Computational chemistry
Hammett σ from composition predicts SN2 barriers
Open access · cc by · source: Europe PMC
A data-enhanced Hammett model maps SN2 barrier heights across substituent and leaving-group space using additive σ values that depend only on group identity and distance to the reacting carbon.
Key findings
Substituent effects are largely additive without resonance; SN2 σ values follow composition and distance to the reaction center (6 parameters vs 20 in a dummy encoding). Their robust fit beats a single-reference Hammett model (errors ≤3.8 vs up to 5.2 kcal mol−1).
Methodology
The authors compressed quantum SN2 potential-energy changes into reaction constants ρ and substituent constants σ, tested additivity versus resonance, compared Theil–Sen averaging with the classical one-reference Hammett fit, and benchmarked against kernel ridge regression.
Limitations
The linear model cannot capture strong resonance coupling of substituents, and barriers are electronic-structure estimates rather than experimental condensed-phase rates.
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