Dear all,
I was trying to calculate the Hammett constant of some special groups (no experiment data). while the reviewer told me it could be done by software. what he/she said was 'approach described by the authors looks very similar to what is done by ACD software Sigma module'. I obtain the sigma through it's correlation with charges. Is anyone aware of how the soft does it ? which one is more rational ?
what are the soft commonly known to be calculating sigma? In case of being criticized by the reviewer next time I submit it, I should be aware of the whole picture. Any hint ?
thanks~
PS before I calculated the Hammett constants through charge, I have optimized the structure by DFT.
Dear Rina,
Th following describes the use of the required software. For ,ore details use the following link:
http://www.acdlabs.com/products/percepta/predictors/pka/
ACD/pKa
Predict accurate acid/base dissociation constants from structure—the industry standard
The acid dissociation constant, Ka, is a measure of the tendency of a molecule or ion to keep a proton (H+) at its ionization center(s). It is related to the ionization ability of a chemical species and is a core property that defines chemical and biological behaviour. The pKa prediction model from ACD/Labs offers the following functionality:
Calculation of accurate acid and base pKa constants (pKa = -logKa) under standard conditions (25°C and zero ionic strength) in aqueous solutions for every ionizable group within organic structures
Confidence intervals for all predicted pKa values, indicating their accuracy
Available Algorithms
The pKa prediction model incorporates two different predictive approaches—Classic and GALAS
Classic
The ACD/pKa Classic calculation algorithm may be trained by adding experimental data and is widely accepted as the industry standard for pKa prediction.
The algorithm uses Hammet-type equations and electronic substituent constants (σ) to predict pKa values for ionizable groups. Effects considered by the software include tautomeric equilibria, covalent hydration, and resonance effects in α, β-unsaturated systems.
Hammet-Type Equations—every ionizable group is characterized by several Hammet-type equations that have been parameterized to cover the most popular ionizable functional groups.
Sigma Constants—the internal training set contains >3000 derived experimental electronic constants. When the required substituent constant is not available from the experimental database, one of four algorithms are used to describe electronic effect transmissions through the molecular system.
Other technical characteristics of this algorithm include:
GALAS
Estimation of ionization constants using the GALAS algorithm is a multi-step procedure involving estimation of pKamicroconstants for all possible ionization centers in the hypothetical state of an uncharged molecule ("fundamental microconstants"), with numerous corrections to these initial pKa values according to the chemical environment of the reaction center, and calculation of charge influences of ionized groups to neighbouring ionization centers. The Calculation routine utilizes a database of 4600 ionization centers, a set of ca. 500 various interaction constants and four interaction calculation methods for different types of interactions, producing a full range of microconstants from which pKa macroconstants are obtained. This allows for the simulation of a complete distribution plot of all protonation states of the molecule under different pH conditions.
Other technical characteristics of this algorithm include:
ACD/Labs Product Suites
The Percepta prediction modules are available as bundles to offer cost savings for multiple modules, and provide related modules as a package.
Request more information on ACD/Percepta Predictor Suites
Other Products
ACD/pKa Model predictions are available as a browser-based thin client application on ACD/Percepta Portal; a thick client product (ACD/Percepta for the Desktop); and a batch calculator (ACD/Percepta Batch).
Hoping this will be helpful,
Rafik
Hi Rina,
You can contact at the following for asking on prices:
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Rafik
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