The A value of the hypothetical ligand X is the absolute value of standard free energy change of the following equilibrium:
The A value of a ligand is a measure of steric strain it causes when interacting with a given ligand (the higher the A value, the more severe the steric strain), which in turn is roughly a measure of the size of the ligand (the more severe the steric strain, the larger the ligand).
eg:
The A value of the tert-butyl group is larger than that of the isopropyl group, implying that the former is bulkier than the later.
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