Hammett σ from composition predicts SN2 barriers
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.
Source
Data enhanced Hammett-equation: reaction barriers in chemical space
Inspect source passages
Hammett σ from composition predicts SN2 barriers
These passages occur verbatim in the stored paper text. They support the points listed below; they do not verify every statement in the explanation.
The enhanced model’s error is at most 3.8 kcal mol−1 versus up to 5.2 for the original approach.
This can give MAE of up to 5.2 kcal mol −1 , while our model has an error of 3.8 kcal mol −1 at most.
Twelve different SN2 reactions were considered.
In total, we consider 12 different S N 2 reactions, one for each combination of Leaving Group LG and nucleophile Nu, shown on the axis of and .
What they did
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.
What they found
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).
The limits
What it doesn't show
The linear model cannot capture strong resonance coupling of substituents, and barriers are electronic-structure estimates rather than experimental condensed-phase rates.
Key terms
- Hammett equation
- Linear free-energy relation log(K/K0)=ρσ separating reaction and substituent effects.
- σ
- Substituent constant encoding electron-donating or -withdrawing power.
- ρ
- Reaction constant measuring sensitivity of the reaction to substituents.
- Theil–Sen regressor
- Robust linear fit used to avoid overfitting a single reference reaction.
- KRR
- Kernel ridge regression ML baseline for the same SN2 energies.
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Quiz yourself
Hammett σ in this SN2 space depends mainly on:
Common questions
How many SN2 reactions were treated?
12 combinations of nucleophile and leaving group.
What substituents sat at R1–R4?
–H, –NO2, –CN, –NH3, –CH3.
How many parameters describe molecular σ with distance decay?
NG+1 = 6.
Worst-case MAE vs original Hammett?
3.8 vs up to 5.2 kcal mol−1.
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