approximately 80% of tested cycloaddition reactions and unimolecular reactions follow MHP.

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Area of Science:

  • Theoretical Chemistry
  • Chemical Reactivity Theory
  • Conceptual Density Functional Theory (CDFT)

Background:

  • Chemical hardness is a key concept in understanding chemical reactivity.
  • The maximum hardness principle (MHP) suggests reactions proceed towards states of maximum hardness.
  • Previous studies have raised questions about the universal applicability of MHP.

Purpose of the Study:

  • To quantitatively assess the applicability of the MHP.
  • To investigate the influence of different computational approximations on MHP validity.
  • To analyze MHP's reliability across diverse reaction types.

Main Methods:

  • Utilized two reaction datasets: 5269 dipolar cycloadditions and 449 BH9 reactions.
  • Employed Kohn-Sham orbital based frontier molecular orbital (FMO) and finite difference approximation (FDA) for hardness calculations.
  • Examined the impact of various definitions for average hardness in bimolecular reactions.

Main Results:

  • Hardness values are sensitive to the level of theory, definitions, and approximations used.
  • Approximately 80% of the 5269 cycloaddition reactions adhere to the MHP.
  • MHP is better supported for unimolecular reactions in the BH9 dataset; its validity in bimolecular reactions depends heavily on the hardness definition.

Conclusions:

  • The validity of MHP is influenced by computational methodology and reaction characteristics.
  • Reactions with significantly different reactant hardness values are less likely to obey MHP.
  • As a qualitative principle, MHP's universality is not expected, emphasizing the need for careful interpretation.