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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.
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What classical linear free-energy relation did they generalize?
The Hammett equation separating substituent (σ) and reaction (ρ) effects.
Source: Data enhanced Hammett-equation: reaction barriers in chemical space · DOI · quote verified
To what reaction class was the enhanced model applied computationally?
An SN2 reaction space with multiple substituents.
Source: Data enhanced Hammett-equation: reaction barriers in chemical space · DOI · quote verified
Are substituent effects additive when resonance is absent?
Yes—largely additive as long as no resonance occurs.
Source: Data enhanced Hammett-equation: reaction barriers in chemical space · DOI · quote verified
How many distinct SN2 reactions (Nu/LG pairs) were included?
12.
Source: Data enhanced Hammett-equation: reaction barriers in chemical space · DOI · quote verified
Which groups occupied positions R1–R4?
–H, –NO2, –CN, –NH3, –CH3.
Source: Data enhanced Hammett-equation: reaction barriers in chemical space · DOI · quote verified
How many parameters describe σ with a distance-decay law versus dummy encoding?
6 versus 20.
Source: Data enhanced Hammett-equation: reaction barriers in chemical space · DOI · quote verified
What worst-case MAE improvement is quoted versus a brittle Hammett fit?
3.8 kcal mol−1 versus up to 5.2 kcal mol−1.
Source: Data enhanced Hammett-equation: reaction barriers in chemical space · DOI · quote verified
Which robust regressor replaced ordinary Hammett linear fits?
The Theil–Sen regressor, plus averaging over reference reactions.
Source: Data enhanced Hammett-equation: reaction barriers in chemical space · DOI · quote verified
What is the sign of ρ for all SN2 reactions here, and why is that chemical?
ρ > 0; the negatively charged TS benefits from electron-withdrawing groups.
Source: Data enhanced Hammett-equation: reaction barriers in chemical space · DOI · quote verified
What ML baseline was compared on the same SN2 energies?
Kernel ridge regression (KRR).
Source: Data enhanced Hammett-equation: reaction barriers in chemical space · DOI · quote verified
What core limitation remains?
A large share of the variance must still be linear trends; resonance needs extra pair σ values.
Source: Data enhanced Hammett-equation: reaction barriers in chemical space · DOI · quote verified